text: EFFECT OF ALTITUDE ON ENDUR- 
  ANCE, PROPELLER DRIVEN AIRPLANES. 
  Since the fuel flow of the propeller driven air- 
  to power required, the 
  plane is proportional 
  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 
  that 
  maximum endurance of the reciprocating en- 
  gine airplane is obtained at the lowest practical 
  altitude. 