Flight and gravity

[swellman]

Question from a third grader's science homework - does an airplane need gravity to fly? There seem at least three possible answers:

1. An atmosphere would not form or stay contained if gravity was absent. (the wisea$$ answer)

2. No, because lift is a function of only velocity and air density. (this is the book answer)

3. Yes, because a standard aircraft balances lift, drag, thrust, velocity and pitching moments to maintain stable flight. Remove the gravity vector and the aircraft becomes unstable without radical redesign/trim.

Any predictions on the stability margin of an aircraft in condition number 3? Or is this answer out to lunch?

[Houngan]

Sure, and I have a real-world example. Make a paper airplane, then cut elevators into the backs of the wings. Bend them up, and give it a toss. It should almost immediately flip nose-up, then buffet and stall. Of course, without gravity, the plane would never stall, but it would still have WAY too much lift.

[Oso]

What happens when you throw a glider in the International Space Station? The only difference would be that you need less velocity to generate lift. #2 sounds pretty good to me.

[Andonyx]

Originally posted by Houngan
Sure, and I have a real-world example. Make a paper airplane, then cut elevators into the backs of the wings. Bend them up, and give it a toss. It should almost immediately flip nose-up, then buffet and stall. Of course, without gravity, the plane would never stall, but it would still have WAY too much lift.

Wouldn't it continually fly in loops until it ran out of energy to fight drag?

[no one in particular]

Number two is a ridiculous, incomplete answer. Without gravity (or an enclosure) there is no air density, atmosphere dissipates to something like a few molecules every few feet. Certainly there is nothing to provide lift. Number one is the only answer. The only exceptions to this are the rare microgravity-in-atmosphere situations (the “Vomit Comet”, a vessel in orbit, or, say, an elevator with the cable cut). Unless you are flying something inside of something else that is in freefall, it is a pointless question. If the book expands on the answer then it is fine, if it does not then it is misleading.

[Oso]

Originally posted by no one in particular
Number two is a ridiculous, incomplete answer. Without gravity (or an enclosure) there is no air density, atmosphere dissipates to something like a few molecules every few feet. Certainly there is nothing to provide lift. Number one is the only answer. The only exceptions to this are the rare microgravity-in-atmosphere situations (the “Vomit Comet”, a vessel in orbit, or, say, an elevator with the cable cut). Unless you are flying something inside of something else that is in freefall, it is a pointless question. If the book expands on the answer then it is fine, if it does not then it is misleading. It appears you were correct swellman, #1 is the wisea$$ answer.
You win the million.

[rwald]

Without gravity, airplanes would need some sort of feature to send them back down to the Earth. I mean, they could use normal aerodynamic lift to move upwards (assuming that the atmosphere didn't dissipate), but they couldn't get back down. That's the real problem, as I see it.

[Ladewig]

2. No, because lift is a function of only velocity and air density. (this is the book answer)

If there is no gravity, why would an airplane need lift?

Heck, if there were no gravity but enough dense air, a running outboard motor could propel itself forward (with a little rebalancing).

[Oso]

OK, let's start over. An airplane typically has wings, flaps, ailerons, elevators, a vertical stabilizer and a rudder. They all would perform, and would be needed for, the exact same functions with or without gravity. As I mentioned above think about what would happen in the ISS.

[rwald]

I don't think so. Without gravity to counter lift, the airplane would just fly in a large vertical circle (as someone else pointed out). Also, it would be impossible for the airplane to ever descend, short of aiming downwards and then aiming straight. Flying without gravity would be very different than with.

[Oso]

Originally posted by rwald
I don't think so. Without gravity to counter lift, the airplane would just fly in a large vertical circle (as someone else pointed out). Also, it would be impossible for the airplane to ever descend, short of aiming downwards and then aiming straight. Flying without gravity would be very different than with. Here's a link. Go down to the model and play with the controls, and think about what they do.
http://travel.howstuffworks.com/airplane6.htm

A plane here on earth would also fly in vertical circles (at least until it broke up). Stunt pilots do it all the time.

Yeah, it would probably be harder to land, also harder to stop, but the question wasn't about landing.

[Walter Wayne]

I think there is a lot more to figuring this out.

For instance, when the plane is in a stable position torque from gravity, thrust, lift (wings and tail) and drag equal out.

Now without gravity, the wings and tail will impart a large upward component and change the angle of attack and drag vector significantly. Assuming the downward (wrt plane) component of drag is primarily due to the surface area of the wings and tail, they may actual be distributed similarly to the lift. However, given the angle of attack would the wings, elevators and ailerons work as advertised?

Also, in terms of the vertical circle, many modern aerobatic and fighter planes are more than capable of pulling -1 g, and supersonic planes have different wing profiles than we are normally taught.

Walt

P.S. Given that one can build a wing using a flat board and angling it with respect to air velocity, I believe we can make a balanced plane for any gravity. However, my mind is curious about a conventional wing in 0 g with air density as normal.

[davefoc]

I think Walter Wayne is absolutely right that a plane could easily be designed to fly in zero gravity (assuming the little problem about the atmosphere leaking into space can be overcome).

I'm not sure that a plane designed for an environment with gravity could fly level, climb and descend in a zero g environment.

I believe the issue is whether at some speed and elevator setting the plane can develop more lift out of the tail assembly than the wings. My guess is yes, because it just seems that at some speed with the elevators pointed down the rear of the plane will have more lift than the front of the plane and the plane will pitch forward.

[swellman]

Originally posted by Walter Wayne

P.S. Given that one can build a wing using a flat board and angling it with respect to air velocity, I believe we can make a balanced plane for any gravity. However, my mind is curious about a conventional wing in 0 g with air density as normal.

I agree. This is the most intriguing part of the problem. If one imagines a huge volume of air with no gravity, why build an entire airplane? All that is really needed is a propulusion system. But actually designing an aircraft that can operate in such an environment and then controlling it calls for a fresh approach.

How about a circular wing built around the fuselage containing the engine? Or if you want to be economical, a square with four lifting/control surfaces?

[MRC_Hans]

Mmm

Premise 1: Despite the lack of gravity, there is an air-pressure (so we must be inside something).

Premise 2: We are talking about a conventional plane.

Premise 3: By "flying" in this environment we mean move around in the air in a controllable way (since, in zero gravity, anything can fly).

Basically, the answer is yes, a conventional plane might be steered around in such an environment. However, ther would be a number of difficulties (some of which have already been mentioned):

1) The lift generated by the wings would send it off course, and would have to be countered by steering in the opposite directions. Some types of planed might not be capable of generating enough "down" elevator.

2) In order to be steerable, for its control surfaces to work properly, the plane would have to travel at near normal flying speed, even if this is not needed for lift. This would give problems when you wanted to stop.

3) Related to the #2, landing, or docking, would be impossible without special equipment. While the plane could be steered into contact with a landing strip and braked with spoilers or the like, once the speed fell below that where the control surfaces worked, it would float away out of control.

4) Since the directional stability of a normal plane is an interaction between lift and gravity, most planes would be quite unstable and would have to be steered constantly.

5) Last, but not least, a lot of practical problems would exist, in the area of jubrication, fuel feed, carburettors, etc. etc. Even aerobatic planes are built to fly upside up most of the time.

Hans

[Houngan]

Originally posted by Oso
OK, let's start over. An airplane typically has wings, flaps, ailerons, elevators, a vertical stabilizer and a rudder. They all would perform, and would be needed for, the exact same functions with or without gravity. As I mentioned above think about what would happen in the ISS.

True enough, except that the wings generate the majority of lift inherently, just by their shape. So all the control surfaces would be inadequate to counter that lift. If you wanted to make a plane to fly in a theoretical zero-G-with-atmosphere environment, you'd need to make the wings symmetric on the cross-section so that they didn't generate lift one way or the other. Stunt planes are made this way, because their engines are so grossly overpowered, they can fly by control surfaces alone. (Think horizontal helicopter.)

H.

[MRC_Hans]

True enough, except that the wings generate the majority of lift inherently, just by their shape. So all the control surfaces would be inadequate to counter that lift. No, that is not correct. The wings are (usually) shaped to give lift, but on most planes, the elevator will be able to change the angle of attack (AOA) sufficiently to remove the lift of the wings. Thats what you do when you dive.

Some plane types, especially types optimized to be easy to fly may not have sufficient elevator effect to do this, but any plane with (even moderate) aerobatic capabilities, and fighter planes can do it. In general, I would expect that any plane that is capable of flying inverted will be able to "fly" in zero gravity.

Hans

[Walter Wayne]

Originally posted by MRC_Hans
2) In order to be steerable, for its control surfaces to work properly, the plane would have to travel at near normal flying speed, even if this is not needed for lift. This would give problems when you wanted to stop.I'm trying to think if the control surfaces do work properly. Whenever wings are discusses their limits are described in terms of an angle where the bottom of the wing is plowing through the air.

In our thought experiment, the top of the wing is plowing through the air. What is the stall angle in this direction and how well to the control surfaces work?

Thats what I meant original about a wierd angle of attack.

Walt

[Walter Wayne]

Originally posted by Houngan


True enough, except that the wings generate the majority of lift inherently, just by their shape. So all the control surfaces would be inadequate to counter that lift. If you wanted to make a plane to fly in a theoretical zero-G-with-atmosphere environment, you'd need to make the wings symmetric on the cross-section so that they didn't generate lift one way or the other. Stunt planes are made this way, because their engines are so grossly overpowered, they can fly by control surfaces alone. (Think horizontal helicopter.)

H. I believe these plans derive much of the lift from the angle of attack of the wings. Not just the control surfaces.

Walt

[Agammamon]

An airplane won't fly in a vertical circle in 0-g. What would happen is the wings would provide a constant upward force (like normal) with no compensating downward force (gravity); this means the plane would continue to rise until it hit something (you could certainly fly it in a vertical circle simply by pitching the nose up to increase the wings angle of attack). What you could do to counter this is when you reach your desired altitude, alter the shape of the wing to eliminate lift thereby allowing straight and level flight. To descend you merely have to alter the wing's shape again to that it is basically upside down and provides "lift" in a downward direction. All of this assumes that you have designated up and down somehow, since in microgravity these directions are pretty much arbitrary.

Of course in 0-g you don't need wings to stay aloft anyway. A small reaction motor would be sufficient to lift you.

[DrMatt]

Originally posted by swellman
Question from a third grader's science homework - does an airplane need gravity to fly? There seem at least three possible answers:

1. An atmosphere would not form or stay contained if gravity was absent. (the wisea$$ answer)

2. No, because lift is a function of only velocity and air density. (this is the book answer)

3. Yes, because a standard aircraft balances lift, drag, thrust, velocity and pitching moments to maintain stable flight. Remove the gravity vector and the aircraft becomes unstable without radical redesign/trim.

Any predictions on the stability margin of an aircraft in condition number 3? Or is this answer out to lunch?

None of these answers is correct.

The correct answer is: Yes, because without gravity, it would just be coasting, not flying.
:D

[CurtC]

Boy, there's a lot of misinformation in this thread. Of course an airplane would be able to fly a stable course with no gravity. It would just be flying at a somewhat nose-down attitude, making for a zero angle of attack on the wings. Actually, it wouldn't need to be quite this nose-down, because the wings would need to provide just a little lift to compensate for the down force of the fuselage's pointing down. Any plane which has enough elevator control movement to suddenly go into a zero-g dive could handle this.

[Walter Wayne]

Originally posted by CurtC
... It would just be flying at a somewhat nose-down attitude, making for a zero angle of attack on the wings... Manyconventional wings are assymetric, and produce lift with a zero angle of attack. I think it would be difficult to get a conventional wing to have a zero angle of attack in zero g.

Walt

[swellman]

It would seem that some combination of low/zero/negative angle of attack on the wings with a properly sized tail control surface in opposing force/moment could achieve stable flight in zero g, assuming the power plant is configured appropriately.

Now the hard part - how do you steer it (in a stable fashion)? Is a coordinated turn easier or harder than in a gravity environment?

I cannot see an obvious answer, but then this stupid problem is making me lose sleep...

[Thumbo]

Originally posted by davefoc
I think Walter Wayne is absolutely right that a plane could easily be designed to fly in zero gravity [...]

Err - zero gravity. Nothing falls. A brick can fly in zero gravity.

[rwald]

Fly, in that you can control its direction of motion by altering its control surfaces and speed. And that you can make it head any direction you choose, including down.

[LucyR]

rwald,

It's not a big deal, but presumably you realize that there is no naturally preferred direction in this gravity-free environment? Hence the word 'down' is meaningless.

[rwald]

Maybe "down with respect to the plane" would be more accurate? The plane does have its own coordinate system, so that's the one I was referring to.

Also, this is the third time in so many days I've had my posts corrected on scientific or mathematical grounds. Maybe I should read them over before submitting them...

[CurtC]

WalterWayne wrote:
Many conventional wings are assymetric, and produce lift with a zero angle of attack. I think it would be difficult to get a conventional wing to have a zero angle of attack in zero g.Almost all wings are asymmetric! There are a few wings, such as on stunt planes, with a symmetric airfoil. But I think they all need a positive angle of attack to produce lift. I guess it depends on precisely how angle of attack is defined, but if the wing doesn't throw air downwards as it passes, it can produce no lift.

For the example standard plane in zero g, you just need to tilt it over until it no longer throws air downwards.

A plane in zero g would also require much less power to maintain cruise speed, since its energy isn't being spent throwing all that air down. And it wouldn't be too hard to coordinate turns - to make a level turn, just turn your wings 90 degrees and pull up on the elevator.

This really isn't that hard.

[rwald]

The point is (and I think that most would agree with this), that airplanes designed for zero-g would be structurally different from most one-g planes (with symmetric wings, elevators which push down as often as up, etc.), and would also be piloted differently.

[Beausoleil]

One thing that stands out is that this is a very poor question to include in homework. The better the student understands flight, the more confused they'll get trying to answer it. It might be useful as a moderated class discussion, but the idea that there is a scientifically 'right' answer is quite disturbing.

My response would be that the question is badly formed - the notion of flying involves the notion of overcoming the tendency to fall under gravity. You can argue that an aeroplane could move along the same path in an atmosphere with or without gravity, but it wouldn't be flying since a brick with an engine could move along that path too. Bricks can't fly, even with engines. In fact, because of lift the direction of thrust would have to be significantly different for an aeroplane to achieve a given speed in the same direction as under gravity.

[swellman]

Originally posted by Beausoleil
One thing that stands out is that this is a very poor question to include in homework. The better the student understands flight, the more confused they'll get trying to answer it. It might be useful as a moderated class discussion, but the idea that there is a scientifically 'right' answer is quite disturbing.

Agreed. This question really just drives a 3rd grade student into fishing for the yes/no answer. While I posted the problem because it made an intriguing thought problem, it is a pi$$ poor grade school science question.

Might as well ask something like "if humans could breath underwater, could we walk around on the sea bed?" Well if we ignore the generally accepted principals of evolution, assume some sort of ballasting is present and all agree not to swim - sure we could.

[garys_2k]

Originally posted by swellman


Agreed. This question really just drives a 3rd grade student into fishing for the yes/no answer. While I posted the problem because it made an intriguing thought problem, it is a pi$$ poor grade school science question.

Might as well ask something like "if humans could breath underwater, could we walk around on the sea bed?" Well if we ignore the generally accepted principals of evolution, assume some sort of ballasting is present and all agree not to swim - sure we could.
So, was this question out of a textbook, or did the teacher make it up? If it was from a book, which one?

[swellman]

Originally posted by garys_2k

So, was this question out of a textbook, or did the teacher make it up? If it was from a book, which one?

I checked back with my friend whose daughter had this assignment. In his words:

"Good timing.......just got back from the teacher conference.

It was just on a worksheet that had questions on the section they were doing, part of which was gravity.

She (the teacher) also thought it wasn't the greatest question. I gave her the printout of the chat from today and yesterday [i.e. this forum - swellman], she was pretty impressed at all the furvor over the subject. She agreed that it wasn't the greatest question and will cross it off next time."

My misunderstanding - it wasn't a textbook, but a handout worksheet (not authored by the teacher).

Now for my next challenge: convince a friend that she really doesn't need to sleep on a sheet of magnets "for her back"...

[scotth]

Originally posted by swellman

Now for my next challenge: convince a friend that she really doesn't need to sleep on a sheet of magnets "for her back"...

Bring her a new bed. Tell her that this new bed has vastly upgraded magnets. They are made of a new material that holds a much larger magnetic field.

After she sleeps on a couple weeks and reports how well it works, then tell her there isn't a magnet in it at all.

Then, she might be ready to listen to why there is no reason to believe that magnets have any effect on bio tissue and that the only people who says it does have a bunch of magnets they want to sell.

Or, she might just be ready to kick your ass.

[Agammamon]

Wings generate lift by being flat on the botom and curve on the top. This generates an area of pressure on top of the wing that is lower than air pressure on the bottom. The higher air pressure below the wing pushes it up.

[CurtC]

How does being curved on top create lower pressure?

[garys_2k]

Originally posted by CurtC
How does being curved on top create lower pressure?
Bernoulli's principle. The pressure that a stream of fluid applies against a surface at right angles to it decreases as the speed of the fluid increases.

Putting a convex curve onto the top surface of the wing forces the air travelling over the top to move a greater distance (in the same amount of time) as air moving over the flatter bottom of the wing. That "higher speed on top" creates less pressure there compared to the bottom, so the higher pressure on the bottom forces the entire wing UP.

Pretty cool, eh?

[roger]

But of course that is not the only way wings generate lift. After all, planes can fly upside down, and some, notably the Wright's machine, have flat wings.

[garys_2k]

Originally posted by roger
But of course that is not the only way wings generate lift. After all, planes can fly upside down, and some, notably the Wright's machine, have flat wings.
These situations usually pitch the wings to cause the same effect, faster moving air over the upper surface.

[CurtC]

Why does the air on top of the wing have to move faster?

[garys_2k]

Originally posted by CurtC
Why does the air on top of the wing have to move faster?
Imagine two "parcels" of air, one just above the other. They're sitting there, just touching, when a wing slams into them and separates them. The one that was on top travels over the curved top of the wing, the other along the bottom, straight face. After traveling along their opposite faces the two parcels rejoin at the trailing edge.

In order for the top parcel to just meet its neighbor it would have to move faster to travel the farther distance.

But, why does it HAVE to move faster? Because if it didn't there would be less air above the wing than below it. Imagine they moved at the same speed and the bottom parcel got to the back of the wing while the top one was only 90% of the way there. What would happen then? We need as much air entering the wing as leaving it, but if both parcels didn't get to the back at the same time we'd be "losing" air on that top surface.

Couldn't the air from the bottom just race around to the top and rejoin it up there? No, because then it would be going against the direction of the flow and there's nothing pushing it that way. It would have to move WITH the wing, in the same direction, but faster than the wing. Remember, the air in this illustration is really only getting pushed out of the way by the moving wing, it is trying to stay stationary.

Real wings generate all sorts of turbulence and real parcels of air almost never neatly rejoin their former neighbors at the back. This rejoining would require laminar flow, a type of flow rarely seen in real applications, but the effect is the same whether there's turbulent flow or not. To conserve mass the top air has to speed up a little to get to the back when it has to.

Did this help?

[CurtC]

Not really, because wind tunnel tests always show that the parcel of air going over the top of the wing does not rejoin the parcel that went under the wing. Not even close.

[garys_2k]

Originally posted by CurtC
Not really, because wind tunnel tests always show that the parcel of air going over the top of the wing does not rejoin the parcel that went under the wing. Not even close.
Right. Remeber when I said this:

Real wings generate all sorts of turbulence and real parcels of air almost never neatly rejoin their former neighbors at the back. This rejoining would require laminar flow, a type of flow rarely seen in real applications, but the effect is the same whether there's turbulent flow or not. To conserve mass the top air has to speed up a little to get to the back when it has to.

It's a mass conservation issue -- the total amount of air passing over the wing cannot be changed, and neither can the total amount of air passing under it. The only way those totals can be conserved is to have the air going over the top, by the longer route, is to go faster. It does this and generates lift.

OK?

[roger]

Originally posted by garys_2k

These situations usually pitch the wings to cause the same effect, faster moving air over the upper surface.
I'm not aware of that explanation. Aren't Newtonian explanations usually favored for explaining lift, such as in this paper from Fermi lab? http://www.aa.washington.edu/faculty/eberhardt/Lift_AAPT.pdf

[garys_2k]

Originally posted by roger

I'm not aware of that explanation. Aren't Newtonian explanations usually favored for explaining lift, such as in this paper from Fermi lab? http://www.aa.washington.edu/faculty/eberhardt/Lift_AAPT.pdf
WOW! Fantastic article! Thanks for posting it, it really helps.

Edit to add: There goes another explanation shot to hell! This does make more sense, thanks. :)

[xouper]

I seem to have come late to this conversation.

swellman: Question from a third grader's science homework - does an airplane need gravity to fly? There seem at least three possible answers:

1. An atmosphere would not form or stay contained if gravity was absent. (the wisea$$ answer)

2. No, because lift is a function of only velocity and air density. (this is the book answer)

3. Yes, because a standard aircraft balances lift, drag, thrust, velocity and pitching moments to maintain stable flight. Remove the gravity vector and the aircraft becomes unstable without radical redesign/trim.

Any predictions on the stability margin of an aircraft in condition number 3? Or is this answer out to lunch? Answer #3 is totally out to lunch. But how many third graders would know that? Or how many adults, for that matter?

I'd like to add my vote that the question is ill-formed, specifically in the use of the word "fly". But I think answer #2 gives some insight into the intent of the question. The lift generated by an airplane wing is not a function of gravity, but that wasn't how the question was phrased.

Nonetheless, I would go with "no" (answer 2), as being the most reasonable of the three, given my interpretation of the intent of the question. However, the justification given in answer 2 is incomplete (as was already pointed out.) Lift is a function of more than just velocity and air density.

However, to complicate the issue, a glider cannot "fly" without gravity. It uses gravity to produce forward motion, without which there would be no lift.

One thing I didn't see mentioned in this thread was a comparison with submarines (which could be thought of as "flying" in a "zero g" environment).

As a pilot, I must concur with Curt that there is a lot of misinformation in this thread. I would be curious to know how anyone reading this thread can identify the bogus from the real if they didn't already know which was which. However, some of the misinformation is countered by the excellent link posted by Roger.

[swellman]

Originally posted by xouper
I seem to have come late to this conversation.

...

However, to complicate the issue, a glider cannot "fly" without gravity. It uses gravity to produce forward motion, without which there would be no lift.

Very true. Though the problem doesn't specify if the "airplane" is powered or not.


One thing I didn't see mentioned in this thread was a comparison with submarines (which could be thought of as "flying" in a "zero g" environment).

Guess I would think of a submarine as being more analogous to a blimp with control surfaces than a winged aircraft.


As a pilot, I must concur with Curt that there is a lot of misinformation in this thread. I would be curious to know how anyone reading this thread can identify the bogus from the real if they didn't already know which was which. However, some of the misinformation is countered by the excellent link posted by Roger.

By misinformation, do you mean the old Bernoulli vs. Newtonian explanations of lift? The performance of stunt aircraft? Or the predictions of how a standard aircraft would fly in zero g?

I think the best part of this problem is it's inate pointlessness. :D