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Old 05-26-2011, 08:33 PM   #1 (permalink)
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Busting an aerodynamic myth -- Planes don't fly due to Bernoulli Effect

I think this topic deserves its own thread because it of how widespread the myth is and how important this is to aerodynamics, including car aerodynamics.

It's one of those "facts" that we were all taught in grade school-- that airplanes fly because the wing is curved on top and flat on the bottom, so air has to move faster over the "longer path" along the top and is therefore lower pressure than air flowing below the wing. Well, I've had trouble picturing this in my mind since I was a kid, and I was finally vindicated a couple years ago when I came across an article that explained that the "longer path" theory is a myth, and that airplanes fly due to the wings' angle of attack redirecting air downward.

Suddenly it made sense how a vehicle moving through the air can generate enough lift to hoist itself into the atmosphere. The bottom of the wing directs air downward, applying positive pressure. Airflow along the top of the wing stays attached and is also redirected downward, applying negative pressure to the top of the wing. That's how you get lift. I believe the flow attachment is the source of the "Bernoulli Effect" myth. I think Bernoulli predicted attached flow, which is responsible for the top of the wing directing air downward as the wing moves through the atmosphere.

Anyway-- flow redirection--- it's simple, it makes sense, and there's no hocus pocus about molecules being compelled by magic to meet each other at the trailing edge (they don't).

And here is the most definitive and clear article, complete with animations and a web app, from NASA themselves.

Incorrect Lift Theory

{Lifting airfoils are designed to have the upper surface longer than the bottom.} This is not always correct. The symmetric airfoil in our experiment generates plenty of lift and its upper surface is the same length as the lower surface. Think of a paper airplane. Its airfoil is a flat plate --> top and bottom exactly the same length and shape and yet they fly just fine. This part of the theory probably got started because early airfoils were curved and shaped with a longer distance along the top. Such airfoils do produce a lot of lift and flow turning, but it is the turning that's important, not the distance. There are modern, low-drag airfoils which produce lift on which the bottom surface is actually longer than the top. This theory also does not explain how airplanes can fly upside-down which happens often at air shows and in air-to-air combat. The longer surface is then on the bottom!

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