P-factor, also known as asymmetric blade effect and asymmetric disc effect, is an
aerodynamic phenomenon experienced by a moving
propeller with a
high angle of attack that produces an asymmetrical ceter of thrust
Causes:
When an aircraft is in straight and level flight at cruise speed, the propeller disc will be normal (i. e. perpendicular) to the airflow vector. As airspeed decreases and wing angle of attack increases, the engines will begin to point up and airflow will meet the propeller disc at an increasing angle, such that horizontal propeller blades moving down will have a greater angle of attack and relative wind velocity and therefore increased thrust, while horizontal blades moving up will have a reduced angle of attack and relative wind velocity and therefore decreased thrust. (Vertical blades are not affected). This asymmetry in thrust displaces the center of thrust of the propeller disc towards the blade with increased thrust, as if the engine had moved in or out along the wing. The engine with the down-moving blades towards the wingtip produces more yaw and roll than the other engine, because the moment (arm) of that engine's thrust about the aircraft center of gravity is greater. Thus, the engine with down-moving blades towards the fuselage will be critical, because its failure will require a larger rudder deflection by the pilot to maintain straight flight.
So this is the factor that normaly determines which engine is critical when the aircraft is in the air. So what determines a critical engine on the ground.??