NAVWEPJ OO-ROT-80  AIRPLANE PERFORMANCE  OFl4X ’ TIMUM  RANGE  AND  ENDUR-  ANCE  airplane will have a maximum specific endur-  ance at 35,ooO ft. which is at least 40 percent  greater than the maximum value at sea level.  If the turbojet airplane is at low altitude and  it is necessary  to hold for a considerable time,  maximum time in the air will be obtained by  beginning a climb to some optimum altitude  dependent upon the fuel quantity available.  Even though fuel is expended during the climb,  the higher altitude will provide greater total  endurance. Of course, the use of afterburner  for the climb would produce a prohibitive re-  duction in endurance.  to airplane factors. The turboprop power-  plant prefers operation at low inlet air tem-  peratures and relatively high power setting to  produce low specific  fuel consumption.  While  an increase in altitude will increase the mini-  mum power required for the airplane, the  powerplant achieves more efficient operation.  As a result of these differences,  maximum en-  durance of the multiengine turboprop airplane  at low altitudes may require shutting down  some of the powerplants in order to operate  the remaining powerplants at a higher, more  efficient power setting.  EFFECT OF ALTITUDE  ON  ENDUR-  ANCE, TURBOJET AIRPLANES.  Since the  fuel flow of the turbojet powered airplane is  proportional to thrust required, the turbojet  airplane will achieve maximum specific endur-  ance when operated at minimum thrust re-  quired  or  (L/D),.  In  subsonic flight,  (L/D)m~  occurs at a specific value of lift  coefBcient  for a given airplane and is essentially  independent of weight or altitude.  If a given  weight an~d  configuration of airplane is oper-  ated at various altitudes, the value of the  minimum thrust required is unaffected by the  curves of thrust required versus velocity shown  in figure 2.27.  Hence, it is apparent that the  aerodynamic configuration has no prefeience  for altitude (within  compressibility limits)  and specific endurance is a function only of  engine performance.  There are many conditions of flying oper-  ations in which optimum range or endurance  conditions are not possible or practical. In  many instances,  the off-optimum conditions  result from certain operational requirements  or simplification of operating procedure. In  addition, off-optimum performance  may be the  result of a powerplant malfunction or failure.  The most important conditions are discussed  for various airplanes by powerplant type.  RECIPROCATING  POWERED  AIR-  PLANE. In the majority of cases, the recipro-  cating powered airplane is operated at ’ an engine  dictated cruise. Service  use will most probably  define some continuous power setting which  will give good service life and trouble-free  operation of the powerplant.  When range or  endurance is of no special interest, the simple  expedient is to operate the powerplant at the  recommended  power setting and accept what-  ever speed, range, or endurance that results.  While such a procedure greatly simplifies the  matter of cruise control, the practice does not  provide the necessary  knowledge required for  operating a high  performance, long range  airplane.  The specific fuel consumption of the turbojet  engine is strongly affected by operating RPM  and altitude.  Generally, the turbojet engine  prefers the operating range near normal rated  engine speed and the low temperatures of the  stratosphere to produce low specific fuel con-  sumption.  Thus, increased altitude provides  the favorable lower inlet air temperature and  requires a greater engine speed to provide the  thrust required at (L/D)-.  The typical  turbojet airplane experiences an increase in  specific endurance with altitude with the peak  values occurring at or near the tropopausc.  For example, a typical single-engine turbojet  The failure of an engine on the multiengine  reciprocating powered airplane has interesting  ramifications. The first problem appearing is  to produce sufficient power from the remaining  engines to keep the airplane airborne. The  172 