The dynamic stall is one of the hazardous phenomena on helicopter rotors, which can cause the onset of large torsional airloads and vibrations on the rotor blades.[1][2] Unlike fixed-wing aircraft, of which the stall occurs at relatively low flight speed, the dynamic stall on a helicopter rotor emerges at high airspeeds or/and during manoeuvres with high load factors of helicopters, when the angle of attack(AOA) of blade elements varies intensively due to time-dependent blade flapping, cyclic pitch and wake inflow. For example, during forward flight at the velocity close to VNE, velocity, never exceed, the advancing and retreating blades almost reach their operation limits whereas flows are still attached to the blade surfaces. That is, the advancing blades operate at high Mach numbers so low values of AOA is needed but shock-induced flow separation may happen, while the retreating blade operates at much lower Mach numbers but the high values of AoA result in the stall (also see advancing blade compressibility and retreating blade stall).
^Tarzanin, F. J. (1972-04-01). "Prediction of Control Loads Due to Blade Stall". Journal of the American Helicopter Society. 17 (2): 33–46. doi:10.4050/JAHS.17.33.
^McCroskey, W. J.; Fisher, Richard K. (1972-01-01). "Detailed Aerodynamic Measurements on a Model Rotor in the Blade Stall Regime". Journal of the American Helicopter Society. 17 (1): 20–30. doi:10.4050/JAHS.17.1.20.
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