Understanding Propeller Torque and P-Factor
This is an attempt to answer the frequent question "Why is my aircraft turning left all the time?"
This occurs only in aircraft with propellers at the front of the aircraft. Several distinct phenomena cause the effect, all causing the aircraft to turn in the same direction. They are:
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 clockwise (when viewed from the cockpit), imparts a tendency for the aircraft to rotate counterclockwise. Since most single engine aircraft have propellers rotating clockwise, 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).
On aircraft with contrarotating propellers (propellers that rotate in opposite directions) the torque from the two propellers cancel each other out, so that no compensation is needed.
Further Reading: Propeller Torque 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. This occurs only when the propeller is not meeting the oncoming airflow head-on, for example when an aircraft is moving down the runway at a nose-high attitude (i.e. at a high angle of attack), as is the case with tail-draggers. Aircraft with tricycle landing gear maintain a level attitude on the takeoff run, so there is little P-factor during takeoff. In all cases, though, the effect is weaker than prop wash.