text: not restrained to a particular altitude, main- 
  the same lift coeAicient and engine 
  taining 
  speed would allow the airplane to climb as the 
  gross weight decreases. Since altitude gen- 
  erally produces a beneficial effect on range, the 
  climbing C&SC implies a more efficient flight 
  path. 
  
  The cruising flight of the turbojet airplane 
  will begin usually at or above the tropopause 
  in order to provide optimum range conditions. 
  If flight is conducted at (a/&)-, 
  optimum 
  range will be obtained at specific values of lift 
  coefficient and drag coefficient. When the air- 
  plane is fixed at these values of CL and C, and 
  the TAS is held constant, both lift and drag are 
  directly proportional 
  to the density ratio, (T. 
  Also, above the tropopause, the thrust is pro- 
  portional to .J when the TAS and RPM are con- 
  stant. As a result, a reduction of gross weight 
  the 
  by the expenditure of fuel would allow 
  airplane to climb but the airplane would re- 
  main in equilibrium because lift, drag, and 
  thrust all vary in the same fashion. This re- 
  lationship is illustrated by figure 2.26. 