NAVWEPS oo-801-80 
AIRPLANE PERFORMANCE 
where 
condition (1) applies to some known condi- 
tion of weight, fuel flow, and specific 
range at some original basic altitude 
during cruise climb. 
con&&r (2) applies to some new values of 
weight, fuel flow, and specific range at 
some different altitude along a partic- 
ular cruise path. 
and 
V= velocity, knots 
M = Mach number 
W= gross weight, lbs. 
FF=fuel flow, lbs./hr. 
JR= specific range, nmi./lb. 
e=altitude density ratio 
Thus, during a cruise-climb flight, a 10 percent 
decrease in gross weight from the consumption 
of fuel would create: 
no change in Mach number or ‘TAS 
a 5 percent decrease in EAS 
a 10 percent decrease in C, i.e., higher 
altitude 
a 10 percent decrease in fuel flow 
an 11 percent increase in specific range 
An important comparison can be made between 
the constant altitude cruise and the cruise- 
climb with respect to the variation of specific 
range. From the previous relationships, a 
2 percent reduction in gross weight durmg 
cruise would create a 1 percent increase in 
specific range in a constant altitude cruise but 
a 2 percent increase in specific range in a cruise- 
climb at constant .Mach number. Thus, a 
higher average specific range can.be maintained 
during the expenditure of a given increment of 
fuel. If an airplane begins a cruise at optimum 
conditions at or above the tropopause with a 
given weight of fuel, the following data 
provide a comparison of the total range avail- 
able from a constant altitude or cruise-climb 
0.0 Loo0 
.I 1.026 
.2 1.057 
.3 1.92 
.4 1.136 
.5 1.182 
.6 1.248 
.7 1.331 
For example, if the cruise fuel weight is 50 per- 
cent of the gross weight, the climbing cruise 
flight path will provide a range 18.2 percent 
greater than cruise at constant ,altitude. This 
comparison does not include consideration of 
any variation of specific fuel consumption dur- 
ing cruise or the effects of compressibility in 
defining the optimum aerodynamic conditions 
for cruising flight. However, the comparison 
is generally applicable for aircraft which have 
subsonic cruise. 
When the airplane has a supersonic cruise for 
maximum range, the optimum flight path is 
generally one of a constant Mach number. 
The optimum flight path is generally-but not 
necessarily-a climbing cruise. In this case of 
subsonic. or supersonic cruise, a Machmeter is 
of principal importance in cruise control of the 
jet airplane. 
The @ct of wind on nznge is of considerable 
importance in flying operations. Of course, 
a headwind will always reduce range and a 
tailwind will always increase range. The 
selection of a cruise altitude with the most 
favorable (or least unfa:vorable) winds is a rel- 
atively simple matter for the case of the 
propeller powered airplane. Since the range of 
the.propeller powered airplane is relatively un- 
affected by altitude, the altitude with the most 
favorable winds is selected for range. However, 
the range of the turbojet airplane is greatly 
affected by altitude so the selection of an op- 
timum altitude will involve considering the 
wind profile ‘with the variation of range with 
altitude. Since the turbojet range increases 
168