text: to produce the required thrust with the highest 
  possible mass flow and lowest possible velocity 
  change. 
  
  The graph of figure 2.5 shows the variation 
  of propulsion efficiency, qP, with 
  the ratio of 
  flight speed to jet velocity, VJV,. To achieve 
  a propulsion efficiency of 0.85 requires that the 
  flight velocity be approximately 75 percent of 
  the slipstream speed relative to the airplane. 
  Such a propulsive efficiency could be typical 
  of a propeller powered airplane which derives 
  its thrust by the propeller handling a large 
  mass flow of air. The typical turbojet power- 
  plant cannot achieve such high propulsive 
  ethciency because the thrust is derived with a 
  relatively smaller mass flow and larger vcloc- 
  ity change. For example, if the jet velocity is 
  1,200 ft. per sec. at a flight velocity of 600 ft. 
  per sec., the propulsion efficiency is 0.67. The 
  ducted fan, bypass jet, and turboprop are vari- 
  aCon -which impiove tliC propulsive efIiciency 
  of a type of powerplant which has very high 
  power capability. 
  
  When the conditions of range, endurance, or 
  economy of operation are predominant, high 
  propulsion efhciency is necessary. Thus, the 
  propeller powered airplane with 
  its inherent 
  high propulsive efliciency will always find ap 
  plication. 
  The requirements of very high 
  speed and high altitude demand very high 
  propulsive power from relatively small powcr- 
  plants. When there are practical limits to the 
  increase of mass flow, high output is obtained 
  by large velocity changes and low propulsive 
  efficiency is an inevitable consequence. 