WHAT IS “P” FACTOR?
There are several forces working on control of the aircraft, related to the operation of the engine and the rotation of the propeller. On take off, (with tail dragger configuration) the aircraft has a tendency to yaw to the left. This is due to the swirling propeller slip stream rotating around the fuselage and pushing on the left side of the rudder. In addition, the angle of attack of the propeller blades, relative to the incoming air flow, also causes a left yaw moment (“P” factor)..
In the three point tail dragger attitude, the thrust line axis of rotation causes a differential in the angle of attack (AOA) of the left and right side propeller blade. The rotating blades have a greater AOA on the RH side of the rotating thrust line and a lesser AOA on the LH side of the thrust line. This causes a yawing moment to the left that goes away when the tail is lifted and the relative wind is now in line with the axis of propeller rotation. This is why on take off, some right rudder is required during the initial run and is dissipated as the tail rises. The rudder control authority also increases as more air passes over the rudder surface with increased airspeed.
In a forth coming article on twin engine operation, opposite rotating engines eliminate the “P” factor on take off since the forces cancel each other out.
With a tricycle gear configuration, the thrust line on take off is already in line with the relative wind and the “P” factor is eliminated. There is still a left yaw component that requires some right rudder input due to the slip stream swirl around the fuselage pushing on the left side of the rudder.
In full scale operation during take off, fixed wing pilots anticipate the left yaw tendency as power is applied, with an automatic application of right rudder. Throughout the aviation industry, there is an attempt to keep aircraft control operations similar so that pilots see similar aircraft handling characteristics, minimizing surprises and changes in control inputs. The major exception is in helicopter operations with regard to yaw.
As power is applied in a US designed single rotor helicopter, the nose of the helicopter wants to yaw to the right. This is not due to “P” factor (since there isn’t any), but to the torque generated by the standard LH rotation of the main rotor. This is one major area of change to pilot control input, between fixed and rotary wing operation. This difference in rudder application with application of power is easily learned and anticipated, but does violate a standard of attempting to keep all piloting responses the same. This anomaly is not present in European designed helicopters since the main rotor direction is opposite and the torque generated by turning the rotor is to the left, requiring standard application of right rudder when power is applied. The direction of main rotor rotation was probably determined originally by the normal rotational direction of the initial power plants in early designs. US engines rotate CCW while others rotated CW. It probably all came down to the simplicity of the main rotor transmission design!
“What’s up Prop”? Marty