Aerodynamics of Flight

87 percent, depending on how much the propeller “slips.” P t p h i r e t o c p p h e r o i l s l p e e t r h l s l e l e i r p t h a i e n s o d t r h i e e t t s i d c e i a f f f f l e e d c r i e t s i n v t c a e e n p c b i e e tc t a w h . p e [ r e F o n i p g t e h u l e l r e e g r e 5 s o - h 4 m o 5 e u ] t l r G d ic e a o p d m i v tc a e h n tr c o i e c f Greater travel distance—very high speed— i a n s T n l d o h i v p o e s a p n l r n i a e e p c g a p r e e s e a o s i v g . n n o e T a l , t h u h p b u t e r u i s o o a t , p n i a g r e ; c . l e e l t o e u f r m f a e i l s e c o t t “ r i r t v i w c e e f i o f s p e r t i e c t t d c t h i h ” v e i o e i s s r p t e h t i t h t a i c c e t h a t d h l i i e n p s o c t i a t l u c u n t h d c e e r e i s s p i a t p b r a r a t o s c s t p e o u d e f a l t l l o h l e n y e r Modera S h t o e rt tr t a ra ve v l e d l i d s i t s a t n a c n e c — e— slo m w o s d p e e e d rate speed — 2 5 9 k n o t s 389 kn o ts propeller blades, like all things that turn about a central point, — 1 t b r l a a v d e e l s f a h s a t d e r t h th e a s n a t m he e p g o e r o t m io e n t s r i n c e p a i r t t c h h e t h h u ro b u . [ g F h i o g u u t r t e h 5 ei - r 4 l 6 e ] n I g f t t h h s e , 2,5 2 0 0 0 i r n p . m stonk 9 2 portions near the hub could have negative AOAs while the propeller tips would be stalled at cruise speed. Twisting or 40 in. variations in the geometric pitch of the blades permits the 2,500 rpm propeller to operate with a relatively constant AOA along its 60 in. length when in cruising flight. Propeller blades are twisted 2,500 rpm to change the blade angle in proportion to the differences in speed of rotation along the length of the propeller, keeping Figure 5-46. Propeller tips travel faster than the hub. thrust more nearly equalized along this length. high, and with the low aircraft speed, there is maximum thrust. Usually 1° to 4° provides the most efficient lift/drag ratio, After liftoff, as the speed of the aircraft increases, the constant- but in flight the propeller AOA of a fixed-pitch propeller speed propeller automatically changes to a higher angle (or varies—normally from 0° to 15°. This variation is caused pitch). Again, the higher blade angle keeps the AOA small by changes in the relative airstream, which in turn results and efficient with respect to the relative wind. The higher from changes in aircraft speed. Thus, propeller AOA is the blade angle increases the mass of air handled per revolution. product of two motions: propeller rotation about its axis and This decreases the engine rpm, reducing fuel consumption its forward motion. and engine wear, and keeps thrust at a maximum. A constant-speed propeller automatically keeps the blade After the takeoff climb is established in an aircraft having a angle adjusted for maximum efficiency for most conditions controllable-pitch propeller, the pilot reduces the power output encountered in flight. During takeoff, when maximum power of the engine to climb power by first decreasing the manifold and thrust are required, the constant-speed propeller is at a pressure and then increasing the blade angle to lower the rpm. low propeller blade angle or pitch. The low blade angle keeps the AOA small and efficient with respect to the relative wind. At cruising altitude, when the aircraft is in level flight and At the same time, it allows the propeller to handle a smaller less power is required than is used in takeoff or climb, the mass of air per revolution. This light load allows the engine to pilot again reduces engine power by reducing the manifold turn at high rpm and to convert the maximum amount of fuel pressure and then increasing the blade angle to decrease the into heat energy in a given time. The high rpm also creates rpm. Again, this provides a torque requirement to match the maximum thrust because, although the mass of air handled reduced engine power. Although the mass of air handled per per revolution is small, the rpm and slipstream velocity are revolution is greater, it is more than offset by a decrease in slipstream velocity and an increase in airspeed. The AOA is Slip still small because the blade angle has been increased with an increase in airspeed. Torque and P-Factor To the pilot, “torque” (the left turning tendency of the Effective pitch airplane) is made up of four elements that cause or produce Geometric pitch a twisting or rotating motion around at least one of the airplane’s three axes. These four elements are: Figure 5-45. Propeller slippage. 1. Torque reaction from engine and propeller 5-30