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  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  which contributes the most rapid accumulation  of creep and fatigue damage.  In fact, in some  instances, the arbitrary time standards can be  set to suit the particular requirements of a  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.  (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.  provide a convenient limit to sustained high  speed flight.  128 