P-factor
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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 center of thrust[1]
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[edit] Causes
The p-factor is caused by the difference in angle of attack between the ascending and descending blades of a rotating propeller blade. Specifically, in a "nose-up" situation where the propeller disk is inclined to the flight path, the descending blade has a higher angle of attack relative to the ascending blade. The propeller blade with the higher angle of attack will provide more force, in this case, measured as thrust[2]. With the descending blade providing more thrust than the ascending blade, the center of thrust is moved away from the center of the propeller disk and out towards a parallel point somewhere along the descending blade.
[edit] Effects
Single engine propeller aircraft
The aircraft has a tendency to yaw to port if using a right-hand propeller, and yaw to starboard with a left-hand propeller. The right-hand propeller is by far the most common. The effect is noticeable during take off[3] and in straight and level flight with high power and high angle of attack[4].
Multi engine propeller aircraft (clockwise rotation)
With engines rotating in the same direction the p-factor will affect VMC (minimum control speed) in asymmetric flight.
Considering right-hand tractor engines (lines projecting from propeller discs represent the p-factor induced thrust lines:
At low speed flight with the port engine failed, the off-centre thrust produced by the starboard engine creates a larger yaw-couple to port than the opposite case. The port engine in this scenario is the critical engine, namely the engine whose failure brings about the more adverse result. In the case of using handed engines (i.e. not rotating in the same direction) the p-factor is not considered in determining the critical engine.
The asymmetric blade effect is dependent on thrust and proportional to forward velocity (specifically TAS) and, while generally insignificant during the initial ground roll for tail-wheel aircraft, will give a pronounced nose-left tendency during the later stages of the roll, particularly if the thrust axis is kept inclined to the flight path vector (i.e. tail-wheel in contact with runway.) If a high angle of attack is used during the rotation (or indeed straight and level flight with high power and high angle of attack) the effect can also be apparent. The effect is not so apparent during the landing rollout and flare given the relatively low power setting, however should the throttle be suddenly advanced with the tail-wheel in contact with the runway then anticipation of this nose-left tendency is prudent.
[edit] Sources
- [1997] (2004) in Willits, Pat: Guided Flight Discovery - Private Pilot, Abbot, Mike Kailey, Liz, Jeppesen Sanderson, Inc.. ISBN 0-88487-333-1.
- Ramskill, Clay. Prop Effects. Retrieved on 2006-12-05.
