torsdag 2 augusti 2012

Lift Not Generated by Circulation

                            The canonical picture of lift by circulation, which does not describe actual physics.

The central thesis of the current text book theory of flight by Kutta-Zhukovsky-Prandtl is that lifting flow around airfoil can be represented as non-lifting potential flow modified by circulation around the airfoil section determined to as to generate a smooth flow at the sharp trailing edge (Kutta condition)

The New Theory of Flight shows that this picture does not describe the actual physics of flight. To see that lift does not originate from circulation, it is instructive to consider potential flow over a surface-mounted  half-cylinder, which has a lift coefficient of 1.67 (as shown here). We see substantial lift (of the same size as that of an airfoil) without circulation around any section. Lift is instead generated by acceleration of air without circulation. 

It is also instructive to recall that the lift of a rotating circular cylinder is generated by unsymmetrical separation and not by large scale circulation around the cylinder section. 

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