P-factor: Difference between revisions

From AviationSafetyX Wiki
Jump to navigation Jump to search
← Older edit
VisualWikitext
No edit summary
No edit summary
 
(6 intermediate revisions by the same user not shown)
Line 1: Line 1:
{{Short description|Yawing force caused by a rotating propeller}}
{{Distinguish|text=the [[p-value|''p''-value]]}}
{{About|the aerodynamic phenomenon|the p factor of psychopathology|p factor (psychopathology)}}


{{multiple images
[[File:Propeller blade AOA.png|thumb|300px|right|Illustration of rotational and forward velocities contributing to angle of attack.]]
| image1      = Propeller blade AOA.png
[[File:Propeller blade AOA versus pitch.png|thumb|300px|right|Effect of pitch on blade load asymmetry.]]
| image2      = Propeller blade AOA versus pitch.png
| footer      = Propeller blade angle of attack (left) and propeller blade angle of attack change with aircraft pitch change, demonstrating asymmetrical load (right)
| total_width  = 600
}}


'''P{{Non breaking hyphen}}factor''', also known as asymmetric blade effect and asymmetric disc effect, is an [[aerodynamic]] phenomenon experienced by a moving [[propeller (aircraft)|propeller]],<ref name="Willits">{{cite book|editor=Willits, Pat|others=Abbot, Mike Kailey, Liz|title=Guided Flight Discovery: Private Pilot|orig-year=1997|publisher=Jeppesen Sanderson, Inc.|year=2004|isbn=0-88487-333-1|page=3-49}}</ref> 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 [[Aircraft principal axes|yaw]] to one side. Rudder input is required to counteract this tendency.
'''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 ==
== Causes ==
[[File:Tilted propeller.png|thumb|alt=Change of forces at increasing Angle of Attack|P-factor, showing relative speed and thrust asymmetry at increasing angle of attack]]
* **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.


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:
These effects combine to move the center of thrust to one side, resulting in yaw.
 
* **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.<ref>{{Cite web|url=http://www.av8n.com/how/htm/yaw.html#sec-p-factor|title=8 Yaw-Wise Torque Budget}}</ref>
 
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.<ref>{{cite book|last=Stowell|first=Rich|title=Emergency Maneuver Training|year=1996|publisher=Rich Stowell Consulting|isbn=1-879425-92-0|pages=26–28}}</ref><ref>{{Cite web|url=http://www.meretrix.com/~harry/flying/notes/pfactor.html|title=P Factor?}}</ref>
 
P-factor is most pronounced during high AOA and high power, such as takeoff.<ref name="Willits"/><ref name="Ramskill">{{cite web | last = Ramskill| first = Clay| title = Prop Effects| work = page 4| publisher = SMRCC| date = June 2003| url = http://www.smrcc.net/Newsletters/2003_SMRCC_Jun_News.pdf | access-date = 2009-04-27}}</ref>


== Effects ==
== Effects ==
=== Single-engine aircraft ===
=== Single-engine aircraft ===
For clockwise propellers (viewed by pilot):
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.
* Climbing → yaw left
* Descending → yaw right


Countered via rudder. Spiral slipstream also contributes.
Tailwheel aircraft are more susceptible to P-factor during ground roll due to a greater propeller tilt.
 
[[Conventional landing gear|Tailwheel aircraft]] show more P-factor on takeoff due to disc tilt. Less noticeable on landing due to low power settings.


=== Multi-engine aircraft ===
=== Multi-engine aircraft ===
If propellers rotate in the same direction:
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.
* 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]] ({{V speeds|V<sub>MC</sub>}}), which are based on the failure of the critical engine.<ref>{{cite book|title=Airplane Flying Handbook FAA-H-8083-3|year=2016|publisher=Federal Aviation Administration|url=https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/airplane_handbook/ | page=Chapter 12 Addendum}}</ref>


[[File:criticalengine1.jpg|frame|none|Fig. 1. Operating right engine produces more yaw when left engine fails, making it the critical engine]]
Counter-rotating props cancel each other’s P-factor effects.


=== Helicopters ===
== Helicopters ==
In forward flight:
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.
* 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]]").<ref>{{cite book|title=Rotorcraft Flying Handbook|year=2019|publisher=Federal Aviation Administration|page=2–20|url=https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/helicopter_flying_handbook/}}</ref><ref>Watkinson, John: "The Art of the Helicopter" (2011), Pg 90.</ref>
== Safety Considerations ==
* 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 ==
* [[Blohm & Voss BV 141]]
* [[Propeller walk]]
* [[Propeller walk]]
* [[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 23:51, 4 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

Retrieved from "https://wiki.alsresume.com/index.php?title=P-factor&oldid=11343"