I learn physics myself and sorry if this is a very simple question

  1. Why is it that we can apply Bernoulli on above and below the plane even if the are not in the same streamline?

  2. Why do we ignore height difference when doing so?

Any help whatsoever is highly appreciated.


Bernoulli shouldn't be used to try and explain the workings of an airfoil, as the NASA page I linked to clearly explains.

It's basically an 'urban myth' that the Bernoulli principle can explain airfoil lift; it isn't true.

  • $\begingroup$ Please consult the title of the question. $\endgroup$ – Gert Nov 30 '20 at 20:30

Your first question finds an answer in this Wikipedia article :

if the fluid flow is irrotational, the total pressure on every streamline is the same and Bernoulli's principle can be summarized as "total pressure is constant everywhere in the fluid flow". It is reasonable to assume that irrotational flow exists in any situation where a large body of fluid is flowing past a solid body. Examples are aircraft in flight, and ships moving in open bodies of water

And for your second question, I would say that the effect of the height difference is negligible compared to the velocity and pressure terms.

  • $\begingroup$ I don't understand that wiki article .how can total pressure be constant every where then how is lift produced ? $\endgroup$ – Glowingbluejuicebox Nov 30 '20 at 12:00
  • 4
    $\begingroup$ By "total pressure", the wiki article presumably means the Bernoulli constant $(1/2) v^2 +P$. $\endgroup$ – mike stone Nov 30 '20 at 12:53

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