P-factor

Not to be confused with the p-value.

This article is about the aerodynamic phenomenon. For the p factor of psychopathology, see p factor (psychopathology).

Propeller blade angle of attack (left) and propeller blade angle of attack change with aircraft pitch change, demonstrating asymmetrical load (right)

P‑factor, also known as asymmetric blade effect and asymmetric disc effect, is an aerodynamic phenomenon experienced by a moving propeller, wherein the propeller's center of thrust moves off-center when the aircraft is at a high angle of attack. This shift in thrust location creates a yawing moment, causing the aircraft to yaw to one side. Rudder input is required to counteract this tendency.

Causes

P-factor, showing relative speed and thrust asymmetry at increasing angle of attack

At cruise speed and level flight, the propeller disc is perpendicular to airflow, creating balanced thrust. In nose-high attitudes (e.g., climb), the disc tilts, and blades descend forward or ascend backward:

• **Down-going blade** (clockwise rotation): more forward speed → higher airspeed → more thrust
• **Up-going blade**: less forward speed → lower airspeed → less thrust

This asymmetry shifts thrust center toward the more thrustful blade.

Disc tilt also alters blade AOA:

• Down-going blade → increased AOA → more thrust
• Up-going blade → decreased AOA → less thrust

Even though higher forward speed would reduce AOA, the tilt effect dominates, resulting in increased AOA and thrust for the down-going blade.

P-factor is most pronounced during high AOA and high power, such as takeoff.

Effects

Single-engine aircraft

For clockwise propellers (viewed by pilot):

• Climbing → yaw left
• Descending → yaw right

Countered via rudder. Spiral slipstream also contributes.

Tailwheel aircraft show more P-factor on takeoff due to disc tilt. Less noticeable on landing due to low power settings.

Multi-engine aircraft

If propellers rotate in the same direction:

• The engine with down-moving blades toward the wingtip causes stronger yaw
• The other engine becomes the "critical engine"

P-factor impacts minimum control speeds (Template:V speeds), which are based on the failure of the critical engine.

Fig. 1. Operating right engine produces more yaw when left engine fails, making it the critical engine

Helicopters

In forward flight:

• Advancing blade → more lift
• Retreating blade → less lift (dissymmetry of lift)

Compensated by cyclic pitch adjustment. Otherwise, imbalance causes roll and backward pitch ("flap back").

See also

• Blohm & Voss BV 141
• Propeller walk
• Dissymmetry of lift