text: The relationship of lift, drag, and thrust is 
  convenient for, in part, it justifies the condi- 
  tion of a constant velocity. Above the tropo- 
  pause, rhe speed of sound is constant hence a 
  the cruise-climb 
  constant velocity during 
  would produce a constant Mach number. 
  In 
  this case, the optimum values of (&,/CD), C, 
  and C, do not vary during the climb since the 
  Mach number is constant. The specific fuel 
  consumption is initially 
  constant above the 
  tropopause but begins to increase at altitudes 
  much above the tropopause. 
  the specific 
  fuel consumption is assumed to be constant 
  during the cruise-climb, the following 
  rela- 
  tionships will apply: 
  
  If 
  
  During cruise flight of the turbojet airplane, 
  the decrease of gross weight from expenditure 
  of fuel can result in two types of cruise control. 
  During a constant altitlrdc C&SC, a reduction in 
  require a reduction of air- 
  gross weight will 
  speed and engine thrust ‘to maintain the opti- 
  mum lift coefhcient of subsonic cruise. While 
  such a cruise may be necessary to conform to 
  the flow of traffic, it constitutes a certain in- 
  If the airplane were 
  efficiency of operation. 