The Momentum Explanation of Lift
According to this explanation, the slight downward deflection of the air leaving the trailing edge of a wing is what produces lift. Since for every action there is an equal and opposite reaction, the downward push on the air must result in an upward push on the wing. But this “downwash” results from air pressure differences on the wing—it is an effect of lift, not the cause of it. The wing cannot “feel” the airstream behind it being deflected downward; but it does feel the pressure of air molecules colliding against its surface.
The Circulation Theory of Lift
This is the oldest and most complex theory of lift. It explains how the difference in air speed over and under the wing results from a net “circulation” of air. Above the wing, the circulatory flow adds to the overall speed of the air; below the wing, it subtracts. But the actual force on the wing results from the difference in air pressure caused by the difference in airflow speeds.