Closed-Loop Aeromechanical Stability of Hingeless Rotor Helicopters with Higher Harmonic Control

The development of a state-space formulation for a multi-input/multi-output (MIMO) higher-harmonic-control (HHC) system is described. Results are also presented of a numerical investigation into closed-loop performance and stability of an HHC system, implemented in the rotating system, based on the simulation of a hingeless rotor helicopter. The results show that the HHC controller reduces the 4/rev accelerations at the center of gravity. The percentage reductions obtained in the simulations are in excess of 80-90%. The vibration attenuation occurs within 5-7 s after the HHC system is turned on. This is equivalent to a frequency of around 1 rad/s, where flight control systems and human pilots tend to operate. Therefore, interactions and potential adverse effects on the stability and control characteristics of the helicopter should be explored. The HHC problem is intrinsically time periodic if the HHC inputs include frequencies other than the frequency one wishes to attenuate. This is true even if the rest of the model is assumed to be time invariant. In these cases, the closed-loop stability results obtained using constant coefficient approximations can be incorrect even at lower values of the advance ratio m, where constant coefficient approximations of the open-loop dynamics are accurate.