Autorotation:
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An autorotation is the exchange of altitude for energy in order to maintain the rotor RPM within a safe margin to allow for a controlled decent and landing. It is an exercise in converting potential energy into kinetic energy. The potential energy is stored in the altitude of the helicopter above the ground. This energy is converted into kinetic energy as the helicopter descends. As altitude decreases, airflow through the rotor disc increases creating kinetic energy to overcome blade drag and turn the rotor blades.
When a rotor disc is in a state of autorotation it can be divided into three regions: driven region, driving region, and stall region.
Driven Region: The total aerodynamic force is aft of the axis of rotation resulting in the generation of both lift and drag.
Driving Region: The total aerodynamic force is forward of the axis of rotation causing the rotor disc accelerate.
Stall Region: The angle of attack of the rotor disc has exceed its ability to produce lift causing drag.
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