NAVWEPS OO-EOT-80  AIRPLANE PERFORMANCE  disc.  In this idealized propeller disc, the pres-  sure difference  is uniformly distributed over the  disc area but the actual case is rather different  from this.  The final velocity of the propeller slipstream,  V,, is achieved some distance behind the pro-  peller.  Because  of the nature of the flow pat-  tern produced by the propeller, one half of the  total velocity change is produced as the flow  reaches the propeller disc. If the complete  velocity increase  amounts to Za, the flow veloc-  ity has increased by the amount II at the pro-  peller disc.  The propulsive  e$icien~, vp, of the  ideal propeller could be expressed  by the fol-  lowing relationship:  and fatigue damage. By minimizing  the  amount of total time spent at high power  setting, greater overhaul life of the powerplant  can be achieved. This should not imply that  the-takeoff rating of the engine should not be  used. Actually, the use of the full maximum  power at takeoff will  accumulate less total  engine wear than a reduced power setting at  the same RPM because  of less time required to  climb to a given altitude or to accelerate to a  given speed.  The most severe rate of wear and fatigue  damage occurs at high RPM and low MAP.  High RPM produces high centrifugal loads  and reciprocating iuertia loads. When the  large reciprocating inertia loads are not cush-  ioned by high compression pressures,  critical  resultant loads can be produced. Thus, op-  erating time at maximum RPM and MAP must  be held to a minimum and operation at mari-  mum RPM and low MAP must be avoided.  where  Since the final velocity, Vs, is the sum of total  velocity change 2a and the initial  velocity,  V,, the propulsive efliciency rearranges to a  form identical to that for the turbojet.  .The aircraft propeller functions to convert  the powerplant shaft horsepower into propul-  sive horsepower. The basic principles of pro-  pulsion apply to the propeller in that thrust is  produced by providing the airstream a mo-  mentum change. The propeller achieves high  propulsive ef?iciency  by processing a relatively  large mass flow of air and imparting a rela-  tively small velocity change.  The momentum  change created by propeller is shown by the  illustration of figure 2.18.  AIRCRAFT PROPELLERS  output power  ?%I=  .  mput power  TV  ‘ I ’ =  T(V+a)  v,=propulsive efficiency  T=thrust, lbs.  V=fligkt velocity, knots  IJ = velocity increment at the  propeller disc, knots  2  VP’  1+ k  0  So, the same relationship exists as with the  turbojet engine in that high efficiency is de-  veloped by producing thrust with the highest  possible mass flow and smallest necessary  velocity change.  The actual propeller must be evaluated in a  more exact sense to appreciate the effect of  nonuniform disc loading, propeller blade drag  forces, interference flow between blades, etc.  With these differences  from the ideal Propeller,  The action of the propeller can be idealized  by the assumption that the rotating propeller  is simply an actuating disc.  As shown in fig-  ure 2.18, the inflow approaching the propeller  disc indicates converging streamlines with an  increase  in velocity and drop in pressure.  The  converging streamlines leaving the propeller  disc indicate a drop in pressure  and increase in  velocity behind the propeller. The pressure  change through the disc results from the distri-  bution of thrust over the area of the propeller  145 