87 percent, depending on how much the propeller “slips.”  Propeller slip is the difference between the geometric pitch of  the propeller and its effective pitch.  [Figure 5-45]  Geometric  pitch is the theoretical distance a propeller should advance  in one revolution; effective pitch is the distance it actually  advances. Thus, geometric or theoretical pitch is based on  no slippage, but actual or effective pitch includes propeller  slippage in the air.  The reason a propeller is “twisted” is that the outer parts of the  propeller blades, like all things that turn about a central point,  travel faster than the portions near the hub.  [Figure 5-46]  If the  blades had the same geometric pitch throughout their lengths,  portions near the hub could have negative AOAs while the  propeller tips would be stalled at cruise speed. Twisting or  variations in the geometric pitch of the blades permits the  propeller to operate with a relatively constant AOA along its  length when in cruising flight. Propeller blades are twisted  to change the blade angle in proportion to the differences in  speed of rotation along the length of the propeller, keeping  thrust more nearly equalized along this length.  Usually 1° to 4° provides the most efficient lift/drag ratio,  but in flight the propeller AOA of a fixed-pitch propeller  varies—normally from 0° to 15°. This variation is caused  by changes in the relative airstream, which in turn results  from changes in aircraft speed. Thus, propeller AOA is the  product of two motions: propeller rotation about its axis and  its forward motion.  A constant-speed propeller automatically 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 because, although the mass of air handled  per revolution is small, the rpm and slipstream velocity are  At cruising altitude, when the aircraft is in level flight and  less power is required than is used in takeoff or climb, the  pilot again reduces engine power by reducing the manifold  pressure and then increasing the blade angle to decrease the  rpm. Again, this provides a torque requirement to match the  reduced engine power. Although the mass of air handled per  revolution is greater, it is more than offset by a decrease in  slipstream velocity and an increase in airspeed. The AOA is  still small because the blade angle has been increased with  an increase in airspeed.  To the pilot, “torque” (the left turning tendency of the  airplane) is made up of four elements that cause or produce  a twisting or rotating motion around at least one of the  airplane’s three axes. These four elements are:  After the takeoff climb is established in an aircraft having a  controllable-pitch propeller, the pilot reduces the power output  of the engine to climb power by first decreasing the manifold  pressure and then increasing the blade angle to lower the rpm.  high, and with the low aircraft speed, there is maximum thrust.  After liftoff, as the speed of the aircraft increases, the constant- speed propeller automatically 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 decreases the engine rpm, reducing fuel consumption  and engine wear, and keeps thrust at a maximum.  Figure 5-46.  Propeller tips travel faster than the hub.  Figure 5-45.  Propeller slippage.  1.  Torque reaction from engine and propeller  Torque and P-Factor  5-30  60 in.  40 in.  20 in.  S h o r t $_{r}$a v e l d i s t a n c e$_{—}$ s l o w s p e e d  — 1 2 9 k n o t s  M o d e r a $_{t}$e t r a v e l d is tan c e$_{—}$$_{m}$ o d e r a t e s p e d — 2 5 9 k n o t s  G r e a t e r tr a ve l di sta nc e$_{—}$ v e r y h i g h s p e d — 3 8 9 k n o t s  2 , 5 0 r p m  2 ,5 0 r p m  2 ,50 0 rp m  Effective pitch  Geometric pitch  Slip 