An experimental study of hierarchical autopilot for untrimmed hingeless helicopters

Different from previous works that require prior trim conditions on the helicopter, this paper proposes a hierarchical PD controller that is robust in controlling untrimmed and therefore critically unstable helicopters. This controller can yield asymptotic stability of the helicopter in horizontal motion control, which can be proven by the linear stability analysis. And this controller can flawlessly engage with traditional dual loop autopilot by using auto-varying references in an inner stabilizing loop. Moreover, to facilitate the controller design, this paper derives the dynamics of hingeless helicopters with an emphasis on gyroscopic effect. Finally, the stability and superior performance of the proposed controller are empirically demonstrated on an instrumented JR Voyager GSR helicopter.