Flight control design for rotorcraft with variable rotor speed

Flight control design issues for rotorcraft with variable rotor speed are investigated, and new design methodologies are developed to deal with the challenges of variable rotor speed. The benefits of using variable rotor speed for rotorcraft are explored with a rotor speed optimization study using a modified GENHEL model of UH-60A Blackhawk. The optimization results recommend to use the optimal rotor speed in each flight condition to improve the helicopter performance. The efforts are made to accommodate the optimal rotor speed schedule into the flight control system design and also address the stability issues due to the rotor speed variation.;The rotor speed optimization results show significant performance improvements can be achieved with moderate reductions in rotor speed. 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 speed degrees of freedom. Rotor speed 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 effects of reduced rotor speeds on stability margins, torque required, and stability issues are also studied.;A power command system in the vertical axis is designed to incorporate variable rotor speed while handling torque limits and other constraints. The vertical axis controller uses a fixed nonlinear mapping to find the combination of collective pitch and rotor speed to optimize performance in level flight, climbing/descending flight, and steady turns. In this scheme, the controller is open-loop, making it an inexpensive and reliable solution. The mapping is designed to produce a desired power level for a given pilot input. Thus the mapping can take into account the performance limits associated with the vertical axis such as power limits, torque limits, and maximum rate of descent. A model following controller is implemented for the pitch, roll, and yaw axes. The piloted simulation was performed to evaluate the controller.;The impact of variable rotor speed on closed loop stability of rotorcraft is discussed. The model following controller can provide high bandwidth control and improve performance using variable rotor speed. However, reduction of rotor speed can result in rotor body coupling and even instability. A rotor state feedback control law can be designed independently and easily integrated into the baseline model following control architecture. Simulations were performed to verify the effectiveness of RSF control to stabilize the rotorcraft dynamics or improve the command tracking performance in the presence of reduced rotor RPM.