NAWEPS 0040140  AlRPLANE PERFORMANCE  to produce the required thrust with the highest  possible mass flow and lowest possible velocity  change.  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.  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.  The resulting expression for propulsion effi-  ciency, v,,, shows a dependency on the flight  velocity, V,, and the jet velocity, VZ. When  the flight velocity 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.  Actually, it would not be possible to produce  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  The turbojet engine has foundwidespread USC  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  TURBOJET ENGINES  where  Pa  vp=Pa+Pw  zv,  ')p= v*+v1  trp  = propulsion efficiency  9=“eta”  Pa = propulsive power available  = TCZV~  Pw= power wasted  106 