Disturbance Observer-Based Control to Suppress Air Resonance for the EC135 ACT/FHS Research Helicopter

Abstract

Increased safety and reduced workload are two main goals for helicopter pilot assistance systems. Feedback control is one possible mean to get towards these goals. In conjunction with hinge- and bearingless main rotor concepts further improvement of the handling qualities are expected. But this rotor concept suffers from air resonance: a self-excited oscillation caused by the coupling of the rotor blades' (regressive) lead-lag especially with body roll motion. Excellent handling quality ratings may require aggressive maneuvering with high agility that is often realized using relatively large feedback gains. Especially the roll rate feedback gain was found to excite air resonance. This paper shows a possible solution consisting in suppressing air resonance by means of a disturbance observer-based control. That allows high-agility maneuvering flight with excellent handling qualities. Three major topics are discussed: The state-space model derived by system identification, the design of the disturbance observer and compensation, and the testing with a highly modified research helicopter that is called ACT/FHS (Active Control Technology / Flying Helicopter Simulator). The results are compared to previously derived and flight-tested filters.