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While using the Bernoulli's equation to deduce the upthrust on an aircraft , a statement is always made that

The air particles going above th wing have to cover a greater distance in same time than those going below, so they travel at a higher speed which causes the pressure difference.

But why do they have to cover greater distance?
Why do the particles have to meet at the ends of wing again?

Qmechanic
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Divyanshu Agarwal
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    They cover a greater distance because they are traveling along the shape of the wing, which has a longer arc length on top than on bottom. As for the assumption that the time is equal this is perhaps due to continuity. Keep in mind that the analysis is probably assuming laminar flow, no turbulence. That is the ideal scenario that a wing is designed for. –  Dec 16 '18 at 19:01
  • But in actual the air has turbulent flow especially when the aircraft travels at very high speed. – Divyanshu Agarwal Dec 16 '18 at 19:03
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    That is neither her nor there. The point of the analysis is to illustrate how the flow behaves in the ideal situation. You seem to be mixing methods, and states. –  Dec 16 '18 at 19:04
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    Using all capitals on the internet is considered to be like shouting rudely. – StephenG - Help Ukraine Dec 16 '18 at 19:11
  • If you read beyond popular accounts you'll find much better (but rather harder) treatments. There are physicists (and I presume also aerodynamics experts) who really loathe the usual popular treatment (which is incomplete in several ways). – dmckee --- ex-moderator kitten Dec 16 '18 at 19:18
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    Possible duplicate of How much effect does the Bernoulli effect have on lift? – RedGrittyBrick Dec 16 '18 at 19:23
  • @ggcg The assumption that the travel time is equal above and below the wing isn't based on anything. It's complete nonsense. A simple experiment with smoke in a wind tunnel will demonstrate that it's not even approximately correct. Just because people "always" say something on the internet doesn't make it true! – alephzero Dec 16 '18 at 19:34
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    @alephzero, then answer the question. I was offering a possible explanation, not an answer. Please, get your points. –  Dec 16 '18 at 19:39
  • @ggcg The explanation has nothing to do with whether the flow is laminar or turbulent. It can be modeled without considering viscosity at all. The link suggested by RedGrittyBrick covers the matter fairly well. – D. Halsey Dec 16 '18 at 21:36

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You are right to ask why it is true, because it is not true.

Many people have heard it and repeated it, but it has a name: The "equal time fallacy".

There is an excellent discussion here about why it is not true, and airplanes would not work if it were.

Mike Dunlavey
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