Definition
The portion of a helicopter rotor disc, during a power-off descent, in which the relative wind striking the rotor blades produces an aerodynamic force with a forward (driving) component. This forward component keeps the rotor turning without engine power. The autorotation region lies between the inner stall region (near the hub) and the outer driven region (near the blade tips), and typically covers roughly the middle portion of each blade's span.
Plain English
It is the part of a helicopter's spinning rotor that keeps the blades turning when the engine has quit. As the helicopter falls, air flowing up through this part of the rotor pushes the blades around, like wind turning a pinwheel.
Context Anchor
Seen in helicopter aerodynamics, especially when learning engine-out descents, autorotation, and rotor speed control.
Derivation
Autorotation comes from the Greek autos meaning 'self' and the Latin rotare meaning 'to turn.' Literally 'self-turning' — the rotor keeps spinning on its own, powered by the air flowing through it rather than by the engine.
Why Pilots Care
It tells the pilot whether an engine failure at the current height and speed will allow a survivable landing or result in an uncontrolled impact.
Grounding Statement
Picture the rotor disc divided into three rings: the inner ring is stalled, the middle ring drives the rotor, and the outer ring is driven by it. The middle ring is the autorotation region.
Intuition Check
Do not think of the autorotation region as the whole rotor blade. It is one working area of the blade; other areas may be slowing the blade or may not be producing useful driving force.
Example Sentence 1
After the engine failed, the instructor lowered the collective immediately so airflow through the autorotation region would keep the rotor RPM in the green.
Example Sentence 2
At 200 feet and 30 knots the aircraft sat outside the autorotation region, so the instructor climbed to gain a safer margin.