Flight Control Design for Rotorcraft with Variable Rotor Speed

A flight control design methodology for rotorcraft with variable rotor speed capability is developed, and preliminary results using the GENHEL simulation model are presented. Variation in trim rotor RPM can be used to optimize performance and reduce power required. A rotor speed optimization study was performed using GENHEL, and the results show significant performance improvements can be achieved with moderate reductions in rotor RPM. The objective of the control design is to accommodate these rotor speed variations, while achieving desired flying qualities and maneuver performance. A gain scheduled model following/model inversion controller is used to control the roll, pitch, yaw, heave, and rotor RPM degrees of freedom. Rotor RPM is treated as a redundant control effector for the heave axis, and different control allocation schemes are investigated. The controllers are evaluated based on step responses and the ADS-33E height response requirement. Results show that dynamic variation in rotor speed can improve maximum climb rate and flying qualities for moderate to large commands in vertical speed, but that non-linear effects present significant challenges when integrating control of the aircraft and the engine. The effect of reduced rotor speeds on stability margins and closed loop stability is also studied.