text: large increase in drag and rearward shift of the 
  airfoil aerodynamic center, compressibility ef- 
  fects on the helicopter increase the power te- 
  quired to maintain rotor RPM and cause rotor 
  roughness, vibration, stick shake, and an un- 
  desirable structural twisting of the blade. 
  
  Since compressibility effects become more 
  severe at higher lift coefficients (higher blade 
  angles of attack) and higher Mach numbers, 
  the following operating conditions represent 
  the most adverse conditions from the stand- 
  point of compressibility: 
  1. High airspeed 
  2. High rotor RPM 
  3. High gross weight 
  4. High density altitude 
  5. Low temperature-the speed of sound 
  is proportional to the square root of 
  the absolute temperature. Therefore, 
  sonic velocity will be more easily 
  obtained at low temperatures when 
  the sonic speed is lower. 
  
  6. Turbulent air-sharp 
  
  gusts momen- 
  increase the blade angle of 
  tarily 
  the critical 
  attack and thus lower 
  Mach number to 
  the point where 
  compressibility effects may be en- 
  countered on the blade. 
  
  Compressibility effects will vanish by de- 
  in 
  creasing the blade pitch. The similarities 
  the critical conditions 
  for retreating blade 
  stall and compressibility should be noticed 
  but one basic difference must be appreciated- 
  compressibility occurs at HIGH RPM while 
  retreating blade stall occurs at LOW RPM. 
  Recovery technique is identical for both with 
  the exception of RPM control. 