Variable Speed Transmission Using a Planetary Gear System for High Speed Rotorcraft Application

High-speed helicopter flight poses many challenges to designers. Many of these challenges result from the additive nature of the helicopter speed and the rotor rotation on the helicopter's main rotor advancing side. These high speeds result in increased wave drag, impulse noise and vibrations. These disadvantages can be lessened by slowing the main rotor during high speed flight. This paper presents a transmission design that slows the main rotor by up to 50% during high-speed forward flight while maintaining high rotational speed during low-speed flight and hover and maintaining engine efficiency. The transmission design uses a standard planetary gear system with the sun gear as the input and the planetary carrier as the output. It accomplishes its speed variation by allowing the ring gear to rotate at variable speeds controlled by a controller and an electric motor. This design was demonstrated on a theoretical coaxial helicopter. This paper also covers the sizing and synthesis methodology used to perform preliminary design of this transmission.