Understanding Propeller Torque and P-Factor
This is an attempt to answer the frequent question "Why is my aircraft turning left all the time?"
Propeller Torque Effect
Torque effect is the influence of engine torque on aircraft movement and control. It is generally exhibited as a left turning tendency in piston single engine propeller driven aircraft.
According to Newton's law, "for every action there is an equal and opposite reaction," such that the propeller, if turning counterclockwise (when viewed from the front), imparts a tendency for the aircraft to rotate clockwise. Since most single engine aircraft have propellers rotating counterclockwise, they rotate to the left, pushing the left wing down.
Typically, the pilot is expected to counter this force through the control inputs. To counter the aircraft roll left, the pilot applies right aileron.
It is important to understand that torque is a movement about the roll axis. Aileron controls roll. Prop torque is not countered by moving the rudder or by setting rudder trim. It is countered by moving or trimming the aileron.
This correction induces adverse yaw, which is corrected by moving or trimming the rudder (right rudder).
Aircraft with contrarotating propellers (propellers that rotate in opposite directions) counteract the torque effect, so that no compensation is needed.
Further Reading: Propeller Torque Factor
P-Factor
P-factor is the term for asymmetric propeller loading, causes the airplane to yaw to the left when at high angles of attack.
The descending right side of the propeller (as seen from the rear) has a higher angle of attack (than the upward-moving blade on the left side) and provides more thrust.
According to What You Need to Know About Aerodyanics... by Franklin Gutierrez.
Gyroscopic Precession
This is the tedency of a spinning object to precess or move about its axis when disturbed by a force. The engine and propeller act as a big gyroscope. However, gyroscopic precession is likely to be minimal in a typical aircraft.
Gyroscopic precession is frequenly confused with p-factor.
One author maintains p-factor is caused by a combination of factors unrelated to gyroscopic precession http://home.earthlink.net/~x-plane/FAQ-Theory-PFactor.html