P-factor

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

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, the descending blade has a higher angle of attack, relative to the ascending blade. In accordance with the Bernoulli Principle, the propeller blade with the higher angle of attack will provide more force, in this case, measured as thrust. With the descending blade providing more thrust than the ascending blade, the center of thrust is moved away from the center and out towards a parallel point somewhere along the descending blade.

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 more common. The effect is noticeable during take off and in straight and level flight with high power and high angle of attack.

Multi engine propeller aircraft

With engines rotating in the same direction the p-factor will affect V_MC (minimum control speed) in asymmetric flight.

Considering right-hand tractor engines (lines projecting from propeller discs represent the p-factor induced thrust lines:

File:Pfactor me.png

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.

Sources

• Willits, Pat, ed. (2004). Guided Flight Discovery - Private Pilot. Abbot, Mike Kailey, Liz. Jeppesen Sanderson, Inc. ISBN 0-88487-333-1. {{cite book}}: Unknown parameter |origdate= ignored (|orig-date= suggested) (help)
• Ramskill, Clay. "Prop Effects". Retrieved 2006-12-5. {{cite web}}: Check date values in: |accessdate= (help)