|Click to Enlarge|
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.