Autorotation

The autorotation ability for a helicopter is one of the features that separates rotary wing from fixed wing aircraft. It is a process of producing lift with the rotor blades, without engine power, as they freely rotates from the air that flows through the rotor system.

In case of an engine failure the rotor system will disengage from the engine, allowing the rotor move freely in its original direction and produce lift, which allows the helicopter to glide.
During normal operations with the engine power engaged, the airflow through the rotor is downward. During an autorotation the flow of the air is upward through the rotor. It is because of this upward flow of air, the rotor will continue to turn.
During autorotation the most important factor is that the rotor RPM is controlled and held within the limits. When the aerodynamic forces of thrust and drag equalize on the rotor blades the RPM of the rotor will stabilize. In case of an sudden upward flow of air through the rotor, the RPM would increase and the pilot would have to compensate with a higher angle of attack on the rotor blades, which in turn will cause a higher drag to decrease rotor RPM. The opposite will happen if the rotor is caught in a downdraft, the pilot decrease the angle of attack on the rotor blades, tending to accelerate the rotor back to normal speed.
Autorotation is very dependent of forward speed. The actual height in which successful autorotation can be performed is directly dependent of the forward speed of the helicopter. An autorotation may be successfully completed at a 300 foot hover, but it would not be possible at a 100 foot hover due to inertia of the blade. At a height of 300 feet the blades will carry enough inertia to allow autorotation. Most helicopters have an area between approximately 12 feet to 300 feet where an autorotation cannot be performed safely without forward speed. It should be noted that autorotation characteristics are also affected by the same elements that alter flight performance, such as density, altitude and weight.