NAVWEPS 00-801-80 
AIRPLANE PERFORMANCE 
have a fuel weight which is a large part of the 
gross weight, cruise control procedures will be 
necessary to account for the changes in opti- 
mum airspeeds and power settings as fuel is 
consumed. 
The effect of altitude on the range of the 
turbojet airplane is of great importance be- 
cause no other single item can cause such large 
variations of specific range. If a given con- 
figuration of airplane is operated at constant 
gross weight and the lift coefficient for 
(JCL/CDL, a change in altitude will produce 
the following relationships: 
vz - -= 3 
J VI .Y* 
Tr=constant (neglecting compressibility 
effects) 
JR.2 - -= 
J 
3 
JR1 rJ* 
(neglecting factors affecting en- 
gine performance) 
where 
condition (I) applies some known condition 
of velocity, thrust required, and specific 
range for (&QCD),, at some original, 
basic altitude. 
condition (2) applies to some new values of 
velocity, thrust required, and specific 
range for (fi/CD)mm at some different 
altitude. 
and 
V= velocity, knots (TAX, of course) 
Tr= thrust required, lbs. 
JR= specific range, nmi/lb. 
a=altitude density ratio (sigma) 
Thus, if flight is conducted at 40,000 ft. 
(u=O.246), the airplane will have: 
a 102 percent higher velocity 
the same thrust required 
a 102 percent higher specific range 
(even when the beneficial effects of altitude 
on engine performance are neglected) 
than when operating at sea level. Of course, 
the greater velocity is a higher TAJ and the 
same thrust required must be obtained with a 
greater engine RPM. 
At this point it is necessary to consider the 
effect of the operating condition on powerplant 
performance. An increase in altitude will im- 
prove powerplant performance in two respects. 
First, an increase in altitude when below the 
tropopause will provide lower inlet Gr tem- 
peratures which redqce the specific fuel con- 
sumption (c~). Of course, above the tropo- 
pause the specific fuel consumption tends to 
increase. A; low altitude, the engine RPM 
necessary to produce the required thrust is low 
and, generally, well below the normal rated 
value. Thus, a second benefit of altifude on 
engine performance is due to the increased 
RPM required to furnish cruise thrust. An 
increase in engine speed to the normal rated 
value will reduce the specific fu,el consumption. 
The increase in specific range with altitude 
of the turbojet airplane can be attributed to 
these three factors: 
(1) An increase in altitude will increase the 
proportion of (V/Tr) and provide a greater 
TAS for the same TY. 
(2) An increase in altitude in the tropo- 
sphere will produce lower inlet air temperature 
which reduces the specific.fuel consumption. 
(3) An increase in altitude requires in- 
creased engine RPM to provide cruise thrust 
and the specific fuel consumption reduces as 
normal rated RPM is approached. 
The combined effect of these three factors de- 
fines altitude as the one most important item 
affecting the specific range of the turbojet air- 
Pl ane. As an example of this combined’effect, 
the typical turbojet airplane obtains a specific 
range at 40,ooO ft. which is approximately 150 
percent greater than that obtained at sea leirel. 
The increased TAS accounts for approxi- 
mately two-thirds of this benefit while in- 
creased engine performance (reduced cJ ,~ ‘ac- 
counts for the other one-third of the benefit. 
For example, at sea level the maximum spe- 
cific range of a turbojet airplane may be 0.1 
nmi/lb. but at 40,000 ft. the maximum specific 
range would be approximately 0.25 nmi/lb. 
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