often used interchangeably. An increase or decrease in one is  usually associated with an increase or decrease in the other.  The pitch of a propeller may be designated in inches. A  propeller designated as a “74–48” would be 74 inches in  length and have an effective pitch of 48 inches. The pitch  is the distance in inches, which the propeller would screw  through the air in one revolution if there were no slippage.  When specifying a fixed-pitch propeller for a new type of  aircraft, the manufacturer usually selects one with a pitch  that operates efficiently at the expected cruising speed of the  aircraft. Every fixed-pitch propeller must be a compromise  because it can be efficient at only a given combination of  airspeed and revolutions per minute (rpm). Pilots cannot  change this combination in flight.  When the aircraft is at rest on the ground with the engine  operating, or moving slowly at the beginning of takeoff,  the propeller efficiency is very low because the propeller is  restrained from advancing with sufficient speed to permit  its fixed-pitch blades to reach their full efficiency. In this  situation, each propeller blade is turning through the air at  an AOA that produces relatively little thrust for the amount  of power required to turn it.  To understand the action of a propeller, consider first its  motion, which is both rotational and forward. As shown by  Fixed-pitch and ground-adjustable propellers are designed  for best efficiency at one rotation and forward speed. They  are designed for a given aircraft and engine combination. A  propeller may be used that provides the maximum efficiency  for takeoff, climb, cruise, or high-speed flight. Any change in  these conditions results in lowering the efficiency of both the  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  to brake horsepower. Propeller efficiency varies from 50 to  The blade angle is also an excellent method of adjusting the  AOA of the propeller. On constant-speed propellers, the blade  angle must be adjusted to provide the most efficient AOA at  all engine and aircraft speeds. Lift versus drag curves, which  are drawn for propellers as well as wings, indicate that the  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.  Thrust can be considered also in terms of the mass of air  handled by the propeller. In these terms, thrust equals mass  of air handled multiplied by slipstream velocity minus  velocity of the aircraft. The power expended in producing  thrust depends on the rate of air mass movement. On average,  thrust constitutes approximately 80 percent of the torque (total  horsepower absorbed by the propeller). The other 20 percent  is lost in friction and slippage. For any speed of rotation,  the horsepower absorbed by the propeller balances the  horsepower delivered by the engine. For any single revolution  of the propeller, the amount of air handled depends on the  blade angle, which determines how big a “bite” of air the  propeller takes. Thus, the blade angle is an excellent means of  adjusting the load on the propeller to control the engine rpm.  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  of the propeller, which is mounted in a vertical instead of a  horizontal plane, the area of decreased pressure is in front of  the propeller, and the force (thrust) is in a forward direction.  Aerodynamically, thrust is the result of the propeller shape  and the AOA of the blade.  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.  Figure 5-44.  Propeller blade angle.  Figure 5-43.  Airfoil sections of propeller blade.  5-29  Relative wind  Chord line  Thrust  Forward velocity  a t $_{t}$a c k A n g $_{l}$e  o f  Rotational veloc P i t ity b l a d e a n g $_{l}$e c h o r 