Robust Approach for Ground Resonance Aeroservoelastic Stability of Helicopters

This paper discusses the use of robust analysis for the sizing of critical rotorcraft components like lead-lag dampers taking into account several types of uncertainties in the vehicle model without undue simplifications. The problem is reformulated as a feedback loop, with the uncertain part of the system is isolated in the return path. The so-called μ-analysis is used to determine whether a prescribed set of uncertain parameters, whose value is bounded, guarantees the stability of the system. The approach is applied to a simplified ground-resonancemodel first, made of the main rotor cyclic lead-lag multiblade coordinates and the airframe roll mode, and to a complete and detailed aeroservoelastic model of the helicopter. The feasible space of stiffness and damping characteristics of linear viscoelastic lead-lag dampers applied to a certain model is computed first. Subsequently, using the μ-analysis, the stability of the system for damper parameters within a design subspace of the feasible space is verified for a given range of uncertainty on other system parameters, including the modal parameters of the roll mode and some properties of a simplified stability and control augmentation system.