Flight Theory of the US Airforce Academy

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The text book theory describing the generation of lift by a wing is the Circulation Theory (CT) developed by Kutta-Zhukovsky more than 100 year ago. The New Theory of Flight shows that CT is unphysical and incorrect:

  1. Lift is not generated by large scale circulation around a wing section.
  2. There is no physical mechanism generating large scale circulation around a wing section.
  3. The sharp trailing edge required by CT to generate circulation is not necessary. Real wings with rounded trailing edges generate lift.

These fact has been (more or less known) since the advent of the theory, but in the absence of a correct physical theory of lift, they have been suppressed and the incorrect unphysical CT has survived into our days.

It is instructive to see how CT is presented in the text book Aerodynamics for Engineers , 5th Ed., 2009, by J. Bertin and R. Cummings, United States Airforce Academy. We read in the central Section 6.2 Circulation and the Generation of Lift, with my comments in parenthesis:

  • For a lifting airfoil, the pressure on the lower surface of the airfoils, on the average, greater than the pressure on the upper surface. (Triviality).
  • Bernoulli’s equation for steady incompressible flow leads to the conclusion that the velocity over the upper surface is on the average, greater than that past the lower surface. (Incorrect conclusion: the assumptions of Bernouilli’s law are not satisfied).
  • Thus the flow around the airfoil can be represented by the combination of a translational flow and a circulating flow. (Note “can be represented”, which indicates the unphysical character).
  • Circulation is necessary to produce lift. (Incorrect).

This is the essence of CT. We see the logical fallacy of confirming a hypothesis (circulation) by observing a consequence (lift). We see the cover-up that the flow “can be represented” as a flow with circulation, as a clever way of coping with the fact that there is no physical mechanism for the generation of circulation which thus is not actually present as the generator of lift.

 

2 Comments

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  1. Russell Cummings

    Interesting that you rebuff a theory without offering any proof that it is wrong. Put simply, all theories are wrong (to some extent), but sometimes theories are useful. So I would like to see your proof that circulation theory is wrong. If you provide the proof I would be happy to change the book.

  2. claesjohnson

    Good to hear from you. Yes, we have evidence that circulation computed close to the wing surface is small, which shows that it is not the origin of lift. You find this evidence in my book Computational Turbulent Incompressible Flow. If you choose the contour to compute circulation in a very special way, then you can get the lift you want, but this is just hokus pokus. If you think of it, it is inconceivable that there would be a large scale circulation around the wing created by the wing itself, as if the wing was a powerful rotating fan. The idea of circulation was invented because lift was a mystery, but we show that it is an invention without physical basis, by showing the true physical mechanism of the generation of lift. Have you studied our explanation?

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