COUPLED ROTOR FUSELAGE MODE SHAPES - A TOOL IN UNDERSTANDING HELICOPTER RESPONSE

The Westland Coupled Rotor-Fuselage Model (CRFM) is under development, in a collaborative project with the UK Defence Research Agency (DRA), to provide a prediction tool for rotor loads and fuselage vibration in both level and manoeuvring flight, as well as application to stability, performance and handling qualities. Modal degrees of freedom for CRFM are generated in the form of complex coupled rotor-fuselage (CRF) mode shapes, with associated natural frequencies, by the CRFD (Coupled Rotor-Fuselage Dynamics) program. The definition of CRF mode shapes is described, and the method of their application within a response analysis. The requirement for explicit modal orthogonality is examined, and a method is presented for enabling orthogonality conditions to be extracted from the transfer matrix form of the CRFD analysis by re-casting into a dynamic stiffness matrix form. The role of CRF mode shapes as a tool in not only calculation but enhancement of understanding of the rotor-fuselage system response and stability is considered, with examples of calculated mode shapes and their significant characteristics. Results from a correlation exercise are given, for CRF modes calculated by CRFD compared with those from a method using blade modes as intermediate degrees of freedom.