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Understanding Propeller Torque and P-Factor

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Revision as of 01:13, 29 July 2009 by Tom-cat (Talk | contribs) (Gyroscopic Precession)

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This is an attempt to answer the frequent question "Why is my aircraft turning left all the time?"

This occurs only in aircraft with propellers at the front of the aircraft. And, yes, it does occur in real life. Three distinct phenomena cause the effect, all causing the aircraft to turn in the same direction. They are:

Prop wash

A propeller pushes air not just horizontally to the back, but more in a twisting helix around the fuselage (clockwise as seen from the cockpit). As the air whirls around the fuselage it pushes against the left side of the vertical tail, causing the plane to yaw to the left. The prop wash effect is at its greatest when the airflow is flowing more around the fuselage than along it, i.e., at high power and low airspeed, which is the situation when starting the takeoff run.

Propeller Torque Effect

Torque effect is the influence of engine torque on aircraft movement and control. It is generally exhibited as a left turning tendency in piston single engine propeller driven aircraft.

According to Newton's law, "for every action there is an equal and opposite reaction," such that the propeller, if turning clockwise (when viewed from the cockpit), imparts a tendency for the aircraft to rotate counterclockwise. Since most single engine aircraft have propellers rotating clockwise, they rotate to the left, pushing the left wing down.

Typically, the pilot is expected to counter this force through the control inputs. To counter the aircraft roll left, the pilot applies right aileron.

It is important to understand that torque is a movement about the roll axis. Aileron controls roll. Prop torque is not countered by moving the rudder or by setting rudder trim. It is countered by moving or trimming the aileron.

This correction induces adverse yaw, which is corrected by moving or trimming the rudder (right rudder).

On aircraft with contrarotating propellers (propellers that rotate in opposite directions) the torque from the two propellers cancel each other out, so that no compensation is needed.


Further Reading: Propeller Torque Factor

P-Factor

P-factor is the term for asymmetric propeller loading, causes the airplane to yaw to the left when at high angles of attack.

The descending right side of the propeller (as seen from the rear) has a higher angle of attack than the upward-moving blade on the left side and provides more thrust. This occurs only when the propeller is not meeting the oncoming airflow head-on, for example when an aircraft is moving down the runway at a nose-high attitude (i.e. at a high angle of attack), as is the case with tail-draggers. Aircraft with tricycle landing gear maintain a level attitude on the takeoff run, so there is little P-factor during takeoff. In all cases, though, the effect is weaker than prop wash.

According to What You Need to Know About Aerodyanics... by Franklin Gutierrez.

Gyroscopic Precession

This is the tendency of a spinning object to precess or move about its axis when disturbed by a force. The engine and propeller act as a big gyroscope. However, gyroscopic precession is likely to be minimal in a typical aircraft.

Gyroscopic precession is frequently confused with p-factor.

One author maintains p-factor is caused by a combination of factors unrelated to gyroscopic precession http://home.earthlink.net/~x-plane/FAQ-Theory-PFactor.html

External Links

  • Engine effects and aerodynamic phenomena Recreational Aviation Australia Ground School article on "prop wash," prop torque, gyro precession, p-factor and other similar factors affecting taildraggers.