NAVWEPS 00401-60 
AIRPLANE PERFORMANCE 
greatly with altitude, the turbojet can tolerate 
less favorable (or more unfavorable) winds 
with increased altitude. 
In some cases, large values of wind may 
cause a significant change in cruise velocity to 
maintain maximum ground nautical miles per 
lb. of fuel. As an example of an extreme con- 
dition, consider an airplane flying into a head- 
wind which equals the cruise velocity. In this 
case, ““9 increase in velocity would improve 
range. 
To appreciate the changes in optimum speeds 
with various winds, refer to the illustration of 
figure 2.26. When zero wind conditions exist, 
a straight line from the origin tangent to the 
curve of fuel flow versus velocity will locate 
maximum range conditions. When a head- 
wind condition exists, the speed for maximum 
ground range is located by a line tangent drawn 
from a velocity offset equal to the headwind 
velocity. This will locate maximum range at 
some higher velocity and fuel flow. Of course, 
the range will be less than when at zero wind 
conditions but the higher velocity and fuel flow 
will minimize the range loss due to the head- 
wind. In a similar sense, a tailwind will re- 
duce the cruise velocity to maximize the 
benefit of the tailwind. 
The procedure of employing different cruise 
velocities to account for the effects of wind is 
necessary only at extreme values of wind 
velocity. It is necessary to consider the 
change in optimum cruise airspeed when the 
wind velocities exceed 25 percent of the zero 
wind cruise velocity. 
ENDURANCE PERFORMANCE 
The ability of the airplane to convert fuel 
energy into flying time is an important factor 
in flying operations. The “specific endurance” 
of the airplane is defined as follows: 
specific endurance==1 
specific endurance= 1 
fuel flow, Ibs. per hr. 
The specific endurance is simply the reciprocal 
of the fuel flow, hence maximum endurance 
conditions would be obtained at the lowest 
fuel flow required to hold the airplane in steady 
level flight. Obviously, minimum fuel flow 
will provide the maximum flying time from a 
given quantity of fuel. Generally, in subsonic 
performance, the speed at which maximum en- 
durance is achieved is approximately 75 per- 
cent of the speed for maximum range. 
While many different factors can affect the 
specific endurance, the most important factors 
at the control of the pilot are the configuration 
and operating altitude. Of course, for maxi- 
mum endurance conditions the airplane must 
be in the clean configuration and operated at 
the proper aerodynamic conditions. 
EFFECT OF ALTITUDE ON ENDUR- 
ANCE, PROPELLER DRIVEN AIRPLANES. 
Since the fuel flow of the propeller driven air- 
plane is proportional to power required, the 
propeller powered airplane will achieve maxi- 
mum specific endurance when operated at mini- 
mum power required. The point of minimum 
power required is obtained at a specific value 
of lift coefficient for a particular airplane con- 
figuration and is essentially independent of 
weight or altitude. However, an increase in 
altitude will increase the value of the minimum 
power required as illustrated by figure 2.27. 
If the specific fuel consumption were not in- 
fluenced by altitude or engine power, the spe- 
cific endurance would be directly proportional 
to ji, e.g., the specific endurance at 22,000 ft. 
(a=O.498) would be approximately 70 percent 
of the value at sea level. This example is very 
nearly the case of the airplane with the recipro- 
cat&g engine since specific fuel consumption and 
propeller efficiency are not directly affected by 
altitude. The obvious conclusion is that 
maximum endurance of the reciprocating en- 
gine airplane is obtained at the lowest practical 
altitude. 
The variation with altitude of the maximum 
endurance of the turboprop airplane requires 
consideration of powerplant factors in addition 
im