text: flow and negative stall regions similar to the 
  powered flight condition. 
  
  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 
  if an external 
  disturbance causes the rotor to speed up, the 
  propeller region automatically expands and 
  tends to accelerate the rotor to the original 
  the stable 
  equilibrium condition. Actually 
  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. 
  
  Autorotation 
  
  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 case of engine failure, 