Definition
The rotational speed of a helicopter's main rotor system, measured in revolutions per minute. Rotor RPM must be maintained within a specific operating range published by the manufacturer to ensure the rotor produces sufficient lift and remains structurally and aerodynamically safe. During an autorotation, rotor RPM is sustained by airflow moving up through the rotor disc rather than by engine power.
Plain English
How fast the main rotor blades are spinning, counted as full turns per minute. Keeping this speed in the right range is what keeps the helicopter flying safely.
Context Anchor
Seen in helicopter autorotation training, engine-failure procedures, and cockpit checks of the rotor speed indicator.
Derivation
Rotor comes from a Latin word meaning “to turn.” RPM means “revolutions per minute,” or how many full turns happen in one minute. Together, rotor RPM means the turning speed of the rotor.
Why Pilots Care
Proper rotor RPM in autorotation preserves rotor energy needed for a controlled flare and touchdown.
Intuition Check
Do not confuse rotor RPM with engine speed. In an autorotation, the engine may not be driving the rotor, but the rotor must still keep turning fast enough for a safe landing.
Example Sentence 1
After the engine failure, the pilot lowered the collective immediately to keep rotor RPM from decaying.
Example Sentence 2
If rotor RPM drops too low the helicopter loses lift and the flare becomes ineffective.