text: NAWEPS 0040140 
  AlRPLANE PERFORMANCE 
  
  Of course, the development of thrus,t with 
  some finite mass flow will require some finite 
  velocity change and there will be the inevita- 
  ble waste of power in the airstream. 
  In order 
  to achieve high efficiency of propulsion, the 
  thrust should be developed with a minimum 
  of wasted power. 
  
  The propulsion efficiency of the jet power- 
  plant can be evaluated by comparing 
  the 
  propulsive output power with the input power. 
  Since the input power is the sum of the output 
  power and wasted power, an expression for 
  propulsion efficiency can be derived. 
  
  Pa 
  
  vp=Pa+Pw 
  
  zv, 
  
  ')p= v*+v1 
  
  where 
  
  trp = propulsion efficiency 
  
  9=“eta” 
  
  Pa = propulsive power available 
  
  = TCZV~ 
  
  Pw= power wasted 
  
  The resulting expression for propulsion effi- 
  ciency, v,,, shows a dependency on the flight 
  velocity, V,, and the jet velocity, VZ. When 
  the 
  is zero, the propulsion 
  efficiency is zero since all power generated is 
  wasted in the slipstream and the propulsive 
  power is zero. The propulsion efliciency would 
  be I.00 (or 100 percent) only when the flight 
  velocity, Vi, equals the 
  jet velocity, Vz. 
  it would not be possible to produce 
  Actually, 
  thrust under such conditions with a finite mass 
  flow. While 100 percent efficiency of propul- 
  sion can not be attained practically, some 
  insight is furnished to the means of creating 
  high values of propulsion efficiency. To ob 
  tain high propulsion efficiency it is necessary 
  
  flight velocity 
  
  in aircraft propulsion because of the relatively 
  high power output per powerplant weight and 
  size. Very few aircraft powerplants can com- 
  pare with the high output, flexibility, simplic- 
  ity, and small size of the aircraft gas turbine. 
  The coupling of the propeller and recipro- 
  cating engine is one of the most efficient means 