Re: supersonic airfoils
Because you are dealing with 2 different mechanisms of lift generation. In the subsonic case all of the lift can be related back to the vorticity generated at the airfoil surface. With a symmetric airfoil at zero angle of attack the net vorticity generated is zero, hence zero lift. Add in a slight asymmetry via camber and the net vorticity generation is no longer zero and you have lift. With a supersonic airfoil the principal lift mechanism is the pressure difference created by shock asymmetry. This is why most supersonic airfoils have sharp edges (a no-no in low subsonic flows). Camber will still have an effect in supersonic flow, but the effect is so small as to be ignored.
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