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 1The 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 2Gyroscopic 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 3Asymmetric 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.
