Wind powered prop cart goes directly downwind 2.5x Wind Speed?

[recursive prophet]

A thread that began at JREF back on 11/9/09, Downwind faster than the wind (http://forums.randi.org/showthread.php?t=128483) may finally be approaching it's finale after hundreds of pages of debate.

When the discussion began on JREF, much of it centered on whether or not a small prop cart a member-spork-built that could advance up a treadmill was proof that under the right conditions it could go directly downwind faster than the wind. According to his Galilean transformations, all the vector forces were the same. So counter intuitive was this claim, that even one Nobel winning physicist, Dan Kammen, stated he believed it would violate conservation of energy.

As was announced in the original thread last October, spork found a sponsor-Joby Energy-and has now built and tested a drivable wind cart. Since then Google has also become a sponsor, and in late March spork took it to the North American Land Sailing Association (NALSA) meet at Ivanpah Lake in the California desert. They made 3 different runs, one of them self starting, which by their GPS calculations achieved 2.5x wind speed. NALSA official Bob Dill-former holder of the land yacht speed record-observed the test, and they have agreed to certify the cart's performance at their next meet coming later this spring.

Below are links to videos of the 3 trial runs of the sporkmobile. Notice the direction of the streamers attached to a movable bar, and how it out-distances some dust from the truck used to run along with the cart for tracking it's speed and making the videos.

First run: http://www.vimeo.com/10476453
Second run: http://www.vimeo.com/10477373
Last run: http://www.vimeo.com/10476216

For those wishing to learn more about the genesis of this ongoing debate, below are the 3 threads started on various tangents it of it posted here on JREF. Below them are the 7 parts posted on RDF. For a detailed description of the cart's construction including more videos and lot of pictures, there is a link to spork's blog in my sig, along with one to TalkRational where most of the current battle still rages. In the immortal words of Papa John, "What a long, strange trip it's been."

JREF Threads:
http://forums.randi.org/showthread.php?t=128483
http://forums.randi.org/showthread.php?t=131646
http://forums.randi.org/showthread.php?t=130705

RDF Threads:
http://forum.richarddawkins.net/viewtopic.php?t=73123
http://forum.richarddawkins.net/viewtopic.php?t=74154
http://forum.richarddawkins.net/viewtopic.php?t=74988
http://forum.richarddawkins.net/viewtopic.php?t=75643
http://forum.richarddawkins.net/viewtopic.php?t=76400
http://forum.richarddawkins.net/viewtopic.php?t=77451
http://forum.richarddawkins.net/viewtopic.php?t=80402

[Modified]

The tests were impressive, but even more impressive is the fact that humber is still at it. When it comes to misunderstanding basic physics, inability to learn, and complete confidence in one's understanding with no basis whatsoever, he is truly an unstoppable machine. I am in awe.

[macdoc]

I agree....

My sense is it's rather inefficient wind sailer looking at the blade shape and the tower shape.
The design allows it to effective sail close to the wind as the blades sort of act as continuous tacking energy pickup.
But it would be interesting to see without those vertical sails what the performance difference is.

any lateral gusts get picked up as additive vectors.and preserved by the mass of the machine...sort of the way albatrosses and others soar dynamically...

[casebro]

Next logical step ought to be an airplane with one prop to power another prop that pulls the plane through the air. Tow it to launch, like a glider, then no fuel for the rest of the flight... Instead of powering a wheel, power an extra prop. hmm., ought to be prototype-able in a land sailer.

Next logical step, merge the two props into one.... needs wing/sail design work...use an engine to get it flying, then shut down engine .

Ya know, this post might have just abrogated a jillion dollars worth of patents. I've made the ideas open source, protected only by my copyrights, if any. If any of this works, I may have just become the most philanthropic man ever. :)

[Folly]

Ha. Yes. I suppose awe isn't entirely inappropriate.

I know that everyone had their own favourite explanation, but I really truly felt that the little LEGO car was a 100% guaranteed argument stopper. It's soooo simple that it took me less than 5 minutes to build a replica after seeing it, and it's pretty hard to disbelieve after actually playing around with a real model (even if it's hand/gear powered rather than wind powered.) I still have the little LEGO car at work, and I show it to people who drop by my office :p

[macdoc]

Next logical step o
Sort of called a gyro-copter but TANSTAAFL eh

Some have tried dynamic soaring in a sailplane - not sure if anyone actually has succeeded .....been a while since I flew :(

[sol invictus]

A thread that began at JREF back on 11/9/09,

11/9/08. Don't sell it short!

The tests were impressive, but even more impressive is the fact that humber is still at it. When it comes to misunderstanding basic physics, inability to learn, and complete confidence in one's understanding with no basis whatsoever, he is truly an unstoppable machine. I am in awe.

Seconded.

Next logical step ought to be an airplane with one prop to power another prop that pulls the plane through the air.

You do understand why that doesn't work (but this cart does), right?

[macdoc]

(but this cart does), right?

explain away....:popcorn1

[sol invictus]

explain away....:popcorn1

Start with a windmill. Put it on wheels. Feed the power generated by the turbine to the wheels. Now you can drive in any direction, including upwind - which increases the speed with respect to both air and ground.

Start with the cart moving at windspeed. Feed the power generated by the wheels to the propeller, and use it to drive the cart in the direction that increases its speed with respect to both air and ground - which is downwind in this case.

If the first is possible the second obviously is too, the physics is exactly the same. The only hard part is engineering (air is softer than ground).

Something flying in air without contact with the ground cannot do this (unless there are layers in the atmosphere moving at different speeds).

[BenBurch]

...

Something flying in air without contact with the ground cannot do this (unless there are layers in the atmosphere moving at different speeds).

Drogue chute on a long carbon-fiber tether...

[technoextreme]

Someone PM'd me this the other day and I've been meaning to say this. I was wrong. I don't remember what the hell I said but I pretty sure I was wrong.

[macdoc]

which is downwind in this case.

:dl:

and just what happens to said turbine when ground speed = windspeed ?

TANSTAAFL

[Myriad]

and just what happens to said turbine when ground speed = windspeed ?


It (the prop motor) consumes power generated by the wheel turbines to propel the vehicle relative to the wind in some direction (including downwind).

Just as in the low-ground-speed case, the wheel motor consumes power generated by the prop turbine to propel the vehicle relative to the ground in some direction (including upwind).

TANSTAAFL


Indeed. If the wind dies down (relative to the ground), you're not going anywhere, in either case.

Respectfully,
Myriad

[sol invictus]

:dl:

and just what happens to said turbine when ground speed = windspeed ?

TANSTAAFL

What turbine?

The cart doesn't have a turbine, it has a propeller.

[Pantaz]

Don't know if it is applicable in this case, but is there anything like a free body diagram to help explain things? I don't know how to do it, myself.

[Brian-M]

The tests were impressive, but even more impressive is the fact that humber is still at it.

Oh god, he's still at it? :bwall He's not still trying to claim that balloons can't travel at wind speed because friction through the air slows them down, is he? :scared:

Next logical step ought to be an airplane with one prop to power another prop that pulls the plane through the air.

Won't work. The Down Wind Faster Than The Wind cart works by exploiting the difference in velocity of two mediums, in this case the ground and the wind. An airplane/glider only has wind-speed to work with.

ETA:

and just what happens to said turbine when ground speed = windspeed ?

Since the theoretical maximum speed of the cart in ideal conditions is a fixed multiple of windspeed (relative to the ground), when windspeed equals zero the theoretical maximum possible speed of the cart equals zero.

[macdoc]

What turbine?

The cart doesn't have a turbine, it has a propeller SIGH :boggled:

your're at THAT stage of misunderstanding??

turbine = propeller...see myriad:rolleyes:

••

a sailplane exploits rising air masses to climb - there is also theoretical dynamic soaring which is hard to sketch out - basically the attitude and ( ideally the wing configuration ) works in such a way as to gain altitude from swooping

( move downwind in a low friction dive, then loop very smoothly into the wind ( preferably a gust ) and trade speed for altitude in a high lift, low speed attitude, repeat.

See albatross et al..

any turbulent medium can find these pockets of energy gain....kayakers do it as well in a different manner.

BUT
TANSTAAFL

When down wind speed and land speed are identical....no energy...turbine/propeller/windmill stops...

Vector gains from angled surfaces do gain which is why ice boats for instance can gain incredible speeds...there MUST be a vector at play no matter how small the angle.

all else is woo..

••

BrianM - my question was rhetorical/sarcastic. ;)

[billw]

In the immortal words of Papa John Robert Hunter, "What a long, strange trip it's been."

Fixed that for you.

[recursive prophet]

Before I forget again, it appears the BBC is interested in doing a segment on the Sporkmobile,* and the Discovery Channel is sending out a crew to spork's shop this weekend. I have a strong feeling after the NALSA certification this fascinating saga will get a lot more press.

It began with a 16g model prop cart that could go up a treadmill, and for those new to this topic I would recommend watching this 7 minute YT video spork posted about 2 years ago. It does a good job of laying out the the history of DDWFTTW going back to Bauer in the sixties, and shows the cart advancing on the treadmill. http://www.youtube.com/watch?v=xHsXcHoJu-A

*This was the name used for the sporks model here, but at TR in was dubbed by a denier Harold Bricer (who is amazingly similar to Christoph from jref) as the Big Ugly F****** Cart, or BUFC. Now they have abandoned that handle; likely at the bequest of their sponsors.:D There was an article on their Ivanpah runs in Sail Magazine (http://sailmagazine.com/racing/running_faster_than_the_wind/) and the explanation for BUFC probably didn't amuse them. So spork and jb have dropped that name and haven't decided on new one yet.

Wanted to respond to some replies to several here, but something weird is up with the quote feature-I keep getting the same one I began with not the one I'm clicking. Anyway, humber truly is unique. As Sol pointed out over a year ago-and thanks for catching my typo on 09 dude-humber was wrong way to consistently. Even a broken watch is right twice a day after all. He was either a master troll, or spork! Remember this topic is on part 15 and has over 45 thousand replies there!! This on a site that sees about one sixth the traffic that JREF does.

I'm sure Sol recalls his theory that humber was actually spork. It certainly would provide a motive for his tenacity. The thread quickly died on RDF after humber was banned. Only the shadow knows...

Did I ever tell you about the connection between spork and George Burdell, sol? Spork got his BS at Georgia Tech. Do Google that name if you don't remember. I'll have more on this later. It makes for an interesting backdrop for your initial theory. It still seems pretty unlikely, but would be a classic MF if true. I love this whole circus, and only wish I could make up stuff like this. I'm basically just the thread text janitor, as I have little else to offer other than my appreciation of humber, spork, and virtually ALL the mad carters.

Thanks to those sharing your thoughts, and I will soon post a link to this thread on the one at TalkRational. It truly has become a recursive cyber-strand (http://www.urbandictionary.com/define.php?term=cyber-strand)

@billw-thanks for the correction. I just always think of walking in the coastal jungles of northern Colombia listening to Truckin on my headphones when I think of those words. :)

[ArmillarySphere]

TANSTAAFL

When down wind speed and land speed are identical....no energy...turbine/propeller/windmill stops...

Vector gains from angled surfaces do gain which is why ice boats for instance can gain incredible speeds...there MUST be a vector at play no matter how small the angle.

all else is woo..

Anddddddd.... here we go again :D


Sorry, you're flat wrong here. The propeller of the cart isn't powering the wheels - it's the other way around.

You're going downwind at windspeed, and your wheels are generating power to spin the propeller, increasing the cart's speed wrt the ground, and so increasing the available power. The wheels are breaking, the propeller is pulling. Generally speaking, the propeller will win.

Eventually, drag forces and wheel losses equal the propeller's forward force, and you reach equilibrium. i.e. top speed.

If you want to talk physics, any sail craft is extracting energy and momentum by slowing a mass of air wrt the ground. A propeller is much more effective at this than a sail, which is why a spinnaker approach loses out to the prop cart.

There are no wind direction changes, no turbulence vortices, nothing but an even, smooth wind to extract energy from.

[Myriad]

Vector gains from angled surfaces do gain which is why ice boats for instance can gain incredible speeds...there MUST be a vector at play no matter how small the angle.

all else is woo..


That's why propellers have angled surfaces.

To get from the ice boats to the downwind cart without woo, just follow these simple steps.

Step 1 is to acknowledge that ice boats can not only sail at speeds faster than the wind speed, they can sail steady-state (without exploiting gusts, momentum from turns, etc) such that the downwind component of their velocity is faster than the wind. This can be verified from ice-sailing sources.

Once that's understood, all that's needed is to transform an ice boat into the downwind "cart" in a series of steps, each of which preserves the essential mechanics of the scenario.

So, step 1 is an iceboat sailing at a downwind angle in a steady wind on an infinite ice surface. The directly downwind component of its velocity is faster than the wind; it's speed of course is faster still.

In step 2, wrap the ice surface into a tube (larger in radius than the height of the iceboat's mast), with the iceboat inside. The wind flows continuously down the tube; the boat on its slanted downwind course now sails a helix in the tube. (Ignore gravity; if you must, assume zero gravity and that the centrifugal force of the spiral course is sufficient to keep the boat's blades pressed against the ice enough for them to work as normal.)

In step 3, add a second ice boat. It is identical to the first, and it sailing the same way, but it is positioned 180 degrees around the tube from the first one. If either ice sailor looks straight "up" he's looking straight down on the other boat.

In step 4, lengthen the two boats' masts so that they connect in the center, rigidly attaching the two boats together.

In step 5, add a threaded rod running down the center axis of the ice tube. This rod passes through a hole in the two masts where they join in the center. The threads have the same pitch (number of turns per meter) as the helical path the ice boats are sailing along.

In step 6, add screw threads to the hole in the masts, that match the threaded rod. Since the hole threads and the rod threads have the same pitch as the path the the boats were traveling anyhow, this will not (aside from adding a bit of friction which we can ignore for the moment, for reasons explained in the next step) alter the courses or impede the movement of the boats.

In step 7, we remove the runners from the ice boats. They no longer contact the ice. However, the screw threads serve the same function that the runners used to, of keeping the boats' course on the most efficient angle. There is no longer any runner friction, but the friction of the central bearing replaces it. Since in principle either can be minimized, call it even.

In step 8, we remove the ice tube, since it no longer plays any part (except maybe to contain the wind in a zero gravity environment, so if it helps, assume the tube is replaced with a much larger tube, still containing the same wind velocity along it.)

In step 9, we introduce a long strip of flat pavement parallel to the tube. The pavement is far enough from the threaded rod that the two boats stay about half a meter away from it at their closest approach. At the same time, we also re-introduce a gravitational field that attracts the boats toward that pavement. Since the boats are still held "up" by the threaded rod, and they counter-balance one another, this does not affect their motion.

In step 10 we add a three-wheeled cart that is in contact with the pavement. We also add some struts that extend from the cart to a bearing around the central threaded rod. The bearing where the threaded rod goes through the boats' masts pushes against the cart's bearing, pushing the cart along at the same downwind speed as the boats.

In step 11, we remove the hulls of the boats, since the pilots can now ride in the cart instead.

In step 12, we remove the threaded rod. Instead, we add linkages and gears from the main axle of the cart to the bearing around which the boats' masts rotate, with the gearing arranged so that the angle that the boats' mast rotates for a given rotation angle of the cart's wheels is the same as it was before.

And we now have the downwind cart. Note that from the start to the finish, the fundamental device that makes it work remains the same: a surface moving laterally at an angle to the wind, whose downwind movement is constrained to maintain a certain ratio to its lateral movement. At the start the surface is an iceboat sail and at the end it's a propeller blade. At the start the mechanism that enforces the ratio of lateral to downwind movement is the iceboat's blades and at the end it's the gearing between the wheels and the propeller. But the basic dynamics stay the same.

If you still have difficult accepting this, please point out in which step I introduced the woo.

Respectfully,
Myriad

[macdoc]

:dl: :dl:

when ground = windspeed = zero energy to be harvested...

this is getting hilarious.

tell us oh wise one....does the cart move when there is no wind`?

[macdoc]

Myriad

You mean that's why wind turbines have angled surfaces....propellers have them to maximize thrust and reduce tip vortice losses - not vector gain.

Answer the question....will the cart move with no wind?

Does a ice boat go faster downwind or across the wind...?

then you will know where you introduced the woo.

[Kapyong]

Gday all,

Very interesting

But one thing strikes me as odd - perhaps I misunderstood, or something, but...

The wind is apparently driving the angled-blade of the propeller to turn it and thus drive the wheels, right?

But looking closely at the video #2 at e.g. 1:25, the angled-blade of the propeller appears to be turning the wrong way - as if it is driving against the wind - implying it is powered from the cart, not the wind.

Or have I completely missed something :-)
?

But then, in video #3, after they lose the chain apparently - it free-wheels the OTHER way - the way I would expect.


K.

[Christian Klippel]

Gday all,

Very interesting

But one thing strikes me as odd - perhaps I misunderstood, or something, but...

The wind is apparently driving the angled-blade of the propeller to turn it and thus drive the wheels, right?

But looking closely at the video #2 at e.g. 1:25, the angled-blade of the propeller appears to be turning the wrong way - as if it is driving against the wind - implying it is powered from the cart, not the wind.

Or have I completely missed something :-)
?


K.

Yes, you missed something. The wheels turn the prop. The bluff body area of the cart and prop is large enough so that the wind can initially push it forward. That causes the wheels to spin, which in turn spin the prop.

Greetings,

Chris

[Kapyong]

Ah, thanks....

Silly me.
And somehow I missed ArmillarySphere's clear explanation too :-(

K.

[Myriad]

Myriad

You mean that's why wind turbines have angled surfaces....propellers have them to maximize thrust and reduce tip vortice losses - not vector gain.


That would be my cue to answer with the laughing dog, except I don't do that.

A flat propeller produces no thrust.


Answer the question....will the cart move with no wind?


If by no wind you mean zero wind speed relative to the ground, no, it will not move. Who ever said it would?

If by no wind you mean zero wind speed relative to the cart, if the wind is moving relative to the ground the wheels are resting on, the cart will move. For instance, on a treadmill in still air, the cart will begin to move in the direction opposite the treadmill. There are many videos showing this with small carts.

Does a ice boat go faster downwind or across the wind...?


An ice boat goes fastest when angled downwind. As I said, in such configurations, high-performance ice boats go fast enough that the downwind component of their velocity is faster than the wind speed. Such an ice boat can get from a starting point to a point directly downwind faster than a balloon in the same wind.

I encourage you to verify this from ice boating sources.

Then you will know where you introduced the woo.


Which step, please.

Respectfully,
Myriad

[Christian Klippel]

:dl: :dl:

when ground = windspeed = zero energy to be harvested...


You are aware of the concept of frames of reference? You know that all motion is relative? You know how a lever works?

Do you think that if there is difference in speed between a surface and a medium, that as soon as an object reaches the speed of the medium everything comes to an halt, relative to the object?


this is getting hilarious.


This topic tends to, yes. But not for the reasons you might think.

tell us oh wise one....does the cart move when there is no wind`?

No one ever claimed that. The cart works because it has interfaces to two mediums (road and air) which are at different velocities.

Maybe it would help if you read a little about the topic.

Greetings,

Chris

Edit: For a simple equivalent of the situation check out this video:

k-trDF8Yldc

Tell us, how can it move faster than the thing driving it, if what you claim would be true? If you accept that video, then tell us why the situation in the wind should be different?

[casebro]

11/9/08.

You do understand why that doesn't work (but this cart does), right?

It's just drag on the prop vs drag on the 'wheel'. Substitute another prop for the 'wheel', add wings.

Wind direction to ground shouldn't matter. Once the cart reaches wind speed, it ought to be free to change direction. Might need to twist the 'prop tower' to do it, keep the prop axis paralell to the apparent wind direction. Haven't you ever been on a sail boat? As cross wind speed picks up, the apparent wind moves closer to the bow. Sails need to be adjusted accordingly, that is when the 'wing' function of the sail becomes most visible. A variable pitch prop makes an airplane more efficient, propably need one to make anything practical in this application.

[Kapyong]

macdoc -

I am pretty sure you are stuck in the very same misunderstanding I was.

(Because I used to insist that this was absolutely impossible, and I was 100% sure of it. I forced myself into being tentative in my post above, but I was thinking that I had caught them out with a hidden power source in the cart :-)

There IS a power source in the cart - the WHEELS.

The gearing is the OPPOSITE to what you think it is - step back, unclench, look and think - the 'aha' moment should come :-)

The WHEELS drive.
NOT the prop.


K.

[Brian-M]

But one thing strikes me as odd - perhaps I misunderstood, or something, but...

The wind is apparently driving the angled-blade of the propeller to turn it and thus drive the wheels, right?

But looking closely at the video #2 at e.g. 1:25, the angled-blade of the propeller appears to be turning the wrong way - as if it is driving against the wind - implying it is powered from the cart, not the wind.

Or have I completely missed something :-)


The propellers do turn against the wind (the opposite direction the wind is trying to turn them). They're pushing the wind backwards. (Well, I haven't watched that video, but I'm assuming it works the same as the models in the original thread.)

The wind does not turn the propeller. The wind pushes the cart forward, and the forward motion of the cart turns the propeller. The propeller pushes the wind backwards, which pushes the cart forwards even more, which makes the propeller turn even faster, which pushes the wind back harder, which pushes the cart forward even more.

ETA: Looks like I was a little late on this answer. But I got distracted by something else while writing my reply.

[Christian Klippel]

The wind does not turn the propeller. The wind pushes the cart forward, and the forward motion of the cart turns the propeller. The propeller pushes the wind backwards, which pushes the cart forwards even more, which makes the propeller turn even faster, which pushes the wind back harder, which pushes the cart forward even more.

Whereas it is important to clarify that the power taken up at the wheels is more than the power expended at the prop. This is due to prop efficiency, gearing losses, etc. For that reason it is required that there is a difference in velocity between the ground and the air. Otherwise it would run in still air, once pushed, which of course is impossible.

Greetings,

Chris

[Kapyong]

Gday,

The propellers do turn against the wind (the opposite direction the wind is trying to turn them). They're pushing the wind backwards. (Well, I haven't watched that video, but I'm assuming it works the same as the models in the original thread.)

The wind does not turn the propeller. The wind pushes the cart forward, and the forward motion of the cart turns the propeller. The propeller pushes the wind backwards, which pushes the cart forwards even more, which makes the propeller turn even faster, which pushes the wind back harder, which pushes the cart forward even more.

ETA: Looks like I was a little late on this answer. But I got distracted by something else while writing my reply.

Thanks Brian,
another clear explanation :-)

I really hope some others get to share my experience to day - that was a real blast. To be wrong, and then to clearly realize I was wrong, and exactly HOW and in such a clear and obvious fashion - that was a fantastic emotional ride - it's hardly EVER so clear-cut (right now I'm in morning caffeine mode.)

It's one thing to argue, and maybe be wrong about some abstract meanings in Paul, or the future of oil, or the origins of the Bible, or the politics of XXX.

But to be so clearly wrong, and to fail to understand so many times - that really really shook me. All those smart people so convinced there are right - nah, somehow they are ALL wrong.

Honestly - I was thinking I was the first one to catch them out faking a video - I was gunna be hailed as a clear-thinking hero, Brian and Hans and bluskool would be in awe of my insight, James Randi would congratulate me personally ... maybe there'd be cars and women and fame (obviously there is more than caffeine at work here - Hey - it's the weekend :-)


So,
to anyone who keeps coming back to this :

"once you reach the speed of the wind - that's it, there's no more energy or whatever"

Stop right there, and imagine the propeller going the OTHER way - just think that, and see if the 'aha' happens for you like it did for me.

(You're subtracting the wind speed, when you should be adding it.)

(back to Battleground europe now, I hear grenades nearby :-)


K.

[Brian-M]

I really hope some others get to share my experience to day - that was a real blast. To be wrong, and then to clearly realize I was wrong, and exactly HOW and in such a clear and obvious fashion - that was a fantastic emotional ride - it's hardly EVER so clear-cut (right now I'm in morning caffeine mode.)


I had the same experience in the original DDWFTTW thread. Spork's explanations based on ice-boats made no sense to me. It's not until I actually saw a video of a model cart running up a treadmill that I began to think about it in a different way, and realize that I was wrong.

[ArmillarySphere]

Glad to be of service :)

Some other clues:

If the prop was the power source, you'd expect top power when the cart was still (maximum wind wrt the cart) and zero power at windspeed. Just the opposite occurs, since it's the groundspeed wrt the cart that is of interest. At standstill, the prop-driving force is zero, since the wheels aren't turning. At windspeed, we have plenty of "engine" power, since while the wind is still wrt the cart, the wheels are spinning like mad and have plenty of power to turn the prop.

Which increases the turn rate, which increases the ground speed, which increases the wheel spin, which turns the prop faster...

Another simile:

Disconnect the wheels from the prop. Add a pair of pedals driving the prop, and Pepe the Mad Pedalier. You now have a man-powered prop cart.
Pepe will always crank the pedals at the same rate the wheels are turning. What will happen at windspeed? Why should Pepe care what speed the wind is going - he only cares about the wheels!
Obviously, with this setup, Pepe will be going to hit terminal velocity, horizontally.

Now add a handbrake, which Pepe will pull in rough proportion (~v^2 or thereabouts, I think) to how much he has to crank. Turns out this puts a limit to how fast he can go, when the breaking is equal to the prop's pull.

Replace Pepe with a differential connecting the wheels to the prop and there you go.

[Modified]

Oh god, he's still at it? :bwall He's not still trying to claim that balloons can't travel at wind speed because friction through the air slows them down, is he? :scared:

I didn't read enough of the threads to find out, but I think he's made that claim too often and too clearly to pull a "that's not what I really meant" (an "I was wrong" is, of course, completely out of the question).

[Modified]

I had the same experience in the original DDWFTTW thread. Spork's explanations based on ice-boats made no sense to me. It's not until I actually saw a video of a model cart running up a treadmill that I began to think about it in a different way, and realize that I was wrong.

I'd say that because I knew that sailboats can go upwind, I started off thinking this is possible and probably true. The wind and water are both streams; "upwind" equals "downstream faster than the stream"; if you can outrace one stream, you can probably do so for the other.

[Floyt]

So what's the theoretical limit on overall speed - is it bounded by friction? I guess you would achieve that with a hypothetical frictionless geartrain mechanism, since there always needs to be friction between wheels and ground to have something to push against.

ETA: 'nother question - could you achieve the same effect by deriving power for the propeller not from the wheels, but from a second passive (i.e. feeder) propeller of different efficiency to the first one?

[recursive prophet]

First, hi to Chris, Brian, and Modified. I remember each of you well from the original thread. Welcome to all others, some I recall posted on the earlier reincarnation but weren't among the regular punters. :D

Kudos on remembering that MichaelC spool video Chris. I should have included it in the OP. For a good chuckle, take a look at the comments under Michael's YT posts. He has developed a fan club of people in love with his voice and classic Oxford accent. They want him to keep talking no matter what the subject. Reading any book has been suggested-nay begged. It's a hoot. Michael is still very active on the TR thread, and I rib him about perhaps being able to convince humber with his melodious tones.

@Macdoc: Here's a link to a long comment today by Harold Bricer at TR: http://talkrational.org/showthread.php?p=894554#post894554 Along with humber he supports your over unity POV, so you will probably find this post of interest. ;)

Rats!! I forgot to mention we have a poll going on TR on what the NALSA test will reveal wrt percentage of windspeed. I'm one of the most conservative guessers, with a prediction of just 1.2 * ws. Spork's prediction was PI, and after Ivanpah he was far more confident it would be at least 3X ws. That seems unrealistically high to me, as even ice boats only exceed 3X ws by a bit going downwind, and then not directly but on a broad reach. I'd like to hear what others here at JREF think about this. And thanks to all for your input. :)

This just in: Here's a link to an interview spork did Thursday on The Bloomberg Report: http://www.facebook.com/video/video.php?v=382130502502 This one is about his day job, but I thought given he is in the center of the ddwfttw debunk the myth campaign some would find it of interest. I'm not actually a sports fan, but all here who are should give it a look.

[macdoc]

3X ws by a bit going downwind, and then not directly but on a broad reach.that would be correct

he supports your over unity POV, I'm not clear on why you think I support over unity....unless you are saying he supports my view of "over unity" being impossible without a vector.
Your post is not clear and I have no idea nor do I care who Humber is.

from your link
There is NO way around this and anyone who tells you differently is a perpetual motion, over unity crank! There is NO way for a cart to go directly down wind faster than the wind powered by the wind!this is correct
That said, birds and fish exploit turbulence all the time with micro-vectoring but they are so incredibly efficient at extracting energy they can do that...

•••

and to others....don't peddle pseudo Eureka moments just because you still don't get the very basic physical principals. :garfield:

•••

Floyt
So what's the theoretical limit on overall speed - is it bounded by friction? assuming you throw out the directly downwind absurdity you then get into sailing theory with length of the hull in water as a major factor in speed ( the longer the vectoring edge the more energy gain ) against drag and sail efficiency.

This has implications for the directly downwind nonsense and should be required reading.
http://web.mit.edu/2.972/www/reports/sail_boat/sail_boat.html

[Thabiguy]

Free advice, macdoc: don't word your posts in a way that will make it embarrassing for you when you find out you've been wrong.

[macdoc]

No interest in your advice.

Let us know when your Nobel arrives:rolleyes:

[Christian Klippel]

and to others....don't peddle pseudo Eureka moments just because you still don't get the very basic physical principals. :garfield:

So, again, care to explain why it should not work? Have you watched the ruler video? If there is a reason why you think that what works with the ruler should not work with the wind, you surely can explain that to us.

Since you never explained why, one can only assume that you are either unwilling or unable, or both, to explain it.

Here's another hint for you: What is the path the prop blades travel, compared to the COG of the vehicle? It is definitely not the same path.

Greetings,

Chris

[macdoc]

Zero wind ... Does it move..whatever Magical device. it is...no.

The rest is horsepucky.

There will always be vector.

[Christian Klippel]

Zero wind ... Does it move..whatever Magical device. it is...no.

The rest is horsepucky.

There will always be vector.

Care to show us where anyone said that the cart works without wind? You know, "wind" as we usually define it: motion of the air relative to the ground. If there is wind, and the cart reaches windspeed, do you think that the wind relative to the ground suddenly stops? Or is it more likely that at this moment, in the reference frame moving with the cart, the ground is moving?

That you simply deny things and are unable to explain that denial is quite telling. Or do you think that you are infallible, and that your knowledge is perfect, and thus you don't need to spend time thinking about stuff?

Greetings,

Chris

[sol invictus]

Zero wind ... Does it move..whatever Magical device. it is...no.

The rest is horsepucky.

There will always be vector.

macdoc, the cart does not work in zero wind. No one is saying it does. If it did, it would indeed be a perpetual motion machine. But it doesn't.

With that said, whether or not there is wind has nothing to do with the speed of the cart. Wind is a local difference in the velocity of the surface of the earth and the air above it. If that difference is there, it's there regardless of how fast or in what direction the cart is moving, and it can always be exploited.

If you will read my first response to you, you will see that the cart accelerating down wind from windspeed is precisely no more and no less strange or hard to understand than a windmill that uses the power it generates to move upwind. The two situations are exactly analogous. Do you agree a windmill on wheels can accelerate into the wind?

Your questions about how the cart can work at windspeed are as absurd as questioning how a windmill can work when it's fixed to the ground, moving at groundspeed.

And by the way, if you actually understood how dynamic soaring (the ability of birds, gliders, etc. to exceed windspeed) works, you would understand this cart. The principle is very similar.

[sol invictus]

SIGH :boggled:

your're at THAT stage of misunderstanding??

turbine = propeller...see myriad:rolleyes:

Dial down the arrogance; you'll look less like a fool when you admit you're wrong.


a sailplane exploits rising air masses to climb - there is also theoretical dynamic soaring which is hard to sketch out - basically the attitude and ( ideally the wing configuration ) works in such a way as to gain altitude from swooping

( move downwind in a low friction dive, then loop very smoothly into the wind ( preferably a gust ) and trade speed for altitude in a high lift, low speed attitude, repeat.

See albatross et al..

any turbulent medium can find these pockets of energy gain....kayakers do it as well in a different manner.

All of those examples work on the same principle. They all work because there are two layers of atmosphere moving at different velocities. By finding a way to slow one layer with respect to the other, you can extract energy that you can use for anything you like, including acceleration in any direction.


Vector gains from angled surfaces do gain which is why ice boats for instance can gain incredible speeds...there MUST be a vector at play no matter how small the angle.


There is a "vector". The propeller on the cart moves through the air in a way that's very similar to the sail on an iceboat as it tacks into its apparent headwind. In fact you can view its motion as one continuous tack, with the enormous advantage that it never has to turn through the wind.


all else is woo..


Sadly for you, the cart has been proven beyond a shadow of a doubt to work exactly as predicted by the basic physics we're trying to explain to you.

[sol invictus]

Next logical step ought to be an airplane with one prop to power another prop that pulls the plane through the air. Tow it to launch, like a glider, then no fuel for the rest of the flight... Instead of powering a wheel, power an extra prop.

It's just drag on the prop vs drag on the 'wheel'. Substitute another prop for the 'wheel', add wings.

The only circumstances under which that could possibly work are if there are two layers to the atmosphere moving at different speeds, and one prop is immersed in one, the other in the other. Otherwise it's impossible, and if you do the analysis you will always find that the net force is zero when the plane is at windspeed.

What allows sailboats, the cart, and anything else wind powered to work is the interface between air and ground, between two masses in relative motion. Planes don't have access to that (again, unless they can exploit an interface in the atmosphere).

[D'rok]

Zero wind ... Does it move..whatever Magical device. it is...no.

The rest is horsepucky.

There will always be vector.

Here's some horsepucky from Mark Drela, professor of fluid dynamics at MIT.

[Snixtor]

Have you watched the ruler video?
Isn't he just pushing down on one end of the ruler? So it's not the force pushing the ruler to the right that is moving the cart to the right, but the force pushing down on the left end of the ruler that is moving the cart to the right. In much the same way an angular force moves a sailboat faster than the wind when tacking. Except instead of a keel, you've got the ground.

Or have I missed something and that's entirely the point the ruler kart video is trying to prove? The version I found on YouTube seemed to be implying that the rulers movement to the right was what was actually moving the kart to the right.

[sol invictus]

Isn't he just pushing down on one end of the ruler? So it's not the force pushing the ruler to the right that is moving the cart to the right, but the force pushing down on the left end of the ruler that is moving the cart to the right. In much the same way an angular force moves a sailboat faster than the wind when tacking. Except instead of a keel, you've got the ground.

Or have I missed something and that's entirely the point the ruler kart video is trying to prove? The version I found on YouTube seemed to be implying that the rulers movement to the right was what was actually moving the kart to the right.

I assume you're talking about this (http://www.youtube.com/watch?v=k-trDF8Yldc) video?

If so, I can't be sure whether or not he pushed down on the end, but I can tell you that no such push is necessary to produce the effect the video shows. You can easily work out for yourself what speed the cart should move at - as a function of the radius of the spools, the wheel, and the velocity of the ruler - and it's faster than the ruler.

[Christian Klippel]

Isn't he just pushing down on one end of the ruler? So it's not the force pushing the ruler to the right that is moving the cart to the right, but the force pushing down on the left end of the ruler that is moving the cart to the right. In much the same way an angular force moves a sailboat faster than the wind when tacking. Except instead of a keel, you've got the ground.

Or have I missed something and that's entirely the point the ruler kart video is trying to prove? The version I found on YouTube seemed to be implying that the rulers movement to the right was what was actually moving the kart to the right.

No, he's not just pushing down one end of the ruler. It is really the movement of the ruler that makes the cart move. There is a second video that shows the same, and in addition he moves the paper (=ground) where the cart is on. The result is the same.

The point is that indeed it is possible that an object can go faster than the thing moving it. If the ruler can do it, through a proper interface (in this case the blue wheel), then there is no reason why it should not work with the wind (in which case the proper interface would be propeller).

Another example of a similar behavior is a spool of wire. Place it on the ground, so that the wire leaves the spool at the bottom. Now pull the wire at a constant rate, but do not jerk on it. The whole spool will move faster towards you than you are pulling the wire.

Greetings,

Chris

Edit: Since the experiment with the ruler is so simple, you can easily replicate it for yourself, if you don't trust the video. You will get the exact same results.

[sol invictus]

Anyone that's having trouble understanding this, try this series of videos:

g8bxXRQtcMY

Ufk6HVWdSzE

H_OKNr120t4

zPFzHoubQzg

za_rPKSwiyc

[Snixtor]

No, he's not just pushing down one end of the ruler. It is really the movement of the ruler that makes the cart move.
Just first impressions. Hence wording it as a question, not a statement. Pushing down on one end of the ruler would move the cart, and on first inspection that seemed likely.

Another example of a similar behavior is a spool of wire. Place it on the ground, so that the wire leaves the spool at the bottom.
This at first seemed counter-intuitive, but I found a video of the same chap demonstrating the cart starting with a spool and "wire". In many ways its a better place to start, since its the size difference between the spindle and outer radius of the spool that makes it all work. Something that wasn't clear when looking at the cart under the ruler. A few basic sketches and sums and it starts to make more sense.

Since the experiment with the ruler is so simple, you can easily replicate it for yourself
I will actually. But not for a matter of distrust, I trust the maths so trust the outcome, but because at midnight after dinner with wine, my visualisation skills are a bit wonky.

[RossFW]

Zero wind ... Does it move..whatever Magical device. it is...no.

The rest is horsepucky.

There will always be vector.

Zero wind but the surface moving backwards?: Yes it does (see the treadmill videos). It moves when there is a differential velocity between surface and air.

Are the tips of the prop travelling directly downwind, or is their velocity vector at an angle, vis a vee the sail on an ice-cart?

[Christian Klippel]

Just first impressions. Hence wording it as a question, not a statement. Pushing down on one end of the ruler would move the cart, and on first inspection that seemed likely.


Yes, i understood it as a question. Maybe my wording isn't the best, since english isn't my native language [lame excuse mode off] ;)


This at first seemed counter-intuitive, but I found a video of the same chap demonstrating the cart starting with a spool and "wire". In many ways its a better place to start, since its the size difference between the spindle and outer radius of the spool that makes it all work. Something that wasn't clear when looking at the cart under the ruler. A few basic sketches and sums and it starts to make more sense.


In fact, the whole downwind cart saga started as a brain-teaser. As such it is meant to be counter-intuitive. It just happens that the cart is somewhat on the extreme end of "counter-intuitiveness". The wire/spool has the drawback that some people will complain "but hey, the wire is permanently connected to the spool!" and thus don't recognize as a more simple, albeit still valid example of the principle.

There are other, similar brainteasers. For example, two boats are on a river on a calm day. That is, there is no wind relative to the ground. The river has a current, as usual. Both boats are at the same positions. Someone yells at them "Free beer two miles down the river!". One of the boatsmen hoists it's sail. Who will get to the free beer first?


I will actually. But not for a matter of distrust, I trust the maths so trust the outcome, but because at midnight after dinner with wine, my visualisation skills are a bit wonky.

Now that i can understand :D

Greetings,

Chris

[roger]

This is the series of thought experiments that made me realize how this works:

1) imagine an ice boat tacking downwind. It can go faster than the wind.
2) imagine an ice boat who's hull faces directly downwind, but the sail is on a long arm that allows it to tack back and forth. boat still goes faster than wind.
3) turn that sail into 1 propeller arm. Still goes faster than wind.
4) rotate that propeller arm, effectively tacking it. Still goes faster than wind.
5) add a couple more propeller arms on for good measure. Still goes faster than wind.

As others have already said, the wind is not pushing on the cart, but on the propeller tips (well, the whole propeller arm, but it's easiest to just visualize that tip as a little sail out there). The propeller tips describe a 3D spiral - they tack across the wind. Tacking allows your downwind vector component to be faster than the wind itself.

[sol invictus]

I will actually. But not for a matter of distrust, I trust the maths so trust the outcome, but because at midnight after dinner with wine, my visualisation skills are a bit wonky.

Take a wine glass and lay it down on the table. Put your finger under the stem. Push forward.

The glass rolls forward, downfinger faster than the finger.

[D'rok]

When down wind speed and land speed are identical....no energy...turbine/propeller/windmill stops. ;)
The propeller is geared to the wheels. The wheels, not the wind, turn the propeller. The only way for the propeller to stop is if the cart stops- i.e., if the wheels stop turning. Is that what happens at wind speed?

[RossFW]

When down wind speed and land speed are identical....no energy...turbine/propeller/windmill stops.

Try this-

Next time it's a windy day, sail a land yacht directly downwind (Not 100% windspeed, but pretty close).

Now sail into a brick wall- No energy available???

[rwguinn]

Zero wind ... Does it move..whatever Magical device. it is...no.

The rest is horsepucky.

There will always be vector.
I think what you're saying here is that the cart is going downwind, but the prop has to be on a "reach" of some sort?
I could understand that, but if everything is going downwind-no *********** way it will go faster than the wind...

[RossFW]

I think what you're saying here is that the cart is going downwind, but the prop has to be on a "reach" of some sort?
I could understand that, but if everything is going downwind-no *********** way it will go faster than the wind...

Already has, witnessed by many knowledgeable people. Almost three times windspeed before the mechanism broke.

They are heading back in a few weeks to set an official world record in the newly created Direct Down Wind catagory, officiated by the North American Land Sailing Association.

Track the path of the tips of the prop and yes, it is on a "Reach".

[sol invictus]

I think what you're saying here is that the cart is going downwind, but the prop has to be on a "reach" of some sort?
I could understand that, but if everything is going downwind-no *********** way it will go faster than the wind...

Everything isn't going directly downwind. The prop blades aren't. Neither are the wheels for that matter.

[ynot]

A propeller spinning in still air creates directional thrust.

When the cart reaches the speed of the wind and is effectively in still air, the propeller is (has to be) spinning creating thrust in the direction of the wind.

This thrust moves the cart faster than the wind.

[sol invictus]

A propeller spinning in still air creates directional thrust.

When the cart reaches the speed of the wind and is effectively in still air, the propeller is (has to be) spinning creating thrust in the direction of the wind.


True.


This thrust moves the cart faster than the wind.

Also true, but it's not quite as easy as that.

If the cart was moving on a day with no wind (airspeed=groundspeed), the prop would be spinning (although though not in still air) and generating thrust, but never enough thrust to compensate for the drag on the wheels it induces.

But on a day with wind, a cart geared in the appropriate way can produce more thrust than drag while going downwind at wind speed.

[ynot]

True.

Also true, but it's not quite as easy as that.

If the cart was moving on a day with no wind (airspeed=groundspeed), the prop would be spinning (although though not in still air) and generating thrust, but never enough thrust to compensate for the drag on the wheels it induces.

But on a day with wind, a cart geared in the appropriate way can produce more thrust than drag while going downwind at wind speed.
It is that easy. “This thrust moves the cart faster than the wind“ defines that there is a wind.

How could a cart that is powered solely by wind move on a day with no wind? The cart isn’t being powered by the thrust of the propeller (it‘s being “leveraged” by it).

I’m describing how it does work. Not how it doesn’t.

[Michael Mozina]

I assume you're talking about this (http://www.youtube.com/watch?v=k-trDF8Yldc) video?

If so, I can't be sure whether or not he pushed down on the end, but I can tell you that no such push is necessary to produce the effect the video shows. You can easily work out for yourself what speed the cart should move at - as a function of the radius of the spools, the wheel, and the velocity of the ruler - and it's faster than the ruler.

Only you would note the last part and compensate for the observer. :) Man you could clean up in the Nobel dept if I could just bring you over from the 'dark' side. :)

[Uncayimmy]

I'm a bit confused about the discussion at this point, macdoc. Are you saying it cannot work on general principles? Look at the cart and the ruler. Clearly it moves faster than the ruler. So, what if instead of the hand moving the rule we built a second cart with an ordinary sail and a ruler mounted on an arm. So long as we had enough ruler we could make that contraption go faster than the cart being pushed by the sail. It's just a matter of engineering at that point.

[macdoc]

How could a cart that is powered solely by wind move on a day with no wind? exactly, some sanity at least....:clap:

Many phenomena occur that seem to defy physical laws but when broken down do not...
There IS a vectoring component no matter how many twists and variations you want to stick in the mix.

If someone wants to claim a useful sailing device - fine..there are myriad odd approaches to extracting energy from mediums such as moving water and air.

The energy will always involved a gradient....no wind, no gradient, no motion.

There will always be a vector to allow the energy to accumulate.

THIS is what is being claimed

http://en.wikipedia.org/wiki/Laminar_flow

There could be any number of vectoring forces when there is no laminar flow (pure laminar is rare or even in actually impossible tho wing designers do their best ). Think about how harnessing wing tip vortices improve fuel mileage in a jumbo.

When there is laminar flow .....

An object moving through a gas or liquid experiences a force (http://en.wikipedia.org/wiki/Force) in direction opposite to its motion. Terminal velocity (http://en.wikipedia.org/wiki/Terminal_velocity) is achieved when the drag force is equal in magnitude but opposite in direction to the force propelling the object. Shown is a sphere (http://en.wikipedia.org/wiki/Sphere) in Stokes flow, at very low Reynolds number (http://en.wikipedia.org/wiki/Reynolds_number)Laminar flow, sometimes known as streamline (http://en.wikipedia.org/wiki/Streamlines,_streaklines_and_pathlines) flow, occurs when a fluid flows in parallel layers, with no disruption between the layers. In fluid dynamics (http://en.wikipedia.org/wiki/Fluid_dynamics), laminar flow is a flow regime characterized by high momentum diffusion (http://en.wikipedia.org/wiki/Momentum_diffusion) and low momentum convection (http://en.wikipedia.org/wiki/Convection). It is the opposite of turbulent flow (http://en.wikipedia.org/wiki/Turbulent_flow). In nonscientific terms laminar flow is "smooth," while turbulent flow is "rough."
When you have turbulent flow then micro-vectors could be harnessed ( and are in nature by albatrosses and others ) to accumulate energy and hence velocity/efficiency.

A sail plane does a slow motion version of this harnessing convective or land form turbulence in the atmosphere - it goes slow in lifting air with maximum extraction ( height gain ) by altering it's wing shape accumulating energy and fast in descending air to minimize loss.

Without the turbulence the sail plane simply follows it's designed glide path. In this case the goal is air duration rather than velocity.

Mountain out of mole-hill .... TANSTAAFL. :garfield:

[ynot]

exactly, some sanity at least....:clap:
But do you realise and agree that a cart moving relative the ground at the speed of a wind (with a wind), and a cart not moving relative to the ground on a day with no wind (air also not moving relative to the ground), are very different scenarios?

[sol invictus]

If someone wants to claim a useful sailing device - fine..there are myriad odd approaches to extracting energy from mediums such as moving water and air.

That's all this is.


When there is laminar flow .....

This cart would work perfectly in a purely laminar flow (of air over ground) with zero turbulence.

When you have turbulent flow then micro-vectors could be harnessed ( and are in nature by albatrosses and others ) to accumulate energy and hence velocity/efficiency.

A sail plane does a slow motion version of this harnessing convective or land form turbulence in the atmosphere - it goes slow in lifting air with maximum extraction ( height gain ) by altering it's wing shape accumulating energy and fast in descending air to minimize loss.

Without the turbulence the sail plane simply follows it's designed glide path. In this case the goal is air duration rather than velocity.

Mountain out of mole-hill .... TANSTAAFL. :garfield:

Again, the operation of this cart has nothing to do with turbulence. It works just like a windmill or sailboat. Do you think those require turbulence to function?

[Earthborn]

Anyone that's having trouble understanding this, try this series of videos:I thought I finally got it after watching the ruler videos many moons ago, especially the second one in which looking at it from a different frame of reference made the cart's behaviour suddenly look like the only behaviour that would make sense. But I have no clue what the videos you link to represent, without a narrator explaining what is one is supposed to see.

I also note a distinct lack of cute animals, which is the death knell for any YouTube video.

[sol invictus]

The cart is essentially a lever. Think of the propeller as the end of the lever that's pushing on the air and propelling the cart forward, while the wheels provide the force on the other end.

Those videos show a slightly simplified and abstracted version of that. They're supposed to make clear how one could, at least in principle, leverage the relative flow of two fluids or surfaces to move faster than either one.

But I agree with you that they suffer from a shortage of cute animals.

[Snixtor]

Take a wine glass and lay it down on the table. Put your finger under the stem. Push forward.
Not feasible, the glass would need to be empty. Tried it with a ribbon reel though.

Moving back to the propeller-cart... Talking about *directly* down-wind. The propeller spins due to wind travelling parallel to the carts direction of travel. Once the cart is travelling at equal to wind speed, isn't the propeller effectively in slack air? The tailwind and headwind speeds are equal, relative to the cart there is no air movement, thus no force acting on the propeller. macdoc, is this the scenario you're trying to relate to people with "no wind"?

If the cart were to be travelling faster than the wind, completely parallel to the direction of the wind, wouldn't it have a headwind stronger than its tailwind? Meaning the headwind would work to slow the rotation of the propeller. Otherwise, wouldn't we be talking about pushing a propeller moving forward in slack air, and having it turn the wrong way?

sol_invictus, the 4th video you linked piqued my interest, is this showing airflow with a downward direction? ie. The wind is blowing from left to right, and top to bottom?

[Modified]

Moving back to the propeller-cart... Talking about *directly* down-wind. The propeller spins due to wind travelling parallel to the carts direction of travel. Once the cart is travelling at equal to wind speed, isn't the propeller effectively in slack air? The tailwind and headwind speeds are equal, relative to the cart there is no air movement, thus no force acting on the propeller. macdoc, is this the scenario you're trying to relate to people with "no wind"?

If the cart were to be travelling faster than the wind, completely parallel to the direction of the wind, wouldn't it have a headwind stronger than its tailwind? Meaning the headwind would work to slow the rotation of the propeller. Otherwise, wouldn't we be talking about pushing a propeller moving forward in slack air, and having it turn the wrong way?

There is still a "tailwind" with respect to the effective surface of the prop. Imagine that the prop and all parts of the cart were very rigid. You begin pushing straight forward with your finger from behind, near the front edge of one prop blade. The gearing to the wheels will cause the blade to turn and slide against your finger until it slides off the back end of that blade, so for that short time, the cart will move forward faster than your finger. To make it more clear, you could replace the prop with a large auger. If you began pushing from behind near the front of the auger, it would turn against your finger until the cart had outdistanced you by the length of the auger.

[Brian-M]

I suppose now would be a good time to throw in a crude GIF-animation I made for the original DDWFTTW thread. (Just substitute "ruler" for the overhead conveyor belt. Same thing.)

http://forums.randi.org/imagehosting/265734bca6603a5ed4.gif

[Brian-M]

The propeller spins due to wind travelling parallel to the carts direction of travel.
No. The wind does not turn the propeller. The wind pushes the cart along, the same way the wind can push an empty tin-can along a road. It's the turning of the wheels that makes the propeller spin, and it spins in the opposite direction to the way the tailwind is trying to push it.

Once the cart is travelling at equal to wind speed, isn't the propeller effectively in slack air? The tailwind and headwind speeds are equal, relative to the cart there is no air movement, thus no force acting on the propeller.
Pretty much. But the propeller is spun by the wheels, not the wind, so the propeller is still spinning when it's traveling at wind speed. And the spinning of the propeller pushing air backward causes the cart to travel even faster.

If the cart were to be travelling faster than the wind, completely parallel to the direction of the wind, wouldn't it have a headwind stronger than its tailwind? Meaning the headwind would work to slow the rotation of the propeller.

You don't have both headwind and tailwind at the same time. There's headwind, no tailwind, when traveling faster than the wind. The headwind is trying to turn the propeller in the same direction it's already turning, but at a slower rate, so yes, it does try and slow down the motion of the propeller. (If it sped the propeller up, the cart would become a perpetual motion machine. :) )

ETA: It basically works in exactly the same way as my little animated cart in the previous post, but with a propeller pushing backwards on the air instead of a wheel pushing backwards on an overhead conveyor belt (or ruler).

[Snixtor]

The wind does not turn the propeller. The wind pushes the cart along, the same way the wind can push an empty tin-can along a road. It's the turning of the wheels that makes the propeller spin, and it spins in the opposite direction to the way the tailwind is trying to push it.
Ah so the propeller is genuinely acting as a propeller, not a turbine (the two terms seem to be getting thrown back and forth a bit, and even interchanged). It's the wheels that are effectively the "turbine", their rotation generating the power to turn the propeller.

Pretty much. But the propeller is spun by the wheels, not the wind, so the propeller is still spinning when it's traveling at wind speed. And the spinning of the propeller pushing air backward causes the cart to travel even faster.
Have I paraphrased this correctly above?

You don't have both headwind and tailwind at the same time. There's headwind, no tailwind, when traveling faster than the wind.
I was sort of, mixing my frames of reference there. But basically, when the cart is travelling at a ground speed equal to the winds speed relative to the ground, the cart has an effective air speed of zero. So I think we're on the same page here. Certainly a "wait a minute" moment if you're considering the propeller as a wind turbine, totally different if its a propeller used for propulsion.

The headwind is trying to turn the propeller in the same direction it's already turning, but at a slower rate, so yes, it does try and slow down the motion of the propeller. (If it sped the propeller up, the cart would become a perpetual motion machine. :) )
Definitely part of the "wait a minute".

The point I come back to though is:
The wind pushes the cart along, the same way the wind can push an empty tin-can along a road.
If that's the case, then that "push" no longer exists once the vehicle is travelling at the speed of the wind relative to the ground does it? And at faster than wind speed, it has a headwind that is pushing in the opposite direction.

Modified - Is your interpretation different to Brian-M's? Are you viewing the propeller as a propeller or turbine?

Are the basic facts of this machine actually known? Like, which way the propeller is spinning, or is this a case of a machine being demonstrated and everybody speculating as to how it works?

[RossFW]

Are the basic facts of this machine actually known? Like, which way the propeller is spinning, or is this a case of a machine being demonstrated and everybody speculating as to how it works?

http://www.fasterthanthewind.org/

[Snixtor]

http://www.fasterthanthewind.org/
Is there a particular post that offers explanation? Most of them are "attached part A to part B, here's a photo, doesn't it look great?".

[Brian-M]

Ah so the propeller is genuinely acting as a propeller, not a turbine (the two terms seem to be getting thrown back and forth a bit, and even interchanged). It's the wheels that are effectively the "turbine", their rotation generating the power to turn the propeller.


Have I paraphrased this correctly above?

Yes.

If that's the case, then that "push" no longer exists once the vehicle is travelling at the speed of the wind relative to the ground does it? And at faster than wind speed, it has a headwind that is pushing in the opposite direction.

Yes, but at that point the propeller is spinning fast enough to propel the cart by pushing air back against the wind (from the ground's frame of reference), or in the same direction as the headwind (from the cart's frame of reference).

The wind is traveling at X mph (relative to the ground), attempting to keep the cart at x, pushing it forward (tailwind) when it slows down, and pushing it backward (headwind) when it speeds up. But because the wheels are connected to the propeller, the propeller is pushing air backwards at y mph, allowing the cart to maintain forward motion against a headwind.

Are the basic facts of this machine actually known? Like, which way the propeller is spinning, or is this a case of a machine being demonstrated and everybody speculating as to how it works?


The basic facts are known, and there's nothing secret or unknown about how it works.

Here's a link to Spork's 2008 design for a small working model (I think it's the same one he demonstrates running the "wrong way" along treadmills in many of the YouTube videos), along with a parts list and construction instructions so you can make your own...

http://www.rtfa.net.nyud.net/wp-content/uploads/2008/12/dwfttw_build_plans_147.pdf

I got the link from this site (http://boingboing.net/2008/12/12/downwind-faster-than-1.html), where Spork and others are discussing the cart.

ETA:
Is there a particular post that offers explanation? Most of them are "attached part A to part B, here's a photo, doesn't it look great?".

It works exactly the same way my little animated cart would work in real life, except with a propeller connecting to the air instead of a wheel connecting to a belt/ruler.

If you can understand the principles of how that works, the propeller cart is the same. It's just a matter of gearing/leverage.

ETAA: If the belt is moving to the right, pushing the top wheel clockwise, it won't turn clockwise. Instead, the whole cart moves to the right fast enough that the top wheel turns counter-clockwise. The velocity of the cart equals the velocity of the belt plus the counter-clockwise motion of the wheel.

(There are videos where you can see carts built on the same principle in action.)

[Snixtor]

I can see how the treadmill cart with the propeller works. Power is applied to the wheels through the treadmill, which is converted to wind power by the propeller. The treadmill moves at say, 10mph, and the gearing forces air through the propeller at say, 12mph. Pardon my poor terminology, I know "speed" is probably not the right measure, it's acceleration, force, etc. But I'm simplifying.

For frames of reference, the ground is moving from right to left, the air is moving right to left, and the car is moving left to right. The air at one speed, the ground at another.

The device converts wheel power to wind power. The alternative use of it would be converting wind power to wheel power, but as configured for the treadmill experiment, this would need a headwind to generate wheel power. The wind moves right to left, the ground moves right to left at a slower speed.

I can see what you're suggesting by the wind "pressing" against the cart to push it forward, thus rotating its wheels, which in turn rotate the propeller for thrust. But I don't view this as being the same thing as demonstrated on the treadmill. The missing element in the treadmill demonstration is the "tailwind". What would happen if we generated this by placing a fan behind the treadmill and cart? Wouldn't this actually slow the cart down by countering the thrust generated by the propeller?

[recursive prophet]

@Snixtor: The answer is no, it doesn't slow it down but speeds it up. Imagine you are on a long treadmill going 10mph. You will feel a steady apparent wind of that speed as you move along, just as if you were sitting still in a steady 10mph wind.

If you or anyone else wants to look through all the many posts of tests spork has conducted on ddwfttw, just type in spork33 on YouTube search. The one where he blows on it with a fan is called "The Harold Bricer Test," as Harold also believed the fan would slow the cart down. It didn't.

Oh god, he's still at it? :bwall He's not still trying to claim that balloons can't travel at wind speed because friction through the air slows them down, is he? :scared:

Indeed he is Brian. Interesting you should ask, as a few days ago at TR I referred to your explanation well over a year ago regarding the molecular pocket around the balloon still being pushed en mass by the wind until it achieved wind speed. He has so many suitors on TR I think he missed my question, so I will soon repost it during one of the threads rare lulls.

My primary interest in this cyber-strand (http://www.urbandictionary.com/define.php?term=cyber-strand) is from an anthropological perspective. I can tell you based on my observations the only threads that achieve real traction are ones where there are opposing positions. I have a feeling in years to come cyber-cultural anthropologists will hold entire conferences based on the humber phenomenon. One way or another, I think it will be acknowledged he had a rare genius in his ability to keep so many bright people who claimed they considered him an idiot coming back for more for a year and a half long period-and counting. You have to see it to believe it. :boggled:

As humber didn't rejoin the discussion till over 3 months after it started at TR, it took part 1 114 days to reach the 2.5k cut off point. Since then, with humber in tow, the thread has reached that limit on average every 16 days. It's now on part 15, half way to 16.

Personally I find humber's arguments quite intriguing and instructive. They play off all the ways intuition and over-thinking can lead one astray on this subject, while generating a lot of interest and drawing in some very knowledgeable people. All think they can show humber and Harold where they are mistaken. None have succeeded after thousands of pages. :D

I'm not clear on why you think I support over unity....unless you are saying he supports my view of "over unity" being impossible without a vector.

Exactly. :) Sorry my presentation was unclear. :(

@Sol Invictus: You never answered my question about your theory spork and humber were the same person, or whether you remembered the story of George P. Burdell ("http://en.wikipedia.org/wiki/George_P._Burdell). Also, you and others haven't told me if you think 3x ws and beyond is probable?

[383LQ4SS]

I never followed the original threads since I am a latecomer to JREF...but I read this one and many of the attached links...as well as viewed the videos. Simply awesome! Well done!

Thanks also to those who gave concise yet simple explanations. The main point to make it click for me was that the wheels drove the propeller and not vice versa.

The machine in the vids did seem to have some trouble getting moving. More surface area could be added to the frame in the form of a traditional sail to get the initial push...but of course that would also hinder top speed once the cart was exceeding the wind speed. Of course you could make that sail retractable. Hoist the traditional sail for a fast take off from a stop....lower it for max speed when traveling faster than the wind.

[macdoc]

My primary interest in this cyber-strand (http://www.urbandictionary.com/define.php?term=cyber-strand) is from an anthropological perspective. I can tell you based on my observations the only threads that achieve real traction are ones where there are opposing positions.

ain't that the truth :boggled:..you should have to wade through the AGW denier swill....:rolleyes:

[Snixtor]

Imagine you are on a long treadmill going 10mph. You will feel a steady apparent wind of that speed as you move along, just as if you were sitting still in a steady 10mph wind.
Could you rephrase or elaborate on that? It sounds like you're telling me that if I'm running on a treadmill at 10mph I will feel a 10mph headwind.


If you or anyone else wants to look through all the many posts of tests spork has conducted on ddwfttw, just type in spork33 on YouTube search. The one where he blows on it with a fan is called "The Harold Bricer Test," as Harold also believed the fan would slow the cart down. It didn't.
I've found a youtube video from spork33 that depicts a "Harold Bricer Test", but it doesn't show what I'm describing. In this they use a fan (with propulsion well in excess of the small propeller on the vehicle) to provide a headwind. I'm talking about putting a fan behind the vehicle, and not one that's so overpowered that it just blows the vehicle in whatever direction its blowing.

[Uncayimmy]

Could you rephrase or elaborate on that? It sounds like you're telling me that if I'm running on a treadmill at 10mph I will feel a 10mph headwind.

I feel comfortable enough with this whole discussion to say that RP means that if you stood "still" on the treadmill and let it carry you, you'd feel a 10 mph headwind. There were some that argued that a moving platform and still aire were not equivalent to a stationary platform and moving air at the same speed.

[Snixtor]

I feel comfortable enough with this whole discussion to say that RP means that if you stood "still" on the treadmill and let it carry you, you'd feel a 10 mph headwind. There were some that argued that a moving platform and still aire were not equivalent to a stationary platform and moving air at the same speed.
Mm, yeah, so running on a treadmill is equivalent to having a constant tailwind. If you start to move forward on the treadmill, relative to the ground, you start moving faster than this "tailwind", and experience a headwind.

[383LQ4SS]

I wonder if the prop is variable pitch. I would think it needs to be to reach peak performance.

ETA: actually...a variable pitch prop would do alot for this contraption. With the blades set flat...it would accelerate from a stop much easier and faster...but would do no better than a regular flat sail in terms of top speed in relation to the wind. Once moving....slowly increasing pitch will enable it to move faster and faster beyond the surrounding wind speed until too much pitch is added where it would begin to slow again. I am sure with any prop...there will be a "sweet spot" of blade angle for max speed. There should also be one for max acceleration...although that would likely be different at different speeds.

[recursive prophet]

Could you rephrase or elaborate on that? It sounds like you're telling me that if I'm running on a treadmill at 10mph I will feel a 10mph headwind.
No, just standing on the treadmill, facing in the opposite direction as the cart does. Then the wind becomes a tailwind. That's what keeps the cart moving up the belt. It wouldn't in a vacuum. Same on the big cart. The prop is blowing into the tailwind, and the wheels turn the prop moving it beyond the wind's speed. Don't confuse force and velocity here; F=ma.

I've found a youtube video from spork33 that depicts a "Harold Bricer Test", but it doesn't show what I'm describing. In this they use a fan (with propulsion well in excess of the small propeller on the vehicle) to provide a headwind. I'm talking about putting a fan behind the vehicle, and not one that's so overpowered that it just blows the vehicle in whatever direction its blowing.

:o DOH. Sorry. I conflated it with a test they did to show Harold the cart wasn't really functioning as a turbine and blew into it from the front. But there is a test video on spork33-humber suggested I think-where they blew on the prop from behind and it doesn't really slow it down. Both humber and Harold have also argued it is the ke created from holding the cart against the belt that allows it to advance for a brief period.

A user from here-y'not-made a circular treadmill to show this wasn't the case. It is in the 3rd link for JREF posts in the OP. http://forums.randi.org/showthread.php?t=130705 On this one humber maintained the arm holding the cart on the mill acted as a tether so there was still a transfer from the motor powering the TM. If humber doesn't get into that there, he likely did in the second link which is still on moderated status after the first reincarnation here. As always, humber managed to really draw a crowd, which soon became quite irate. http://rationalia.com/forum/images/smilies/angrymob.gif

JMHO, but I think humber has an incredible sense of humor. Hardly a day goes by he doesn't provide a howler. If this thread survives I'll post links from TR to some of the pictures he's photo-shopped. Several show spork in clown outfits working on the big cart and all kinds of other hilarious pictures. I don't see how he keeps coming up with his new material, and he averages like 50 posts a day. Meanwhile spork has become a master of ironic sarcasm and also regularly cracks me up. I truly meant what I had in my earlier sig here-Best Show in the Metaverse. http://i5.photobucket.com/albums/y171/charlou/cheesygrin-1.gif

AND....

http://forums.randi.org/imagehosting/68904b3f689adcc60.jpg

[Brian-M]

I can see how the treadmill cart with the propeller works. Power is applied to the wheels through the treadmill, which is converted to wind power by the propeller. The treadmill moves at say, 10mph, and the gearing forces air through the propeller at say, 12mph.

Yes. That's exactly how the cart works, on the treadmill or off it.

For frames of reference, the ground is moving from right to left, the air is moving right to left, and the car is moving left to right. The air at one speed, the ground at another.

I think you might be mixing up frames of references here (unless by "the air is moving right to left" you're referring to the air coming from the propeller, in which case you're right, but I'll throw in the explanation anyway).

Take a huge treadmill in a room with no air movement.

For someone hovering over the treadmill in the balloon, the air is still and the "ground" (treadmill belt) is moving from right to left.

But for someone standing on the treadmill, the "ground" (treadmill belt), the "ground" (treadmill belt) is still and the air is moving from left to right.

It would be equally correct to say that the "ground" is moving and the air is still as it would be to say the "ground" is still and the air is moving, depending on whether you're looking at the system using the "ground" or the air as your frame of reference.

If you step outside and stand on an actual road with the wind blowing, the same thing applies. It would be equally correct to say the ground is moving and the air is still as it would be to say the ground is still and the air is moving, depending on whether you're looking at the system using the ground or the air as your frame of reference.

As far as the laws of physics are concerned, all frames of references are equally valid. Just think of the earth itself as a giant treadmill.

(Of course, the fact that the earth is curved and the surface is moving in a circle changes things a little, but not enough to have any effect on the scale we're talking about.)

What would happen if we generated this by placing a fan behind the treadmill and cart? Wouldn't this actually slow the cart down by countering the thrust generated by the propeller?

Since the propeller is pushing back against the tailwind in order to push itself forwards (every action has an equal and opposite reaction), the more wind behind the cart the faster it goes. The cart is powered by the difference in the velocities of the air and the "ground". You're just giving it more power.

Putting the cart on a treadmill in a room with still air is exactly the same as putting it in a wind tunnel. Putting an object on a treadmill moving 10km/h right to left is exactly the same as putting that object in a wind tunnel with the wind moving 10km/h left to right.

Putting a fan behind the cart on the treadmill is the same as turning up the speed of the treadmill.

[Brian-M]

Indeed he is Brian. Interesting you should ask, as a few days ago at TR I referred to your explanation well over a year ago regarding the molecular pocket around the balloon still being pushed en mass by the wind until it achieved wind speed.

*sigh* It strikes me as bizarre that he can't grasp such a simple concept. I'm almost tempted to think that he actually does understand, but refuses to admit it out of pure stubbornness.

(I can't remember how the subject came up. Was it about whether or not any cart could, in theory on a frictionless surface, achieve wind-speed? Or was it about using bubbles as a measure of windspeed, and that the cart could be demonstrated as traveling faster than the wind by out-racing them?)

One way or another, I think it will be acknowledged he had a rare genius in his ability to keep so many bright people who claimed they considered him an idiot coming back for more for a year and a half long period-and counting. You have to see it to believe it. :boggled:

I believe it. I spent far too long arguing with him as it is. What finally prompted me to stop was when the internet filter at work started blocking JREF, and as I didn't have a connection at home back then I had to get on through an over-priced internet cafe. I realized I was paying a lot of good money to argue with him, and that really brought home how absurd it was spending so much time arguing with him. Like a very bad version of a Monty Python sketch...

kQFKtI6gn9Y

The "getting hit on the head lessons" would have been less painful.

[recursive prophet]

Sure wish I'd known you were going to post such a clear reply to Snixtor's questions, Brian. As you can see I really suck at explaining this stuff. Anyway, I'll leave you in Brian's capable hands Snixtor. I will tell you you're not alone in having trouble seeing the total equivalence between the treadmill and a cart in the wind. I don't believe my_wan, who started this discussion here-first link in OP-ever fully accepted that, and he's pretty sharp. Humber used to just say sporks cart had a wind allergy. :D

Heh, yeah humber really has a gift for frustrating people. The trick to appreciating him is suspension of disbelief. It's a lot like enjoying sci-fi. I'm happy to consider that despite all the evidence humber might be right. Hey, as far as I know only one Nobel physicist has weighed in on this-Dan Kammen. And HE said in an email to one of sporks fans that it would violate conservation of energy; same as humber and Christoph. Yes, I realize he's a busy guy and probably gave it about 2 minutes thought, is all. Still... See what I mean? Let others do the teaching and enjoy watching them get frustrated. spork mostly does that any more. He even makes funny sarcastic remarks like..."Ya know-I have good feeling about this. I think you've almost got humber convinced." :rofl:

[Uncayimmy]

Mm, yeah, so running on a treadmill is equivalent to having a constant tailwind. If you start to move forward on the treadmill, relative to the ground, you start moving faster than this "tailwind", and experience a headwind.

There is no tailwind. Think of a people mover in an airport. The track is moving 5 mph relative to the air. When you stand on it, you start moving 5 mph relative to the air. Likewise, one could do that math and say that you and the track were stationary and the air was moving. Same results in the calculations.

If you turn around and start walking at 5 mph in the opposite direction of the track, you are stationary relative to the air. There is no headwind or tailwind.

[Snixtor]

equivalence between the treadmill and a cart in the wind
Which leads well to my next question...

On the treadmill, the power is being provided by the treadmill, through the wheels, to the propeller, pushing against the air. To swap the reference frame, to say that the air is moving and the ground is "still" would mean that the air pushes against the propeller which turns the wheels. In this case, it's a headwind that drives the carts wheels.

In the treadmill experiment, what is the tailwinds effect on the cart? My thinking is that it's reducing the drag, but it's not providing any thrust. The volume of air behind the treadmill isn't aiding the cart in moving forward, it's just providing some resistance from it moving backwards.

[Snixtor]

There is no tailwind. Think of a people mover in an airport. The track is moving 5 mph relative to the air. When you stand on it, you start moving 5 mph relative to the air. Likewise, one could do that math and say that you and the track were stationary and the air was moving. Same results in the calculations.

If you turn around and start walking at 5 mph in the opposite direction of the track, you are stationary relative to the air. There is no headwind or tailwind.
I understand what you're saying, but I think you've misinterpreted what I've said. Running on a treadmill going 10mph in the gym you have no airflow relative to your body, but a 10mph relative to the ground. It's the equivalent of running outside with a 10mph "tailwind". If you're on the gym treadmill and run off the front of it (oops), you outrun this effective tailwind and experience a headwind. I'm sure we're on the same page there, just mixing terms and examples.

[Christian Klippel]

Which leads well to my next question...

On the treadmill, the power is being provided by the treadmill, through the wheels, to the propeller, pushing against the air. To swap the reference frame, to say that the air is moving and the ground is "still" would mean that the air pushes against the propeller which turns the wheels. In this case, it's a headwind that drives the carts wheels.


The cart does not care where the power comes from. It interfaces the belt and the air, nothing else. It works on the difference of velocity between belt(ground) and air. On the treadmill, the treadmills motor provides the difference. Outside, the big fusion reactor in the sky (aka. the sun) provides it. If the cart would be stationary on the belt (that is, the belt dragging it through the air), it would experience a tailwind, not a headwind.


In the treadmill experiment, what is the tailwinds effect on the cart? My thinking is that it's reducing the drag, but it's not providing any thrust. The volume of air behind the treadmill isn't aiding the cart in moving forward, it's just providing some resistance from it moving backwards.

Note that the treadmill experiments show the situation where the cart is brought up to windspeed already, and then released. It then accelerates further. The reason for that is that a treadmill is simply too short to allow for a self-start scenario. The equivalent outside would be a car or truck pushing the vehicle to windspeed, on the treadmill it is a hand holding the cart until the wheels/prop are up to speed.

Greetings,

Chris

[D'rok]

ain't that the truth :boggled:..you should have to wade through the AGW denier swill....:rolleyes:

As a lurker in these and the AGW threads, I gotta say...you're in with the swill on this one.

Honestly. Not trying to put you down. Take a step back and examine your righteous certainty in this context. You're not dealing with Poptech here - these are real scientists and real engineers doing real work. Take a look at Spork's youtube videos and other related videos, read Drela's paper, and, if you can stomach it, wade through some of the threads.

Or don't. Just a friendly prod, is all. You do good work in the AGW threads.

[macdoc]

and if you read what I wrote you will see that I don't deny the phenomena - I deny the statement of what it is describing...

a) any use of a treadmill or turntable is useless crap as you have an outside power source of practically unlimited torque.

directly downwind 2.5x Wind Speed? (http://forums.randi.org/showthread.php?p=5840087#post5840087)
That's the TANSTAAFL statement.

b) you won't be getting a laminar flow.

Harnessing turbulence is no great breakthrough..it's "interesting" to play with....no real world significance that I can ascertain.

Tell us, why the sail construction on the vertical poles?? :rolleyes:

There are all sorts of designs to capture wind energy

http://blog.modernmechanix.com/2007/11/07/new-rotor-ship-sails-in-lightest-wind/

http://openlearn.open.ac.uk/mod/resource/view.php?id=254231

http://i106.photobucket.com/albums/m269/macdoc/T307_1_026i.jpg
:garfield:

[sol invictus]

@Sol Invictus: You never answered my question about your theory spork and humber were the same person

I wouldn't have bet on that at straight odds even before, and it seems considerably less probable now. Spork appears to be a real person with a real job and real interests; to maintain a humberian persona over such a long time he'd have to be psychopathic beyond the point where I think he could function efficiently. But anything is possible...

or whether you remembered the story of George P. Burdell ("http://en.wikipedia.org/wiki/George_P._Burdell). Also, you and others haven't told me if you think 3x ws and beyond is probable?

I'd forgotten about Burdell, but this would go well beyond that. As for 3x... that's purely a question of engineering. Directly dw vehicles have a major advantage (over sailcarts or boats) in that the apparent wind they see at Nx wind speed is only (N-1)x. So 3x wind speed may well be possible; the cart will see only a 2x apparent wind, which makes air resistance less of an issue.

So I'd say yes, I think it's possible, and probably will be achieved soon (if it hasn't already) by spork and co.

[sol invictus]

and if you read what I wrote you will see that I don't deny the phenomena - I deny the statement of what it is describing...

Your posts are generally incoherent to the point that it's very difficult to determine what you think.

a) any use of a treadmill or turntable is useless crap as you have an outside power source of practically unlimited torque.


You mean, like the wind?

That's the TANSTAAFL statement.

b) you won't be getting a laminar flow.


Nonsense.


Harnessing turbulence is no great breakthrough..it's "interesting" to play with....no real world significance that I can ascertain.

More nonsense. To my knowledge there are no wind-powered devices that operate that way. Turbulence is usually a problem for them; for example that's why windmill turbines are generally positioned upstream of the tower they are attached to.

As for "real world" applications, you don't consider this cart to be an object in the real world?

[D'rok]

and if you read what I wrote you will see that I don't deny the phenomena - I deny the statement of what it is describing...

a) any use of a treadmill or turntable is useless crap as you have an outside power source of practically unlimited torque.


That's the TANSTAAFL statement.

b) you won't be getting a laminar flow.

Harnessing turbulence is no great breakthrough..it's "interesting" to play with....no real world significance that I can ascertain.

Tell us, why the sail construction on the vertical poles?? :rolleyes:

There are all sorts of designs to capture wind energy

http://blog.modernmechanix.com/2007/11/07/new-rotor-ship-sails-in-lightest-wind/

http://openlearn.open.ac.uk/mod/resource/view.php?id=254231

http://i106.photobucket.com/albums/m269/macdoc/T307_1_026i.jpg
:garfield:

You're applying the wrong expertise to the wrong problem. Turbulence has nothing to do with what is happening. And the treadmill is a valid proof of concept. Where's the turbulence there? Why do you think it is different in principle than a steady wind? Why does the cart move up the treadmill? As silly as it is, the faster than the ruler video shows why.

The science types can explain it better than I.

As an aside, the treadmill experiments helped my liberal arts brain get a basic grasp of relative motion.

[macdoc]

No a treadmill is not "proof of concept" and no I'm not going to explain nor wade into the nonsense.
You are on your own if you choose to be deluded.

hint - put said cart on floor....does it move? ..no
even with a push does it continue? no
start from there

TANSTAAFL

:garfield:

This from the 50s
http://www.dcss.org/bauer_cart.jpg

[D'rok]

No a treadmill is not "proof of concept" and no I'm not going to explain nor wade into the nonsense.
You are on your own if you choose to be deluded.

hint - put said cart on floor....does it move? ..no
even with a push does it continue? no
start from there

TANSTAAFL

:garfield:

This from the 50s
http://www.dcss.org/bauer_cart.jpg
Yes the cart requires wind (or a treadmill) to move. Duh.

As to the rest. Well...arrogant ignorance rocks!

Best.

[sol invictus]

No a treadmill is not "proof of concept" and no I'm not going to explain nor wade into the nonsense.
You are on your own if you choose to be deluded.

Evidently by spending your time arguing with AGW deniers, you have adopted their way of thinking - be unwilling to consider any evidence, ignore the laws of physics as understood for the last 400 years (that is what you are denying by ignoring the equivalence of the cart on the treadmill to a cart in the wind), then salt liberally with unwarranted arrogance.

hint - put said cart on floor....does it move? ..no
even with a push does it continue? no
start from there

And there's a tasty strawman for desert. No one disagrees with that - the cart does not work without wind.

[Modified]

No a treadmill is not "proof of concept" and no I'm not going to explain nor wade into the nonsense.
You are on your own if you choose to be deluded.

hint - put said cart on floor....does it move? ..no
even with a push does it continue? no
start from there


The equivalent situation would be the cart on the treadmill with a treadmill-speed headwind (no wind with respect to the surface). The cart would behave exactly the same with respect to the surface there as in your example: it would stay at surface speed if at surface speed and return to surface speed after being pushed.

[Christian Klippel]

Imagine science would work like macdoc. Then we would still have the sun revolving around a flat earth. Because anything else would be TANSTAAFL.

Lucky for us that this is not how science works.

Greetings,

Chris

[Myriad]

I don't understand macdoc's point about turbulence either. While the operation of the propeller (and the cart itself, in the wind) will inevitably create some turbulence, that is not to the cart's advantage, and no turbulence is necessary (or desirable) in the initial conditions either.

Since all of the cart's mechanisms will, as far as I can tell, work at any Reynolds number, it might help to imagine starting with a 1 mm/sec flow of oil down a duct, and a slow moving cart with a slow moving propeller that goes down-oil faster than the oil. No turbulence before or after. (Traction for the wheels against the duct would be an engineering issue but doesn't affect the principles. I'd suggest making the oil stationary, a simple oil bath, and simulate "down-oil motion in moving oil with wheel traction against a stationary duct" by replacing the wheels with a spool with thread wrapped around it, and pulling the thread backward from outside at the same 1 mm/sec. Down-oil faster than the oil propulsion would be demonstrated by the device moving away from the thread puller, while unwinding more thread from its spool than is taken up by the pulling spool. All with no fluid turbulence. But that might revive all those tiresome treadmill arguments.)

Respectfully,
Myriad

[Myriad]

This from the 50s
http://www.dcss.org/bauer_cart.jpg


So, it worked in the 50s but it wouldn't work today?

Respectfully,
Myriad

[ynot]

At the very beginning the wind pushes against the frame of the cart and propeller but never directly spins the propeller as a turbine.

The wind indirectly spins the propeller via the rotation of the wheel(s) to create thrust against the wind. This is possible using appropriate gearing.

When the cart reaches and exceeds the speed of the wind the propeller is still thrusting against air in the direction of the wind.

Effectively the propeller acts as a sail with thrust.

[Snixtor]

The cart does not care where the power comes from.
The cart may not, but we clearly do.

It interfaces the belt and the air, nothing else. It works on the difference of velocity between belt(ground) and air.
Absolutely, but what is that difference? On the treadmill, the cart wheels are turned by the treadmill, the wheels turn the propeller, the propeller pulls air past the cart. If we change our frame of reference, its not the ground that is moving, but the air that is moving. But in which direction is the air moving? Massively from the front of the cart to the back of the cart. To "reverse" the reference, you need a headwind, relative to the carts direction on the ground, not a tailwind.

The important thing to derive from "under the ruler faster than the ruler", is that when he holds the ruler still and moves the ground, the ground is going in the same direction, relative to the cart, as when he moved the ruler. Ruler to the right, ground to the left.

On the treadmill, relative to the cart, in which direction is the air moving? It is moving from the front of it, to the back, thanks to its propeller. If we are not moving the ground from the front of the cart to the back of the cart, we must instead move the air from the front of the cart to the back of the cart. Much as with "under the ruler", he could move the ruler to the right, or the ground to the left.

Note that the treadmill experiments show the situation where the cart is brought up to windspeed already, and then released. It then accelerates further.
And what provides that acceleration? The rotation of the wheels. And what provides the rotation of the wheels? Its forward motion. And what provides its forward motion? The wind pushing on the back of it. Once it is going faster than that wind, its no longer pushing on the back of it. The physics then enter a closed system where:
What is making the cart move forward? the propeller
What is making the propeller spin? the wheels, due to the carts forward motion.
What is making the cart move forward? - etc.

It would effectively be boot-strapping itself (many of us will have heard this term, whereby a man stands on the ground, then pulls upwards on the laces of his boots, thus pulling himself into the air and levitating).

I'm convinced there's something wrong with using the ruler above the cart to represent the air. My first clue is that the cart under the ruler doesn't need a boost to get going. The system doesn't change the rules at a certain speed, the same principles apply at any speed.

[Brian-M]

No a treadmill is not "proof of concept" and no I'm not going to explain nor wade into the nonsense.
You are on your own if you choose to be deluded.

I'd like to hear your reason as to why the treadmill is not "proof of concept". It should be good for a laugh. Come on, tell us why you think that such a very basic and long-standing principle of physics as "there are no preferred frames of reference" is invalid, and why we are deluded if we think it applies.

Motionless surface with moving air exactly the same thing as motionless air and moving surface. It's the same thing from different frames of reference. Just because you can't grasp this simple fact doesn't make it "nonsense".

hint - put said cart on floor....does it move? ..no
even with a push does it continue? no
start from there

The cart on the treadmill has a difference between ground speed and air speed to exploit. Put the cart on the floor where the same difference between air speed and ground speed exists and it will move... exactly the same way it does on the treadmill. It's been demonstrated to do this in real life. It's a proven fact, not just some wild theory.

[r-j]

Some people never let the facts get in the way of the truth.

[macdoc]

:dl:
Not sure bm understands the irony of his statement :rolleyes:

[Brian-M]

Absolutely, but what is that difference? On the treadmill, the cart wheels are turned by the treadmill, the wheels turn the propeller, the propeller pulls air past the cart.

It would be equally valid to say that on the street, the cart wheels are turned by the earth moving under the cart, so no difference between that and the treadmill there.

If we change our frame of reference, its not the ground that is moving, but the air that is moving. But in which direction is the air moving? Massively from the front of the cart to the back of the cart. To "reverse" the reference, you need a headwind, relative to the carts direction on the ground, not a tailwind.

If the cart is facing in the right direction to move to the right, then by placing it on the ground, you have a tailwind moving from left to right, relative to the ground. From the back of the cart to the front of the cart until the cart reaches windspeed.

If the cart is facing in the right direction to move to the right, then by placing it on the treadmill, you have a tailwind moving from left to right, relative to the belt. From the back of the cart to the front of the cart until the cart reaches windspeed.

Exactly the same physics, no reversal of wind needed. Of course, as the cart starts to go over wind-speed, then you do get a headwind, from front to back relative to the cart in both cases, which reduces the maximum speed of the cart.

The important thing to derive from "under the ruler faster than the ruler", is that when he holds the ruler still and moves the ground, the ground is going in the same direction, relative to the cart, as when he moved the ruler. Ruler to the right, ground to the left.

Moving the ground under a still ruler is the same difference as moving the ground (treadmill belt) under still air.

And what provides that acceleration? The rotation of the wheels. And what provides the rotation of the wheels? Its forward motion. And what provides its forward motion? The wind pushing on the back of it. Once it is going faster than that wind, its no longer pushing on the back of it. The physics then enter a closed system where:
What is making the cart move forward? the propeller
What is making the propeller spin? the wheels, due to the carts forward motion.
What is making the cart move forward? - etc.

But remember, the air is not stationary relative to the ground, and the propeller is "pushing" back against the wind. If the wind is traveling 10km/h to the right, and the propeller is pushing air 5km/h to the left, then you have the same effect on the moving prop as would a 15km/h wind on an immobile prop.

I'm convinced there's something wrong with using the ruler above the cart to represent the air. My first clue is that the cart under the ruler doesn't need a boost to get going. The system doesn't change the rules at a certain speed, the same principles apply at any speed.

The difference between the ruler-cart and the propeller cart is friction. The ruler driven cart has a very high level of friction and energy transfer between the ruler and the top-wheel even when stationary. The propeller driven cart has a very low level of friction and power transfer between the air and the propeller when stationary.

Using a very slippery ruler would be a better analogy for the wind.

[recursive prophet]

hint - put said cart on floor....does it move? ..no
even with a push does it continue? no
start from there.
Of course after a couple thou pages most possible arguments have already been made ad nauseum mac, and this one has been mentioned many times by deniers. You have to accept that from the RF of the cart on the TM that everything in the room is at WS. Hope I can find the humber photo-shop gem where he puts signs on objects in spork’s shop saying “At Wind Speed.”

I won’t take the time to recap that discussion, but will hopefully find some links where it is explained far better than I could even approach and post them for you. I really would like to see you follow up on this, and would be willing to bet you’ll see the light if you do. You can’t imagine how many I’ve seen even more adamantly positive than you that ddwfttw is an over unity farce who became converts in a few pages. I for one would be extremely grateful if you’d continue to post your viewpoints, just to see if it happens with you. You’re a smart guy, and I’m always on the lookout for more support for humber’s claims. I’m a dull bulb on a very bright marquee. While many of his diatribes seem to contain elements of truth, the Humberverse is still difficult for me to grok even though spork thinks I summer there. :D Help me understand your reasoning and we all might learn in the process.

I would ask everyone to notch up the civility just a bit wrt any who question the claims put forth in the OP. I’m trying to get Kammen and Drela to join this discussion, which I wouldn’t dream of doing at TR. If you want to be part of a gloves off slugfest, take it there. (Link on my sig) It’s the wild west of cyberspace, and loads of fun. But academia wouldn’t even want to be mentioned there, let along participate. So take a deep breath and the high road. He who ignores snide remarks always scores more points. It’s all pure glasperlenspiel after all. :rolleyes:


I wouldn't have bet on that at straight odds even before, and it seems considerably less probable now. Spork appears to be a real person with a real job and real interests; to maintain a humberian persona over such a long time he'd have to be psychopathic beyond the point where I think he could function efficiently. But anything is possible...

Especially in the realm of virtual reality, eh sol? I definitely agree it’s a long shot, and that spork is the real deal. I had an hour long Skype chat with him a year or so back, and no question he’s a heavy hitter. Yet I know next to nothing about humber or Harold, though I’ve exchanged many emails with humber and PM’s with Harold/Christoph. At some point I’ll reveal my different theories, including a few tidbits I know about the whole scene that others don’t. But I’m pretty sure the whole George Burdell legend plays a more significant role than you suspect due to spork’s connection with it. Many possibilities. :)

I don't understand macdoc's point about turbulence either. While the operation of the propeller (and the cart itself, in the wind) will inevitably create some turbulence, that is not to the cart's advantage, and no turbulence is necessary (or desirable) in the initial conditions either.

I think what macdoc has in mind is that under the right conditions the cart could ride the transients between gusts. humber made a good case-at least for the great unwashed like me-that this was exactly what happened in Bauer's experiments.

The wind has a 20 second gust at 25mph, then drops back to say 10mph. At that point the cart's mass will have enough ke to exceed that speed for how long? Beats me, but I'm sure sol could maths it out. This is one of the reasons spork build the treadmill model; a steady state apparent wind. There is a video of it running on the ground in the YT spork33 collection, including a race with a turbine cart. These tests were mostly in response to arguments made by Harold and humber, who claimed the turbine would win. It didn't, but of course H&H found some ostensible flaws in how the test was conducted. Laurel and Hardy Do a Science Test was the title on one of humbers more hilarious parodies. :D

I really hope someday a lot more people read this saga. You learn a lot of interesting science from those trying to explain to humber how he fails to grasp reference frames; relative motion; air foils; prop vector analysis; GPS tracking; Galileo; Newton;.... You get the picture. And you get to laugh as you learn!! There are a few silly seasons, but I've never read anything anywhere that elicited as many belly laughs. :p

I like your oil analogy Myriad, and something like it has been discussed before. However, after two thousand pages of replies between the 3 sites everything starts to get fuzzy and hard to find for the search challenged like me. I'm in my dotage, and tend to conflate a lot; to wit the Harold Bricer Test. But I know at least one chemistry buff posted a fluids model. If I come upon it I'll send a link.

Liking your exchanges with Brian, Snixtor; keep asking questions. And hi to the yguy. Glad to see you here. Your dialogue at TR with cold one and spork on billiard balls was a classic. Definitely admire your style. :cool:

[macdoc]

Originally Posted by Myriad http://forums.randi.org/helloworld2green/buttons/viewpost.gif (http://forums.randi.org/showthread.php?p=5840360#post5840360)
I don't understand macdoc's point about turbulence either. While the operation of the propeller (and the cart itself, in the wind) will inevitably create some turbulence, that is not to the cart's advantage, and no turbulence is necessary (or desirable) in the initial conditions either.

I think what macdoc has in mind is that under the right conditions the cart could ride the transients between gusts. humber made a good case-at least for the great unwashed like me-that this was exactly what happened in Bauer's experiments.


Under all real world wind conditions - there will always be vectors and using a treadmill for anything involving this is hilariously wrongheaded..

The turbulence is not from the vehicle tho prop tip vortices may have some sort of impact on efficiency...
..think about vectors and how energy is gained to drive the speed above the wind speed.....and why nothing happens sitting on the floor.

Visualize instantanteous high efficiency tacking as to why turbulence allows energy harvesting.
How is that ship I posted working? ..it can absorb energy from various directions...

In a laminar flow it simply will not happen in the same manner as that flow is smooth and uni-directional....

for starters the windspeed in the real world is an average of the gusts....it's one reason they state .....60 kph wind gusting to 80 kph....well it's ALWAYS gusting up and down....there is likely no laminar flow in nature.

Unless you can see why the treadmill with its ..for practical purposes in these kind home experiments...infinite torque...you'll flounder.

[jsfisher]

So what would be a valid experiment to distinguish between your explanation and others?

[Myriad]

And what provides that acceleration? The rotation of the wheels. And what provides the rotation of the wheels? Its forward motion. And what provides its forward motion? The wind pushing on the back of it. Once it is going faster than that wind, its no longer pushing on the back of it. The physics then enter a closed system where:
What is making the cart move forward? the propeller
What is making the propeller spin? the wheels, due to the carts forward motion.
What is making the cart move forward? - etc.

It would effectively be boot-strapping itself (many of us will have heard this term, whereby a man stands on the ground, then pulls upwards on the laces of his boots, thus pulling himself into the air and levitating).

I'm convinced there's something wrong with using the ruler above the cart to represent the air. My first clue is that the cart under the ruler doesn't need a boost to get going. The system doesn't change the rules at a certain speed, the same principles apply at any speed.


E pur si muove (ocius quam ventus)!

Let's look again at the situation when the cart is moving at just below wind speed. The wheels are turning, so the propeller is turning. If the propeller blades were flat, with zero pitch (making the propeller just a funny-shaped sail, whether it's spinning or not), then the wind is (just barely) pushing on the propeller and the cart frame, the drag from the road and the propeller mechanism acting on the wheels is (just barely) pushing back on the wheels. At that point, just below the wind speed, we're at equilibrium, so the cart does not accelerate.

(If the same condition were simulated on a treadmill, the cart would drift slowly backward on the treadmill. The force applied by the still air against the cart frame and the propeller-shaped sail would be not quite enough to hold the cart in place, just as outside in the wind, the wind force is not quite enough to push the cart all the way to wind speed.)

Now we add some pitch to the propeller, so that (relative to the cart) it pushes air backward when it spins. The propeller must continue spinning because the wheels are spinning. So the propeller is adding some additional thrust, accelerating the cart toward windspeed... and beyond...

And this is where a lot of people's woo meters, understandably, but mistakenly, go off! ALERT! ALERT! IMPOSSIBLE! OVER UNITY! WOO!

Because of course, for the cart to accelerate past wind speed, it has to gain kinetic energy. Specifically, it has to gain more kinetic energy from the thrust generated by the spinning propeller that it loses from the wheels doing the work of spinning the propeller. And that's impossible.

Except, it isn't.

Because spinning the propeller, which is what the wheels do, isn't what actually provides the thrust. What provides the thrust is the tail wind, the same tail wind that pushed the cart from a standing start, pushing against the spinning propeller.

What the spinning of the propeller does is provide a dynamic surface that's moving at the right speed for the wind to push against. For the wind to push the cart, it needs a surface to push against that's effectively moving slower than the wind. That means, when the cart is moving downwind near, at, or beyond wind speed, the dynamic surface must be moving slower than the cart with respect to the ground, and backward with respect to the cart. An angled surface moving across the wind -- such as a propeller blade or an ice boat's sail -- can do that.

[Lengthy digression:] There are other less elegant ways to do it. Here's a cart that uses simple sail drag (represented by the parachutes) to move faster than the wind:

http://forums.randi.org/imagehosting/13012493204e1d7668.jpg

When this cart moves to the right relative to the ground, the parachute at the top also moves to the right relative to the ground. But it doesn't move to the right as fast as the cart does, because it's being wound to the left (relative to the cart) by the movement of the wheels and rope.

Pushing the parachutes forward moves the cart forward (faster than the parachute) in exactly the same way that, in the "faster than the ruler" video, the ruler pushing forward on the top of the large wheel moves the cart forward faster than the top of the wheel.

So, when the cart is moving at wind speed, the parachutes are moving backward with respect to the cart, and therefore slower than the wind. So the wind can still push forward on the parachutes, accelerating the cart.

That's a different (and cruder) kind of "dynamic surface moving at the right speed for the wind to push against when the cart is moving at or beyond wind speed" than the propeller, but it serves the same function. [end of lengthy digression]

In still air, if you propel a vehicle with a propeller, the kinetic energy of the vehicle derives from the energy put into the propeller, which creates thrust acting over a distance. Given inevitable inefficiencies, the vehicle's resulting kinetic energy be less than the energy put in. That's why you can't just link the wheels to the propeller, give it a push, and expect it to accelerate or even keep going.

But in the downwind cart, the thrust is provided by the wind, not the energy used to spin the propeller. So one cannot assume (and it turns out it is not correct to say) that the propeller must take away more energy from the cart's KE (via the wheels) than can possibly be imparted to the cart's KE from the propeller's thrust. When the cart is moving near wind speed, the power needed to spin the propeller is less than the power of the wind pushing on the spinning propeller. The cart actually does gain kinetic energy and hence goes faster. The wind is slowed and loses kinetic energy.

Respectfully,
Myriad

[recursive prophet]

http://3.bp.blogspot.com/_XtHg_q76Tgs/S8tnQjyHCII/AAAAAAAAA6o/NvvdzIw1sUU/s1600/36b.JPG

This is spork's ready for prime time cart: The Blackbird. It was just posted on his blog. I think any interested in this topic really should check it out via the link in my sig, and sign on to follow it. They've done a terrific job documenting the entire construction process, including test videos. A great read also. :)

@Myriad: Some intriguing posts here; thanks for sharing. I look forward to reading mac and Sniks response to your latest. I'm also hoping sol, Brian, and others will comment on it it. Kinetic energy transfer seems to be at the core of this debate, along with drag and thrust. My initial impression is that some of your thinking circumvents the aggregate consensus, but I'll leave that for the cognoscenti here to analyze. In my chosen roles as janitor, facilitator, and observer, I've learned not to overreach my limited capabilities. Very interesting explanation, no matter.

@staff: I have permission from spork to post this picture from his blog and can document this if required. Thanks.

[Robo Sapien]

I hear they are using this as the basis for a prototype pizza delivery drone in Chicago..

[383LQ4SS]

This is spork's ready for prime time cart: The Blackbird. It was just posted on his blog. I think any interested in this topic really should check it out via the link in my sig, and sign on to follow it. They've done a terrific job documenting the entire construction process, including test videos. A great read also. :)



Thanks for pointing out the link. It appears they do have a variable pitch system and are set to use it soon. That should improve acceleration performance from a stop as well as top speed.

Next thing they should do is put the entire prop tower on a swivel so they can keep it pointed in the wind regardless of the direction of the cart. That would allow them to go DDWFTW as well as sideways...and directly upwind...basically moving in any direction in a straight line with no tacking. That would be kinda neat. Maybe even a first for a land based wind powered vehicle.

Thanks for the updates.

[RossFW]

383,

No, a swiveling tower wouldn't work. The prop is a PROP not a turbine. It provides thrust to propulse the cart. If the cart were facing into the wind, with the prop facing downwind, it would be trying to drive the cart backwards.

[Snixtor]

It would be equally valid to say that on the street, the cart wheels are turned by the earth moving under the cart, so no difference between that and the treadmill there.
Right, so the earth moves, spins the wheels, turns the propeller, forces air from the front of the cart to the back.

Or...

The air moves from the front of the cart to the back, which turns the wheels, which moves the earth beneath it.

In the full size, what is moving the air from the front of the cart to the back? Its forward motion. What is causing its motion? The wheels, what are making the wheels go faster than "air push" speed? The propeller, and you're back in the closed system.

[383LQ4SS]

383,

No, a swiveling tower wouldn't work. The prop is a PROP not a turbine. It provides thrust to propulse the cart. If the cart were facing into the wind, with the prop facing downwind, it would be trying to drive the cart backwards.



With a variable pitch prop it seems you could do either. Propulse the cart...as you say...or drive the wheels. It seems a 45 degree blade angle change would turn the prop from PROP to turbine.

[Ririon]

Great thread!

Fun physics, cool hardware and entertaining science deniers all in one. :)

I kind of see how you would accept, say, a nuclear power plant but deny evolution for religious reasons, but how can you accept AGW and then turn around to deny classical physics? :confused:

[recursive prophet]

Thanks for pointing out the link. It appears they do have a variable pitch system and are set to use it soon. That should improve acceleration performance from a stop as well as top speed.

Next thing they should do is put the entire prop tower on a swivel so they can keep it pointed in the wind regardless of the direction of the cart. That would allow them to go DDWFTW as well as sideways...and directly upwind...basically moving in any direction in a straight line with no tacking. That would be kinda neat. Maybe even a first for a land based wind powered vehicle.

Thanks for the updates.

You're more than welcome 383. I see Ross has already responded to your supposition, and he can explain the physics involved far better than I. He's an airline pilot and one of the regulars in Cartville from the start here at JREF in '08. :cool:

Glad to see you back Ross. I haven't mentioned this thread yet at TR as I'm afraid it might overheat again. Figured I'd give others a little time to answer any questions from newbies and skeptics. As you can see, many of the original participants are still around, so I knew things could move forward without assistance from our swat unit at TR for now. May as well keep this off moderated status for as long as possible. :D

[RossFW]

With a variable pitch prop it seems you could do either. Propulse the cart...as you say...or drive the wheels. It seems a 45 degree blade angle change would turn the prop from PROP to turbine.

383,

Yes, that would work. The current prop is only (AFAIK) variable in the positive range, but a fully reversible one would enable turbine or prop mode.

[Thabiguy]

Next thing they should do is put the entire prop tower on a swivel so they can keep it pointed in the wind regardless of the direction of the cart. That would allow them to go DDWFTW as well as sideways...and directly upwind...basically moving in any direction in a straight line with no tacking. That would be kinda neat. Maybe even a first for a land based wind powered vehicle.

Actually, pointing the propeller in the opposite direction (or equivalently, reversing the transmission) will not make the cart move backwards. It will still move forwards, only slower and "stronger". To go backwards, the prop needs to move air in the same direction as in the "faster than the wind mode" (wrt vehicle) but faster (for example by making the prop turn faster wrt wheels).

Looking at it from the other side, turning the propeller to accomodate different wind directions would also simultaneously require changing the transmission or prop pitch (or both, for optimum efficiency).

Also, automatically keeping the prop facing the right direction would be non-trivial; for example, when the DDWFTTW cart breaks the wind speed, it goes through a period when there's no apparent wind, and then there's apparent wind in the opposite direction, but still the prop must be facing the same way the whole time.

To sum up, a "general direction" operation would be much more complex, and perhaps the simplest approach would be manual control of prop orientation/pitch/transmission by an operator. Automatic electronic control would be tricky, and automatic mechanical control extremely tricky.

[Jack by the hedge]

Right, so the earth moves, spins the wheels, turns the propeller, forces air from the front of the cart to the back.

Or...

The air moves from the front of the cart to the back, which turns the wheels, which moves the earth beneath it.

In the full size, what is moving the air from the front of the cart to the back? Its forward motion. What is causing its motion? The wheels, what are making the wheels go faster than "air push" speed? The propeller, and you're back in the closed system.

No. Did you read Myriad's post #119? There were some great alternative explanations there.

Try this version: Imagine first it was a cart with a fixed sail. It could sail downwind at just a little less than the speed of the wind, right? If the sail was travelling at the same speed as the wind, then the accelerating force would drop to zero. Yes?

OK. Now imagine it was a long cart, and the sail's mast was fixed to some kind of sliding mount which could move the mast forwards and backwards along the cart. A chain drive from the wheels moves the mast backwards as the cart rolls forwards.

Neglecting for a moment the absurdity that the mast would very quickly run out of travel, do you see that this would (briefly) allow the cart to go faster than before? The sail, mechanically connected to the wheels, is moving backwards so the windspeed relative to the sail is more than the windspeed relative to the cart.

Right. Now all you need to do is replace the unsustainable constantly-retreating sail with a real possibility - an airscrew mechanically connected to the wheels and pitched so that its surface is constantly retreating from the air's point of view, even when the cart is travelling at windspeed.

[sol invictus]

Right, so the earth moves, spins the wheels, turns the propeller, forces air from the front of the cart to the back.

Or...

The air moves from the front of the cart to the back, which turns the wheels, which moves the earth beneath it.

In the full size, what is moving the air from the front of the cart to the back? Its forward motion. What is causing its motion? The wheels, what are making the wheels go faster than "air push" speed? The propeller, and you're back in the closed system.

Everything you said here is precisely as valid when applied to a windmill. Do you believe that windmills work?

The system is not closed. After the cart has passed, the air behind it is moving slower relative to the earth's surface than it was before. That energy had to go somewhere, and it goes into accelerating the cart, or if it's at top speed, into friction in its components.

[Snixtor]

Did you read Myriad's post #119? There were some great alternative explanations there.
I've had to go back to it, and there's some interesting points made there. The alternative machine in the diagram is food for thought, and I'll be giving it some consideration.

I still see a critical flaw in the cart under the ruler analogy though, in that its principles apply at any speed. It doesn't need a boost to get to "faster than the ruler", the system itself is geared such that it is the only way for it to move. The faster than wind cart though, appears to need an external boost to get it to that stage. Clearly it's something different.

entertaining science deniers
...
deny classical physics
Completely counter-productive Ririon. I don't think I've seen anybody "deny" classical physics on either side of this argument. What we've seen is different interpretations of the scenario, and in what ways the known laws of physics apply. This isn't a case of - you can get a free lunch, no you can't, yes you can. This is a case of - that scenario would be a free lunch, no it wouldn't, yes it would.

[sol invictus]

I still see a critical flaw in the cart under the ruler analogy though, in that its principles apply at any speed. It doesn't need a boost to get to "faster than the ruler", the system itself is geared such that it is the only way for it to move. The faster than wind cart though, appears to need an external boost to get it to that stage. Clearly it's something different.


That's a very minor difference. The friction between the ruler and the wheel is so high that the ruler doesn't slip, whereas it's easy for air to slip over a propeller as the cart accelerates. If you can imagine making the ruler and the wheel it contacts fairly slippery, so that you'd have to start with a long ruler and keep passing it above the wheel for a while before the wheel comes up to speed, that's a closer analogy.

The cart is "geared" to the air, it's just that the gear can slip quite easily, like a car with its clutch in most of the way.

Completely counter-productive Ririon. I don't think I've seen anybody "deny" classical physics on either side of this argument. What we've seen is different interpretations of the scenario, and in what ways the known laws of physics apply. This isn't a case of - you can get a free lunch, no you can't, yes you can. This is a case of - that scenario would be a free lunch, no it wouldn't, yes it would.

The fact that the cart advances up a treadmill plus the laws of physics proves beyond any doubt that it works as advertised. In fact the cart on the treadmill is a cart in a wind going directly downwind faster than the wind. Anyone that denies that either doesn't understand the laws of physics or doesn't accept them as valid. There isn't any middle ground.

[Skwinty]

I watched a video of this cart and the ribbons on the horizontal mast spell it out very clearly.
You can see when the cart is travelling at less than wind speed, equal to wind speed and faster than windspeed.

If that doesn't convince you, then what will?:confused:

[Ririon]

(...)

Completely counter-productive Ririon. I don't think I've seen anybody "deny" classical physics on either side of this argument. What we've seen is different interpretations of the scenario, and in what ways the known laws of physics apply. This isn't a case of - you can get a free lunch, no you can't, yes you can. This is a case of - that scenario would be a free lunch, no it wouldn't, yes it would.
I will grant you that it is counter-productive. It is also not nice, and should classify as name-calling. :o I am not perfect. It is probably more a case of "persistently misunderstand or fail to grasp" than "deny". As I said, I fail to see any motive to actually denying classical physics except perhaps trolling. And I don't really understand the point of that, either. :)

[Myriad]

This is spork's ready for prime time cart: The Blackbird. It was just posted on his blog. I think any interested in this topic really should check it out via the link in my sig, and sign on to follow it. They've done a terrific job documenting the entire construction process, including test videos. A great read also. :)


Aha! They've covered it in black to hide the motor!

@Myriad:My initial impression is that some of your thinking circumvents the aggregate consensus...


I agree. The best way to analyze the system is in terms of all the forces acting on it (in various conditions such as at a standing start, near windspeed, and faster than windspeed).

However, that's difficult to do accurately, and difficult to follow when someone else is doing it, so there's a tendency to try to shift the analysis into the energy domain, which is trickier than it seems and leads to wrong conclusions.

The problem is that kinetic energy of individual components of the system is reference frame dependent. So you have to stay consistent with regards to what frame you're in.

With the cart at windspeed, in the ground reference frame, the power to spin the propeller must come from the cart's kinetic energy. But the wind pushing on the propeller, not the propeller pushing air backwards, provides the thrust. So the kinetic energy gained from thrust (speeding up the cart) can exceed the amount converted into spinning the propeller (slowing down the cart).

With the cart at windspeed, in the cart's reference frame (most easily appreciated in the treadmill model), the propeller does push air backwards to provide the thrust. But the kinetic energy to spin the propeller comes from the ground (the cart is stationary and so has none) which has an essentially unlimited supply. So the kinetic energy gained from thrust (speeding up the cart) can exceed the amount converted into spinning the propeller (slowing down the earth/treadmill).

Neither way of analyzing it violates conservation of energy. However, if you muddle the two (as in, "you can't gain energy from the wind because at windspeed the wind is stationary, and the energy gained from thrust from the propeller can't possibly exceed or equal the kinetic energy the cart loses at the wheels from driving the propeller to generate that thrust") it's easy to erroneously conclude an "over unity" impossibility.

Respectfully,
Myriad

[I Ratant]

Powering the drive wheels with some energy source was solved 150 years ago.
This particular variation, while amusing/interesting appears less practical than internal combustion/electric motors, which can move the conveyance when there's no wind.
And the usual shapes for the usual conveyances are a bit more practical, with no large spinning thing placed above the cargo container.
Navigating a standard urban street environment with the DWFTTW thingy might be awkward, considering the vertical clearances and lateral clearances needed, to say nothing of the need for a wind to move it at all.
As an exercise in "yes, it can be done" it's a good example, but the utility might only become of interest when the usual motive sources become impossible to use.. no gas, no electricity.
The clipper ship Sovereign of the Seas held the speed record of 22 knots for at a 100 years.
The DWFTTW power system might be a more practical power source for a clipper ship, but the very clipper ship idea is long obsolete.

[ThinAirDesigns]

Powering the drive wheels with some energy source was solved 150 years ago..

Yes it was, but in this particular vehicle the "drive wheels" aren't "powering" the vehicle forwards as most think they are.

To put it another way, the "drive wheels" are actually "braking wheels" and are applying a braking force that is trying to slow the vehicle down

JB

[3point14]

This makes my head hurt.

I think I'm getting it. I still don't quite get the bit where the drive comes from the prop not the wheels. Does it throw air out of the back of the prop faster than the air is coming at it from the front?

If you connected the fan/turbine to a (theoretical) resistance free generator and used a motor on the wheels would you get the same thing? I suspect not. Would the motor just become the generator and the generator the motor at one point?

Have I just demonstrated an appalling lack of understanding of the whole thing?

[ThinAirDesigns]

If you connected the fan/turbine to a (theoretical) resistance free generator and used a motor on the wheels would you get the same thing?

No, you need the generator on the wheels and the motor on the prop and then you would get the same thing.

BTW ... it's not a "fan/turbine" .. it's a garden variety propeller acting as a garden variety propeller acts.

JB

[3point14]

No, you need the generator on the wheels and the motor on the prop and then you would get the same thing.

BTW ... it's not a "fan/turbine" .. it's a garden variety propeller acting as a garden variety propeller acts.

JB


Thank you. That's the bit that gets me, when it's driven more like a hovercraft than a car. Still confused, but getting there. The toy animals were cool though.

[I Ratant]

Whichever way the power goes, the concept though possible, isn't practical.

[Modified]

With the ruler-driven cart (and a long ruler), you could easily get a generator/motor configuration to outpace the ruler.

[Brian-M]

In the full size, what is moving the air from the front of the cart to the back? Its forward motion. What is causing its motion? The wheels, what are making the wheels go faster than "air push" speed? The propeller, and you're back in the closed system.


Some of kinetic energy of the cart's forward motion is sacrificed to turn the propeller to push the air back. But since the wind is moving forward (relative to the ground), pushing it back on it adds more kinetic energy to the cart than is required to push the air back. If the propeller is pushing the air backwards at 5km/h, and the wind is blowing forwards at 10km/h, this has almost the same effect as the propeller pushing the air back at 15km/h with the same force.

The cart is being powered by the difference in velocities of the air and the ground, transferring kinetic energy that exists between the air and the earth to the cart. The end result being that the air and the earth have less kinetic energy relative to each-other, and the cart has more (with a little bit of energy lost as heat and sound).

I still see a critical flaw in the cart under the ruler analogy though, in that its principles apply at any speed. It doesn't need a boost to get to "faster than the ruler", the system itself is geared such that it is the only way for it to move. The faster than wind cart though, appears to need an external boost to get it to that stage. Clearly it's something different.

But the cart doesn't need a boost to go from slower than the wind to faster than the wind, so I don't see how this is different. In fact, if the wind is strong enough to overcome starting friction, the cart can go from being stationary to faster than the wind without any help.

[Snixtor]

The fact that the cart advances up a treadmill plus the laws of physics proves beyond any doubt that it works as advertised. In fact the cart on the treadmill is a cart in a wind going directly downwind faster than the wind. Anyone that denies that either doesn't understand the laws of physics or doesn't accept them as valid. There isn't any middle ground.
I'm still not entirely sure of what I'm seeing in the treadmill experiment, and whether it really is a proper analogue of the cart on the ground in the wind. Maybe it is as simple as "doesn't understand the physics", but that's not as scathing a comment as it would seem. I've seen comments from physicists on either side of the fence, so if it's a case of misunderstanding the physics, it's obviously not a case of misunderstanding something simple and concise. Spork himself, one of the prime advocates of the cart has even said comments to the effect of "we do still have many physicists, PHDs and other well educated people disagreeing with us".

Be careful of claiming "it works and I understand the physics" though, as they're two separate things. Just because something works and you think you know why, doesn't mean your understanding of the reason it works is correct.

I see three distinct claims here:
1 - DDWFTTW is possible.
2 - The cart, as designed, is capable of it, and this is what we're seeing it do in videos.
3 - The physics used to describe how the cart is working, is correct.

Obviously, truth of any of the lower claims affirms those above, and disagreeing with any of the top points disagrees with the lower points.

I watched a video of this cart and the ribbons on the horizontal mast spell it out very clearly.
You can see when the cart is travelling at less than wind speed, equal to wind speed and faster than windspeed.
This falls into the category of the 2nd point above. The question to ask being "what exactly are we seeing here?". I've seen it too, and the flag flapping on it does clearly shift from flapping forwards and flapping backwards relative to the carts direction of travel. What's not easy to ascertain from the video though, is that if it's going directly downwind. You could put a windsock on a sailboat and it would point to the back of a sailboat tacking into the wind. That doesn't mean the sailboat is travelling directly downwind, in fact, it's not.

Is that what's happening in this video? I'm not really sure. I'd like to see more quantitative analysis, the measurements themselves, not just a video that's too far open to interpretation.

I will grant you that it is counter-productive. It is also not nice, and should classify as name-calling. :o I am not perfect. It is probably more a case of "persistently misunderstand or fail to grasp" than "deny".
Maybe so, and there's a lot of intelligence on either side of the consideration. Neither side, I think, being worthy of scorn simply for the direction they are considering things from.

The best way to analyze the system is in terms of all the forces acting on it (in various conditions such as at a standing start, near windspeed, and faster than windspeed).
And I'll have to come back later to re-read your analysis of it in this manner.

[Rasmus]

I watched a video of this cart and the ribbons on the horizontal mast spell it out very clearly.
You can see when the cart is travelling at less than wind speed, equal to wind speed and faster than windspeed.

If that doesn't convince you, then what will?:confused:

I've only been reading some of this, and I simply cannot grasp the ideas that a lot of you are advancing.

At no point does the ground begin to suddenly power the wheels of my bicycle, no matter how fast I hit the pedals. Yet in all these explanations this is what I am supposed to believe: All of a sudden the ground is transferring energy to the wheels.

What would convince me? A proper test.

A test on a level surface, with a measurement for wind speed taken once every 100m along the track and the vehicle giving a proper record of it's own speed - compared to a second measurement of speed taken from the outside every time it passes one of the aforementioned 100m marks.

Not some guys shouting out various values at random.

And then, make the difference meaningful: 2.5 times the windspeed would be pretty good, actually. Don't give me something that could be explained away with simple measuring errors.

I am not buying the treadmill-experiments, e.g. I am not sure how it works, but too much is wrong with it. The vehicle is pressed on the track to begin with, giving the propeller a chance to speed up and exceed the speed of the track, e.g. with just a little nudge. (And no, I can't explain what I see there, either.)

Would the experiment with the ruler cart still work if I went and used cogwheels with a fixed, unbreakable transmission and then used a rules with matching cogs as well?

And even then I would not be convinced: I can move a bicycle from the ground by turning the pedals - for as long as I can reach the pedals. That's what the ruler does. The ruler will no longer reach the car or be able to push the big wheel forward as soon as the car is as fast as the ruler, though.

[rwguinn]

Some of what I'm reading here sounds a lot like "if you hover a helicopter 5000 feet above New York Harbor, 5 hours later it will crash into the Grand Teton"
In the treadmill test, the unlimited torque (comparatively) of the treadmill turns the wheels, which turn the prop.
Off the treadmill, what turns the wheels?

[Myriad]

In the treadmill test, the unlimited torque (comparatively) of the treadmill turns the wheels, which turn the prop.
Off the treadmill, what turns the wheels?


The relative motion of the ground and the vehicle.

Which comes about because the wind is pushing on, and being slowed down by, the propeller.

This is possible when the cart is at or beyond wind speed because the moving propeller presents, with respect to any given wind stream line impinging on it, a surface that's moving slower than the cart, and slower than the wind.

Respectfully,
Myriad

[rwguinn]

The relative motion of the ground and the vehicle.

Which comes about because the wind is pushing on, and being slowed down by, the propeller.

This is possible when the cart is at or beyond wind speed because the moving propeller presents, with respect to any given wind stream line impinging on it, a surface that's moving slower than the cart, and slower than the wind.

Respectfully,
Myriad
I beg your pardon?
so Eout<=2.5*Ein, at Least?
(Assuming the mass of the cart=mass of impinging wind...)

[Brian-M]

I beg your pardon?
so Eout<=2.5*Ein, at Least?
(Assuming the mass of the cart=mass of impinging wind...)


V≠E
Eout<Ein
Vout>Vin

[ThinAirDesigns]

Whichever way the power goes, the concept though possible, isn't practical.

Well, that all depends on what you define as the task.

No one here is suggesting that human transportation will be sprouting wheel driven propellers anytime soon, but the fact remains that harnessing the power of the wind through a turbine is subject to the Betz Limit while doing the same through a propeller is not. This means that more energy can be harvested from the same wind through this method over turbines. Who knows where that leads.

JB

[ThinAirDesigns]

A test on a level surface, with a measurement for wind speed taken once every 100m along the track and the vehicle giving a proper record of it's own speed - compared to a second measurement of speed taken from the outside every time it passes one of the aforementioned 100m marks.

Not some guys shouting out various values at random.

Just stay tuned to www.fasterthanthewind.org and you'll get almost exactly what your asking for.

NALSA (the org that ratified the recent Greenbird wind powered land speed record) will be administering.

JB

[sol invictus]

I'm still not entirely sure of what I'm seeing in the treadmill experiment


You're seeing the cart steadily advance up the tread.

and whether it really is a proper analogue of the cart on the ground in the wind.

It's not an analogue - it is a cart on the ground in the wind. It is as solid a proof as a cart going directly downwind faster than the wind in a long wind tunnel.

I've seen comments from physicists on either side of the fence


I doubt that. Link?

so if it's a case of misunderstanding the physics, it's obviously not a case of misunderstanding something simple and concise. Spork himself, one of the prime advocates of the cart has even said comments to the effect of "we do still have many physicists, PHDs and other well educated people disagreeing with us".

Spork has a strong motive for this cart to be as controversial as possible.

Be careful of claiming "it works and I understand the physics" though, as they're two separate things. Just because something works and you think you know why, doesn't mean your understanding of the reason it works is correct.

That's true - see macdoc's posts. In this case however, it works and I understand the physics (which really aren't very complex).


At no point does the ground begin to suddenly power the wheels of my bicycle, no matter how fast I hit the pedals. Yet in all these explanations this is what I am supposed to believe: All of a sudden the ground is transferring energy to the wheels.

As I said in my first response to macdoc, think about a windmill. It's at rest with respect to the ground, gathering energy from a moving airmass that passes over it.

Now invert the situation, and imagine something at rest with respect to the air, gathering energy from the ground moving beneath it. That's precisely what the cart does (when it's at windspeed).

If the first is possible, so is the second. The first is possible.

[Myriad]

I beg your pardon?
so Eout<=2.5*Ein, at Least?
(Assuming the mass of the cart=mass of impinging wind...)


Huh? Your question is a dimensional nightmare.

Please define Ein and Eout, explain why anyone would expect that the limiting speed would have anything to do with any such ratio (as opposed to being whatever speed at which Ein equals Eout), and how much impinging wind you're talking about (which would seem to require a time dimension).

Respectfully,
Myriad

[rwguinn]

V≠E
Eout<Ein
Vout>Vin
E=1/2*M*V2
Ein=Mwind*Vwind2
Eout=Mcart*Vcart2

[sol invictus]

I beg your pardon?
so Eout<=2.5*Ein, at Least?
(Assuming the mass of the cart=mass of impinging wind...)

Power is force times velocity. If there is wind, the velocity of the air relative to the cart is lowe than the velocity if the ground relative to the cart. Therefore the thrust the air exerts on the cart can be greater than the drag the ground exerts on the cart, even if some power is lost to friction. Therefore the cart can accelerate from wind speed.

[rwguinn]

Power is force times velocity. If there is wind, the velocity of the air relative to the cart is lowe than the velocity if the ground relative to the cart. Therefore the thrust the air exerts on the cart can be greater than the drag the ground exerts on the cart, even if some power is lost to friction. Therefore the cart can accelerate from wind speed.
What is moving the ground relative to the cart (we'll use the cart as a frame of reference, since you phrased it that way)

[Brian-M]

I've seen it too, and the flag flapping on it does clearly shift from flapping forwards and flapping backwards relative to the carts direction of travel. What's not easy to ascertain from the video though, is that if it's going directly downwind. You could put a windsock on a sailboat and it would point to the back of a sailboat tacking into the wind. That doesn't mean the sailboat is travelling directly downwind, in fact, it's not.

Imagine a giant frame surrounding a sailboat or iceboat, attached to to the boat by an elastic tether from the back of the boat to the back of the frame. Put guide-rails to either side of the frame, so the frame can only move forward or backward, not side to side.

The frame itself is now truly moving DDWFTTW powered (ultimately) only by the wind, while the boat inside the frame, dragging it along, continues tacking back and forth in order to move faster downwind than the wind. So DDWFTTW becomes theoretically possible as soon you accept that wind powered devices can travel downwind faster than the wind by tacking side to side.

The cart is like the frame, and the parts moving side-to-side, (up and down, forward and back,) such as the wheels and propeller, are like the boat tacking side to side.

[Modified]

Would the experiment with the ruler cart still work if I went and used cogwheels with a fixed, unbreakable transmission and then used a rules with matching cogs as well?

It would work even better - less slippage.

That's what the ruler does. The ruler will no longer reach the car or be able to push the big wheel forward as soon as the car is as fast as the ruler, though.Yes it will. From the moment you move the ruler, the cart is moving faster. This continues until it runs off the end of the ruler or the table. The same thing will happen with the DWFFTW cart - eventually you will come to an area with no wind or run into some trees.

[sol invictus]

What is moving the ground relative to the cart (we'll use the cart as a frame of reference, since you phrased it that way)

Objects in motion remain in motion unless acted on by an external force (Newton I).

The earth is in motion in that frame; nothing is required to keep it in motion, although the acceleration of the cart will slow it down ever so slightly.

What keeps the air moving relative to a windmill?

[ThinAirDesigns]

I've seen it too, and the flag flapping on it does clearly shift from flapping forwards and flapping backwards relative to the carts direction of travel. What's not easy to ascertain from the video though, is that if it's going directly downwind. You could put a windsock on a sailboat and it would point to the back of a sailboat tacking into the wind. That doesn't mean the sailboat is travelling directly downwind, in fact, it's not.

You bring up an excellent point Snixtor.

Now, I know that the following sequence of screen grabs won't answer 100% of our question, but it it pretty solid evidence for both 'faster than the wind' (the dust is trailing to the rear while the vehicle goes downwind) and also for *directly* downwind (the dust trails out *straight* behind the vehicle).

These grabs are taken from this video:

http://www.youtube.com/watch?v=c5VlX-xEk00


Take particular note of the pilots helmet relative to the propeller pylons -- you can use these as a way to know when the camera shot is to the left, right or straight down the pipe.

I think you'll agree that it's pretty evident that it's line up pretty good with the wind justing by the dust trail.

http://forums.randi.org/attachment.php?attachmentid=17631&stc=1&d=1271725244

http://forums.randi.org/attachment.php?attachmentid=17632&stc=1&d=1271725335

http://forums.randi.org/attachment.php?attachmentid=17633&stc=1&d=1271725335

[Snixtor]

Link?
Actually, I suppose I gave a half truth. The only comment I've seen from any physicist is this one:

http://scienceblogs.com/dotphysics/2008/12/dwfttw-the-saga-continues.php

Associate Professor
Ph.D., North Carolina State University, 2001
Physics Education Research

I'm not entirely sure what those credentials mean. Does that make him a physicist? I'll gladly concede to misinterpreting.

It's probably another half truth saying "side of the fence". His comments don't seem to fall on either side of the prime question here "is it possible", much as mine don't. I think like many, he's not completely satisfied with any of the proposed explanations, and that the physics of it don't seem to add up.

[rwguinn]

Objects in motion remain in motion unless acted on by an external force (Newton III).

The earth is in motion in that frame; nothing is required to keep it in motion, although the acceleration of the cart will slow it down ever so slightly.
yep. Hover over NY, crash into the Tetons...
Did it ever occur to you that the cart is not inertialess (Hello, Grey Lensman!) WRT the Earth? There is no relative motion.

What keeps the air moving relative to a windmill?
pressure differences due to differential Solar (i.e., outside energy) heating

[sol invictus]

yep. Hover over NY, crash into the Tetons...
Did it ever occur to you that the cart is not inertialess (Hello, Grey Lensman!) WRT the Earth? There is no relative motion.

What?


pressure differences due to differential Solar (i.e., outside energy) heating

That's what got it going - relative to the earth - in the first place. That's not really what keeps it going. Once an air mass is moving, it has a lot if inertia. A few windmills aren't going to stop it (although they do slow it down locally).

Did you expect the earth to stop rotating because the cart accelerates? Nothing needs to "move the ground" - the idea is absurd.

[ThinAirDesigns]

For those concerned whether the vehicle is going directly downwind, here is a point of consideration:

It's easily shown that this vehicle's performance is greatest when running DDW. Any angle off the wind and less energy is extracted from the wind. Turn the vehicle away from the wind and it slowly loses speed to the point it will simply come to a stop.

Think about a sailboat gybing back and forth from reach to reach -- as long as the average course is DDW, the same sail and heading settings work perfectly in both directions. Make that average course off the wind however and now the boat needs to use different settings each direction or its performance suffers.

The vehicle in the video has no variable gearing and no cyclic rotor head. thus it is constrained to the same settings as the airfoil goes both directions.

JB

[rwguinn]

What?



That's what got it going - relative to the earth - in the first place. That's not really what keeps it going.

Did you expect the earth to stop rotating because the cart accelerates? Nothing needs to "keep the earth going" - the idea is utterly absurd.Whereinhell do you get that?
So little understanding of basic physics, so little time...sigh

[ThinAirDesigns]

Actually, I suppose I gave a half truth. The only comment I've seen from any physicist is this one:

http://scienceblogs.com/dotphysics/2008/12/dwfttw-the-saga-continues.php

Associate Professor
Ph.D., North Carolina State University, 2001
Physics Education Research

I'm not entirely sure what those credentials mean. Does that make him a physicist? I'll gladly concede to misinterpreting.

It's probably another half truth saying "side of the fence". His comments don't seem to fall on either side of the prime question here "is it possible", much as mine don't. I think like many, he's not completely satisfied with any of the proposed explanations, and that the physics of it don't seem to add up.

Rhett Allain is indeed a professor of physics and wrote this piece claiming concept to be impossible:

http://blog.dotphys.net/2008/12/physics-and-directly-downwind-faster-than-the-wind-dwfttw-vehicles/#more-902

To my knowledge he has yet to change his position.

JB

[sol invictus]

So little understanding of basic physics, so little time...sigh

You're talking about yourself?

You asked
What is moving the ground relative to the cart (we'll use the cart as a frame of reference, since you phrased it that way)

The question is absurd. If we start in a situation where the cart is moving relative to the earth and use a frame in which the cart is initially at rest, the earth has a ridiculously huge inertia. The motion of the cart is clearly not going to affect it in any noticeable way (although if you work out the math you'll find the cart actually does extract energy from slowing it down slightly). So nothing is "moving the ground", and nothing needs to - not according to Newton's laws of motion, at least.

[Snixtor]

Now, I know that the following sequence of screen grabs won't answer 100% of our question, but it it pretty solid evidence for both 'faster than the wind' (the dust is trailing to the rear while the vehicle goes downwind) and also for *directly* downwind (the dust trails out *straight* behind the vehicle).
The dust trail is a good indicator, but the more I have to pause, rewind and squint to get a good look, the less weight I give to it as evidence. It's a glimpse. An overhead shot would be more compelling. A helicopter would have some influence on the airflow though. Which does bring back the interesting consideration that the wind may not be directly parallel to the ground, for accurate measurement the wind direction would need to be asessed in three dimensions.

[jsfisher]

You're talking about yourself?

You asked


The question is absurd. If we start in a frame where the cart is moving relative to the earth, the earth has a ridiculously huge inertia. The motion of the cart is clearly not going to affect it in any noticeable way (although if you work out the math you'll find the cart actually does extract energy from slowing it down slightly). So nothing is "moving the ground", and nothing needs to - not according to Newton's laws of motion, at least.


I think rwginn may be confusing the initial conditions for his question. In the cart/treadmill scenario, the treadmill is moving when the experiment begins. Now, if you want to translate that to the equivalent cart/ground case, the ground is already moving when the experiment begins.

Perhaps rwginn was thinking of the cart starting from a dead stop, which isn't where the treadmill experiment begins.

[sol invictus]

Rhett Allain is indeed a professor of physics and wrote this piece claiming concept to be impossible:

http://blog.dotphys.net/2008/12/physics-and-directly-downwind-faster-than-the-wind-dwfttw-vehicles/#more-902

To my knowledge he has yet to change his position.

JB

After reading his second post on it - including the comments - I don't think it's a fair statement of his position to say he's still claiming it's impossible. He just sounds confused.

[ThinAirDesigns]

The dust trail is a good indicator, but the more I have to pause, rewind and squint to get a good look, the less weight I give to it as evidence. It's a glimpse.

That's why I present the screen grabs -- no 'pause, rewind, squint' necessary.

An overhead shot would be more compelling.

I suggest that I could run a vehicle further off the wind if you were watching me from overhead than if you were aligned looking right down my rig from front to back. There is a reason that we sight rifles down the barrel rather than from far above.

JB

[rwguinn]

I think rwginn may be confusing the initial conditions for his question. In the cart/treadmill scenario, the treadmill is moving when the experiment begins. Now, if you want to translate that to the equivalent cart/ground case, the ground is already moving when the experiment begins.

Perhaps rwginn was thinking of the cart starting from a dead stop, which isn't where the treadmill experiment begins.
It matters not. Energy input is required, both to start motion wrt the ground, and to keep moving wrt the ground.
Why would the card decouple from the frame of reference of the Earth? The Earth's inertia is not the issue, the cart's inertia with respect to the Earth is.
You guys are proposing a <unity machine.
Show me some free body diagrams, and/or some math on how the sucker works. I see hand waving and "The earth is already moving" ********. Give us some SCIENCE!

[Brian-M]

At no point does the ground begin to suddenly power the wheels of my bicycle, no matter how fast I hit the pedals. Yet in all these explanations this is what I am supposed to believe: All of a sudden the ground is transferring energy to the wheels.

It's not that simple. In these explanations, it's because the ground is moving relative to the wind. Peddling a bicycle is different, because windspeed is irrelevant.

But remember, objects tend to remain in motion unless energy is applied to them to change their motion. The wind applies energy to your bike to change its motion. Unless countered by an opposing force of energy, your bike would travel at windspeed constantly.

Let's say it's a an extremely windy day, and the wind is blowing very hard from behind you, pushing you along a level road with enough force that you don't need to peddle.

You're moving at a different speed from the ground, and at a different speed to the wind, and not providing any energy to the peddles.

Without the wind (ie. in a vacuum), the speed of bike would become the same speed as the earth, because of the energy loss from the friction with the ground. So where is the energy to overcome this coming from? The energy is coming from the wind.

But...

Without the earth (ie, hovering with an anti-gravity device), the speed of the bike would become the same speed of the wind, because of the energy loss from friction with the wind. So where is the energy to overcome this coming from? The energy is coming from the earth.

In that case, the earth is providing your bike with the energy it need to prevent it from traveling at the same speed as the wind. So the earth actually is providing your bike with energy all the time.

Drawing energy from the ground is commonplace, especially with braking. It takes exactly the same energy to stop a moving object as it does to get it moving at that speed in the first place. If you have a ten tonne truck moving at 60mph, where does the huge amount of energy required to stop the truck from moving come from? It comes from the ground.

Imagine you have a hot air balloon, and a wheel is attached to a generator on the bottom of the basket, and the balloon (racing along at windspeed) is lowered so the wheel touches the ground, and the generator produces power.

Where is this power coming from?

You could say it comes from the wind pushing the balloon along over the ground. But, if you use the air as you frame of reference, it would be equally correct to say the energy comes from the ground rushing along under the balloon.


I am not buying the treadmill-experiments, e.g. I am not sure how it works, but too much is wrong with it. The vehicle is pressed on the track to begin with, giving the propeller a chance to speed up and exceed the speed of the track, e.g. with just a little nudge. (And no, I can't explain what I see there, either.)

I remember seeing videos where they tilted the treadmill up slightly. Not only was the cart traveling against the direction of the treadmill belt, but it was also going uphill. In one video they tilted the treadmill up exactly enough that the cart stayed in one place continuously. In another they tilted it less, but every time the cart reached the top, Spork pushed it downwards (with a plastic spork), so that the cart was moving in the same direction as the belt, but every time he did this the cart managed to regain its lost speed and work it's way up the belt again without any help. It did this repeatedly.

Would the experiment with the ruler cart still work if I went and used cogwheels with a fixed, unbreakable transmission and then used a rules with matching cogs as well?

Yes.

And even then I would not be convinced: I can move a bicycle from the ground by turning the pedals - for as long as I can reach the pedals. That's what the ruler does. The ruler will no longer reach the car or be able to push the big wheel forward as soon as the car is as fast as the ruler, though.

No. The ruler car will still travel faster than the ruler, even with matching cogs on the ruler and top wheel.

Using your bike example, the peddles are like the wheel the ruler is pushing. Doesn't your bike travel along the ground much faster than your feet turn the peddles? It's exactly the same thing, only a matter of gearing.

[ThinAirDesigns]

After reading his second post on it - including the comments - I don't think it's a fair statement of his position to say he's still claiming it's impossible. He just sounds confused.

We have exchanged emails with Rhett -- he has not yet changed his initial position, but of course I admit he's confused.

JB

[MRC_Hans]

:dl: :dl:

when ground = windspeed = zero energy to be harvested...

this is getting hilarious.

tell us oh wise one....does the cart move when there is no wind`?

I'm sorry, it's counterintuitive, it's bloody unfair, but, yes the cart is feasible.

To see how it is possible, think of this construction (it won't work well, because it is ineffective, but it may be easier to understand):

Imagine a large spoked wheel (add support wheels to keep it from toppling as you whish).

On each spoke is a foldable sail.

A mechanism opens the sail when the spoke is pointing towards the ground, and keeps it closed all other times.

Now, since the axle of the wheel is above the sails, it will move faster than the sails. You have geared the thrust from the wind.

Same things happens with the propeller in an autogyro, or a prop driven cart:

The wind drives the the blades round, but since they are coupled to the wheels (or in the autogyro, pulled through the air by an engine), the plades will cut the air at a speed much higher than the wind. So even if the vehicle is moving downwind at a speed higher than the wind, the blades can still cut the wind with an angle of attack that generates a lift, pulling the vehicle. It is a form of gearing.

Hans

[Brian-M]

It matters not. Energy input is required, both to
start motion wrt the ground, and to keep moving wrt the ground.

The only energy required to keep it moving is to overcome friction. But yes, in practice it does require energy to keep it going.

Why would the cart decouple from the frame of reference of the Earth?

What the hell are you talking about? Nothing decouples from any frame of reference, that's nonsensical. Every frame of reference applies all the time. We simply use whatever frame of reference that makes things easier to understand, but it really doesn't matter which one we use, all frames of reference are equally valid.

(You could use the motion of a train running perpendicular to the cart as a frame of reference if you wanted to. It would be absurd to do so, but still equally as valid.)

You guys are proposing a <unity machine.


Well, yes. It is a < unity machine. All machines are < unity machines. Nobody's ever built a working > unity machine.

But assuming that you meant a > unity machine, then no it's not. The cart runs of kinetic energy drawn from the kinetic energy available from the difference in velocities between the ground and the air. Without a difference in velocity between the earth and the air to draw from (ie, no wind) the cart slows down and stops because it no longer has an external source of power.

[Snixtor]

Rhett Allain is indeed a professor of physics and wrote this piece claiming concept to be impossible
I'll agree with Sol that he doesn't seem to be declaring it impossible. But more like "under these principles, I don't see how it could work". I won't agree with Sol that he sounds confused though, what I've read seems more like "questioning".

Are there any other physicists out there passing comment on the feasibility of it, maths, etc? I know there are physicists involved in the practical experiments, but that doesn't really indicate their view of the theory involved. Indeed, good scientists should be willing to conduct an experiment whether or not their theories and calculations say it will work.

That's why I present the screen grabs -- no 'pause, rewind, squint' necessary.
Unfortunately it's still all a bit too blurry and unclear for me. In the middle screen grab it sort of looks like there's more dust to the right of the picture. Or at least more blurring to the right, which could be caused by dust. But that's the problem. I can't tell what bits are dust from the cart, what bits are blurring, what bits are video compression artifacts, etc.

I suggest that I could run a vehicle further off the wind if you were watching me from overhead than if you were aligned looking right down my rig from front to back.
Not sure I understand what you're saying? If observed from overhead, the vehicle will appear to go further?

[sol invictus]

It matters not. Energy input is required, both to start motion wrt the ground, and to keep moving wrt the ground.

Yes, to overcome friction. It comes from the wind. To be precise, it comes from slowing the air with respect to the ground. That energy has to go somewhere, and the only place it can go is into accelerating the cart (or doing work against the friction that prevents the cart from accelerating).


You guys are proposing a <unity machine.

I assume you mean >unity?

This cart operates by wind power. It gains energy by reducing the kinetic energy of the air (or the earth depending on frame) just as all other wind powered devices do (sailboats, windmills, etc.). To show it's over-unity, you have to show it is gaining more energy than is lost by the wind. Be my guest - go ahead and try. You might come to understand how it works in the process.

Show me some free body diagrams, and/or some math on how the sucker works.

That's all been done, many many times, on this forum. In a nutshell, the forces are:

thrust from the prop
drag from the wheels
air resistance if the cart isn't at wind speed.

Power = force*velocity. If the velocity of the cart with respect to the air is less than the velocity of the cart with respect to the ground, then if we set power in = power out, the thrust must be larger than the drag (at least at wind speed), and so the cart will accelerate to faster than wind speed. Taking into account losses, it will accelerate if it's efficient enough and the wind is strong enough.

That's exactly what you see on the treadmill.

[ThinAirDesigns]

Not sure I understand what you're saying? If observed from overhead, the vehicle will appear to go further?

I am suggesting that looking from above is not necessarily a great way to compare a dust trail to the exact direction the vehicle is pointing.

I'm saying that if I were the driver and you were responsible for ensuring that I was always leaving a dust trail directly behind me, I believe I could 'cheat' you more if you were looking from the heli view rather than straight down my nose -- that's why the rifle analogy. It's difficult to tell exactly where a rifle is pointing from directly above, while looking down the barrel has proven to be quite accurate.

JB

[ThinAirDesigns]

Show me some free body diagrams, and/or some math on how the sucker works. I see hand waving and "The earth is already moving" ********. Give us some SCIENCE!

I submit the attached.

JB

[ThinAirDesigns]

I'll agree with Sol that he doesn't seem to be declaring it impossible.

Then perhaps we're not reading the same entry:

From:
http://blog.dotphys.net/2008/12/physics-and-directly-downwind-faster-than-the-wind-dwfttw-vehicles/#more-902

Clearly, there is a problem if a vehicle is wind powered and travels faster than the wind. You see it, I see it. But there are many that do not see this as a problem. Really, the reasoning in this area is the same as for most perpetual motion machines. Energy for nothing. Free energy – it’s not the 70’s. Stuff isn’t free.

Ok – what if I made the air going FASTER after it goes through the wind generator? Well, then momentum would be ok – but not energy. The air would gain energy, where did it come from? Magic? Do you see why this is a problem?

Etc.

JB

[Snixtor]

I'm saying that if I were the driver and you were responsible for ensuring that I was always leaving a dust trail directly behind me, I believe I could 'cheat' you more if you were looking from the heli view rather than straight down my nose -- that's why the rifle analogy. It's difficult to tell exactly where a rifle is pointing from directly above, while looking down the barrel has proven to be quite accurate.
Ah I see what you mean. It sounded like you were saying the vehicle would appear to go further when being observed from far above. Which led me to think you were trying to bring general relativity into it. :p

[Brian-M]

I'm still not entirely sure of what I'm seeing in the treadmill experiment, and whether it really is a proper analogue of the cart on the ground in the wind.

Imagine you're on a planet that has a perfectly smooth, featureless surface. The surface is also frictionless. You're standing on a giant mat that's attached to a cable. The other end of the cable is past the horizon, so you can't see what it's attached to. There is also a strong wind coming from the direction of the cable.

The question is, without using an external references, can you tell whether the cable is keeping the mat tethered in place against a strong wind, or if the cable is dragging the mat rapidly along through still air?

You can't*. There is no mechanical experiment you can do on the mat that will tell you whether the mat is moving or if the air is.

It comes from a basic principle of physics that the laws of physics work the same in all frames of references.

It doesn't matter whether or not the mat is being dragged along the surface of the planet or not. Either way, using the mat itself as a frame of reference, the mat is still and the air is moving.

It doesn't matter whether the air is moving or if the mat is being dragged through it. Either way, using the air itself as a frame of reference, the air is still and the mat is moving.

To say that a cart that works on a moving surface in still air won't work on a still surface in moving air goes against well established and endlessly tested principles of physics.

* Since the surface of the planet is curved you could set up some kind of gyroscope experiment, so for the sake of argument let's assume that the planet is a hollow sphere of infinite size.


In the treadmill test, the unlimited torque (comparatively) of the treadmill turns the wheels, which turn the prop.
Off the treadmill, what turns the wheels?

The unlimited torque (comparatively) of the earth (relative to the air).

[Snixtor]

Clearly, there is a problem if a vehicle is wind powered and travels faster than the wind. You see it, I see it. But there are many that do not see this as a problem. Really, the reasoning in this area is the same as for most perpetual motion machines. Energy for nothing. Free energy – it’s not the 70’s. Stuff isn’t free.
I read that statement as a bit of a devils advocate. Kind of like starting a discussion of superfluids with "clearly there is a problem if a fluid travels up hill, we all see it".

His position would seem to be though: "by these principles, it wouldn't work". Let's just be careful in what we distinguish "it" to be. The principle of DDWFTTW? The cart as it is designed? The physics principles that are claimed to be involved? I'd say his statement rests firmly in the 3rd.

[I Ratant]

I am suggesting that looking from above is not necessarily a great way to compare a dust trail to the exact direction the vehicle is pointing.

I'm saying that if I were the driver and you were responsible for ensuring that I was always leaving a dust trail directly behind me, I believe I could 'cheat' you more if you were looking from the heli view rather than straight down my nose -- that's why the rifle analogy. It's difficult to tell exactly where a rifle is pointing from directly above, while looking down the barrel has proven to be quite accurate.

JB
.
The defensive weaponry on the B-29/B-50/B-36 had the sights offset from the locations of the guns.
A little trigonometry permitted the gunner to track incoming attackers and shoot at them fairly accurately.
In this instance, there's no vertical direction of motion of the vehicle, so a view looking down wouldn't be inaccurate.

[ThinAirDesigns]

.
The defensive weaponry on the B-29/B-50/B-36 had the sights offset from the locations of the guns.

Somehow I do doubt that this "slightly offset" view was from above, looking down on the barrels.

JB

[ThinAirDesigns]

I read that statement as a bit of a devils advocate. Kind of like starting a discussion of superfluids with "clearly there is a problem if a fluid travels up hill, we all see it".

His position would seem to be though: "by these principles, it wouldn't work". Let's just be careful in what we distinguish "it" to be. The principle of DDWFTTW? The cart as it is designed? The physics principles that are claimed to be involved? I'd say his statement rests firmly in the 3rd.

Again, I will state that we have had direct exchanges with Rhett. I'm quite comfortable and confident in my interpretation and assertions regarding his position. To my knowledge his position hasn't changed.

JB

[ThinAirDesigns]

We also have emails from one Dan Kammen, physicst, Nobel winner and Professor at UC Berkeley stating that ddwfttw is impossible and that the treadmill is not a valid test of such.

It's a brainteaser that trips up many smart folks.

JB

[Modified]

We also have emails from one Dan Kammen, physicst, Nobel winner and Professor at UC Berkeley stating that ddwfttw is impossible and that the treadmill is not a valid test of such.

It's a brainteaser that trips up many smart folks.

JB

For most people (maybe not physicists), I think the incorrect intuition is "if the wind hits a prop, then it will act as a turbine, no matter what else is involved" or for the yo-yo/wine glass/ruler cart, "when you push or pull on a wheel, it turns on its axle in the direction you are pushing or pulling, no matter what else is involved".

[quarky]

Great thread. This debate is new to me, as I'm not a sailor or physicist. After perusing some of the links and you-tubes, i can get my brain around it, but now I'm curious about the ice-boats mentioned earlier, which have (evidently) already achieved this to credible satisfaction.

Can anyone explain how that works, or provide a link to the type of ice-craft wherein this is possible? I assume the same principles apply, yet without the prop and the wheel gearing relationship? Or do these crafts also use a prop?

[ThinAirDesigns]

Great thread. This debate is new to me, as I'm not a sailor or physicist. After perusing some of the links and you-tubes, i can get my brain around it, but now I'm curious about the ice-boats mentioned earlier, which have (evidently) already achieved this to credible satisfaction.

Can anyone explain how that works, or provide a link to the type of ice-craft wherein this is possible? I assume the same principles apply, yet without the prop and the wheel gearing relationship? Or do these crafts also use a prop?

The important thing to remember is that while ice-boats and land-yachts have for decade now been able to achieve downwind VMGs of greater than windspeed, they cannot achieve this while going directly downwind but rather through running at an angle to the wind.

That is the key difference between DDWFTTW and what the previous craft have been capable of.

JB

[ThinAirDesigns]

Show me some free body diagrams, and/or some math on how the sucker works. I see hand waving and "The earth is already moving" ********. Give us some SCIENCE!

Here is some more SCIENCE for rwguinn, these treatments from one of the most accomplished and respected aerodynamicists in the world -- MIT's Mark Drela.

Drela's treatments are specifically for a water based craft (a much more difficult task), but he addresses the wheeled vehicle with the following statement: "the DDWFTTW condition V/W > 1 is achievable with a wheeled vehicle without too much difficulty".

Attached:

[Snixtor]

The important thing to remember is that while ice-boats and land-yachts have for decade now been able to achieve downwind VMGs of greater than windspeed, they cannot achieve this while going directly downwind but rather through running at an angle to the wind.

That is the key difference between DDWFTTW and what the previous craft have been capable of.

JB
Is it possible that the propeller cart is "tacking" in the sense that there is a frequency of lateral force against the wheels? Sort of "vibrating" its way down its path?

[DogB]

Evidently by spending your time arguing with AGW deniers, you have adopted their way of thinking…

There are those who would say his thinking hasn’t changed a damn…
;)

I submit the attached.

JB

Nice! To me the idea is intuitively possible but your paper solidified my instinct.

[Modified]

Is it possible that the propeller cart is "tacking" in the sense that there is a frequency of lateral force against the wheels? Sort of "vibrating" its way down its path?

No, the sails (prop blades) are "tacking" continuously.

[ThinAirDesigns]

Is it possible that the propeller cart is "tacking" in the sense that there is a frequency of lateral force against the wheels? Sort of "vibrating" its way down its path?

It's definitely a steady state force from the wheels to the prop, but the 'tacking' of the prop is actually far more literal than one might imagine at first.

Here is an image that helps one visualize the fact that the spinning airfoil of the prop is actually on one long continuous downwind 'tack' (or more accurately, a "reach").

http://forums.randi.org/attachment.php?attachmentid=17639&stc=1&d=1271739032

JB

[Snixtor]

No, the sails (prop blades) are "tacking" continuously.
Eeeeinteresting. So when going "faster than the wind", the "tacking" motion and angle of the blades means that wind is still hitting the back of them?

[ThinAirDesigns]

Eeeeinteresting. So when going "faster than the wind", the "tacking" motion and angle of the blades means that wind is still hitting the back of them?

That is correct.

JB

[383LQ4SS]

All this thread is doing is making me wish I lived in an area that had wide open, unregulated, flat spaces with a constant wind. I would build something and have some fun of my own!

[Brian-M]

Eeeeinteresting. So when going "faster than the wind", the "tacking" motion and angle of the blades means that wind is still hitting the back of them?

That depends on what you mean by wind. When traveling faster than the wind, you have a headwind moving from the front to the back of the cart, but because of the angle of the blades and the speed the prop is turning, the pressure on the blades from the air is on the back of the blades, pushing them (and consequently the entire cart) forward.

(But that's the same way any propeller works.)

[Snixtor]

So the wheels causing the propeller to spin, aren't really providing the "driving force" as such, but spin the propeller in such a way that it can "continuously tack" to allow the air pressure behind it to "push it". Uhh... quote unquote.

ThinAir, could you explain that diagram / graph a little? It looks great, but it's not immediately obvious what all the lines represent.

[ThinAirDesigns]

ThinAir, could you explain that diagram / graph a little? It looks great, but it's not immediately obvious what all the lines represent.

The black dotted line represents the direction of travel of the vehicle chassis (and thus the propeller axle).

The helix portion of the bold red line represents the path of the *tip* of the propeller and the later portion of the line shows it's path conceptually 'unwrapped' into one broad reach.

The lighter red lines represent the same 'unwrapped' path of the prop, but for progressively inward segments of the propeller.

JB

[Snixtor]

The lighter red lines represent the same 'unwrapped' path of the prop, but for progressively inward segments of the propeller.
Ah yes, its "tacking" path, always angular to the direction of travel. There wouldn't need to be a lateral force in the wheels because the propeller is balanced, while one part of it tacks in one direction, another part tacks the other way.

[Andrew Wiggin]

Eppur si muove!

Α

[recursive prophet]

As for 3x... that's purely a question of engineering. Directly dw vehicles have a major advantage (over sailcarts or boats) in that the apparent wind they see at Nx wind speed is only (N-1)x. So 3x wind speed may well be possible; the cart will see only a 2x apparent wind, which makes air resistance less of an issue.

So I'd say yes, I think it's possible, and probably will be achieved soon (if it hasn't already) by spork and co.
But at what point would this go beyond engineering is my question? At 3xws wouldn’t the headwind created by the Blackbird’s forward motion be greater that the tailwind moving it forward? Surely there must be some theoretical limit? How would it be determined?

In a laminar flow it simply will not happen in the same manner as that flow is smooth and uni-directional....

for starters the windspeed in the real world is an average of the gusts....it's one reason they state .....60 kph wind gusting to 80 kph....well it's ALWAYS gusting up and down....there is likely no laminar flow in nature.

Unless you can see why the treadmill with its ..for practical purposes in these kind home experiments...infinite torque...you'll flounder.
If someone has responded to the above comment by mac I missed it. I have seen similar arguments made elsewhere, and would appreciate learning the POV of those here at JREF.
I will grant you that it is counter-productive. It is also not nice, and should classify as name-calling. :o I am not perfect. It is probably more a case of "persistently misunderstand or fail to grasp" than "deny". As I said, I fail to see any motive to actually denying classical physics except perhaps trolling. And I don't really understand the point of that, either. :)
http://forums.randi.org/images/smilies/grouphug4.gif Pickin up some good vibes here Ririon. Just as an experiment if nothing else, I'd like to see if we can keep this a highly civil discussion of this topic on this thread. After all, we will soon have scientific evidence about whether the claims made for ddwfttwss are true or false. A debate that goes back to the middle of the last century will finally be ended. (Mostly. ;))

Neither side denies classical physics as far as I know. The arguments are about how they are applied and what parameters/terminology are relevant. It intrigues me that some remain adamant ddwfttw is impossible in the face of such overwhelming evidence and opposition.

Aha! They've covered it in black to hide the motor!
Close, but no cigar.
The choice of BLACK is very appropriate for the BUFC as it will soon be having its own funeral!
On this forum, his (spork’s) need for a back-up team to handle his technical claims, is the last piece of the gallows.

<snip> Neither way of analyzing it violates conservation of energy. However, if you muddle the two (as in, "you can't gain energy from the wind because at windspeed the wind is stationary, and the energy gained from thrust from the propeller can't possibly exceed or equal the kinetic energy the cart loses at the wheels from driving the propeller to generate that thrust") it's easy to erroneously conclude an "over unity" impossibility.
Nice, succinct summary. But I’m still disappointed none have really commented on your earlier analysis. Oh, and I forgot to mention NASA has invited spork and jb to test Blackbird at one of their facilities that has a runway

Now that jb is here, I won’t even bother to try and answer any more questions. (Though perhaps I’ll still ask a few.) AKA ThinAirDesigns, jb is the guy who put together the blog linked in blue on my sig, along with most of the physical construction of the BB, I suspect. He has also designed hang gliders, and at one time held some world records in the sport. If he can’t convince macdoc and a few other skeptics that ddwfttwss is possible, I doubt anyone can. My work here is thus done, and I can go back to just observing. It’s been a really interesting read so far, and thanks to all who have contributed.

[Snixtor]

http://forums.randi.org/imagehosting/13012493204e1d7668.jpg
ThinAir, could I get you to consider the above diagram poster earlier by Myriad? It seems to be at odds with the understanding I've drawn of the propeller cart based on your descriptions. Is the above parachute cart equivalent? My understanding is that there would be no "tacking" involved in such a design, while that is a critical concept for the propeller cart, thus it is not equivalent.

[Thabiguy]

But at what point would this go beyond engineering is my question? At 3xws wouldn’t the headwind created by the Blackbird’s forward motion be greater that the tailwind moving it forward? Surely there must be some theoretical limit? How would it be determined?

Well, there's the theoretical limit of the speed of light.;) Other than that, I believe the limits are practical rather than theoretical - in theory, you can always increase the equilibrium speed by using "higher gear" and reducing unwanted friction and drag; it's practical considerations that prevent it. I mean, if you could afford to construct gigantic high-tech tracks in outer space (and were liberal enough with the definition of "cart" and "wind"), this could stay within the realm of engineering (and financing) even for ridiculously high speeds.

(On the other hand, if you actually had resources of such magnitude, spending them on this would be really silly.)

[Jack by the hedge]

So the wheels causing the propeller to spin, aren't really providing the "driving force" as such, but spin the propeller in such a way that it can "continuously tack" to allow the air pressure behind it to "push it".

Yes.

If you stood behind the cart and pushed on the back of the propeller with your finger (pretending to be a streamline of wind) then, if the prop wasn't connected to the wheels, you'd expect the prop to turn aside like a windmill and let your finger move past.

With the prop mechanically linked to the wheels, that doesn't happen. Instead, when your finger presses forward, the whole cart rolls away from you and the prop actually turns the opposite way to before, seeming to "climb away" from your finger (it has to, because that's the way it's geared to the wheels). The point of contact of the prop blade sliding across your fingertip is gradually moving backwards relative to the body of the cart. So the cart actually moves forward a little faster than your fingertip - it's moved the distance your finger has moved plus the depth of the pitch of the propeller.

Instead of lots of pushing fingertips we have streamlines of wind. Each one which bears on a point on the propeller blade finds that, as the blade slides past, its surface appears to move back towards the wind.

So if the wind and the cart were both travelling at 10mph relative to the ground, the surface of the propeller which the wind pushes on appears to be moving more slowly than that.

[Andrew Wiggin]

Am I the only one who just got this intuitively and doesn't have a ginormous problem with this concept? It's just a logical extension of the sailboat or autogyro. I think this is an elegant and clever solution. I don't think it's especially practical but perhaps for wind powered trains or such that didn't need to do a lot of direction changing... Seems to me that one could reverse the gearing of the prop, or reverse its pitch and make a cart that sailed into the wind, with the prop driving the wheels, in order for the train car to make the return trip.

A

[H'ethetheth]

Am I the only one who just got this intuitively and doesn't have a ginormous problem with this concept? It's just a logical extension of the sailboat or autogyro. I think this is an elegant and clever solution. I don't think it's especially practical but perhaps for wind powered trains or such that didn't need to do a lot of direction changing... Seems to me that one could reverse the gearing of the prop, or reverse its pitch and make a cart that sailed into the wind, with the prop driving the wheels, in order for the train car to make the return trip.

AI don't know, but I didn't get it intuitively. That is, it took me about 4 pages of reading the original thread before realizing I had the whole thing figured the wrong way around.

[Jack by the hedge]

Am I the only one who just got this intuitively and doesn't have a ginormous problem with this concept?

I couldn't get my head round it for quite a while. I got the plane on a conveyor belt puzzle much faster. :)

[sol invictus]

But at what point would this go beyond engineering is my question? At 3xws wouldn’t the headwind created by the Blackbird’s forward motion be greater that the tailwind moving it forward?

I don't understand your question. If you're talking about the "true wind" (i.e. the relative velocity of air to ground), that has nothing to do with the cart's motion. If you're talking about "apparent wind" (i.e. the relative velocity of air to cart), there's never a "tailwind" once the cart is above wind speed.


Surely there must be some theoretical limit? How would it be determined?


There is no theoretical limit of any relevance. In an ideal (no friction) Newtonian world, the steady state speed of the cart could be any multiple of wind speed. However the net thrust available to it below that speed will get smaller and smaller as the multiplier gets larger. With friction there's of course a top speed, but it will depend strongly on the design and materials, and it's very difficult to predict what it will be. I'd guess the best multiple they will reach will be around 4x.

[sol invictus]

Am I the only one who just got this intuitively and doesn't have a ginormous problem with this concept?

I can't understand why anyone has a problem with it. The essential mechanism is literally no harder to understand than a windmill.

Seems to me that one could reverse the gearing of the prop, or reverse its pitch and make a cart that sailed into the wind, with the prop driving the wheels, in order for the train car to make the return trip.


You don't have to reverse anything. You just have to change the pitch a bit.

[Thabiguy]

Seems to me that one could reverse the gearing of the prop, or reverse its pitch and make a cart that sailed into the wind, with the prop driving the wheels, in order for the train car to make the return trip.

Again, this is not the case. If the transmission or the propeller is reversed, the cart will still move in the direction of the wind, only slower and "stronger" (with equilibrium speed below windspeed). To move against the wind, the propeller needs to turn the same way (from the cart's point of view) as in the "faster than the wind mode", but faster.

The behavior of an idealized cart is described by a few simple relationships. The key value is the "advance ratio" f, given by the construction and gearing (i.e. constant regardless of cart or wind speed), which can be expressed in two ways:

fcart = vairscrew / vground (in cart frame)
fground = vairscrew / vcart (in ground frame)

The relationship between the two is fcart = 1 - fground (or vice versa). (Airscrew is the moving virtual helical surface that the propeller blades approximate a part of.)

The equilibrium speed of an idealized cart is then

videal = vwind / fground (in ground frame)

(At equilibrium speed, the airscrew of an idealized cart is at rest wrt air.)

From this, it can be seen that to move against the wind, fground must be < 0, therefore fcart > 1, as compared to "faster than the wind mode", in which 0 < fcart < 1.

[macdoc]

Mr. Wiggin....

Windmill Sailboat: Sailing Against the Wind (http://www.treehugger.com/files/2007/02/windmill_sailbo.php)

by Christine Lepisto, Berlin (http://www.treehugger.com/author/christine-lepisto-berlin-1/) http://www.treehugger.com/images_site/feed-icon-10x10.png (http://www.treehugger.com/feeds/authors/Christine%20Lepisto.xml) on 02.18.07



http://www.treehugger.com/files/2007/02/windmill_sailbo.php

[Myriad]

ThinAir, could I get you to consider the above diagram poster earlier by Myriad? It seems to be at odds with the understanding I've drawn of the propeller cart based on your descriptions. Is the above parachute cart equivalent? My understanding is that there would be no "tacking" involved in such a design, while that is a critical concept for the propeller cart, thus it is not equivalent.


I can answer that (though I certainly invite ThinAirDesigns to provide his own answer). It is not equivalent in that it does not use tacking. Every component is oriented directly parallel to the wind.

There is, however, still a lever or "wedge" involved, comparable in principle to the tack angle of an iceboat sail or the pitch angle of the Blackbird's propeller. It's hidden in the center wheel. Imagine a line running vertically from the wheel's axle down to the contact point on the road. When the 'chute cart moves a small distance along the ground, the wheel turns by a small angle; this pulls the rope (in the direction that pulls the parachutes back toward the rear of the cart) by a smaller proportional amount, determined by the ratio of the two radii.

While the 'chute cart does work in principle, it would be fraught with practical difficulties and I would not make a wager I couldn't afford to lose on being able to build a working model at any scale that would successfully run downwind faster than the wind. (It's kind of like imagining a heavier than air aircraft using a giant vertical conveyor belt with parachutes along it that furl on the way up and unfurl on the way down, instead of wings and propellers. Not physically impossible in principle, but making the framework and moving parts sufficiently light, strong, low-drag, and mechanically reliable might be beyond the limits of available materials.)

Respectfully,
Myriad

[ThinAirDesigns]

Mr. Wiggin....



http://www.treehugger.com/files/2007/02/windmill_sailbo.php

While certainly an interesting boat with it's 'direct upwind' capabilities, that boat isn't capable of ddwfttw. It's blades are designed for and operate only in turbine mode.

JB

[ThinAirDesigns]

ThinAir, could I get you to consider the above diagram poster earlier by Myriad? It seems to be at odds with the understanding I've drawn of the propeller cart based on your descriptions. Is the above parachute cart equivalent? My understanding is that there would be no "tacking" involved in such a design, while that is a critical concept for the propeller cart, thus it is not equivalent.

As Myriad correctly shows, theoretically, ddwfttw can be done with pure drag (no lifting surfaces) if the drag force is properly located and then mitigated when returning the drag device (going the other direction). There are dozens if not scores of ideas that have been sketched to do this -- some with parachutes, some with collapsing flaps, etc., , etc.

The method of using a propeller has many advantages -- few moving parts, rather high efficiency, etc., but it would certainly be fun to see someone build a 'non-prop' version.

Snixtor, you'll have to explain your confusion between the two in more detail for me to formulate a response of much value -- but do remember that they are two entirely different methods of accomplishing the same thing.

JB

[Modified]

Eeeeinteresting. So when going "faster than the wind", the "tacking" motion and angle of the blades means that wind is still hitting the back of them?

Yes, but not literally. In the simplest sense, when determining the winds "push" in one direction, you should consider the intersection of a line parallel to the wind and a prop blade (same goes for any sail). Because the blade is turning, that intersection point moves backward wrt to the cart. As long as that point (and all or most such points) are moving slower than the wind wrt to the ground, there is a forward push. As I mentioned before, the same thing would happen if you pushed straight forward on the prop blade with your finger, as long as doing so didn't bend it too much.

[ThinAirDesigns]

But it would be interesting to see without those vertical sails what the performance difference is.

Going back to the very start of this thread ...

The "vertical sails" that macdoc refers to and shown in the videos are 2.25" thick and 16" long and are completely rectangular in shape. Of course anyone with a modicum of aero knowledge (and even most without) will recognized the impossibility of generating any usable lift from those rectangles in this application.

Those "vertical sails" to which macdoc refers are simply prop tower structural members.

JB

[I Ratant]

... (It's kind of like imagining a heavier than air aircraft using a giant vertical conveyor belt with parachutes along it that furl on the way up and unfurl on the way down, instead of wings and propellers. Not physically impossible in principle, but making the framework and moving parts sufficiently light, strong, low-drag, and mechanically reliable might be beyond the limits of available materials.)

Respectfully,
Myriad
.
This is the overshot waterwheel idea of perpetual motion in essence.
Dat ol' debbil friction extracts its due.
Perpetual motion machines always have their hearts ripped out on the altar of friction.
.
ETA...added pichur.
.
And the link to the history of such..
http://www.lhup.edu/~dsimanek/museum/unwork.htm

[Thabiguy]

This is the overshot waterwheel idea of perpetual motion in essence.

Not at all. What Myriad described here

(It's kind of like imagining a heavier than air aircraft using a giant vertical conveyor belt with parachutes along it that furl on the way up and unfurl on the way down, instead of wings and propellers. Not physically impossible in principle, but making the framework and moving parts sufficiently light, strong, low-drag, and mechanically reliable might be beyond the limits of available materials.)

is an aircraft with its own power source, and the "vertical conveyor belt with parachutes" is a device that lets it hover in the air (spending energy, rather than extracting energy). It is in no way like a PMM.

The purpose of that hypothetical contraption was obviously to illustrate that while such an aircraft might work in principle, the construction is so clumsy that perhaps a working model could not be constructed with available technology at all.

[Michael C]

.
This is the overshot waterwheel idea of perpetual motion in essence.

No, there is a fundamental difference. A "perpetual motion machine" is supposed to work without an external energy source. This machine has an energy source: the wind. As long as the wind blows, any wind-powered machine, including this one, will keep working. That doesn't put them in the category of "impossible perpetual motion machines".

This particular wind-powered machine only surprises people because it goes faster than the wind that powers it: many think that in order to achieve this, it would have to be putting out more energy than it takes in. It doesn't. It's no more surprising than a bicycle, really: the wheels turn faster than the feet that power them.

[I Ratant]

It's just a variation on the theme...
From that source..
And note the blunder in the construction.. :)

[sol invictus]

It's just a variation on the theme...
From that source..
And note the blunder in the construction.. :)

No, you're wrong. Myriad's design does not constitute a perpetual motion machine any more than a windmill does (and please note that windmills actually work, unlike the gravity mill your image shows). While I think Myriad's design would be hard to construct for various practical reasons, there's absolutely nothing wrong with it in principle.

People in this thread keep forgetting that these devices only work when there is a clear and obvious source of power - the wind.

[3point14]

Yes.

If you stood behind the cart and pushed on the back of the propeller with your finger (pretending to be a streamline of wind) then, if the prop wasn't connected to the wheels, you'd expect the prop to turn aside like a windmill and let your finger move past.

With the prop mechanically linked to the wheels, that doesn't happen. Instead, when your finger presses forward, the whole cart rolls away from you and the prop actually turns the opposite way to before, seeming to "climb away" from your finger (it has to, because that's the way it's geared to the wheels). The point of contact of the prop blade sliding across your fingertip is gradually moving backwards relative to the body of the cart. So the cart actually moves forward a little faster than your fingertip - it's moved the distance your finger has moved plus the depth of the pitch of the propeller.

Instead of lots of pushing fingertips we have streamlines of wind. Each one which bears on a point on the propeller blade finds that, as the blade slides past, its surface appears to move back towards the wind.

So if the wind and the cart were both travelling at 10mph relative to the ground, the surface of the propeller which the wind pushes on appears to be moving more slowly than that.

NOW I've got it! Thank you!

[Snixtor]

'chute cart
I think I follow now. The cart can move forward, wrt the ground faster than the wind, but the 'chute itself, at least when "deployed" at the top, does not?

[Myriad]

I think I follow now. The cart can move forward, wrt the ground faster than the wind, but the 'chute itself, at least when "deployed" at the top, does not?


Correct, within a certain range of cart speed. The chute along the top moves backward (right to left) relative to the cart at about 2/3 of the cart's (left to right) ground speed. That also means it is going forward (left to right) relative to the ground at about 1/3 of the cart's (left to right) ground speed. So if the cart were going downwind left to right at, say, 150% of the the wind speed, the chute would be going left to right at only 1/3 * (150%) or 50% of the wind speed, so the wind would still be pushing on it.

The crude drawing is supposed to suggest that there are several chutes along the rope, and that just as one is being pulled around the rear (left) pulley and collapsing and being drawn into a tube in the process, another is emerging from the other end of the tubes at the front (right) pulley and self-deploying. That's one of several "engineering details" that, though they don't affect the principle of the thing, would be very difficult to get working smoothly in practice.

By comparison to Jack by the Hedge's "finger on propeller" discussion a few posts back, if you were to push on the top 'chute with your finger (or let's say, if there were no wind filling the chute, the knot where the chute's lines attach to the rope), for every 1 inch you push (left to right), the cart frame would move about 3 inches (left to right) so moving ahead of your finger, and the knot would "climb against your finger" (right to left relative to the cart) toward the rear (left) pulley by about 2 inches.

Respectfully,
Myriad

[I Ratant]

I really can't finger this one out as you describe it.
It's like lifting yourself with your own bootstraps.
If the parchute/line direction is changed to the same direction as the cart/wind, I can almost see it working in my mind's eye.
Looking at your drawing and description, I see it evolving this way....
I'm minded of a device, say a kite/balloon, attached to a line, which is on a reel.
Let the kite/balloon free in a wind, and let it pull out the line unrestricted.
It will go downwind at or slightly less than wind speed due to the drag of the line until
the line runs off the reel.
There's little reaction at the reel from the pull on the line.
Now add a drag to the reel, slowing the progress of the kite downwind.
This places a load on the reel's support.
When the reel's support is on a platform with wheels, the platform can then be
observed to go downwind reacting to the pull from the load.
Prevent the reel from turning, and the coupled device will then move downwind
as fast as the friction in the reel's supporting wheels (boat hull) and the drag of the line to
the device permit.
This will be less than the wind speed.
With the reel -just- capable of turning, and feeding more kites/balloons on the line,
paying out the line to a certain distance, and then pulling it back around a something,
and retrieving those kites/balloons back to the moving support, an infinite length of tether
need not be provided for.
If the drag on the reel is provided by a link to the support wheels (boat drive), these can turn and drive
the support downwind adding to the force the kite/balloon less the drag of the line provide for motive
power.
Pulling the kites/balloons back would require some manner of removing the aerodynamic drag these will
have due to their speed upwind on the return trip.
Or the kites/balloons could be replaced by the parachutes, which can be more easily collapsed than the kites/balloons.
A physical problem is to place the something/turnaround far enough away from the reel to have more than one line loading
device doing the work.
And reducing the friction in the return process, collapsing the parachutes in some manner.
Dragging them through a tube probably won't be feasible.
Too much friction in the system for practicality.
IT would be interesting/amusing to see if it could be made to operate at wind speed plus anything.

[quarky]

Vaguely on topic;

As a fan of Hpv's and the various prizes, which I haven't kept up with, I noticed a boat (I think it was called the "Decavitator") which was like all the other contenders, except the athlete's legs turned a fan instead of an underwater prop. The $ was for 20 knots; this rig managed 19, I believe, and was the fastest of the human powered boats. It was eventually disqualified over arcane definitions of boat.

Imagine a boat that shoots out a missile, of sorts, even if human powered, with a line attached. When the streamlined device falls into the water, it opens, like a parachute, and the power is put into reeling the boat in (or the parachute). A series of such unfolding missiles would allow constant pulling against the water.

In principle, I wonder if this is different than the normal push of a boat prop?

[Modified]

Vaguely on topic;

As a fan of Hpv's and the various prizes, which I haven't kept up with, I noticed a boat (I think it was called the "Decavitator")

http://lancet.mit.edu/decavitator/
On 27 October 1991, Mark Drela pedalled the human-powered hydrofoil, Decavitator ...So he does the grunt work too.

[Thabiguy]

If the parchute/line direction is changed to the same direction as the cart/wind, I can almost see it working in my mind's eye.

It does move in the same direction as the cart/wind in the ground frame. In the cart frame, it moves in the opposite direction.

Let the kite/balloon free in a wind, and let it pull out the line unrestricted.

Pulling the line out will never accomplish faster than wind travel. To go faster than the wind, you must let the wind pull the line into the reel by tugging on the attached kite/balloon.

(Which will only work when turning of the reel is mechanically coupled to its forward motion, for example as in this picture (http://forums.randi.org/imagehosting/15144492613dfb1b89.jpg) by Dan O.)

[Michael C]

If the parchute/line direction is changed to the same direction as the cart/wind, I can almost see it working in my mind's eye.

It does move in the same direction as the cart/wind in the ground frame. In the cart frame, it moves in the opposite direction.

This means that with respect to the ground, the parachutes are moving in the same direction as the cart, but slower than the cart. This is a feature common to all the vehicles that go "down flow faster than the flow", where the flow can be a flow of air, a flow of water, or even a flow of something solid. The trick in making a vehicle that goes faster than the flow is to construct the vehicle so that the part that is being pushed/pulled by the flow is moving forwards slower than the vehicle as a whole.

There are many ways to achieve this. Even a simple rolling wheel has parts that are moving forwards slower than the wheel as a whole. Just use the flow of something solid to drag on part of the wheel that's moving forwards slower than its centre, and you have a "faster than the flow" machine.

E7vcQcIaWSQ

When the flowing substance is air, you need to be a bit more inventive. Since the whole machine is in contact with the air, you need to minimise the contact with the parts of the machine that don't go slower than the machine as a whole and maximise the contact of the air with those parts that do go slower. This leads to such contraptions as Myriad's parachute cart, which might just work, or the Blackbird (http://www.fasterthanthewind.org/2010/03/wait-for-it.html), which certainly does work.

[Jack by the hedge]

NOW I've got it! Thank you!

Yay! And you're welcome.

My work here is done. <dons cape, flies>

[recursive prophet]

But at what point would this go beyond engineering is my question? At 3xws wouldn’t the headwind created by the Blackbird’s forward motion be greater that the tailwind moving it forward? Surely there must be some theoretical limit? How would it be determined?

I don't understand your question. If you're talking about the "true wind" (i.e. the relative velocity of air to ground), that has nothing to do with the cart's motion. If you're talking about "apparent wind" (i.e. the relative velocity of air to cart), there's never a "tailwind" once the cart is above wind speed.
I understand there’s not tailwind; I was referring to the apparent headwind. You have to admit it’s VERY counter-intuitive that at 3-4x ws this headwind wouldn’t quickly become a relevant limitation on bluff force alone.
There is no theoretical limit of any relevance. In an ideal (no friction) Newtonian world, the steady state speed of the cart could be any multiple of wind speed. However the net thrust available to it below that speed will get smaller and smaller as the multiplier gets larger. With friction there's of course a top speed, but it will depend strongly on the design and materials, and it's very difficult to predict what it will be. I'd guess the best multiple they will reach will be around 4x.
So there is NO theoretical maximum speed if you ignore all dissipative losses?

On the subject of theoretical limits, I’d like to get some feedback on a quote from a recent TR post by Nick, below. The screw analogy reminded me of a post here at JREF by John Freestone over a year ago.

http://talkrational.org/showthread.php?t=24499&page=61

My post of max power was not based on prop or turbine, I was trying to post the absolute max limit of power which is taking the velocity all the way to 0. A total theoretical limit.

Showing thrust power at 1000W = 200 N * 5 m/sec

and

showing drag power at 1000W = 100 N * 10 m/sec

confirms that you can get more force with the same power at a lower velocity. When I was showing this to someone and explaining that the wind is what allows the thrust velocity to be lower than ground velocity. This point also shows why it is not PM as in if there is no wind the thrust velocity must be 10 m/sec or higher, which means more thrust power than drag power.

The way they grasped the concept was that the thrust power is pushing back against the wind, recreating the velocity delta, and allowing the wind to put more power (pushing against a pocket of air), and putting >1000W back into the system. That observation seemed to be intuitive, but when I posted it back on here several said it was incorrect, but somehow the wind must be putting in >1000W. It can no longer put anything in by pushing against the surfaces of the vehicle, so that only leaves pushing against the flow created by the prop. The wind can not reach the prop or the vehicle since the vehicle is moving faster than the wind.
The way they grasped the concept was that the thrust power is pushing back against the wind, recreating the velocity delta, and allowing the wind to put more power (pushing against a pocket of air), and putting >1000W back into the system. That observation seemed to be intuitive, but when I posted it back on here several said it was incorrect, but somehow the wind must be putting in >1000W. It can no longer put anything in by pushing against the surfaces of the vehicle, so that only leaves pushing against the flow created by the prop. The wind can not reach the prop or the vehicle since the vehicle is moving faster than the wind.

The secret to this part of the brainteaser is that the vehicle is immersed in the fluid (air) and working its way through it. It is not like a bunch of particles hitting it from behind. If the "air cushion" theory were accurate, it would require that the air cushion elongates at the rate at which the vehicle is going above wind speed. Also, we can see by analysis or experimentation (tufting for example) that the flow is identical to that of a plane pulling itself through the air. There is no "convergence" of flows - although there is clearly a high pressure region behind the prop.

Imagine a wood screw working its way through a block of wood. Now imagine that block of wook happens to be in your car moving along at 60 mph. The screw moving through the wood doesn't know any different, nor can any experiment be performed to show differeng results.

ETA: to make matters just a bit more bizarre perhaps... The air molecules coming from in front of the prop are struck by the back surface of the prop. This is due to the pitch of the prop and the fact that it's spinning. Exactly like that of an airplane pulling itself through the air.

Another ETA: BUT... those air molecules coming from in front are moving relatively slowly (because of the tailwind component). That means they're easier to grab and throw aft. Just like it's hard to keep up with a treadmill that's turning 15 mph, but easy to walk on one that's turning 3 mph.

[RossFW]

I understand there’s not tailwind; I was referring to the apparent headwind. You have to admit it’s VERY counter-intuitive that at 3-4x ws this headwind wouldn’t quickly become a relevant limitation on bluff force alone.


Point is, RP, there's no theoretical limit as to how small you can make the drag the headwind causes. A streamlined shape will go fast. A MORE streamlined shape will go faster. What is the theoretical minimum Co-Efficient of drag? There isn't one, it just will always be greater than 0.

So there is NO theoretical maximum speed if you ignore all dissipative losses?


No, and there is no THEORETICAL limit on how small those losses can be. What is the THEORETICAL limit on the speed a car with a 100hp engine can achieve?

[Uncayimmy]

No, and there is no THEORETICAL limit on how small those losses can be. What is the THEORETICAL limit on the speed a car with a 100hp engine can achieve?

c?

[sol invictus]

I understand there’s not tailwind; I was referring to the apparent headwind. You have to admit it’s VERY counter-intuitive that at 3-4x ws this headwind wouldn’t quickly become a relevant limitation on bluff force alone.

But it does, in the real world. That's why I said they might achieve 4x wind speed, but much higher is going to be very hard.

So there is NO theoretical maximum speed if you ignore all dissipative losses?

Well, that depends on what you include. The curvature of the earth or variations in the wind would be relevant for obvious reasons. Ignoring those, I suppose it's the speed of light as UncaYimmy points out.

On the subject of theoretical limits, I’d like to get some feedback on a quote from a recent TR post by Nick, below.

What about it? The thing about the wind not being able to reach the prop or car body is obvious nonsense; the cart doesn't suddenly find itself in a vacuum when it's going faster than the wind, any more than a sailboat does. The screw analogy is good.

[Ririon]

c?
I think that is the correct answer. Check out the cute video with the simple concept of gearing and relative motion in Michael C's post (about 6 posts up.)

If you have no resistance, you can have insanely high gearing.

I suspect that going supersonic might be a problem even in theory for a wind driven device. (I have not done the physics for this properly). Practical limits will kick in a long time before that.

As for "but this is not practical"... I, for one, do not care. :)

[Skeptical Greg]

If everything were practical, there would be no fun ....

[Michael Redman]

I think there would be limitations as the prop began moving fast enough through the air. There's a reason we don't have propeller diven supersonic aircraft.

Maybe the vehicle could use a turbine instead of a prop, or switch over from a prop to a turbine at sufficient speed. It would take a wind tunnel to try it, though, as I don't think you're going to find sustained winds at anywhere near 1/4 the speed of sound, much less 1/3 or 1/2.

[RossFW]

I think there would be limitations as the prop began moving fast enough through the air. There's a reason we don't have propeller diven supersonic aircraft.


Supersonic propellor driven aircraft aren't impossible, but are highly impractical, as propellor efficiency drops off a cliff in the transonic range.

Maybe the vehicle could use a turbine instead of a prop, or switch over from a prop to a turbine at sufficient speed. It would take a wind tunnel to try it, though, as I don't think you're going to find sustained winds at anywhere near 1/4 the speed of sound, much less 1/3 or 1/2.

I think you are muddling definitions here. A turbine engine is called such because it utilises a turbine (a series of angled planes driven by a gas flow) at the end of the combustion cycle, driven by the exhaust gasses, to drive the compressor. It is termed such to differentiate it from a ram jet or rocket. Turbines don't accelerate air to produce thrust, they accelerate air to take energy FROM thrust.

I think what you are alluding to is a ducted fan. By having a shrouded propellor, with proper inlet and exhaust ducting, you can slow a supersonic flow down, allow the propellor to add energy to it, then re-accelerate it to supersonic speed to match the rest of the airflow as it efluxes.

This is actually what happens in supersonic aircraft. Their jet engines ALSO can't work with supersonic airflows, so it is ducted such that it is presented to the compressor at less than sonic speed.

SCRAMjets (supersonic combustion ram jets) are only now being developed.

[Michael Redman]

I think you are muddling definitions here. Absolutely. Thanks for the clarification.

[mender]

This is actually what happens in supersonic aircraft. Their jet engines ALSO can't work with supersonic airflows, so it is ducted such that it is presented to the compressor at less than sonic speed.


So why don't people refuse to believe that jet aircraft move faster through air than the engines that power them?:jaw-dropp

[RossFW]

Absolutely. Thanks for the clarification.

Most welcome. I love it when I can actually display some knowledge here. I'm usually the slow kid down the back when it comes to this forum!!

[recursive prophet]

So why don't people refuse to believe that jet aircraft move faster through air than the engines that power them?:jaw-dropp

Because laymen, mender, generally only pay attention to what can get them laid. :rolleyes:

It is interesting to note at this juncture that much of the research conducted in science-especially physics-would seem to offer little that on the surface appears practical. Yet every small step forward in our understanding can often trigger epiphanies unimaginable until many years later. People won't be driving wind powered vehicles, but who can foresee what might occur in the mind of some budding genius after reading about traveling directly downwind faster than the wind driving it forward? :boxedin:



http://www.talkrational.org/attachment.php?attachmentid=1918&stc=1&d=1271860073[/QUOTE]

[quarky]

Because laymen, mender, generally only pay attention to what can get them laid. :rolleyes:

It is interesting to note at this juncture that much of the research conducted in science-especially physics-would seem to offer little that on the surface appears practical. Yet every small step forward in our understanding can often trigger epiphanies unimaginable until many years later. People won't be driving wind powered vehicles, but who can foresee what might occur in the mind of some budding genius after reading about traveling directly downwind faster than the wind driving it forward? :boxedin:



http://www.talkrational.org/attachment.php?attachmentid=1918&stc=1&d=1271860073[/QUOTE]

Absolutely.

The pedal-powered vehicle top speed record (on flat road; no wind) is now 81.4 mph, which I find fascinating, though most folks have no idea this has been accomplished, and find it pointless when I tell them about it.
But so much is learned in these attempts that translates into real world advantages.

In the case of the cart of the o.p., much will be learned about side issues, like chain routing methods; tires; mast strength shape and materials, etc.

[I Ratant]

If everything were practical, there would be no fun ....
.
Or, it might marketable!
http://www.youtube.com/watch?v=VFcUjYXPo30&feature=player_embedded

[recursive prophet]

Absolutely.

The pedal-powered vehicle top speed record (on flat road; no wind) is now 81.4 mph, which I find fascinating, though most folks have no idea this has been accomplished, and find it pointless when I tell them about it.
But so much is learned in these attempts that translates into real world advantages.

In the case of the cart of the o.p., much will be learned about side issues, like chain routing methods; tires; mast strength shape and materials, etc.

Not to mention how to plant the seeds for viral discussions in cyberspace. Dont forget that one. Some interesting quotes from this thread I plan to spread around via links. Writer accredited, natch. I posted one of yours recently on TR I Ratant. These threads form a strand that circles the globe in various languages, and have stimulated thousands of pages of discussion. No mean feat, that. :boggled:

If you go to YT quarky there are videos of that record run. It is amazing how we can keep pushing the limits over time. They hold races with turbine driven vehicles in Denmark and Germany. Perhaps this debunking of another physics myth will engender similar races here going downwind instead of up. I do hope Joby and Google do a good PR job with this. It could get a lot of traction as a way to make the public aware of wind as a power source in a way the masses can relate to; a race! Make it a sport. The approach has worked well in Europe. Denmark gets over 40% of its electricity from wind, and the races get a lot of publicity. :)