From the previous analysis, it is apparent 
that the cruise altitude of the turbojet should 
be as high as possible within compressibility 
or thrust limits. Generally, the optimum alti- 
tude to begin cruise is the highest altitude at 
which the maximum continuous thrust can 
provide the optimum aerodynamic conditions. 
Of course, the optimum altitude is determined 
mainly by the gross weight at the begin of 
cruise. For the majority of turbojet airplanes 
this altitude will be at or above the tropopause 
for normal cruise configurations. 
Most turbojet airplanes which have rran- 
sonic or moderate supersonic performance will 
obtain maximum range with a high subsonic 
cruise. However, the airplane designed spe- 
cifically for high supersonic performance will 
obtain maximum range with a supersonic 
cruise and subsonic operation will cause low 
lift-drag ratios, poor inlet and engine perform- 
ance and redute the range capability. 
The cruise control of the turbojet airplane 
is considerably ~different from that of the pro- 
peller driven airplane. Since the specific range 
is so greatly affected by altitude, the optimum 
altitude for begin of cruise should be attained 
as rapidly as is consistent with climb fuel re- 
quirements. The range-climb program varies 
considerably between airplanes and the per- 
formance section of the flight handbook will 
specify the appropriate procedure. The de- 
scent from cruise altitude will employ essen- 
tially the same feature, a rapid descent is 
necessary to minimize the time at low altitudes 
where specific’ range is low and fuel flow is high 
for a given engine speed. 
During cruise flight of the turbojet airplane, 
the decrease of gross weight from expenditure 
of fuel can result in two types of cruise control. 
During a constant altitlrdc C&SC, a reduction in 
gross weight will require a reduction of air- 
speed and engine thrust ‘to maintain the opti- 
mum lift coefhcient of subsonic cruise. While 
such a cruise may be necessary to conform to 
the flow of traffic, it constitutes a certain in- 
efficiency of operation. If the airplane were 
NAVWEPS OO-BOT-RO 
AIRPLANE PERFORMANCE 
not restrained to a particular altitude, main- 
taining the same lift coeAicient and engine 
speed would allow the airplane to climb as the 
gross weight decreases. Since altitude gen- 
erally produces a beneficial effect on range, the 
climbing C&SC implies a more efficient flight 
path. 
The cruising flight of the turbojet airplane 
will begin usually at or above the tropopause 
in order to provide optimum range conditions. 
If flight is conducted at (a/&)-, optimum 
range will be obtained at specific values of lift 
coefficient and drag coefficient. When the air- 
plane is fixed at these values of CL and C, and 
the TAS is held constant, both lift and drag are 
directly proportional to the density ratio, (T. 
Also, above the tropopause, the thrust is pro- 
portional to .J when the TAS and RPM are con- 
stant. As a result, a reduction of gross weight 
by the expenditure of fuel would allow the 
airplane to climb but the airplane would re- 
main in equilibrium because lift, drag, and 
thrust all vary in the same fashion. This re- 
lationship is illustrated by figure 2.26. 
The relationship of lift, drag, and thrust is 
convenient for, in part, it justifies the condi- 
tion of a constant velocity. Above the tropo- 
pause, rhe speed of sound is constant hence a 
constant velocity during the cruise-climb 
would produce a constant Mach number. In 
this case, the optimum values of (&,/CD), C, 
and C, do not vary during the climb since the 
Mach number is constant. The specific fuel 
consumption is initially constant above the 
tropopause but begins to increase at altitudes 
much above the tropopause. If the specific 
fuel consumption is assumed to be constant 
during the cruise-climb, the following rela- 
tionships will apply: 
V, M, CL and C, are constant 
62 wz 
61 w, 
FR 02 
FFI ~1