A Methodology for Evaluation of Coupled Rotor-Body Stability using Reduced Order Linear Time Invariant (LTI) Models

Linear Time Invariant (LTI) models comprising of body and harmonic components of rotor states are seen to provide a convenient framework for exploring integrated flight and rotor control. While stability properties of such LTI model formulations converge to those of linear time periodic models as the number of harmonic components included is progressively increased, a method is needed to assess the fidelity of stability properties captured by LTI formulations with limited number of harmonic components of rotor states. A methodology for assessing the fidelity of reduced order LTI model formulations is presented in this paper. The methodology is evaluated using two types of isolated articulated rotor flapping dynamics models and a generic helicopter model with body, rotor flap and leadlag, and inflow dynamics.