Variable speed rotors will give helicopters several advantages: higher top speed, greater fuel efficiency, momentary emergency over-power, resonance detuning capability and a reduction in noise. The applicant proposes to develop such a system using a doubly-fed planetary gear box with advanced SiC-based power electronics and motors. The new components enabling variable speed will be rated at a fraction of the total helicopter power rating, thus reducing weight dramatically. The resulting hybrid mechanical/electrical power system also gives, in addition to greater fuel efficiency, smooth continuous speed variations with electronics, inherent starting capability (e.g., elimination of the existing starter motor), simplification of the tail rotor system, and the ability to enhance power to the main rotor with batteries and the APU. Feasibility will be determined in Phase I by the applicant with the assistance of a major helicopter manufacturer.