See How It Flies: A New Spin on the Perceptions, Procedures and Principle of Flight by John S. Denker sells the the old Kutta-Zhukovsky circulation theory of lift:
- In real flight situations, precisely enough circulation will be established so that the rear stagnation line is right at the trailing edge, so no air needs to turn the corner there.
- The general rule — called the Kutta condition — is that the air hates to turn the corner at a sharp trailing edge.
- To a first approxmation, the air hates to turn the corner at anysharp edge, because the high velocity there creates a lot of friction.
- For ordinary wings, that’s all we need to know, because the trailing edge is the only sharp edge.
- The funny thing is that if the trailing edge is sharp, an airfoil will work even if the leading edge is sharp, too. This explains why dime-store balsa-wood gliders work, even with sharp leading edges.
- It is a bit of a mystery why the air hates turning a corner at the trailing edge, and doesn’t mind so much turning a sharp corner at the leading edge — but that’s the way it is.
- In any case, the rule is:The air wants to flow cleanly off the trailing edge.