text: 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- 
  thrust re- 
  ance when operated at minimum 
  flight, 
  quired or 
  (L/D),. 
  lift 
  (L/D)m~ occurs at a specific value of 
  coefBcient for a given airplane and is essentially 
  independent of weight or altitude. 
  If a given 
  weight an~d configuration of airplane is oper- 
  the value of the 
  ated at various altitudes, 
  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 
  limits) 
  and specific endurance is a function only of 
  engine performance. 
  
  In  subsonic 
  
  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. 