be expected to vary as the square of the rota- 
tive speed, N. However, since a variation in 
rotative speed will alter airflow, fuel flow, 
compressor and turbine efficiency, etc., the 
thrust variation will be much greater than 
just the second power of rotative speed. In- 
stead of thrust being proportional to iV2, the 
typical fixed geometry engine develops thrust 
approximately proportional to N3.6. Of course, 
such a variation is particular to constant alti- 
tude and speed. 
Figure 2.10 illustrates the variation of per- 
cent maximum thrust with percent maximum 
RPM for a ‘typical fixed geometry engine. 
Typical values from this graph are as follows: 
P<m#r ma%. RPM Pmwit IMX. tlJrw,r 
100 loo (of course) 
99 96.5 
95 83.6 
90 69.2 
80 45.8 
70 28.7 
Note that in the top end of power output, each 
1 percent RPM change causes a 3.5-percent 
change in thrust output. This illustrates the 
power of variation of thrust with rotative 
speed which, iii this example, is N3.“. Also 
note that the top 20 percent of RPM controls 
more than half of the output thrust. 
While the fixed geometry engine develops 
thrust approximately proportional to Na.“, the 
engine with variable geometrywill demonstrate 
a much more powerful effect of rotative speed. 
When the jet engine is equipped with a vari- 
able nozzle, multispool compressor, variable 
stator blades, etc., the engine is more likely 
to develop thrust proportional to rotative 
speed from values of N4.6 to N6.0. For ex- 
ample, if a variable geometry engine develops 
thrust proportional to Ns.‘, each one per cent 
RPM change causes a 5.0-percent thrust change 
at the top end of power output. Also, the 
top 13 percent of RPM would control the top 
50 percent of thrust output. 
The powerful variation of thrust with engine 
speed has certain ramifications which should 
NAVWEPS 00-801-80 
AlR,PlANE PERFORMANCE 
be appreciated. If the turbojet powerplant 
operates at less than the “trimmed” or adjusted 
speed for maximum thrust, the deficiency of 
thrust for takeoff may cause a considerable 
increase in takeoff distance. During approach, 
an excessively low RPM may cause very low 
thrust and produce a very steep glide path. 
In addition, the low RPM range involves the 
much greater engine acceleration time to pro- 
duce thrust for a waveoff. Another compli- 
cation exists when the thrust is proportional 
to some large power of rotative speed, e.g., 
Nb.O. The small changes in RPM produce 
such large variations in thrust that instruments 
other than the tachometer must be furnished 
for accurate indication of thrust output. 
The “specific fuel consumption, ci’ is an 
important factor for evaluating the perform- 
ance and efficiency of operation of a turbojet 
engine. The specific fuel consumption is the 
proportion between the fuel flow (in lbs. per 
hr.) and the thrust (in lbs.). For example, 
an engine which has a fuel flow of 14,000 lbs. 
per hr. and a thrust of 12,500 lbs. has a specific 
fuel consumption of: 
Fuel flow 
“= Thrust 
14,000 lbs./hr. 
‘I= 12,500 lbs. 
c,=1.12 lbs./hr./lb. 
Thus, each unit pound of thrust requires 1.12 
lbs. per hr. fuel flow. Obviously, high engine 
efficiency would be indicated by a low value of 
c,. Typical values for turbojet engines with 
relatively high pressure ratios range from 0.8 
to 1.2 at design operating conditions in sub- 
sonic flight. High energy fuels and greater 
pressure ratios tend to produce the lower values 
of ct. Supersonic flight with the attendant in- 
let losses and high compressor inlet air tem- 
peratures tend to increase the specific fuel con- 
sumption to values of 1.2 to 2.0. Of course, 
the use of an afterburner is quite inefficient