[tgrayson]

Originally Posted by Matt13C

Just to clarify. It seems to me that it is saying that it is not the upper cambered surface being a greater distance than the lower less cambered surface. Instead it is the angle of attack that causes a rotational force that forces the air over the top of the wing at a greater speed and slows the air beneath the wing which actually causes the pressure variation?

The necessity of being at an AOA is true for a symmetrical airfoil; a cambered airfoil will generate lift even at a zero AOA. There's no fundamental difference between the two situations.

This is different than it has been explained to me. I had it explained as a product of the barn door effect and camber, camber being the majority contributor. The upper cambered surface was a greater distance than the lower less cambered surface which caused the air to travel faster above the wing lowering the pressure. Does that still play a part?

First, let me stress that circulation theory is about the most fundamental concept in aerodynamics. It's not a fringe theory or something I just made up. Its discovery revolutionized the study of aerodynamics. You won't find it in any pilot literature, although Aerodynamics for Naval Aviators touches on it. But you see vague references to it for people selling things that reduce the stall speed, which are often called "circulation control" technology.

The mechanism you mentioned is often pooh-poohed by aerodynamics authors when they write towards pilots, because there is no physics which says that the air must speed up when it encounters a longer distance to the trailing edge.

However, there's something that's close: when you obstruct the flow of air, it speeds up. The problem with that as the *sole* explanation of lift is that studies show that the top and bottom of the air speed up the same, generating no lift. You have to somehow explain why the upper airspeed ups *more* than the lower. Circulation theory does that.