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

March 12, 2019|FliteTest

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.