Aerodynamics of Flight

Reaction Action Figure 5-47. Torque reaction. 5-31 Y a w 2. Corkscrewing effect of the slipstream This yawing moment on the takeoff roll is corrected by the pilot’s proper use of the rudder or rudder trim. 3. Gyroscopic action of the propeller 4. Asymmetric loading of the propeller (P-factor) Corkscrew Effect The high-speed rotation of an aircraft propeller gives a Torque Reaction corkscrew or spiraling rotation to the slipstream. At high Torque reaction involves Newton’s Third Law of Physics— propeller speeds and low forward speed (as in the takeoffs for every action, there is an equal and opposite reaction. As and approaches to power-on stalls), this spiraling rotation applied to the aircraft, this means that as the internal engine is very compact and exerts a strong sideward force on the parts and propeller are revolving in one direction, an equal aircraft’s vertical tail surface. [Figure 5-48] force is trying to rotate the aircraft in the opposite direction. [Figure 5-47] When this spiraling slipstream strikes the vertical fin, it causes a yawing moment about the aircraft’s vertical axis. When the aircraft is airborne, this force is acting around The more compact the spiral, the more prominent this force the longitudinal axis, tending to make the aircraft roll. To is. As the forward speed increases, however, the spiral compensate for roll tendency, some of the older aircraft are elongates and becomes less effective. The corkscrew flow rigged in a manner to create more lift on the wing that is being of the slipstream also causes a rolling moment around the forced downward. The more modern aircraft are designed longitudinal axis. with the engine offset to counteract this effect of torque. Note that this rolling moment caused by the corkscrew flow NOTE: Most United States built aircraft engines rotate the of the slipstream is to the right, while the yawing moment propeller clockwise, as viewed from the pilot’s seat. The caused by torque reaction is to the left—in effect one may discussion here is with reference to those engines. be counteracting the other. However, these forces vary greatly and it is the pilot’s responsibility to apply proper Generally, the compensating factors are permanently set so corrective action by use of the flight controls at all times. that they compensate for this force at cruising speed, since These forces must be counteracted regardless of which is most of the aircraft’s operating time is at that speed. However, the most prominent at the time. aileron trim tabs permit further adjustment for other speeds. When the aircraft’s wheels are on the ground during the Gyroscopic Action takeoff roll, an additional turning moment around the vertical Before the gyroscopic effects of the propeller can be axis is induced by torque reaction. As the left side of the understood, it is necessary to understand the basic principle aircraft is being forced down by torque reaction, more weight of a gyroscope. All practical applications of the gyroscope is being placed on the left main landing gear. This results in are based upon two fundamental properties of gyroscopic more ground friction, or drag, on the left tire than on the right, action: rigidity in space and precession. The one of interest causing a further turning moment to the left. The magnitude for this discussion is precession. of this moment is dependent on many variables. Some of these variables are: Precession is the resultant action, or deflection, of a spinning 1. Size and horsepower of engine rotor when a deflecting force is applied to its rim. As can be seen in Figure 5-49, when a force is applied, the resulting 2. Size of propeller and the rpm force takes effect 90° ahead of and in the direction of rotation. 3. Size of the aircraft The rotating propeller of an airplane makes a very good 4. Condition of the ground surface Slipstream Force Figure 5-48. Corkscrewing slipstream.