NAVWEPS 00401-60  AIRPLANE PERFORMANCE  The specific endurance is simply the reciprocal  of the fuel flow, hence maximum endurance  conditions would be obtained at the lowest  fuel flow required to hold the airplane in steady  level flight.  Obviously, minimum fuel flow  will provide the maximum flying time from a  given quantity of fuel.  Generally, in subsonic  performance,  the speed  at which maximum en-  durance is achieved is approximately 75 per-  cent of the speed for maximum range.  greatly with altitude, the turbojet can tolerate  less favorable (or more unfavorable) winds  with increased  altitude.  In some cases, large values of wind may  cause a significant change in cruise velocity to  maintain maximum ground nautical miles per  lb. of fuel.  As an example of an extreme con-  dition, consider an airplane flying into a head-  wind which equals the cruise velocity.  In this  case, ““9  increase in velocity would improve  range.  While many different factors can affect the  specific  endurance, the most important factors  at the control of the pilot are the configuration  and operating altitude.  Of course, for maxi-  mum endurance conditions the airplane must  be in the clean configuration and operated at  the proper aerodynamic conditions.  To appreciate the changes  in optimum speeds  with various winds, refer to the illustration of  figure 2.26. When zero wind conditions exist,  a straight line from the origin tangent to the  curve of fuel flow versus velocity will locate  maximum range conditions. When a head-  wind condition exists, the speed for maximum  ground range is located by a line tangent drawn  from a velocity offset equal to the headwind  velocity.  This will locate maximum range at  some higher velocity and fuel flow.  Of course,  the range will be less than when at zero wind  conditions but the higher velocity and fuel flow  will minimize the range loss due to the head-  wind.  In a similar sense, a tailwind will re-  duce the cruise velocity to maximize the  benefit of the tailwind.  EFFECT OF ALTITUDE  ON  ENDUR-  ANCE, PROPELLER DRIVEN AIRPLANES.  Since the fuel flow of the propeller driven air-  plane is proportional to power required, the  propeller powered airplane will achieve maxi-  mum specific  endurance  when operated at mini-  mum power required. The point of minimum  power required is obtained at a specific value  of lift coefficient for a particular airplane con-  figuration and is essentially independent of  weight or altitude.  However, an increase in  altitude will increase  the value of the minimum  power required as illustrated by figure 2.27.  If the specific fuel consumption were not in-  fluenced by altitude or engine power, the spe-  cific endurance  would be directly proportional  to ji,  e.g., the specific  endurance at 22,000 ft.  (a=O.498) would be approximately 70 percent  of the value at sea level. This example is very  nearly the case of the airplane with the recipro-  cat&g engine  since specific  fuel consumption and  propeller efficiency are not directly affected by  altitude.  The obvious conclusion is that  maximum endurance of the reciprocating en-  gine airplane is obtained at the lowest practical  altitude.  The procedure of employing different cruise  velocities to account for the effects  of wind is  necessary only at extreme values of wind  velocity.  It  is necessary to consider the  change in optimum cruise airspeed when the  wind velocities exceed 25 percent of the zero  wind cruise velocity.  The ability of the airplane to convert fuel  energy into flying time is an important factor  in flying operations. The “specific endurance”  of the airplane is defined as follows:  The variation with altitude of the maximum  endurance of the turboprop airplane requires  consideration of powerplant factors in addition  ENDURANCE PERFORMANCE  specific endurance==1  specific endurance=  1  fuel flow, Ibs. per hr.  im 