US Patent 8020803 Rotorcraft engine and rotor speed synchronization

Systems and methods are provided in which an electrical control system independently effects acceleration of both driven and driving elements of a clutch to engage each other. In preferred embodiments the clutch is not a friction clutch, but a dog clutch, and forms part of a drive drain of a rotorcraft. A second clutch can be used, along with a mechanical interlock to prevent simultaneous engagement of the clutches. Speeds of the driven and driving elements can be sensed, and altered using at least one of a rotor, a brake, a generator, an electric motor, and a combustion motor. In rotorcraft embodiments, the gearbox can have a neutral condition in which no power is transmitted from the engine to the rotor.