A wing passing through air produces lift by deflecting air in the opposite direction from the direction of lift. The component of the down wash behind the wing that is in the opposite direction from the lift is the velocity of the air that has been accelerated from rest, before the wing passed. The lift force is the mass of the air deflected times the acceleration produced by the passing of the wing.

The flow of the air around the airfoil the mechanism which produces the acceleration opposite to lift. Bernouli’s principal is part of the explanation for the behavior of the flow around the airfoil.

In the case of a symmetrical airfoil operating at a positive angle of attack, the stagnation point, where the flow seperates to go over and under the airfoil, is below the nose of the airfoil. As a result the flow over the top goes back and around the nose of the airfoil before proceeding over the top surface. In the case of a flat plate, at an angle of attack, the stagnation point is somewhat aft of the leading edge.