NAVWEPS OO-BOT-RO  AIRPLANE PERFORMANCE  not restrained to a particular altitude, main-  taining the same lift coeAicient and engine  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.  From the previous analysis, it is apparent  that the cruise altitude of the turbojet should  be as high as possible within compressibility  or thrust limits.  Generally, the optimum alti-  tude to begin cruise is the highest altitude at  which the maximum continuous thrust can  provide the optimum aerodynamic conditions.  Of course, the optimum altitude is determined  mainly by the gross weight at the begin of  cruise. For the majority of turbojet airplanes  this altitude will be at or above the tropopause  for normal cruise configurations.  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  by the expenditure of fuel would allow the  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.  Most turbojet airplanes which have rran-  sonic or moderate supersonic performance  will  obtain maximum range with a high subsonic  cruise. However, the airplane designed spe-  cifically for high supersonic performance will  obtain maximum range with  a supersonic  cruise and subsonic operation will cause low  lift-drag ratios, poor inlet and engine perform-  ance and redute the range capability.  The cruise control of the turbojet airplane  is considerably ~different  from that of the pro-  peller driven airplane. Since the specific  range  is so greatly affected  by altitude, the optimum  altitude for begin of cruise should be attained  as rapidly as is consistent with climb fuel re-  quirements. The range-climb program varies  considerably between airplanes and the per-  formance section of the flight handbook will  specify the appropriate procedure.  The de-  scent from cruise altitude will employ essen-  tially the same feature, a rapid descent is  necessary  to minimize the time at low altitudes  where specific’  range is low and fuel flow is high  for a given engine speed.  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  constant velocity during  the cruise-climb  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. If the specific  fuel consumption is assumed to be constant  during the cruise-climb, the following rela-  tionships will apply:  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  gross weight will require a reduction of air-  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-  efficiency of operation. If the airplane were  V, M, CL and C, are constant  62 wz  61 w,  FR  02  FFI  ~1  JR2-W, (cruise climb above tropopause,  x-W9  constant M, c,)  167 