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<templatedata>
[[File:Propeller blade AOA.png|thumb|300px|right|Illustration of rotational and forward velocities contributing to angle of attack.]]
<templatedata>
[[File:Propeller blade AOA versus pitch.png|thumb|300px|right|Effect of pitch on blade load asymmetry.]]
{"description": "Yawing force caused by a rotating propeller."}
</templatedata>


<templatestyles src="Multiple image/styles.css" />
'''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.


{{Short description|Yawing force caused by a rotating propeller}}
== Causes ==
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* **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.
{{About|the aerodynamic phenomenon|the p factor of psychopathology|p factor (psychopathology)}}
* **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.
 
<div class="tmulti">
  <div class="thumb">
    [[File:Propeller blade AOA.png|frameless|alt=Angle of attack on a propeller|400px]]
  </div>
  <div class="thumb">
    [[File:Propeller blade AOA versus pitch.png|frameless|alt=Asymmetrical blade loading due to aircraft pitch|400px]]
  </div>
</div>
<div style="font-size:90%; text-align:center; margin-bottom:1em;">
  Propeller blade angle of attack (left) and propeller blade angle of attack change with aircraft pitch change, demonstrating asymmetrical load (right)
</div>
 
'''P-factor''', also known as '''asymmetric blade effect''' and '''asymmetric disc effect''', is an [[aerodynamic]] phenomenon experienced by a moving [[propeller (aircraft)|propeller]], wherein the propeller's center of [[thrust]] shifts off-center at high [[angle of attack]]. This asymmetry exerts a yawing moment, requiring rudder input to maintain directional control.


== Causes ==
These effects combine to move the center of thrust to one side, resulting in yaw.
[[File:Tilted propeller.png|thumb|alt=Change of forces at increasing Angle of Attack|Illustration of P-factor caused by changing angle of attack]]
At low airspeeds and nose-high attitudes (e.g., during climb), the descending propeller blade moves forward faster than the ascending blade. The increased airspeed and angle of attack of the descending blade generates more thrust than the ascending blade, shifting the center of thrust and producing yaw.


== Effects ==
== Effects ==
=== Single-engine aircraft ===
=== Single-engine aircraft ===
With a clockwise-turning propeller (as seen from the cockpit), the aircraft tends to yaw left during high-power, high-angle-of-attack flight (e.g., takeoff). Pilots apply right rudder to compensate. On descent, the effect reverses due to angle-of-attack changes.
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 exhibit more P-factor on the ground due to increased propeller disc tilt. Tricycle gear aircraft experience less effect during takeoff roll.
Tailwheel aircraft are more susceptible to P-factor during ground roll due to a greater propeller tilt.


=== Multi-engine aircraft ===
=== Multi-engine aircraft ===
Aircraft with same-direction rotating props experience unequal yaw from each engine. The engine whose descending blade is further from the centerline produces more yaw, making the opposite engine the "[[critical engine]]." If that engine fails, increased rudder is needed to maintain control.
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.


=== Helicopters ===
== Safety Considerations ==
In helicopters, P-factor manifests as [[dissymmetry of lift]]: the advancing blade generates more lift than the retreating one. Blade pitch adjustments counteract this. Failure to manage this leads to roll and potentially [[flap back]] due to [[gyroscopic precession]].
* Pilots must anticipate rudder input during high power and high angle of attack conditions.
* Minimum control speeds ([[V speeds|V<sub>MC</sub>]]) are affected by P-factor in multi-engine aircraft.


== See also ==
== See also ==
* [[Propeller walk]]
* [[Propeller walk]]
* [[Blohm & Voss BV 141]]
* [[Critical engine]]
* [[Dissymmetry of lift]]
* [[Dissymmetry of lift]]


== References ==
== References ==
{{Reflist}}
* FAA Airplane Flying Handbook FAA-H-8083-3, Chapter 12
* Rotorcraft Flying Handbook, FAA, 2019
* Rich Stowell, ''Emergency Maneuver Training''


[[Category:Aerodynamics]]
{{Aviation topics}}
[[Category:Aircraft manufacturing]]
{{Aircraft aerodynamics topics}}

Latest revision as of 00:51, 5 April 2025

Illustration of rotational and forward velocities contributing to angle of attack.
Effect of pitch on blade load asymmetry.

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

Template:Aviation topics Template:Aircraft aerodynamics topics

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