To most pilots, P-Factor, also known as asymmetric blade effect, is the most mysterious of left turning forces, though it can be explained with some imaginative visualization. P-Factor is most pronounced at high angles of attack. The angle of attack, explained extremely simply, can be thought of as how much is "demanded" of the wings, so it increases in high g maneuvers, climbs, and slow flight.

P-Factor, Explained Visually

                                          Diagram illustrating P-Factor    

(Photo: Pilot's Handbook Of Aeronautical Knowledge, Page 5-33)

Imagine the arrows in the diagram similar to those in free body diagrams, but the weight of the arrow indicates magnitude. As the "Low angle of attack" portion of shows, the two propeller blades are moving equal amounts of air as the propeller rotates. This should come as no surprise.

However, something interesting happens as the angle of attack of the airplane increases. Propeller blades are not flat; they are in fact shaped as small wings. Therefore as the angle of attack of the airplane increases, the air passing by hits the blades differently:

                                  AOPA Diagram

(Photo: AOPA Pilot Magazine, December 2011)

This means that the blade swinging "down" viewed from the cockpit will now have a larger angle of attack of its own compared to the up-swinging blade! This then means that the down-swinging (right) blade exerts a force that pushes the nose to the left, and vice versa for the up-swinging blade.

    From here, it is a matter of balancing forces: the two act in opposite directions, but the one on the right is slightly larger, so it causes a yawing moment to the left. Once again, since the airplane is suspended in air with little resistive force, the yawing moment is noticeable to the pilot.