Swashplateless Control of a Rotary-Wing UAV using Variable RPM and a Movable CG

This paper presents a new method for swashplateless control of a rotary-wing UAV. In place of the rotor collective and cyclic pitch controls in conventional rotorcraft, rotor RPM and CG travel in the longitudinal and lateral directions are used to trim the aircraft over the range of operational flight speeds. The CG can be moved, for example, by placing the fuel tank or payload on tracks and using an actuator to move them in the fore-aft and lateral directions. The rotor is pitch-indexed, so the RPM variation required is limited enough to be achieved through an engine controller. Results show that using this swashplateless control strategy, the CG is required to move forward as the flight speed increases. The forward movement of the CG creates a nose-down pitching moment, and puts the aircraft in a nose-down attitude which tilts the rotor thrust vector forward to provide the required propulsive force. In the absence of longitudinal cyclic pitch, the rotor tip path plane actually tilts back slightly in the wind, relative to the rotor shaft. The forward CG travel required and the large nose-down pitch attitude at high cruise speeds (vis-à-vis a conventionally controlled helicopter with swashplate) can be reduced by introducing a built-in forward shaft tilt. The required forward CG travel at high speeds can also be reduced substantially by setting the horizontal tail at a nose-up angle of attack relative to the waterline, to produce an upward lift force and a nose-down pitching moment on the aircraft. However, the horizontal tail (without the built-in forward shaft tilt) does not reduce the large nose-down pitch attitude.