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.  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.  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.  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.  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:  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  Generally, the stall-surge and turbine tem-  perature limits predominate to form an ac-  celeration fuel flow boundary typified by curve  (1) compressor  stall or surge  (2) excessive  turbine inlet temperature  (3) excessively rich fuel-air ratio which  may not sustain combustion  121 