NAVWEPS OO-SOT-80 
AIRPLANE PERFORMANCE 
operational factors will define operating pro- 
cedures. 
RANGE, TURBOJET AIRPLANES. Many 
different factors influence the range of the 
turbojet airplane. In order to simplify the 
analysis of the overall range problem, it is 
convenient to separate airplane factors from 
powerplant factors and analyze each item 
independently. An analogy would be the 
study of “horsecart” performance by separat- 
ing “cart” performance from “horse” per- 
formance to distinguish the principal factors 
which affect the overall performance. 
In the case of the turbojet airplane, the 
fuel flow is determined mainly by the thrust 
rather than power. Thus, the fuel flow could 
be most directly related to the thrust required 
to maintain the airplane in steady, level flight. 
.This fact allows study of the turbojet powered 
airplane by analysis of the curves of thrust 
required versus velocity. Figure 2.25 illu- 
strates a typical curve of thrust required versus 
velocity which would be (somewhat) analo- 
gous to the variation of fuel flow versus veloc- 
ity. Maximum endurance condition would 
be obtained at (L/D)- since this would incur 
the lowest fuel flow to keep the airplane in 
steady, level flight. Maximum range condition 
would occur where the proportion between 
velocity and thrust required is greatest and 
this point is located by a straight line from 
the origin tangent to the curve. 
The maximum range is obtained at the aero- 
dynamic condition which produces a maximum 
proportion between the square root of the 
lift coefficient (CJ and the drag coe&cient 
(CD), or (&/CD)-. In subsonic perform- 
ance, (G/C > D - occurs at a particular value 
angle of attack and lift coefficient and is un- 
affected by weight or altitude (within com- 
pressibility limits). At this specific aerody- 
namic condition, induced drag is approxi- 
mately 25 percent of the total drag so the 
turbojet airplane designed for long range does 
not have the strong preference for high aspect 
ratio planform like the propeller airplane. 
On the other hand, since approximately 75 
percent of the total drag is parasite drag, the 
turbojet airplane designed specifically for long 
range has the special requirement for great 
aerodynamic cleanness. 
The effect of the variation of airplane gross 
weight is illustrated by the second graph 
of figure 2.25. The flight condition of 
(mc 1 D IMI is achieved at one value of lift 
coefbcient for a given airplane in subsonic 
flight. Hence, a variation of gross weight will 
alter the values of airspeed, thrust required, 
and specific range obtained at ,(&/CD)-. If 
a given configuration is operated at constant 
altitude and lift coefficient the following re-~ 
lationships will apply: 
SR2 -= 
SRI (constant .altitude) 
where 
condition (1) applies to some! known condi- 
tion of velocity, thrust required, and 
specific range for (&/CD)- at some 
basic weight, Wi 
condition (2) applies to some new values of 
velocity, thrust required, and specific 
range for (&/CD)- at some different 
weight, W, 
and 
V= velocity, knots 
W=gross weight, lbs. 
Tr= thrust required, lbs. 
.SR= specific range, nmi/lb. 
Thus, a 10 percent increase in gross weight 
would create: 
a 5 percent increase in velocity 
a 10 percent increase in thrust required 
a 5 percent decrease in specific range 
when flight is maintained at the optimum con- 
ditions of (&/CD)-. Since most jet airplanes 
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