Suppression of Regressive Lead-Lag to Improve Roll Bandwidth in Helicopter Flight Control Systems

Abstract

Helicopter flight has to be safe and the control of the helicopter has to fulfil specific requirements on static and dynamic stability. A typical design goal for the roll axis is to achieve a stable response with a roll bandwidth larger than approx. 2-3 rad/s for midsize rotorcraft. During the first phase of the EC135 ACT/FHS research flight control design, a low-damped roll oscillation at approx. 1.8 Hz or 12 rad/s was observed. This effect was interpreted as air resonance which was excited by the research flight control system. Two different approaches have been flight-tested to suppress this oscillation: an input filter and disturbance feed-forward. The paper presents the low-order equivalent model, the control system, and flight test results. Both approaches showed a good performance in flight while the disturbance feedforward achieves a slightly higher bandwidth and more phase margin