NAVWEPS OO-ROT-RO  AIRPLANE PERFORMANCE  The governing of the engine-propeller combi-  nation will allow operation throughout a wide  range of power and speed while maintaining  efficient operation.  If the envelope of maximum propeller dfi-  ciency is available, the propulsive horsepower  available will appear as shown in the second  chart of figure 2.19. The propulsive power  available, Pa, is the product of the propeller  efficiency  and applied shaft horsepower.  a single feathered propeller is a relatively small  contribution to the airpfane total drag.  The propellers used on most large reciprocating  engines derive peak propeller efficiencies  on the  order of s,=O.85 to 0.88.  Of course, the peak  values are designed to occur at some specific  design condition.  For example, the selection  of a propel!er for a !ong rasge transport wsuld  require matching of the engine-propeller com-  bination for peak efhciency  at cruise condjtion.  On the other hand, selection of a propeller for  a utility or liaison type airplane would require  matching of the engine-propeller combination  to achieve high propulsive power at low speed  and high power for good takeoff and climb  performance.  Several special considerations must be made  for the application of aircraft propellers. In  the event of a powerplant malfunction or  failure, provision must be made to streamline  the propeller blades and reduce drag so that  flight may be continued on the remaining op-  erating engines.  This is accomplished by  feathering the propeller blades which .stops  rotation and incurs a minimum of drag for the  inoperative engine. The necessity for feather-  ing is illustrated in figure 2.19 by the change  in equivalent parasite area, Af,  with propeller  blade angle, 8, of a typical instaliation.  When  the propeller blade angle is in the feathered  position, the change in parasite drag is at a  minimum and, in the case of a typical multi-  engine aircraft, the added parasite drag from  At smaller blade angles near the Rat pitch  position, the drag added by the propeller is  very large. AC these small blade angles, the  propeller windmilling at high RPM can create  such a tremendous amount of drag that the  airplane may be uncontrollable. The propel-  ler windmilling at high speed in the low range  of blade angles can produce an increase  in para-  site drag which may be as great as the parasite  drag of the basic airplane. An indication of  this powerful drag is seen by the hclieopter  in  autorotation. The windmilling  rotor  is ca-  pable of producing autorotation rates  ofdcscent  which approach that of a parachute  canopy  with the identical disc area laading. THUS,  the propeller windmilling  at high speed  and  small blade angle can produce an  cffccti+e  drag coefficient  of the disc area which compares  with tha~t of a parachute canopy.  The drag  and yawing moment caused by loss of power  at high engine-propeller speed is considerable  and the transient yawing displaccmcnt  of the  aircraft’ may produce critical loads for the  vertical tail.  For this reason, automatic  feathering may be a necessity  rather than a  luxury.  The large drag which can be produced by  the rotating propeller can be utilized to im-  prove the stopping performance of the air-  plane.  Rotation of the propekr blade to  small positive values or negative values with  applied power can produce large drag or re-  verse  thrust.  Since the thrust capability of the  propeller is quite high at low speeds, very  high deceleration can be provided by reverse  thrust alone,  The qs&zg limitatiar of the pmpcllcr are  closely associated with those of the Rower-  plant.  Overspeed conditions are critical be-  cause of the large centrifugal loads and blade  twisting moments produced by an excessive  rotative speed.  In addition, the propeller  blades will have various vibratory modes  and  certain operating  limitations may hc necessary  to prevent exciting resonant  conditions. 