NAVWEPS 00-801-80  AIRPLANE PERFORMANCE  RPM.  Since the variation of thrust with  RPM is quite powerful, the tachometer in-  dication is a powerful reference.  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.  (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.  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 :  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.  (1) The tachometer provides indication of  engine speed, N, by percent of the maximum  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-  tions is absolutely necessary  in order to obtain  TURBOJET OPERATING LIMITATIONS. 