P-factor
P-factor, also known as asymmetric blade effect and asymmetric disc effect, is an aerodynamic phenomenon affecting aircraft with rotating propellers. It occurs when the center of thrust of the propeller shifts off-center due to high angle of attack, causing a yawing moment that the pilot must counteract with rudder input.
Causes
- **Blade Speed Difference**: At nose-high attitudes (common during takeoff or slow flight), the downward-moving blade moves forward faster relative to the oncoming air, producing more thrust than the upward-moving blade.
- **Angle of Attack Asymmetry**: The down-going blade experiences a higher angle of attack than the up-going blade due to the propeller disc's tilt.
These effects combine to move the center of thrust to one side, resulting in yaw.
Effects
Single-engine aircraft
Aircraft with clockwise-turning propellers (from pilot's perspective) tend to yaw left during climb. The pilot must apply right rudder to maintain coordinated flight.
Tailwheel aircraft are more susceptible to P-factor during ground roll due to a greater propeller tilt.
Multi-engine aircraft
If both engines rotate the same way, the engine with its down-going blades farther from the fuselage produces more yaw and roll. This makes one engine the "critical engine"—usually the left engine on clockwise systems.
Counter-rotating props cancel each other’s P-factor effects.
Helicopters
P-factor in helicopters manifests as dissymmetry of lift. The advancing blade produces more lift than the retreating blade. Rotorcraft counteract this by cyclically adjusting blade pitch during rotation.
Safety Considerations
- Pilots must anticipate rudder input during high power and high angle of attack conditions.
- Minimum control speeds (VMC) are affected by P-factor in multi-engine aircraft.
See also
References
- FAA Airplane Flying Handbook FAA-H-8083-3, Chapter 12
- Rotorcraft Flying Handbook, FAA, 2019
- Rich Stowell, Emergency Maneuver Training
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