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I understand why the pressure is reduced above the wings of aircraft, but when I read books and look for information, it says that the air is compressed below the wings, I do not understand why it is? Could you tell me what makes the air compressed below the wing, and how is it compressed?

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The simple answer is that a wing moves through the air generally at a non-zero angle of attack. The air flow below the wing sort of impacts the wing surface, compressing and slowing down as it is deflected.

See this Drawing:

Airfoil

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    $\begingroup$ This is troublesome, because it doesn't show the air being deflected downward, and it says nothing about the air above arriving at the trailing edge well in advance of the air underneath. $\endgroup$ – Mike Dunlavey Dec 18 '13 at 19:51
  • $\begingroup$ ^But it's not necessary for the air at the trailing edge come in advance of the air under the wing. IIRC, the upper and lower currents can even meet at the trailing edge at the same time and lift can still be explained using Bernoulli's theorem. But reaction lift is a much better explanation IMO, although this diagram does not illustrate it properly. $\endgroup$ – shortstheory Dec 29 '13 at 14:38
  • $\begingroup$ @shortstheory: This is my favorite explanation of all that. $\endgroup$ – Mike Dunlavey Jan 8 '14 at 14:45

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