By definition, “torque” is a force, or combination of forces, that produces or tends to produce a twisting or rotating motion of an airplane.

An airplane propeller spinning clockwise, as seen from the rear, produces forces that tend to twist or rotate the airplane in the opposite direction, thus turning the airplane to the left. Airplanes are designed in such a manner that the torque effect is not noticeable to the pilot when the airplane is in straight-and-level flight with a cruise power setting.
 

The effect of torque increases in direct proportion to engine power, airspeed, and airplane attitude. If the power setting is high, the airspeed slow, and the angle of attack high, the effect of torque is greater. During takeoffs and climbs, when the effect of torque is most pronounced, the pilot must apply sufficient right rudder pressure to counteract the left-turning tendency and maintain a straight takeoff path. Several forces are involved in the insistent tendency of an airplane of standard configuration to turn to the left. All of these forces are created by the rotating propeller. How they are actually created varies greatly from one explanation to the next.

Figure 1-18.—Factors which cause left-turning tendency.

Individual explanation of these forces is perhaps the best approach to understanding the reason for the left-turning tendency.
The four forces are:
• Torque Reaction
• Spiraling Slipstream
• Gyroscopic Precession
• Asymmetric Propeller Loading (“P” Factor)