P factor
The P-factor is an effect on a rotating propeller that an aircraft experiences in flight at a high angle of attack . It leads to a shift of the thrust / propulsion center on the propeller and thus to an asymmetrical thrust distribution. Apart from other influences, such as cross winds, the shifting of the thrust / propulsion center takes place on a horizontal straight line that runs through the hub of the propeller.
The P-factor is partially counteracted by using a motor train .
causes
In a normal flight position , the plane of rotation of the propeller blades of a propeller is at right angles to the incoming air, ie to the "airspeed" vector . In this condition, both the relative angle of attack of the blades in relation to the air flowing through the propeller plane is largely the same during the entire revolution, as is the relative speed of the blade tips. This results in a tensile force in the precise alignment of the axis of rotation (= propeller shaft), the vector of which also acts in the center of the circular area, i.e. on the hub.
As soon as the aircraft is in flight conditions with an increased angle of attack, the axis of rotation of the propeller or propellers also points upwards and thus deviates from the angle of attack of the air. The propeller plane is no longer traversed vertically, but is inclined backwards in relation to the air flow. Due to this inclination, the inclined flow component and the peripheral speed of the propeller blades are superimposed differently in the left and right halves of the circular plane; each downward moving leaf has a higher speed relative to the air than the upward moving one, this has a similarly reduced relative speed to the air. Likewise, the amount of the angle of the inclined flow is added to or subtracted from the specified angle of attack (the “pitch”) of the propeller blades. The blade moving upwards has a smaller angle of attack in this segment, the blade moving downwards a larger one. The sheets that are just passing through the upper or lower position show no such change in their thrust generation.
Since the lift or the propulsion generated by each propeller blade is proportionally dependent on its current relative speed and its angle of attack at certain points during the orbit, the upward inclination and the inclined flow caused by it result in different relationships in the left and right parts of the Circular level and thus lead to an asymmetry .
Effects
Single engine aircraft
When the propeller rotates clockwise ("right") when viewed in the direction of flight, the force application point moves horizontally to the right from the center (away from the propeller shaft) with increasing angle of attack (and thus higher pitch and inclined flow). This creates a yaw moment for the entire aircraft around the vertical axis to the left. However, this can be compensated by counter-steering with the rudder . The effect can also be seen, for example, on a rolling tail-wheel aircraft. Aircraft with such a landing gear typically have a higher angle of attack than those with a nose wheel landing gear when operating on the ground .
Twin-engine aircraft
In twin-engine airplanes, the propellers of which rotate in the same direction, the center of propulsion moves away from the fuselage in the case of the motor whose blade is moving downwards towards the end of the wing on the corresponding side , while in the case of the motor on the other side it shifts towards the fuselage. The latter is therefore the “critical” engine, as it requires stronger countermeasures in the event of a failure. In normal flight, both engines have the same lever arm around the vertical axis. So while the lever arm is reduced in the critical engine, it increases in the other engine. In twin-engine aircraft with the engines rotating in the opposite direction, the shifts in the thrust center compensate each other. It is then advisable to select the direction of rotation so that the blades facing the trunk run downwards. In the event of a failure, there is always an engine that works with a reduced lever arm.
General
Due to the direct proportionality of the P-factor to the engine power, the negative influences on the landing approach when floating out ( English flare ) with a high angle of attack shortly before touchdown are negligible. Theoretically, the P-factor exists in relation to its impact strength, but in practice it is subject to the stronger corkscrew effect produced by the twisted propeller jet .