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 the asymmetrical relocation of the propeller's center of thrust when an 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.

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.

P-Factor is sometimes erroneously explained with the word "bite", as in "the descending blade has a bigger bite, or angle of attack, than the ascending blade". This faulty explanation does not take into account the forward motion of the blades as aircraft's angle of attack increases. In order to better understand this concept, imagine a propeller aircraft moving forward with a 90° angle of attack (vertical). This situation is identical to what a helicopter experiences, but a helicopter can reduce or increase the angle of attack of individual blades of the rotor (decreasing the angle of attack on the advancing blade, while increasing the angle of attack on the retreating blade) in order to keep the lift of the rotor disc balanced. Because the force of the air on the blades moving forwards through the arc is greater, they will produce more thrust than the blades that move backwards. If the blades of the rotor were unable to independently change their angle of attack there would be a constant backwards rolling motion due to the increased lift on the side of the rotor disc with the advancing blade. In a fixed-wing aircraft, there is usually no way to adjust the angle of attack of the individual blades of the propellers, therefore the pilot must contend with P-Factor and use the rudder to counteract the shift of thrust.

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