Aerodynamics of Flight

Figure 5-43. Airfoil sections of propeller blade. Thrust Chord line Forward velocity Relative wind 5-29 Rotational velocity the vectors of propeller forces in Figure 5-44, each section of a propeller blade moves downward and forward. The angle at which this air (relative wind) strikes the propeller blade is its AOA. The air deflection produced by this angle causes the dynamic pressure at the engine side of the propeller blade to be greater than atmospheric pressure, thus creating thrust. The shape of the blade also creates thrust because it is cambered like the airfoil shape of a wing. As the air flows past the propeller, the pressure on one side is less than that on the other. As in a wing, a reaction force is produced in the direction of the lesser pressure. The airflow over the wing has less pressure, and the force (lift) is upward. In the case often used interchangeably. An increase or decrease in one is of the propeller, which is mounted in a vertical instead of a usually associated with an increase or decrease in the other. horizontal plane, the area of decreased pressure is in front of The pitch of a propeller may be designated in inches. A the propeller, and the force (thrust) is in a forward direction. propeller designated as a “74–48” would be 74 inches in Aerodynamically, thrust is the result of the propeller shape length and have an effective pitch of 48 inches. The pitch and the AOA of the blade. is the distance in inches, which the propeller would screw through the air in one revolution if there were no slippage. Thrust can be considered also in terms of the mass of air handled by the propeller. In these terms, thrust equals mass When specifying a fixed-pitch propeller for a new type of of air handled multiplied by slipstream velocity minus aircraft, the manufacturer usually selects one with a pitch velocity of the aircraft. The power expended in producing that operates efficiently at the expected cruising speed of the thrust depends on the rate of air mass movement. On average, aircraft. Every fixed-pitch propeller must be a compromise thrust constitutes approximately 80 percent of the torque (total because it can be efficient at only a given combination of horsepower absorbed by the propeller). The other 20 percent airspeed and revolutions per minute (rpm). Pilots cannot is lost in friction and slippage. For any speed of rotation, change this combination in flight. the horsepower absorbed by the propeller balances the horsepower delivered by the engine. For any single revolution When the aircraft is at rest on the ground with the engine of the propeller, the amount of air handled depends on the operating, or moving slowly at the beginning of takeoff, blade angle, which determines how big a “bite” of air the the propeller efficiency is very low because the propeller is propeller takes. Thus, the blade angle is an excellent means of restrained from advancing with sufficient speed to permit adjusting the load on the propeller to control the engine rpm. its fixed-pitch blades to reach their full efficiency. In this situation, each propeller blade is turning through the air at The blade angle is also an excellent method of adjusting the an AOA that produces relatively little thrust for the amount AOA of the propeller. On constant-speed propellers, the blade of power required to turn it. angle must be adjusted to provide the most efficient AOA at all engine and aircraft speeds. Lift versus drag curves, which To understand the action of a propeller, consider first its are drawn for propellers as well as wings, indicate that the motion, which is both rotational and forward. As shown by most efficient AOA is small, varying from +2° to +4°. The actual blade angle necessary to maintain this small AOA varies with the forward speed of the aircraft. Fixed-pitch and ground-adjustable propellers are designed b P la it d c e h a o n r g l e A n g a o l t e f ta c k f a p f th o o r r e e r o r s p t b d e a e e e k c l s s e l o t i e o g n r e f n d f m f e f , i t i d c a i c o l y i f i e n m o b n s r e b c r a , u y e c s s g a r e u i u t v d l t i o e s s t n n h e i , a e n a o t i l r p r r o o c r h w t r o a i a e g v t f i r h i t o i d - n a n s e g n p s a d e t t n h h e e d e d e n e f m f g o f l i i f r a n g i w x c e h i i a t e c m . r n o A d u c m m n s y b p y o i e e c n f f e h a f b d i a t o c . i n o t i T g h e n h n e . t e c h i A y n y e propeller and the engine. Since the efficiency of any machine is the ratio of the useful power output to the actual power input, propeller efficiency is the ratio of thrust horsepower Figure 5-44. Propeller blade angle. to brake horsepower. Propeller efficiency varies from 50 to