P-factor

P-factor, also known as asymmetric blade effect and asymmetric disc effect, is an aerodynamic phenomenon experienced by a moving propeller, that is responsible for asymmetrical relocation of the propeller's center of thrust when aircraft is at a high angle of attack. This shift in the location of the center of thrust will exert a yawing moment on the aircraft, causing it to yaw slightly to one side. A rudder input is required to counteract the yawing tendency.

Causes

When an aircraft is in straight and level flight at cruise speed, the propeller disc is perpendicular to the relative wind. Each of the propeller blades will contact the air at the same angle and speed and thus the thrust produced is evenly centered across the propeller. As the aircraft's angle of attack increases and the propeller disc rotates toward the horizontal, the airflow will meet the propeller disc at an increasing angle. The propeller blades moving down and forward (for clockwise rotation, from the one o'clock to the six o'clock position when viewed from the front) will have a greater relative wind velocity and therefore will produce greater thrust, while propeller blades moving up and back (from the seven o'clock through 12 o'clock position) will have a decreased relative wind velocity and therefore decreased thrust. This asymmetry displaces the center of thrust of the propeller disc towards the blade with increased thrust. In an aircraft with two or more propeller engines, P-Factor is what determines which engine is the critical engine.