Figure 7-6.  Changes in propeller blade angle from hub to tip.  throughout its entire length would be inefficient because as  airspeed increases in flight, the portion near the hub would  have a negative AOA while the blade tip would be stalled.  Small aircraft are equipped with either one of two types of  propellers: fixed-pitch or adjustable-pitch.  A propeller with fixed blade angles is a fixed-pitch propeller.  The pitch of this propeller is set by the manufacturer and  cannot be changed. Since a fixed-pitch propeller achieves  the best efficiency only at a given combination of airspeed  and rpm, the pitch setting is ideal for neither cruise nor  climb. Thus, the aircraft suffers a bit in each performance  category. The fixed-pitch propeller is used when low weight,  simplicity, and low cost are needed.  There are two types of fixed-pitch propellers: climb and  cruise. Whether the airplane has a climb or cruise propeller  When operating altitude increases, the tachometer may not  show correct power output of the engine. For example, 2,300  rpm at 5,000 feet produces less horsepower than 2,300 rpm  at sea level because power output depends on air density. Air  density decreases as altitude increases and a decrease in air  The rpm is regulated by the throttle, which controls the fuel- air flow to the engine. At a given altitude, the higher the  tachometer reading, the higher the power output of the engine.  In a fixed-pitch propeller, the tachometer is the indicator of  engine power.  [Figure 7-8]  A tachometer is calibrated in  hundreds of rpm and gives a direct indication of the engine  and propeller rpm. The instrument is color coded with a green  arc denoting the maximum continuous operating rpm. Some  tachometers have additional markings to reflect engine and/or  propeller limitations. The manufacturer’s recommendations  should be used as a reference to clarify any misunderstanding  of tachometer markings.  The propeller is usually mounted on a shaft, which may be  an extension of the engine crankshaft. In this case, the rpm  of the propeller would be the same as the crankshaft rpm. On  some engines, the propeller is mounted on a shaft geared to  the engine crankshaft. In this type, the rpm of the propeller  is different than that of the engine.  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.  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.  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 i s t a nc 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 t r a v e l d is tan ce —v er 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 00  $_{rp}$m  2 ,50 0 rp m  RPM HUNDREDS  25 3 5 I5 30 20 I0 35 25  5  I5  30  20  I0  HOURS  AVOID  CONTINUOUS  OPERATION  BETWEEN 2250  AND 2350 RPM  I  I0  Figure 7-8.  Engine rpm is indicated on the tachometer.  Figure 7-7.  Relationship of travel distance and speed of various  portions of propeller blade.  7-5  Fixed-Pitch Propeller 