P-factor

P-factor, also known as asymmetric blade effect and asymmetric disc effect, is an aerodynamic phenomenon experienced by a moving propeller,^[1] that is responsible for asymmetrical relocation of the propeller's center of thrust when aircraft is at a high angle of attack.

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.

Effects

Single engine propeller aircraft

(As viewed by the pilot), the aircraft has a tendency to yaw to the left if using a clockwise turning propeller (right hand), and to the right with a counter-clockwise turning propeller (left hand). The right-hand propeller is by far the most common. The effect is noticeable during take off^[2] and in straight and level flight with high power and high angle of attack.^[1]

Multi engine propeller aircraft (clockwise rotation)

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 than if the other engine had failed.

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):

At low speed flight with the left engine failed, the off-centre thrust produced by the right engine creates a larger yaw-couple to left than the opposite case. The left engine in this scenario is the critical engine, namely the engine whose failure brings about the more adverse result. In the case of using counter-rotating 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.

See also

• Propeller walk

References

1. ^ ^{a b} (Willits 3-49)
2. ^ (Ramskill)

Bibliography

• Willits, Pat, ed (2004) [1997]. Guided Flight Discovery - Private Pilot. Abbot, Mike Kailey, Liz. Jeppesen Sanderson, Inc.. ISBN 0-88487-333-1. 
• Ramskill, Clay (June 2003). "Prop Effects". page 4. SMRCC. http://www.smrcc.net/Newsletters/2003_SMRCC_Jun_News.pdf. Retrieved 2009-04-27.