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.

  • $\begingroup$ Read this It's the simplest and best explanation I know of. $\endgroup$ – Mike Dunlavey Jun 30 at 15:33

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.

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  • $\begingroup$ Nice answer, +1 from me. So the Bernoulli story many of us have been told is BS? $\endgroup$ – Gert Jun 26 at 6:29
  • $\begingroup$ @Gert That is what I am lead to believe. The arguments against the Bernoulli seem sound, but I admit that I have a soft spot for the Bernoulli argument for wings because it is indeed what I was raised on. I keep hoping that someone will provide a counterargument that permits my childhood upbringing to be the true story, downvoting my answer like mad, but the best I am able to do is say "it's complicated!" $\endgroup$ – Cort Ammon Jun 26 at 23:20
  • $\begingroup$ Yup, same here: its hard to let go of the BP on wings! $\endgroup$ – Gert Jun 27 at 11:02

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