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Old 09-26-2009, 08:12 PM   #38
Marin
Scraping Paint
 
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Join Date: Oct 2007
Posts: 13,745
RE: how much hp to plane?

There is only one thing a person needs to know about why a wing develops lift and that is that a wing moves air down. An airplane flies not because of the Bernoulli Theorem but because of Newton's law of action and re-action. The Bernoulli Theorem or Principle is just one factor that helps a wing move air down, but the bottom line is that a wing moves air down with a force that opposes the weight of the airplane. You can design a wing to fly with no difference between the upper and lower surfaces at all--- remember the little flat wing, balsa wood, rubber-band powered airplanes we used to build?

A sailboat sail operates the same way. The sail, using a variety of aerodynamic and physical principles, moves air down, or away in this case, and this force in combination with the hull, keel, etc. design moves the boat forward.

We have a terrific piece of film in our library illustrating the moves-air-down principle of lift. It's of the prototype 757 skimming the top of a flat cloud layer. The camera plane was out front of the 757 looking straight back at it and when you see the reaction of the cloud layer behind and under the plane, the whole lift thing instantly becomes crystal clear. NASA and the National Air & Space Museum both use the "moves air down" explanation of lift on their websites.

I have a drawing sent to me by one of the at-the-time chief aerodynamicists at NASA which illustrates what a 747 wing would have to look like to develop enough lift to fly using the Bernoulli Theorem alone. In order to get the required pressure differential, the upper surface of the wing would have to have a curve on it that stuck up higher than the tail. Needless to say, this configuration would generate so much drag the airplane would never be able to get off the ground.

So a plane does not fly because there is lower pressure across the top of the wing, it flies because the wing, for a whole host of reasons, moves air down with the same force required to oppose the weight of the plane.

As FF said, there have been wings designed with no upper surface at all. Many of the early planes, including the Wright's, had only a single surface--- the upper and lower surfaces of the wing were exactly the same. The first successful seaplane, Fabre's Hydravion in France, not only had a single-surface wing but had a thick cross-hatched box beam at the leading edge that stuck up above the wing. Using the Bernoulli Theorem plus the smooth-airflow-over-the-top idea, this wing should not have flown at all, but in fact it flew very well. Because, regardless of how many "rules" it broke, it moved air down with the same force as the weight of the plane.
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