fredag 21 september 2012

Von Karman on Incorrect KZ-theory of Lift

The above pictures from von Karman's Aerodynamics (1954) compares the lift coefficient for different angles of attack (left) and pressure distribution around a NACA 4412 airfoil, between Kutta-Zhukovsky circulation theory of lift and experimental values.

We see that the theoretical lift coefficient is too big and does not include the drop at stall at 16 degrees, and that the high pressure at the trailing edge from theory is not observed in experiments.

Von Karmann comments on p 46:
  • The pictures show that the usefulness of the theory is restricted to a limited range of angles of attack, comprising relatively small angles, positive and negative. Beyond this range the measured values falls far below the values predicted by the theory.
This comment alone would have been sufficient to discard KZ-theory as unphysical (and thus dangerous to use), if there had only been a better theory around. But it was not and so the incorrect KZ-theory has survived into our days as the text-book theory of lift. 

With the advent of the New Theory of Flight as the first the physically correct theory predicting both lift and pressure distribution in close correspondence to observation for all angles of attack including stall and beyond, the KZ-theory can now finally be put into the wardrobe of incorrect physical theories along with flogiston theory. It will take some time before this is realized and acknowledged by AIAA: The World's Forum for Aerospace Leadership, but it will come.

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