Aircraft Systems

installed depends upon its intended use. The climb propeller has a lower pitch, therefore less drag. Less drag results in higher rpm and more horsepower capability, which increases performance during takeoffs and climbs but decreases performance during cruising flight. The cruise propeller has a higher pitch, therefore more drag. More drag results in lower rpm and less horsepower capability, which decreases performance during takeoffs and climbs but increases efficiency during cruising flight. Figure 7-6. Changes in propeller blade angle from hub to tip. The propeller is usually mounted on a shaft, which may be an extension of the engine crankshaft. In this case, the rpm throughout its entire length would be inefficient because as of the propeller would be the same as the crankshaft rpm. On airspeed increases in flight, the portion near the hub would some engines, the propeller is mounted on a shaft geared to have a negative AOA while the blade tip would be stalled. the engine crankshaft. In this type, the rpm of the propeller Small aircraft are equipped with either one of two types of is different than that of the engine. propellers: fixed-pitch or adjustable-pitch. In a fixed-pitch propeller, the tachometer is the indicator of Fixed-Pitch Propeller engine power. [Figure 7-8] A tachometer is calibrated in A propeller with fixed blade angles is a fixed-pitch propeller. hundreds of rpm and gives a direct indication of the engine The pitch of this propeller is set by the manufacturer and and propeller rpm. The instrument is color coded with a green cannot be changed. Since a fixed-pitch propeller achieves arc denoting the maximum continuous operating rpm. Some the best efficiency only at a given combination of airspeed tachometers have additional markings to reflect engine and/or and rpm, the pitch setting is ideal for neither cruise nor propeller limitations. The manufacturer’s recommendations climb. Thus, the aircraft suffers a bit in each performance should be used as a reference to clarify any misunderstanding category. The fixed-pitch propeller is used when low weight, of tachometer markings. simplicity, and low cost are needed. The rpm is regulated by the throttle, which controls the fuel- There are two types of fixed-pitch propellers: climb and air flow to the engine. At a given altitude, the higher the cruise. Whether the airplane has a climb or cruise propeller tachometer reading, the higher the power output of the engine. M o G de re ra a te te t r r S a h t o r r v t a t e r v a l e v d e l i l s d d t i a s is t n a t c a n e n c — c e e — m — v o s e d lo r e y w r a h sp t i e e g e d h s p s e p e e d e — d 2 — 5 9 3 k n 8 o t s 9 k nots W s r a d p h t e m o s h n e w e s a i a n t c t l y e o o 5 v d p r , r e 0 e e e l 0 c r c a b r 0 t e t e i p a f c n e o s a g e e w u t s a s e p l e a r t r s i p o o t u o a u d l d w t u t II p e i c 55 e t u e u r i t n s d o o c e l u f e r t e i t s p 22 n h a s u c e s 00 h t r e e e o d s n a , r e g s s p t e e i h e n s p e n e o a d . t w n a s F c d e o o h r r n a o t e a m h d x i a e r a e n c m d t r e e 2 e r p n a , l 3 m s s e 0 i e , t a 0 y 2 i y n . , r 3 A p n a 0 m o i i 0 r r t — 2,5 2 0 0 0 i r n p . m stonk 9 2 1 II00 H R UN P DR M EDS 2255 I 40 in. 55 I0 3300 2,500 rpm 33 HOU RS 3355 AVOID 60 in. C O O P N E T R IN AT U I O O U N S BETWEEN 2250 AND 2350 RPM 2,500 rpm Figure 7-7. Relationship of travel distance and speed of various portions of propeller blade. Figure 7-8. Engine rpm is indicated on the tachometer. 7-5