constant vibration tends to weaken the blade and eventually  causes failure. To understand the action of a propeller, consider first its  motion, which is both rotational and forward. Thus, as shown  by the vectors of propeller forces in  Figure 7-9,  a section  of a propeller blade moves downward and forward. As far  as the forces are concerned, the result is the same as if the  blade were stationary and the air coming at it from a direction  opposite its path. The angle at which this air (relative wind)  strikes the propeller blade is called angle of attack (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, creating thrust. The shape of the blade also creates thrust because it is  shaped like 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, this difference in pressure produces a reaction force  in the direction of the lesser pressure. The area above a wing  has less pressure, and the force (lift) is upward. The area of  decreased pressure is in front of a propeller which is mounted  in a vertical instead of a horizontal position, and the force  (thrust) is in a forward direction. Aerodynamically, thrust is  the result of the propeller shape and the AOA of the blade. Another way to consider thrust is in terms of the mass of  air handled. In these terms, thrust is equal to the mass of  air handled multiplied by the slipstream velocity minus  the velocity of the airplane. Thus, the power expended in  producing thrust depends on the mass of air moved per  second. On the 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 displaced (moved) depends on the blade angle,  which determines the quantity or amount of mass of air the  propeller moves. Thus, the blade angle is an excellent means  of adjusting the load on the propeller to control the engine  rpm. If the blade angle is increased, more load is placed on  the engine, tending to slow it down unless more power is  applied. As an airfoil is moved through the air, it produces  two forces: lift and drag. Increasing propeller blade angle  increases the AOA and produces more lift and drag; this  action increases the horsepower required to turn the propeller  at a given rpm. Since the engine is still producing the same  horsepower, the propeller slows down. If the blade angle is  decreased, the propeller speeds up. Thus, the engine rpm can  be controlled by increasing or decreasing the blade angle. 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 airplane speeds. Lift versus drag curves,  which are drawn for propellers as well as wings, indicate  that the most efficient AOA is a small one varying from 2°  to 4° positive. The actual blade angle necessary to maintain  this small AOA varies with the forward speed of the airplane.  This is due to a change in the relative wind direction, which  varies with aircraft speed. Fixed-pitch and ground-adjustable propellers are designed  for best efficiency at one rotation and forward speed. In other  words, they are designed to fit a given airplane and engine  combination. A propeller may be used that provides the  maximum propeller efficiency for takeoff, climb, cruising,  or high speeds. Any change in these conditions results in  lowering the efficiency of both the propeller and the engine. A constant-speed propeller, however, keeps the blade  angle adjusted for maximum efficiency for most conditions  encountered in flight. During takeoff, when maximum power  and thrust are required, the constant-speed propeller is at a  low propeller blade angle or pitch. The low blade angle keeps  the AOA small and efficient with respect to the relative wind.  At the same time, it allows the propeller to handle a smaller  mass of air per revolution. This light load allows the engine  to turn at high rpm and to convert the maximum amount  of fuel into heat energy in a given time. The high rpm also  creates maximum thrust. Although the mass of air handled  per revolution is small, the engine rpm is high, the slipstream  velocity (air coming off the propeller) is high, and, with the  low airplane speed, the thrust is maximum. After liftoff, as the speed of the airplane increases, the  constant-speed propeller changes to a higher angle (or pitch).  Again, the higher blade angle keeps the AOA small and  efficient with respect to the relative wind. The higher blade  angle increases the mass of air handled per revolution. This  7-5 Forward velocity Linear velocity Thrust Blade path Chord of section B A Propeller Relative wind of the blade B A = angle of attack (angle formed by relative wind and chord) = pitch or blade angle Figure 7-9. Propeller forces.  Aerodynamic Factors