Evaluation of Control Allocation Techniques for a Medium Lift Tilt-Rotor

In a traditional single main rotor helicopter, only 4 controls are available - lateral cyclic, longitudinal cyclic, main rotor collective pitch and tail rotor collective pitch. As such, redundancy is not available as all four controls are needed to maneuver the aircraft in the four control axes - pitch, roll, yaw and heave. In contrast, high speed rotorcraft configurations (such as tilt-rotors and compound helicopters) have multiple redundant control effectors. The strategy for applying these control effectors, and in which proportion, is called 'control allocation'. This paper provides an evaluation and comparison of different control allocation methods, applied to a medium lift tilt rotor. The control allocation methods are evaluated based on a comprehensive set of time and frequency domain metrics, as well as robustness criteria. In addition to evaluation of standard control allocation methods, several control allocation methods are further developed and evaluated for reducing flap motion of the rotors. A piloted fixed base simulation is performed, and pilot comments and data are presented. The paper concludes with discussion of the results and trade-offs of the most effective control allocation techniques.