Why Kutta-Zhukovsky Lift Theory is Wrong

· theory of flight

How to Check if a Theory is Wrong?

The following criteria can be used as evidence that a certain proposed physics theory is wrong:

  1. Compare with a correct theory and notice fundamental difference.
  2. Compare with experimental observation and notice major difference.
  3. Identify a contradiction within the proposed theory.

Consider as an example the classical circulation theory for lift of a wing proposed by Kutta and Zhukovsky 100 years ago (KZ-theory), which is still the accepted theory propagated in text books and university education. The essence of KZ-theory is captured in the above picture: Inviscid potential flow without lift (top) is modified by large scale circulation (middle) into a flow with lift leaving the sharp trailing edge smoothly (bottom).

We present a New Theory of Flight which is fundamentally different from the Old Theory based on KZ-theory for lift, and Prandtl’s boundary layer theory for drag. Let us now test KZ-theory against 1. – 3.

1.  New Theory has massive support from mathematics, physics, computation and experimental observation,  and thus may be considered to be correct using stringent scientific standards. KZ-theory is fundamentally different from New Theory.

2.  KZ-theory does not predict stall and thus lacks essential observed physics. KZ-theory predicts high pressure at separation which is not observed. KZ-theory gives zero drag which is not observed.

3. The post Why KZ Is Incorrect exhibits a contradiction of KZ-theory with viscosity playing two contradictory roles to ensure that the flow separates at the sharp trailing edge by preventing:

  • (i) flow around the trailing edge (as in inviscid potential flow in top picture),
  • (ii) separation on the upper surface well before the trailing edge.

Viscosity is thus given the double effect of (i) not allowing flow to stick to the surface and flow around the trailing edge and (ii) making the flow stick to the upper surface. KZ-theory flow is both (i) different from potential flow (no flow around the trailing edge), and (ii) similar to potential flow (no separation on the crest of the wing).

Viscosity thus has the double role of not allowing the flow to stick/flow around the trailing edge , and to make the flow stick onto the upper surface. In both cases no reason is given for this seemingly contradictory effect of both causing sticking and non-sticking.

In an attempt to save KZ-theory the following argument could be put forward by proponents that the sharpness of the trailing edge makes a difference (the infinite velocity about a sharp trailing edge in the above picture): Viscosity can make the flow stick to a smooth surface but not a non-smooth. This would then be the reason why airfoils are postulated to have sharp trailing edges, although real wings do not and no criterion is presented making it possible to tell if a given real wing has a sharp trailing or not, as evidenced in communication with Prof of Aeronautics Art Rizzi at KTH.

From scientific point of view this cumbersome: The KZ-theory is claimed to be a real scientific theory, but it is impossible to tell if for a given real wing the theory is applicable. This makes the theory into a pseudo-scientific theory,  that is a theory which is true by its construction but says nothing about reality.

The truth is that a flow with a viscous boundary layer with a no-slip boundary condition will separate on the crest of the wing and thus viscosity/no-slip does not act so as to make the flow stick to the surface.

The truth is that it is inviscid potential flow with slip without boundary layer which sticks to the upper surface because it can only separate at stagnation and until stall this does cannot occur before the trailing edge.

Opposite to the conventional KZ-theory, it is slip which creates lift by sticking to the upper surface, not viscosity with no-slip.


We have found KZ-theory to be an incorrect  theory according to all of the criteria 1. – 3.

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