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Retreating blade stall is a hazardous flight condition in helicopters and other rotary wing aircraft, where the retreating rotor blade has a lower relative blade speed, combined with an increased angle of attack, causing a stall and loss of lift. Retreating blade stall is the primary limiting factor of a helicopter's never exceed speed, V NE. [1]
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).
This includes but its not limited to: Dynamic rollover, Ground resonance, Loss of tail-rotor effectiveness, Retreating blade stall,Dynamic stall, Vortex ring state, Servo transparency, Mast bumping, and Tailstrike. Because the main rotor is vital to keeping a helicopter in the air, any damage to can have disastrous consequences.
[1]: 2–14 Rotor blades are designed to flap: the advancing blade flaps up and develops a smaller angle of attack due to a change in relative wind vectors, thus producing less lift than a rigid blade would. Conversely, the retreating blade flaps down, develops a higher angle of attack due to a change in relative wind vectors, and generates ...
In a stationary hover, each rotor blade will experience the same airspeed at a constant RPM. In forward flight conditions, one rotor blade will be moving into the oncoming air stream while the other moves away from it. At certain airspeeds, this can create a dangerous condition in which the receding rotor blade stalls, causing unstable flight. [5]
The sizes of these regions vary with the blade pitch, rate of descent, and rotor rotational speed. When changing autorotative rotational speed, blade pitch, or rate of descent, the sizes of the regions change in relation to each other. The driven region, also called the propeller region, is the region at the end of the blades.
Single rotor helicopters are limited in forward speed by a combination of sonic tip speed and retreating blade stall.As the advance ratio increases, the relative velocity experienced by the retreating blade decreases so that the tip of the blade experiences zero velocity at an advance ratio of one.
Ground resonance is an imbalance in the rotation of a helicopter rotor when the blades become bunched up on one side of their rotational plane and cause an oscillation in phase with the frequency of the rocking of the helicopter on its landing gear.