text: following 
  stall: 
  
  items depending on severity of the 
  
  blade must operate at a greater angle of attack. 
  If the blade pitch is increased or the forward 
  speed increased the stalled portion of the rotor 
  disc becomes larger with 
  the stall progressing 
  in toward the hub from the tip of the retreating 
  blade. When approximately 15 percent of 
  the rotor disc is stalled, control of the heli- 
  copter will be impossible. Flight 
  tests have 
  determined that control becomes marginal and 
  the stall is considered severe when the outer 
  one-quarter of the retreating blade is stalled. 
  Retreating blade stall can be recognized by 
  rotor toughness, erratic stick forces, a vibration 
  and stick shake with a frequency determined 
  by the number of blades and the rotor speed. 
  Each of the blades of a three-bladed rotor will 
  stall as it passes through the stall region and 
  create a vibration with 
  three beats pet rotor 
  revolution. Other evidence of retreating blade 
  stall is partial or complete loss of control or a 
  pitch-up tendency which can be uncontrollable 
  if the stall is severe. 
  
  1. Decrease collective pitch 
  2. Decrease airspeed 
  3. Increase rotor RPM 
  4. Decrease severity of accelerated ma- 
  
  newer or control deflection 
  If the stall is severe enough to result in pitch-up, 
  forward cyclic to attempt to control pitch-up 
  is ineffective and may aggravate the stall since 
  forward cyclic results in an increase in blade 
  angle of attack on the retreating blade. The 
  recover from 
  helicopter will automatically 
  a severe stall since the airspeed is decreased in 
  the nose high attitude but recovery can be 
  assisted by gradual reduction 
  in collective 
  pitch, increasing RPM, and leveling the heli- 
  copter with pedal and cyclic stick. 