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When is retreating blade stall most likely to occur?

  • A
    At the vortex ring, (sometimes called 'settling with power').
  • B
    At high forward speed, high AUM, high altitude, temperature and humidity.
  • C
    At low forward speed with high AUM.
  • D
    At high AUM.

Refer to figure.
Due to the dissymmetry of lift, the retreating blade must operate at a higher angle of attack to provide a balanced level of thrust across the disc. The retreating blade's angle of attack will be greatest when the blade is half way round on the retreating side. As forward speed increases further, the angle of attack of the retreating blade increases.

The retreating blade reaches maximum angle of attack and airflow separation occurs due to excessive flapping. Perhaps surprisingly, the stall begins not at the root but at the blade tip.

A good way to remember this is to think. More forward airspeed -> more dissymmetry of lift -> more flapping -> higher angle of attack -> stall.

Factors affecting the retreating blade stall are:

  • High Gross Weight
  • High forward airspeed (Vne)
  • Steep turns
  • Turbulence
  • High Density Altitude, temperature and humidity
  • Low Rotor RPM

During the retreating blade stall the following occurs:

  • Airflow separation
  • Turbulent pattern
  • Loss of effective lift area

Retreating blade stall is recognised by:

  • Low frequency vibrations
  • Erratic cyclic forces
  • Flap back
  • Roll to the retreating blade

Retreating blade stall recovery is achieved by:

  • Lower collective (lowering blade pitch and reducing of angle of attack)
  • Reduce forward airspeed
  • RPM top of the green

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