Helicopter Ground Resonance Investigation with Dissimilar Nonlinear Lead-Lag Dampers

This work describes the analysis of helicopter ground resonance when nonlinearity and non-isotropy of the problem are taken into account. Ground resonance is a dynamic instability caused by the interaction between the rotor and the airframe of a helicopter. Sources of nonlinearity can be geometrical (finite blade lead-lag motion) and constitutive (hydraulic lead-lag dampers and shock absorbers). Standard methods use special coordinate transformations that make it possible to cast the problem in linear, time invariant form when considering small oscillations of an isotropic rotor about a reference solution. However, potential non-isotropy of the rotor (e.g.\ resulting from degraded performance of lead-lag dampers) may turn the problem into linear, time periodic. In such cases, the Floquet-Lyapunov method is normally used to study the stability of the coupled system. In this work the problem is investigated using Lyapunov Characteristic Exponents (LCE). The analysis shows that in some cases, characterized by a marked contribution of the nonlinearity of the blade lead-lag dampers, the problem assumes a nearly chaotic behavior. The stability of the system is investigated, and the sensitivity of the LCEs with respect to system parameters is determined, in an attempt to provide a consistent analysis framework and useful design guidelines.