NAVWEPS OO-BOT-BO  APPLICATION OF AERODYNAMICS  TO SPECIFIC PROBLEMS OF FLYl,NG  flow and negative stall regions similar to the  powered flight condition.  Autorotation is essentially a stable flight  condition.  If external disturbances cause the  rotor to slow down, the autorotation region of  the disc automatically expands to restore the  rotor speed to the original equilibrium condi-  tion.  On the other hand, if an external  disturbance causes the rotor to speed up, the  propeller region automatically expands and  tends to accelerate the rotor to the original  equilibrium  condition.  Actually the stable  autorotation condition will exist only when  the autorotational speed is within  certain  limits.  If the rotor speed is allowed to slow  some excessive  amount, then the rotor becomes  unstable and the RPM will  decrease even  further unless the pilot immediately corrects  the condition by proper control action.  during autorotation descent  would be obtained  at the flight condition which produces the  greatest proportion between airspeed and rate  of descent. Thus, a straight line from the  origin tangent to the curve will  define the  point for maximum autorotative glide dis-  tance. This corresponds to Point B of figure  6.18. If the helicopter is being glided at the  speed for maximum glide distance, a decrease  in airspeed would reduce the rate of descent  but the glide distance would decrease. If the  helicopter is being glided at the speed for  minimum rate of descent, the rate of descent  (steady state) can not be reduced but the glide  distance can be increased by increasing the  glide speed to that for maximum distance.  Weight and wind affect the glide character-  istics of a helicopter the same way an airplane  is affected. Ideally, the helicopter autorotates  at a higher equivalent airspeed at higher gross  weight or when autorotating into a headwind.  In case of engine failure, the fixed-wing  airplane will be glided at maximum lift-drag  ratio to produce maximum glide distance. If  minimum rate of descent  is desired in power-off  flight rather than maximum glide distance, the  fixed-wing airplane will  be flown at some  lower airspeed. Actually, the minimum rate  of descent will  occur at minimum  power  required. The helicopter exhibits similar char-  acteristics but ordinarily the best autorota~tion  speed  may be considered  that speed that results  in the minimum rate of descent rather than  maximum glide distance. The aerodynamic  condition of the rotor which produces mini-  mum rate of descent is:  In addition to aerodynamic forces which act  on the rotor during autorotation, inertia forces  are also important.  These effects are usually  associated with the pilot ’ s  response time be-  cause the rate a pilot reacts to a power failure  is quite critical. The time necessary  to reduce  collective pitch and enter autorotation be-  comes  critical if the rotor inertia characteristics  are such as to allow the rotor to slow down to  a dangerous level before the pilot can react.  With power on, the blade pitch is relatively  high and the engine supplies enough torque to  overcome the drag of the blades. At the  instant of power failure the blades are at a high  pitch with high drag. If there is no engine  torque to maintain the RPM, the rotor will  decelerate depending on the rotor torque and  rotor inertia. If the rotor has high rotational  energy the rotor will lose RPM less rapidly,  giving the pilot more time to reduce collective  pitch and enter autorotation. If the rotor has  low rotational energy, the rotor will lose RPM  rapidly and the pilot may not be able to react  quickly enough to prevent a serious loss of  rotor RPM.  Once the collective pitch is at  It is this ratio which determines the auto-  rotation rate of descent. Figure 6.18 illus-  trates the variation of autorotation rate of  descent with equivalent airspeed for a typical  helicopter. Point A on this curve defines the  point which produces autorotation with mini-  mum rate of descent. Maximum glide distance  Maximum ratio of  (Mean blade lift coetIicient)3”  Mean blade drag coefficient 