text: From the previous discussion, it is apparent 
  that there is some degree of retreating blade 
  stall even at moderate airspeeds. However, 
  the helicopter is able to perform satisfactorily 
  until a sufficiently large area of the rotor disc 
  is stalled. Adequate warning of the impend- 
  ing stall is present when the stall condition 
  is approached slowly. There is inadequate 
  warni,ng of the stall only when the blade pitch 
  or blade angle of attack is increased rapidly. 
  Therefore, unintentional severe stall is most 
  likely to occur during abrupt control motions 
  or rapid accelerated maneuvers. 