What is Prop Wash, Torque, and P-Factor?

by FliteTest | March 12, 2019 | (10) Posted in Just Fun

What is all of this stuff, and why does it keep turning my airplane left?!

Certain terms in the hobby tend to allude pilots and builders for years, so here’s an 'Aerodynamics Simplified' type article to clarify three particular concepts all to do with the propeller of your RC airplane.  

Prop Wash

This is the twisting helix of air caused by the propeller that spins around a fuselage.


This can mean that the vertical tail of the airplane can be pushed more by the air attacking from one side than the other, causing the plan to yaw as a result. 


This isn’t all that noticeable in small scale RC airplanes, but it’s worth knowing about, just in case you find yourself flying a model with a very large propeller and an equally large rudder. 

Prop Torque 

This is the influence of motor or engine torque on a plane’s roll axis. On a single motored plane with a clockwise turning prop, the aircraft will have the tendency to roll counterclockwise. 


Remember Newton’s law? 

“For every action there is an equal and opposite reaction”.

Keep this in mind when thinking about the force of your motor and the weight of your prop being turned by it.  


Larger props and larger motors tend to create greater amounts of torque. Some twin aircraft use counter-rotating propellers to cancel out the effect of engine torque. 

P-Factor

Confusing many, P-Factor is all to do with asymmetric propeller loading; it’s where your prop creates more ‘pull’ or lift on one side which causes the airplane to yaw. 


This happens when your plane is traveling at a high angle of attack relative to the oncoming airflow. This means the descending propeller blade experiences a higher airflow and therefore creates more thrust than the ascending blade on the left side. 

Yep! Aerodynamics is a complex yet fascinating subject. If you’d like to learn more, check out this article collection below.

Aerodynamics Simplified Series  

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Article by James Whomsley

Editor of FliteTest.com

Contact: james@flitetest.com

YouTube Channel: www.youtube.com/projectairaviation 

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COMMENTS

Ran D. St. Clair on March 13, 2019
For P-factor, I think what you meant to say is that the downward sweeping blade has a higher angle of attack and therefore generates more thrust. This is especially noticeable for a WW2 tail dragger fighter like a P-51 on takeoff. For CCW rotation (as viewed from the front) the downward sweeping prop blade is on the right so the extra thrust pushes the plane to the left. This is why it is important to hold some right rudder as you begin the takeoff roll. As the plane lifts the tail the gyroscopic procession of the rotating propeller also pushes the plane to the left. All of this can push you off the left side of the runway if you don't get ahead of it.
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hugh.i on October 21, 2020
Actually, you both are correct. The descending blade has both a higher angle of attack AND higher velocity air flow. Both contribute to greater thrust from the descending blade than the ascending blade causing the plane to yaw.
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winglet on April 18, 2019
Thank you for writing this article. You explain it all well. I hope that everyone reads it. Over the years I have become weary of trying to explain this to everyone who complains about their airplane trying to turn left!

Nice work!
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What is Prop Wash, Torque, and P-Factor?