Magnus Effect

The explanation of lift can best be explained by looking at a cylinder rotating in an air stream. The local velocity near the cylinder is composed of the air stream velocity and the cylinder's rotational velocity, which decreases with distance from the cylinder. On a cylinder, which is rotating in such a way that the top surface area is rotating in the same direction as the airflow, the local velocity at the surface is high on top and low on the bottom.

As shown in following figure, at point "A," a stagnation point exists where the air stream line that impinges on the surface splits; some air goes over and some under. Another stagnation point exists at "B," where the two air streams rejoin and resume at identical velocity's. We now have up wash ahead of the rotating cylinder and down wash at the rear.

The difference in surface velocity accounts for a difference in pressure, with the pressure being lower on the top than the bottom. This low pressure area produces an upward force known as the Magnus Effect. This mechanically induced circulation illustrates the relationship between circulation and lift. 

Magnus Effect is a lifting force produced when a rotating cylinder produces a pressure differential. This is the same effect that makes a baseball curve or a golf ball slice.

An airfoil with a positive angle of attack develops air circulation as its sharp trailing edge forces the rear stagnation point to be aft of the trailing edge, while the front stagnation point is below the leading edge.

Air circulation around an airfoil occurs when the front stagnation point is below the leading edge and the aft stagnation point is beyond the trailing edge.