P-factor: Difference between revisions

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{"description": "Yawing force caused by a rotating propeller."}
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{{Short description|Yawing force caused by a rotating propeller}}
{{Short description|Yawing force caused by a rotating propeller}}
{{Distinguish|text=the [[p-value|''p''-value]]}}
{{Distinguish|p-value}}
{{About|the aerodynamic phenomenon|the p factor of psychopathology|p factor (psychopathology)}}
{{About|the aerodynamic phenomenon|the p factor of psychopathology|p factor (psychopathology)}}


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[[File:Propeller blade AOA.png|center|thumb|300px|Pitch of blade angle, chord line, angle of attack]]
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[[File:Propeller blade AOA versus pitch.png|center|thumb|300px|Propeller blade angle of attack change with pitch change (right) showing asymmetrical loading]]
    [[File:Propeller blade AOA.png|frameless|alt=Angle of attack on a propeller|400px]]
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    [[File:Propeller blade AOA versus pitch.png|frameless|alt=Asymmetrical blade loading due to aircraft pitch|400px]]
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  Propeller blade angle of attack (left) and propeller blade angle of attack change with aircraft pitch change, demonstrating asymmetrical load (right)
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'''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 ==
'''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]] moves off-center at high [[angle of attack]]. This asymmetry exerts a yawing moment, requiring rudder input to maintain directional control.
[[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 ==
==Causes==
=== Single-engine aircraft ===
[[File:Tilted propeller.png|thumb|Change of forces at increasing angle of attack]]
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.
At low airspeeds with nose-high attitude, the propeller disc tilts, causing the down-going blade (typically on the right for clockwise propellers) to move forward with greater speed and angle of attack, producing more thrust. The up-going blade moves backward with lower airspeed and produces less thrust, shifting the thrust center off-axis.


Tailwheel aircraft exhibit more P-factor on the ground due to increased propeller disc tilt. Tricycle gear aircraft experience less effect during takeoff roll.
==Effects==
===Single-engine aircraft===
Clockwise-turning propellers (from pilot view) cause left yaw in climb due to increased thrust on the right side. The effect reverses in descent. Rudder input is needed to maintain coordinated flight. Tailwheel aircraft experience greater P-factor during ground roll due to steeper disc angle.


=== 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.
In same-direction twin-engine aircraft, P-factor contributes to determining the [[critical engine]]—the engine whose failure causes more severe yaw. For clockwise props, the left engine is typically critical. Counter-rotating props cancel out P-factor asymmetry.


=== Helicopters ===
==Helicopters==
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]].
Helicopters experience significant P-factor, especially in forward flight. The advancing blade creates more lift than the retreating blade, requiring cyclic pitch adjustments to maintain stability. Without compensation, the aircraft would roll and pitch undesirably.


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


== References ==
==References==
{{Reflist}}
{{reflist}}


[[Category:Aerodynamics]]
[[Category:Aerodynamics]]
[[Category:Aircraft manufacturing]]
[[Category:Aircraft manufacturing]]
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Revision as of 23:41, 4 April 2025

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Not to be confused with p-value.

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This article is about the aerodynamic phenomenon. For the p factor of psychopathology, see p factor (psychopathology).
Pitch of blade angle, chord line, angle of attack
Propeller blade angle of attack change with pitch change (right) showing asymmetrical loading

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 at high angle of attack. This asymmetry exerts a yawing moment, requiring rudder input to maintain directional control.

Causes

Change of forces at increasing angle of attack

At low airspeeds with nose-high attitude, the propeller disc tilts, causing the down-going blade (typically on the right for clockwise propellers) to move forward with greater speed and angle of attack, producing more thrust. The up-going blade moves backward with lower airspeed and produces less thrust, shifting the thrust center off-axis.

Effects

Single-engine aircraft

Clockwise-turning propellers (from pilot view) cause left yaw in climb due to increased thrust on the right side. The effect reverses in descent. Rudder input is needed to maintain coordinated flight. Tailwheel aircraft experience greater P-factor during ground roll due to steeper disc angle.

Multi-engine aircraft

In same-direction twin-engine aircraft, P-factor contributes to determining the critical engine—the engine whose failure causes more severe yaw. For clockwise props, the left engine is typically critical. Counter-rotating props cancel out P-factor asymmetry.

Helicopters

Helicopters experience significant P-factor, especially in forward flight. The advancing blade creates more lift than the retreating blade, requiring cyclic pitch adjustments to maintain stability. Without compensation, the aircraft would roll and pitch undesirably.

See also

References

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