autorotation: one construct, many systems

In plant biomechanics, autorotation is the way a single-winged fruit or seed (a samara, like a maple key) begins to spin about a vertical axis as it falls, the rotation generating lift that slows its descent and lengthens the time the wind has to carry it. Norberg's Autorotation, self-stability, and structure of single-winged fruits and seeds (samaras) analysed how the seed's mass distribution and wing shape make the spin self-stable and set its descent speed. The referent is a passive, self-sustaining spin of a falling winged body that reduces sink rate, and it is the same aerodynamic construct a helicopter relies on in an engine-out descent.[1]

Unit

The self-sustaining spin rate and the descent (sink) speed it produces

Recovery means

Not applicable

Measured by

Characterised by rotation rate, descent velocity, and the resulting dispersal distance of the falling seed

In rotorcraft aerodynamics, autorotation is the state in which a helicopter rotor, with engine power removed, is kept turning by the upward airflow through the disc during descent, so that the rotor extracts energy from the descent to sustain its own rotation and still produce enough lift for a controlled landing. Leishman's Principles of helicopter aerodynamics treats the autorotative descent, the energy balance across the disc, and the flare that converts stored rotor energy into a soft touchdown. It is the identical construct as a spinning samara (a free wing driven to rotate by the air it falls through), realised in an engineered rotor and measured by rotor speed and sink rate.[2]

Unit

The equilibrium rotor speed and sink rate sustained with no engine power

Recovery means

Not applicable

Measured by

Tracked through the height–velocity diagram, steady-state rotor RPM, and descent rate in power-off flight