Definition
The act of spinning a gyroplane's main rotor up to operating speed on the ground, before takeoff, using a mechanical drive from the engine. Once the rotor reaches the required RPM, the drive is disengaged and the rotor turns freely (autorotates) in flight, supplying lift as the aircraft moves forward.
Plain English
Getting the rotor spinning fast enough on the ground so the gyroplane can take off in a short distance. The engine briefly drives the rotor, then releases it to spin on its own once airborne.
Context Anchor
Encountered in gyroplane takeoff procedures and discussions of rotor speed before liftoff.
Derivation
From 'pre-' (before) and 'rotation' (spinning). The name describes exactly what it does: it rotates the rotor before flight begins.
Why Pilots Care
Adequate rotor speed is required for immediate lift generation in autorotation; insufficient prerotation lengthens the takeoff roll and raises the risk of blade flapping or stall.
Intuition Check
Prerotation does not mean the aircraft is already ready to fly. It means the rotor has been spun up before the takeoff roll begins.
Example Sentence 1
After lining up on the runway, the pilot engaged prerotation until the rotor reached takeoff RPM.
Example Sentence 2
After a short pause for prerotation, the gyroplane lifted off cleanly once forward speed was added.