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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

Quote:
{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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Old 05-26-2011, 09:37 PM   #2 (permalink)
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Interesting, considering that I spent several years teaching that to pilots and airplane mechanics
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Old 05-26-2011, 10:01 PM   #3 (permalink)
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Yeah, I read a forum thread a while back where pilots were complaining about being asked about that on tests, and having to lie about lift being created by "the longer path theory".

And this despite everybody in the aviation industry knowing that a plane has to fly at a pitch greater than zero and that in order to increase lift you increase pitch!
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Old 05-26-2011, 10:57 PM   #4 (permalink)
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I too have instructed that gem. After many years of aircraft flying and ownership, I can tell you the bare truth:
The 4 forces of thrust and drag and lift and weight, Bernoulli Effect, even your little tale about directing air downward, none of these make a plane fly.
It has been proven that the wallet is indeed the necessary force. Remove the wallet and the plane falls out of the sky like a toolbox with a broken handle.
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Old 05-26-2011, 11:40 PM   #5 (permalink)
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Skyking touched on what I heard is the current theory. The air is being directed downward, and this "Thrust Vector" is what creates the lift due to the whole action opposite reaction theory. It's tough to visualize it but it has to make sense, if you're pushing air down, something has to be pushing back with equal force.
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Old 05-26-2011, 11:49 PM   #6 (permalink)
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It's not tough to visualize though. It's the most intuitive thing. You can stick your hand out the window of a car and feel it yourself, or watch a bird take advantage of a wind gust to take off just by holding its wings open at the right angle
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Old 05-27-2011, 12:06 AM   #7 (permalink)
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An other dificulty with the bernoulli principal as an explination of lift is the ruder, it is semetrical with repect to length of the sides...
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Old 05-27-2011, 01:29 AM   #8 (permalink)
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1. The way it was explained to me, back in flight training, is that if you "do the math": calculate the total amount of lift from "accelerated air" (Bernoulli) vs the total amount of lift derived from forcing air down (F=MA), you'll wind up with the same number.

2. The air flowing over the upper wing surface is indeed traveling (relative to the airplane) in excess of the surrounding undisturbed air. This is why when planes transition to the "transonic" region of flight (some localized areas of sub- and supersonic flow), they generally go supersonic first on the upper wing surface, causing mainly undesirable flight characteristics (google "Mach tuck.")

2a. Airplanes designed for "high-subsonic" cruise speeds generally have rather convoluted airfoils due to the phenomenon discussed in "2"...to stave off transonic effects as long as possible.

3. The air pressure atop the wing is indeed lower than beneath it. At the tips, the high-pressure air tries to work its way over to the top of the wing, producing "wingtip vortices," which are really cool to look at (when they're visible) and/or disturbing to contemplate (when they aren't visible and you're following a "heavy" for landing.)
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Old 05-27-2011, 01:45 AM   #9 (permalink)
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Old 05-27-2011, 02:21 AM   #10 (permalink)
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If gravity can be explained by Intelligent Falling, then lift could be the effect of Intelligent Ascending, right?

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