Retreating Blade Stall[PDF]
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.
Advancing and Retreating Rotor Blades
A rotor blade that is moving in the same direction as the aircraft is called the advancing blade and the blade moving in the opposite direction is called the retreating blade.
Effect of Increasing Airspeed
In forward flight, the relative airflow in the advancing side of the main rotor is faster than the retreating side. In the advancing side, forward speed is added to the rotor speed and in the retreating side, forward speed is subtracted from rotor speed.
Rotor blades are designed to compensate this lift asymmetry by freely moving up and down, this movement is also called flapping.
Symptoms of Retreating Blade Stall
The symptoms are:
- Rotor roughness and vibration
- Nose pitch up movement
- The helicopter may roll to the left or right depending on the rotor rotation direction
The contributing factors in addition to high forward speed are:
- High weight
- Low rotor rpm
- High density altitude
- Steep, abrupt turns
When pilots are faced with a retreating blade stall, they will notice a nose up attitude accompanied with rotor roughness and vibration.
- VID 522050 - Creation
DATE OF SUBMISSION
- 12:46, 23 February 2021
- This documentation is copyrighted as part of the intellectual property of the International Virtual Aviation Organisation.
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