Spiraling Slipstream

Spiraling Slipstream does not have much in the way of a physics explanation that I can offer in great detail, so here it is explained simply:

Propeller Slipstream (And Its Spiral Tendancy)

The slipstream of the propeller (the air that it pushed toward the tail of the airplane) is not a smooth cylinder of air. Rather, it is a spiral that can actually wrap around the fuselage of the airplane. Some of this spiral ends when it strikes the vertical stabilizer of the airplane:

                      Spiraling Slipstream Diagram

(Photo: Pilot's Handbook Of Aeronautical Knowledge)

Obviously, this current striking a flat surface creates a notable force, which, yet again, causes a left yawing moment.