NAVWEPS OS80140 
AIRPLANE PERFORMANCE 
flow is determined mainly by the shaft poluet 
put into the propeller rather than thrust. Thus, 
the powerplant fuel flow could be related di- 
rectly to power required to maintain the air- 
plane in steady, level flight. This fact allows 
study of the range of the propeller powered 
airplane by analysis of the curves of power 
required versus velocity. 
Figure 2.24 illustrates a typical curve of 
power required versus velocity which, for the 
propeller powered airplane, would be analo- 
gous to the variation of fuel flow versus veloc- 
ity. Maximum endurance condition would be 
obtained at the point of minimum power re- 
quired since this would require the lowest fuel 
flow to keep the airplane in steady, level flight. 
Maximum range condition would occur where 
the proportion between velocity and power re- 
quired is greatest and this point is located by 
a straight line from the origin tangent to the 
curve. 
The maximum range condition is obtained 
at maximum lift-drag ratio and it is important 
to note that (L/D),, for a given airplane 
configuration occurs at a particular angle of 
attack and li5t coefficient and is unaffected by 
weight or altitude (within compressibility 
limits). Since approximately 50 percent of 
the total dra.g a’t (L/D)* is induced drag, the 
propeller powered airplane which is designed 
specifically i3r IJong range will have a strong 
preference for rbe thigh aspect rario planform. 
The effect ,df tihe variation of airplane gross 
weight is illustrated by the second graph of 
figure 2.24. ‘The flight condition of (L/D),., 
is achieved a’t,one-particular value of lift coefIi- 
cient for a given airplane configuration. 
Hence, a variation of gross weight will alter 
the values of airspeed, power required, and spe- 
cific range obtained at (L/D)m.r. If a given 
configuration ‘of airplane is operated at con- 
stant altitude and the lift coefficient for 
WDL the following relationships will 
awb : 
-4 v*- E VI K 
pr* w* s’* 
-=H PC WI 
where 
SRs WI -=- 
SRI W, 
condition (1) applies to some known condi- 
tion of velocity, power required, and 
specific range for (L/D),., at some basic 
weight, WI 
condition (2) applies to some new values of 
velocity, power required, and specific 
range for (L/D),., at some different 
weight, WI 
and, 
V= velocity, knots 
W= gross weight, Jbs. 
Pr=power required, h.p. 
SK= specific range, nmi/lb. 
Thus a 10 percent increase in gross weight 
would create: 
a 5 percent increase in velocity 
a 15 percent increase in power required 
a 9 percent decrease in specific range 
when flight is maintained at the optimum con- 
ditions of (L/D),.,. The variations of veloc- 
ity and power required must be monitored by 
the pilot as part of the cruise control to main- 
tain .(L/D),.+ When the airplane fuel weight 
is a small part of the gross-weight and the range 
is small, then cruise control procedure can be 
simplified to essentially a constant speed and 
power setting throughout cruise. However, 
the long range airplane has a fuel weight which 
is a conside’rable part of the gross weight and 
cruise control procedure must employ sched- 
uled airspeed and power changes to maintain 
optimum range conditions. 
The effect of altitude on the range of the 
propeller powered airplane may be appreciated 
by inspection of the final graph of figure 2.24. 
If a given configuration of airplane is operated 
at constant gross weight and the lift coefficient 
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