NAVWEPS 00-801-80  AlR,PlANE PERFORMANCE  be expected to vary as the square of the rota-  tive speed, N.  However, since a variation in  rotative speed will  alter airflow, fuel flow,  compressor and turbine efficiency, etc., the  thrust variation will  be much greater than  just the second power of rotative speed. In-  stead of thrust being proportional to iV2, the  typical fixed geometry engine develops thrust  approximately proportional to N3.6. Of course,  such a variation is particular to constant alti-  tude and speed.  Figure 2.10 illustrates the variation of per-  cent maximum thrust with percent maximum  RPM for a  ‘ typical  fixed geometry engine.  Typical values from this graph are as follows:  be appreciated. If the turbojet powerplant  operates  at less than the “trimmed”  or adjusted  speed for maximum thrust, the deficiency of  thrust for takeoff may cause a considerable  increase  in takeoff distance.  During approach,  an excessively low RPM may cause very low  thrust and produce a very steep glide path.  In addition, the low RPM range involves the  much greater engine acceleration time to pro-  duce thrust for a waveoff. Another compli-  cation exists when the thrust is proportional  to some large power of rotative speed, e.g.,  Nb.O. The small changes in RPM produce  such large variations in thrust that instruments  other than the tachometer must be furnished  for accurate indication of thrust output.  The “specific fuel consumption, ci’  is an  important factor for evaluating the perform-  ance and efficiency of operation of a turbojet  engine.  The specific fuel consumption is the  proportion between the fuel flow (in lbs. per  hr.) and the thrust (in lbs.).  For example,  an engine which has a fuel flow of 14,000 lbs.  per hr. and a thrust of 12,500 lbs. has a specific  fuel consumption of:  Note that in the top end of power output, each  1 percent RPM change causes a 3.5-percent  change in thrust output.  This illustrates the  power of variation of thrust with rotative  speed which, iii this example, is N3.“. Also  note that the top 20 percent of RPM controls  more than half of the output thrust.  While the fixed geometry engine develops  thrust approximately proportional to Na.“, the  engine with variable geometrywill demonstrate  a much more powerful effect of rotative speed.  When the jet engine is equipped with a vari-  able nozzle, multispool compressor, variable  stator blades, etc., the engine is more likely  to develop thrust proportional to rotative  speed from values of N4.6 to N6.0. For ex-  ample, if a variable geometry engine develops  thrust proportional to Ns. ‘ , each one per cent  RPM change causes  a 5.0-percent  thrust change  at the top end of power output.  Also, the  top 13 percent of RPM would control the top  50 percent of thrust output.  Thus, each unit pound of thrust requires 1.12  lbs. per hr. fuel flow.  Obviously, high engine  efficiency  would be indicated by a low value of  c,. Typical values for turbojet engines with  relatively high pressure ratios range from 0.8  to 1.2 at design operating conditions in sub-  sonic flight.  High energy fuels and greater  pressure  ratios tend to produce the lower values  of ct.  Supersonic  flight with the attendant in-  let losses and high compressor inlet air tem-  peratures tend to increase the specific  fuel con-  sumption to values of 1.2 to 2.0.  Of course,  the use of an afterburner is quite inefficient  The powerful variation of thrust with engine  speed has certain ramifications which should  Fuel flow  “=  Thrust  14,000 lbs./hr.  ‘ I=  12,500 lbs.  c,=1.12 lbs./hr./lb.  P<m#r  ma%.  RPM  Pmwit  IMX.  tlJrw,r  100  loo  (of course)  99  96.5  95  83.6  90  69.2  80  45.8  70  28.7 