Why is pressure over the wing lesser than the pressure on the bottom?

Question

Why is the flow above the wing faster than the lower one? Most people say it's because the pressure above is lesser than the bottom one But for the pressure to be low... The velocity must be high.So its like the chicken and egg question for me. Can't understand which one is the cause and which one is the effect.

Answer 1

The answer to this is "it's complicated." Indeed NASA has a 4 page trail devoted to the many oversimplifications, such as the assumption that the air flowing over the top must move faster to "catch up" to the air moving across the lower side of the wing.

There are certainly two major aspects of this lift. The first is that air flowing laminarly over a curved surface will exert a lower pressure on the surface than it would if it were flat. This accounts for some of the lift. There is also what NASA calls the "skipping stone" argument, which notes that the air leaving an airfoil is directed downward.

In reality, neither model fully captures the effect of air flowing over a wing. To do that, we need to simply integrate the Euler equations (or Navier Stokes, in materials where we cannot ignore viscosity) to get the full description of lift. However, in a pinch, one can recognize that those two models both describe part of the story, like two blind men describing the leg and the trunk of the elephant in the famous story.