 A hydraulically actuated clutch and a transmission are provided to permit the engine to drive the rotor briefly while on the ground to obtain the rotor speed necessary for takeoff.The clutch is released immediately prior to takeoff, and the rotor is not engine-driven in flight.The rotor can be pre-spun up to 370 rpm on the ground with the blades in flat pitch to store energy for take-off (flight rpm typically being in the low 200s).