text: Conditions favorable for the occurrence of 
  retreating blade stall are those conditions that 
  result in high retreating blade angles of attack. 
  Each of the following conditions results in a 
  higher angle of attack on the retreating blade 
  and may contribute to retreating blade stall: 
  
  1. High airspeed 
  2. Low 
  
  low 
  rotor RPM necessitates the use of 
  higher blade pitch 
  to get a given 
  thrust from the rotor, thus a higher 
  angle of attack 
  
  totor RPM-operation  at 
  
  COMPRESSIBILITY EFFECTS. The highest 
  relative velocities occur at the tip of the ad- 
  vancing blade since the speed of the helicopter 
  is added to the speed due to rotation at this 
  point. When the Mach number of the tip 
  section of the advancing blade exceeds the 
  critical Mach number for the rotor blade sec- 
  tion, compressibility effects result. The criti- 
  cal Mach number is reduced by thick, highly 
  cambered airfoils and critical Mach number 
  decreases with increased lift coefficient. Most 
  helicopter blades have symmetrical sections 
  and therefore have relatively high critical 
  Mach numbers at low lift coefhcients. Since 
  the principal effects of compressibility are the 
  
  3. High gross weight 
  4. High density altitude 
  5. Accelerated flight, high load factor 
  6. Flight through turbulent air or gusts- 
  sharp updrafts result in temporary 
  increase in blade angle of attack 
  7. Excessive ot abrupt control deflections 
  
  during maneuvers 
  
  Recovery from a stalled condition can be 
  effected only by decreasing the blade angle of 
  attack below the stall angle. This can be 
  accomplished by one or a combination of the 