NAVWEPS 00-8OT-80  AtRPlANE PERFORMANCE  due to thc~  low combustion pressure  and values  of c, from 2.0 to 4.0 are typical with  aftet-  burner operation.  If the fixed geometry engine is operated at a  constant V (TAS) in subsonic flight and con-  stant N (RPM) the inlet velocity, inlet ram,  and compressor pressure ratio are essentially  constant with altitude.  An increase in alti-  tude then causes the engine air mass flow to  decrease  in a manner very nearly identical to  the altitude density ratio.  Of coutsc, this de-  crease  in mass flow will produce a significant  e&ct on the output thrust of the engine.  Actually, the variation of thrust with altitude  is not quite as severe as the density variation  because favorable decreases in temperature  occut. The decrease  in inlet air temperature  will provide a relatively greater combustion  gas &ergy and allow a greater jet velocity.  The increase in jet velocity somewhat offsets  the decrease  in mass flow.  Of course, an in-  crease  in altitude provides lower temperatures  below the tropopause. Above the tropopause,  no further favorable decrease in temperature  takes place so a more rapid variation of thrust  will  take place. The approximate variation  of thrust with altitude is represented  by figure  2.11 and some typical values at specific alti-  tudes ate as follows :  The turbojet engine usually has a strong  preference  fot high RPM to produce low specif-  ic fuel consumption. Since the normal rated  thrust condition is a particular design point  for the engine, the minimum value of c, will  occur at or near this range of RPM.  The  illustration of figure 2.10 shows a typical vati-  ation of c, with percent maximum RPM where  values of RPM less than 80 to 85 percent pro-  duce a specific fuel consumption much greater  than the minimum  obtainable.  This pref-  erence for high.RPM to obtain low values of  C, is very pronounced in the fixed geometry  engine.  Turbojet engines with  multispool  compressors  tend to be less sensitive in this  respect  and are more flexible in their operating  characteristics.  Whenever low values of cI ate  necessary to obtain range or endurance, the  preference  of the turboiet engine for the design  operating RPM can be a factor of great  influence.  Altitude is one factor which strongly affects  the performance of the turbojet engine.  An  increase in altitude produces a decrease in  density and pressure and, if below the tropo-  pause, a decrease  in temperature.  If a typical  nonaftcrbutning turbojet engine is operated at  a constant RPM and true airspeed, the vatia-  tion of thtust and specific fuel consumption  with altitude can be approximated from figure  221.  The variation of density in the standard  atmosphere is shown by the values of density  ratio at vatious altitudes.  Typical values of  the density ratio at specific altitudes are as  follows:  Since the change in density with altitude is  quite rapid at low altitude turbojet takeoff pet-  formance wil1 Abe  greatly affected at high alti-  tude.  Also note that the thrust at 35,000 ft.  is approximately 39 percent of the sea level  value.  RIrio  of  Tbrvrt  at dri14  Altitude, ft. :  (  )  Thi  ti I,‘ bwl  Scalevel.............................  1.m  5,ooo................................  ,888  lO,ooo...............................  .785  2o,ooo...............................  ,604  35,Mx)...............................  .392  40,Ko.  .315  =Jo,ocQ  ._._..,...._....._,.,.__.,.....  .180  Altitude, ft.:  Dews@  ra#ie  scaleeel . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  Loo0  5,ooo..  :.  .  .a617  lO,coo..............................  .7385  .?2#XQ.  .4976  35,cao . . . . . . . . . . . . . . .  . . . . . .  .3099  40,oal.. . . .  .2462  ~,OUO. .  .  .  .lS32  The thrust added by the afterburner of a  turbojet engine is not affected so greatly by  altitude as the basic engine thrust.  The use of  afterburner  may provide a thrust increase  of 50  percent at low altitude or as much as 100 per-  cent at high altitude.  119 