CFI Brief: Torque

Today’s discussion is on torque. An airplane of standard configuration has an insistent tendency to turn to the left. This tendency is called torque, and is a combination of four forces: reactive force, spiraling slipstream, gyroscopic precession, and P-factor.

Reactive force is based on Newton’s Law of action and reaction. A propeller rotating in a clockwise direction (as seen from the rear) produces a force which tends to roll the airplane in a counterclockwise direction. See Figure 1.

Figure 1

The spiraling slipstream is the reaction of the air to a rotating propeller. (The propeller forces the air to spiral in a clockwise direction around the fuselage.) This spiraling slipstream strikes the airplane’s vertical stabilizer on the left side. This pushes the tail of the airplane to the right and the nose of the airplane to the left. See Figure 2. Weight-shift control and powered parachutes do not have this effect.

Figure 2

Gyroscopic precession is the result of a deflective force applied to a rotating body (such as a propeller). The resultant action occurs 90° later in the direction of rotation. See Figure 3.

Figure 3

Asymmetric propeller loading, called P-factor, is caused by the downward moving blade on the right side of the propeller having a higher angle of attack, a greater action and reaction, and therefore a higher thrust than the upward moving opposite blade. This results in a tendency for the aircraft to yaw to the left around the vertical axis. Additional left-turning tendency from torque will be greatest when the aircraft is operating at low airspeed with a high power setting.

Now lets see if we can answer a few sample FAA knowledge test questions. Answers can be found in the comments section.

1. The left turning tendency of an airplane caused by P-factor is the result of the
A—clockwise rotation of the engine and the propeller turning the airplane counter-clockwise.
B—propeller blade descending on the right, producing more thrust than the ascending blade on the left.
C—gyroscopic forces applied to the rotating propeller blades acting 90° in advance of the point the force was applied.

2. In what flight condition is torque effect the greatest in a single-engine airplane?
A—Low airspeed, high power, high angle of attack.
B—Low airspeed, low power, low angle of attack.
C—High airspeed, high power, high angle of attack.

3 Comments

1. Posted May 12, 2016 at 7:17 am | Permalink

Q1. The downward-moving blade on the right side of the propeller has a higher angle of attack and greater action and reaction than the upward moving blade on the left. This results in a tendency for the airplane to yaw around the vertical axis to the left.

Answer (A) is incorrect because it describes the characteristics involved with torque effect. Answer (C) is incorrect because it describes gyroscopic precession.

Q2. The effect of torque increases in direct proportion to the engine power, airspeed, and airplane attitude. If the power setting is high, the airspeed slow, and the angle of attack high (or a high deck angle for a PPC), the effect of torque is greater.

Answer (B) is incorrect because the least amount of torque effect is produced under these conditions. Answer (C) is incorrect because torque effect is negligible at higher airspeeds due to increased stability generated by more airflow moving over all airfoils.

2. Scott Saunders
Posted June 24, 2016 at 3:40 pm | Permalink

Now and then I will find myself walking a propeller-driven aircraft that has started the engine. (Walking behind to stay away from the prop, and to avoid delaying the crew from beginning their taxi when they are ready.) At times like this I have noticed the prop blast is stronger on one side of the tail than the other. I suppose this is due to spiraling slipstream effect, or perhaps it is P-factor since axis of the crankshaft is slightly above horizontal.

I wonder how spiralling slipstream effect changes with aircraft speed during takeoff roll and after liftoff? Not sure about that.

3. Posted June 27, 2016 at 9:23 am | Permalink

Hey Scott, When looking at the prop from behind (pilots view) it rotates in a clockwise direction. The descending blade has a higher angle of attack, essentially taking a bigger bite of air then the ascending blade is. This is why you might feel more thrust or air being pushed back on the right side of the aircraft. A number of other factors are still at play.