## Pilot's Handbook of Aeronautical Knowledge Aerodynamics of Flight Basic Propeller Principles

 Pilot's Handbook of Aeronautical Knowledge Preface Acknowledgements Appendix Glossary Index Torque and P-Factor To the pilot, "torque" (the left turning tendency of the airplane) is made up of four elements which cause or produce a twisting or rotating motion around at least one of the airplane's three axes. These four elements are: 1. Torque reaction from engine and propeller, 2. Corkscrewing effect of the slipstream, 3. Gyroscopic action of the propeller, and 4. Asymmetric loading of the propeller (P-factor). Torque Reaction Torque reaction involves Newton's Third Law of Physicsâ€” for every action, there is an equal and opposite reaction. As applied to the aircraft, this means that as the internal engine parts and propeller are revolving in one direction, an equal force is trying to rotate the aircraft in the opposite direction. [Figure 4-39] Figure 4-39. Torque reaction. When the aircraft is airborne, this force is acting around the longitudinal axis, tending to make the aircraft roll. To compensate for roll tendency, some of the older aircraft are rigged in a manner to create more lift on the wing that is being forced downward. The more modern aircraft are designed with the engine offset to counteract this effect of torque. NOTE: Most United States built aircraft engines rotate the propeller clockwise, as viewed from the pilot's seat. The discussion here is with reference to those engines. Generally, the compensating factors are permanently set so that they compensate for this force at cruising speed, since most of the aircraft's operating lift is at that speed. However, aileron trim tabs permit further adjustment for other speeds. When the aircraft's wheels are on the ground during the takeoff roll, an additional turning moment around the vertical axis is induced by torque reaction. As the left side of the aircraft is being forced down by torque reaction, more weight is being placed on the left main landing gear. This results in more ground friction, or drag, on the left tire than on the right, causing a further turning moment to the left. The magnitude of this moment is dependent on many variables. Some of these variables are: 1. Size and horsepower of engine, 2. Size of propeller and the rpm, 3. Size of the aircraft, and 4. Condition of the ground surface. This yawing moment on the takeoff roll is corrected by the pilot's proper use of the rudder or rudder trim. Corkscrew Effect The high-speed rotation of an aircraft propeller gives a corkscrew or spiraling rotation to the slipstream. At high propeller speeds and low forward speed (as in the takeoffs and approaches to power-on stalls), this spiraling rotation is very compact and exerts a strong sideward force on the aircraft's vertical tail surface. [Figure 4-40] Figure 4-40. Corkscrewing slipstream. When this spiraling slipstream strikes the vertical fin it causes a turning moment about the aircraft's vertical axis. The more compact the spiral, the more prominent this force is. As the forward speed increases, however, the spiral elongates and becomes less effective. The corkscrew .ow of the slipstream also causes a rolling moment around the longitudinal axis. Note that this rolling moment caused by the corkscrew .ow of the slipstream is to the right, while the rolling moment caused by torque reaction is to the leftâ€”in effect one may be counteracting the other. However, these forces vary greatly and it is the pilot's responsibility to apply proper corrective action by use of the flight controls at all times. These forces must be counteracted regardless of which is the most prominent at the time.

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