eliminated. This drag variation causes a 
shaking force on the rotor system and con- 
tributes to the vibration of the helicopter. 
RETREATING BLADE STALL. Retreat- 
ing blade stall results whenever the angle of 
attack of the blade exceeds the stall angle of 
attack of the blade section. This condition 
occurs in high speed flight at the tip of the 
retreating blade since, in order to develop the 
same lift as the advancing blade, the retreating