How sails work... Bernoulli es kaput

[JSFolk]

Sail Theory Page (http://www.sailtheory.com/index.html)

I spent some time reading the old thread about various ways of looking at how wings work earlier today, and, being a wannabe sailor, got to thinking about how this related to sails. I happen to know two pilots who are also sailors, and they both reacted poorly when I expressed some skepticism as to the Bernoulli effect and wings. Hopefully this will help explain it for them.

[MRC_Hans]

Jeesh! Do we have to go through this AGAIN? Bernoulli effect, down wash, pressure differences are ALL different aspects of the same function.

On the page you link to is a silly picture of a "sail" with a curly lee side, and the idiot who has made the site claims that according to Bernoulli, this should give even better effect. This is of course blatant nonsense; the whole thing requires a LAMINAR flow, which does not exist over an irregular surface.

The laminar flow over differently curved sides give rise to a pressure difference, to a down(or side) wash, and thus to a "lift".

Any experienced yactsman can tell you that the shape and depth of the sail profile is essential in trim.

Hans

[JSFolk]

Yes, I agree that the "curly" ( I would have said "jagged") diagram (if you're talking about the same one I am, the one in the "wrong theory of wing" page) is stupid, but what he's addressing is the "equal transit time theory" of the bernoulli effect, which has apparently been shown to be so much hot air.

If you look at this (http://www.sailtheory.com/tuning.html) page, though, you will see a large amount of information about sail trim and how sail tuning works to effect sail efficiency.

[MRC_Hans]

No, it has not been shown to be hot air. It is part of the picture needed to understand how airfoils, including sails, work. It is not the whole picture, and that is not news.

The trim page is all right, albeit very elementary.

Hans

[JSFolk]

I found the photos of airfoils with the timed smoke puffs showing that the air does not meet up at the trailing edge of the wing to be somewhat illuminating. How does the "equal transit time" theory hold up if it's shown that the "air blocks" (or what have you) do not, in fact, meet up at the trailing edge of the wing? I thought that was the whole point.

Anyway, a sail is not a smoothly curved shape like a cambered wing. It is a smoothly curved plane connected at its leading edge to a round (or obround) cylinder. I would think the turbulence created by the transition from mast to eliminate the laminar flow you mentioned. In a headsail, the leading edge of the sail is the same thickness as the rest of the sail, so you still don't get an airfoil.

BTW, I posted this not as a "new news" link, but because I thought it was interesting and was not mentioned (except in passing) in the earlier wing thread.

[MRC_Hans]

Air does not meet up, that is, if you look at --- well, a smoke puff, being cut by the foil, the two halves do not meet up at the trailing edge. But the air streams at the trailing edge meet up, meaning that they travel at same speed. But in a new direction, creating a "downwash". There is a virtual circulating airstream around the foil.

Sorry if I'm a bit sharp tonite, it's--- well, a long story.

Hans

[ceptimus]

Notice how everyone tends to think of, 'air flowing over a wing', but really it's the other way round.

On a calm day, the air is just sitting there, minding its own business. Suddenly, a wing comes along and slices it into two pieces - 'above the wing' and 'below the wing'. Once the wing has cleared off out of the way, the two pieces of air get back together again, slightly disturbed by their experience.

Assuming the wing is generating lift, then, by Newton's law, some of the air must have been accelerated downwards. But the air must spring back up again somewhere else, or after all these years of planes flying, it would have been compressed down nearer to the ground.

[JSFolk]

The air that has been compressed at ground level by a century of flight is what causes global warming. If we want to save the planet, we must install thousands of inverted helicopter rotors to fluff the atmosphere. If you don't do it for yourself, think of the children!!!
:eek:

[CurtC]

MRC_Hans wrote:
Air does not meet up, that is, if you look at --- well, a smoke puff, being cut by the foil, the two halves do not meet up at the trailing edge. But the air streams at the trailing edge meet up, meaning that they travel at same speed.Haven't we gone over this before? If two adjacent parcels of air are split apart by the edge of the wing, one going over the top and the other going underneath, and the one on the top gets to the trailing edge well before the lower one, then how can you defend the equal transit times assumption? The transit times are clearly not equal.

[xouper]

MRC_Hans: Jeesh! Do we have to go through this AGAIN? Bernoulli effect, down wash, pressure differences are ALL different aspects of the same function.Just to clarify, they are not equivalent aspects.

MRC_Hans: The laminar flow over differently curved sides give rise to a pressure difference, to a down(or side) wash, and thus to a "lift".Um, just to clarify, the pressure differential is mostly caused by angle of attack (AOA), not the curved surfaces. How does a flat plate wing generates lift? Hint - Angle of Attack, not Bernoulli.



Changing topic slightly, here's a semi-trick question about what may be a counter-intuitive result to some:

True or False - Can a vehicle that gets its propulsion from a sail go faster than the speed of the wind?




<table cellpadding="4" cellspacing="1" border="0" bgcolor="#666699" align="center" width="80%"><tr><td bgcolor="#666699"><font size="1" face="Arial, Helvetica, sans-serif" color="#FFFFFF">Spoiler:</font></td></tr><tr><td bgcolor="white"><font size="2" face="Arial, Helvetica, sans-serif" color="white">
True. Ice boats, land yachts, and skate sails, for example, can routinely tack into the wind and generate a "ground speed" faster than the wind speed they are sailing into.

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[MRC_Hans]

Originally posted by xouper
Just to clarify, they are not equivalent aspects.

And, I did write "different" :rolleyes:

Um, just to clarify, the pressure differential is mostly caused by angle of attack (AOA), not the curved surfaces. How does a flat plate wing generates lift?

Answer: Not very well.

Hint - Angle of Attack, not Bernoulli.



Changing topic slightly, here's a semi-trick question about what may be a counter-intuitive result to some:

True or False - Can a vehicle that gets its propulsion from a sail go faster than the speed of the wind?

Of course!

Hans

[xouper]

Perhaps we are having a language difficulty here.

xouper: Just to clarify, they are not equivalent aspects.

MRC_Hans: And, I did write "different" Yes you did. But since "different" can sometimes mean "different but equivalent", it still needed clarifying that your use of "different" means "different and not equivalent".

For example, .25 and 1/4 are "different but equivalent" ways to express the same quantity.

I just wanted to clarify that Bernoulli and downwash are not two different ways of explaning the same thing (how a wing generates lift), just in case anyone might get the mistaken impression that's what you were saying. No need to take offense at my clarification, I was trying to be helpful.

xouper: How does a flat plate wing generates lift?

MRC_Hans: Answer: Not very well. Well enough for an airplane to fly. :)