When the inlet ram and compressor pressure 
ratio is fixed, the principal factor affecting the 
specific fuel consumption is the inlet air temp- 
erature. When the inlet air temperature is 
lowered, a given heat addition can provide 
relatively greater changes in pressure or vol- 
ume. As a result, a given thrust output 
requires less fuel flow and the specific fuel con- 
sumption, c,, is reduced. While the effect of 
altitude on specific fuel consumption does not 
compare with the effect on thrust output, the 
variation is large enough to strongly influence 
range and endurance conditions. Figure 2.11 
illustrates a typical variation of specific fuel 
consumption with altitude. Generally, the 
specific fuel consumption decreases steadily 
with altitude until the tropopause is reached 
and the specific fuel consumption at this point 
is approximately 80 percent of the sea level 
value. 
Above the tropopause the temperature is con- 
stant and altitudes slightly above the tropo- 
pause cause no further decrease in specific fuel 
consumption. Actually, altitudes much above 
the tropopause bring about a general deteriora- 
tion of overall engine efficiency and the~spkific 
fuel consumption begins an increase with 
altitude. The extreme altitudes above the 
tropopause produce low combustion chamber 
pressures, low compressor Reynolds Numbers, 
low fuel flow, etc. which are notconduci,ve to 
high engine efficiency. 
Because of the variation of c, with altitude, 
the majority of turbojet engines achieve maxi- 
mum efficiency at or above 35,000 ft. For this 
reason, the turbojet airplane will find optimum 
range and endurance conditions at. or above 
35,000 ft. provided the aircraft is not thrust 
or compressibility limited at these altitudes. 
The governing apparatus of the turbojet engine 
consists primarily of the, items which control 
the flow of fuel to the engine. In addition, 
there may be included certain functions which 
operate variable nozzles, variable stator vanes, 
variable inlets, etc. Generally, the fuel con- 
trol and associated items should regulate fuel 
NAVWEPS 00-8OT-80 
AIRPLANE PERFORMANCE 
flow, nozzle area, etc. to provide engine per- 
formance scheduled by the throttle or power 
lever. These regulatory functions provided 
must account for variations in altitude, tem- 
perature, and flight velocity. 
One principal governing factor which must 
be available is that a selected power setting 
(RPM) must be maintained throughout a wide 
range of flight conditions. Figure 2.12 illus- 
trates the sariation of fuel flow with RPM for 
a turbojet operating at a particular set of 
flight conditions. Curve 1 depicts the varia- 
tion with RPM of the fuel flow required for 
stabilized, ste,ady state operation of the engine. 
Each point along this curve 1 defines the fuel 
flow which is necessary to achieve equilib- 
rium at a given RPM. The steady state fuel 
flow produces a turbine, power to equal the 
compressor power requirement at a particular 
RPM. The throttle position primarily com- 
mands .a given, engine speed and, as changes 
occur in the ambient pressure, temperature, 
and flight speed, the .steady state fuel flow will . vary. The governing’ apparatus must account 
for these variations in flight conditions and 
maintain the power setting scheduled by 
throtrle position. 
In addition to the maintenance of steady 
state operation, the fuel control and associ- 
ated engine control itemsmust provide for the 
transient conditions of engine acceleration and 
deceleration. In order to accelerate the en- 
gine, the fuel control must supply a fuel flow 
greater than that required for steady state 
operation to ,produce a’ turbine power greater 
than the compressor power requirement. How- 
ever, the additional fuel flow to accelerate the 
engine must be controlled and regulated to 
prevent any one or combination of the follow- 
ing items: 
(1) compressor stall or surge 
(2) excessive turbine inlet temperature