This subject came up in another thread and rather than interrupt that one, I figured a new one might be in order. I don’t know if this might belong in GC, but there are so many misconceptions floating around that it seems like it would fit pretty well here.
Everyone has seen the diagrams in high school textbooks showing a diagram of a stream of air that splits into two, part going above and part going below an asymmetric wing section. The explanation provided by such sources, and in fact believed to be the whole story by many people (even many pilots!) is that some magic (which is never quite explained) requires the air above the wing rejoin with the air it used to be right next to, the air passing over the wing must travel faster, and thus there is lower pressure above the wing, and the plane is “sucked” upwards (so to speak) by low pressure.
There is some truth in such explanations, although much of what they say is flat out wrong as well, and at best, they miss the bigger picture that most people are unaware of. If pressure differential caused by the asymmetric cross section was the only thing going on, then (1) planes with symmetric cross section wings could not fly (but they do), and (2) planes with “normal” wings could not fly upside-down (but they can, at least if their engines and so on are designed for it), and (3) the air right after a plane flys through it would not be moving down (but it is). In fact, Newton’s law says that when there is a force present (as there must be to hold the plane up against it’s weight), there must be a reaction mass accelerated in the opposite direction. The reaction mass in this case is air. The Bernoulli explanation is not “wrong” so much as “only part of the picture, and in a big way, a byproduct of more fundamental things”.
No airfoil can fly in straight and level flight at a zero degree angle of attack, which is a misconception held by many people who have only seen the simplistic Bernoulli explanation in textbooks. In reality, the angle of attack is far more important than the wing cross section. A simple wing, with no control surfaces at all, can fly in what most of us would think of as an inverted condition, if the resulting angle of attack is positive. It may not be as efficient, but it will fly. In fact, even perfectly flat wings can fly (although ineffeciently except at very low airspeeds). So why are most wings made with a more curved surface on top then? To a large extent, to preserve laminar flow. Turbulent flow causes huge drag losses, and it turns out that you can tune the cross section to best preserve laminar flow in the desired speed regime by giving it the sort of camber we’re all familiar with.
Furthermore, it is not in question that airfoils accelerate a mass of air downward. You can see this by standing underneath a large plane as it flys slowly overhead - there is an incredible downwash of air from the wings. The plane has to accelerate something downword. Helicopters, which are just rotating wing aircraft, are exactly the same thing, and most people would agree that they are accelerating a bunch of air downward. They do have lower pressure regions above their wings, just as a fixed wing a/c does, so that explanation is not really wrong so much as missing part of the picture.
The exact manner in which the air is accelerated downwards is sort of complicated and probably not easily discussed in a text forum like this, but there are some web resources that discuss it, and you can find better explanations than I can give here in more advanced texts on aerodynamics, but a decent simplified explanation can be found here: http://www.rz.uni-frankfurt.de/~weltner/Flight/PHYSIC4.htm - this site has a lot of related information such as pictures from smoke pulse generators in wind tunnels where you can see what’s happening. It’s a worthwhile read. There’s another site here: http://www.monmouth.com/~jsd/fly/how/htm/airfoils.html#SECTION00680000000000000000 which says similar things.
Now, is anyone prepared to argue that airplanes do not accelerate air downwards when flying straight and level? If so, let’s debate it 
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peas on earth
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