NAVWEPS 00-801-80 
AIRPLANE PERFORMANCE 
the centrifugal flow engine has relatively large 
acceleration margins and good acceleration 
characteristics result with the low rotational 
inertia. The axial flow compressor must oper- 
ate relatively close to the stall-surge limit to 
obtain peak efficiency. Thus, the acceleration 
fuel flow boundary for the axial flow engine is 
set by these stall-surge limits which are more 
immediate to steady state conditions than tur- 
bine temperature limits. The fixed geometry 
axial flow engine encounters relatively small 
acceleration margins and, when compared to 
the centrifugal flow engine with larger accel- 
eration margins and lower rotational inertia, 
has inferior acceleration characteristics. Cer- 
tain variation of the axial flow engine such as 
variable nozzles, variable stator blades, multi- 
ple-spool compressors, etc., greatly improve 
the acceleration characteristics. 
A note of caution is appropriate at this 
point. If the main fuel control and govern- 
ing apparatus should malfunction or become 
inoperative and an unmodulated secondary or 
emergency system be substitued, extreme care 
must be taken to avoid abrupt changes in 
throttle position. In such a case, very gradual 
movement of the throttle is necessary to ac- 
complish changes in power setting without 
excessive turbine temperatures, compressor 
stall or surge, or flameout. 
There are various instruments to relate irnr 
portant items of turbojet engine performance. 
Certain combinations of these instruments are 
capable of immediately relating the thrust 
output of the powerplant in a qualitative man- 
ner. It is difficult to provide an instrument or 
combination of instruments which immedi- 
ately relate the thrust output in a ~arrantitativ~ 
manner. As a result, the pilot must rely on 
a combination of instrument readings and judge 
the output performance according to standard 
values particular to the powerplant. Some of 
the usual engine indicating instruments are as 
follows : 
(1) The tachometer provides indication of 
engine speed, N, by percent of the maximum 
RPM. Since the variation of thrust with 
RPM is quite powerful, the tachometer in- 
dication is a powerful reference. 
(2) The exhaust gas temperature gauge 
provides an important reference for engine 
operating limitations. While the tempera- 
ture probe may be located downstream from 
the turbine (tailpipe or turbine discharge 
temperature) the instrument should provide 
an accurate reflection of temperatures up- 
stream in the turbine section. The exhaust 
gas temperature relates the energy change 
accomplished by fuel addition. 
(3) The fuel flowmeter can provide a fair 
reflection of thrust output. and operating 
efficiency. Operation at high density alti- 
tude or high inlet air temperatures-reduces 
the output thrust and this effect is related by 
a reduction of fuel flow. 
(4) The’ tailpipe total pressure (p+q in 
the tailpipe) can be correlated with the jet 
thrust for a given engine geometry and set of 
operating conditions. The output thrust 
can be related accurately with various com- 
binations of compressor inlet total pressure, 
tailpipe total pressure, ambient pressure and 
temperature. Hence; pressure differential 
(Ap), pressure ratio, and ,tailpipe total pres- 
sure instruments can provide more accurate 
immediate indications of output thrust than 
combined indications of RPM and EGT. 
This is especially true with variable geom- 
etry or multiple spool engines. 
Many other specialized instruments furnish 
additional information for more detailed items 
of engine performance. Various additional 
engine information is realized from fuel pres- 
sure, nozzle positions, compressor inlet air 
temperature, etc. 
TURBOJET OPERATING LIMITATIONS. 
The operating characteristics of the turbojet 
engine provide various operating limitations 
which must be given due respect. Operation 
of the powerplant within the specified limita-