NAVWEPS OO-BOT-80 
AIRPLANE PERFORMANCE 
the lower temperatures can precipitate this 
water out of solution in liquid or ice crystal 
form. 
High altitude flight produces relatively small 
air mass flow through the engine and the rela- 
tively low fuel flow rate. At these conditions 
a malfunction of the fuel control and governing 
apparatus could cause flameout. If the fuel 
control allows excessively low fuel flow during 
controlled deceleration, the lean blow out limit 
may be exceeded. Also, if the governed idle 
condition allows any deceleration below the 
idle condition the engine will usually continue 
to lose speed and flameout. 
Restarting the engine in flight requires sufli- 
cient RPM and airflow to allow stabilized op- 
eration. Generally, the extremes of altitude 
are most critical for attempted airstart. 
(4) An increased compressor inlet air tcmpcra- 
tare can have a profound effect on the output 
tbLrust of 2 rnrhniet m&n,= ---“-,-- --o---. As shown in 
figure 2.13, an increase in compressor inlet 
temperature produces an even greater increase 
in the compressor discharge temperature. Since 
the turbine inlet temperature is limited to 
some maximum value, any increase in com- 
pressor discharge temperature will reduce the 
temperature change which can take place in 
the combustion chamber. Hence, the fuel flow 
will be limited and a reduction in thrust is 
incurred. 
The effect of inlet air temperature on thrust 
output has two special ramifications. At rakc- 
off, a high ambient air temperature at a given 
pressure altitude relates a high density altitude. 
Thus, the takeoff thrust is reduced because of 
low density and low mass flow. In addition 
to the loss of thrust due to reduced mass flow, 
thrust and fuel flow are reduced further be- 
cause of the high compressor inlet temperature. 
In flight at Sigh Mach number, the aerodynamic 
heating will provide an increase in compressor 
inlet temperature. Since the compressor inlet 
temperature will reflect the compressor dis- 
charge temperature and the allowable fuel 
flow, the compressor inlet air temperature may 
provide a convenient limit to sustained high 
speed flight. 
(5) The effect of engine overspeed or critical vi- 
bration speed ranger is important in the service 
life of an engine. One of the principal sources 
of turbine loads is the centrifugal loads due to 
rotation. Since the centrifugal loads vary as 
the square of the rotative speed, a 5 percent 
overspeed would produce 10.25 percent over- 
stress (1.05*= 1.1025). The large increase in 
stress with rorative speed could produce very 
rapid accumulation of creep and fatigue dam- 
age at high temperature. Repeated overspeed 
and, hence, overstress can cause failure early 
in the anticipated service life. 
Since the turbojet engine is composed of 
many different distributed masses and elastic 
structure, there are certain vibra~tory modes 
and frequencies for the shaft, blades, etc. 
While it is necessary to prevent any resonant 
conditions from existing within the normal 
operating range, there may be certain vibra- 
tory modes encountered in the low power range 
common to ground operation, low altitude 
endurance, acceleration or deceleration. If 
certain operating RPM range restrictions are 
specified due to vibratory conditions, opera- 
tions must be conducted with a minimum of 
time in this area. The greatly increased 
stresses common to vibratory conditions are 
quite likely to cause fatigue failures of the 
offending components. 
The operating limitations of the engine are 
usually specified by various combinations of 
RPM, exhaust gas temperature, and allowable 
time. The conditions of high power output 
and acceleration have relatively short times 
allowable to prevent abuse of the powerplant 
and obtain good service life. While the al- 
lowable times at various high power and 
acceleration condition appear arbitrary, the 
purpose is to reduce the spectrum of loading