Aeroelastic stability analysis of damaged helicopter rotor system

The effects of rotor blade ballistic damages on helicopter airworthiness and blade and hub conditions are investigated using a UMARC helicopter aeroelastic analysis code. A finite element formulation based on Hamilton's principle is used for structural analysis, and aerodynamic loads are calculated using quasisteady aerodynamic theory. Each blade is treated as being composed of elastic beams undergoing flap bending, lag bending, elastic twist, and axial deflections. Dynamic responses of multi-bladed rotor systems are calculated from nonlinear periodic normal mode equations using a finite element in time scheme. Results are calculated for a soft in-plane articulated rotor helicopter for both undamaged and damaged blade configurations. Blade damage effects are determined i n terms of blade mode shapes and frequencies, aeroelastic response and rotor hub loads. Blade dissimilarity due to ballistic damage can induce a large l/rev vibratory conlponent on the rotor hub.