Extracting Scaled Fuselage Mode Shapes from Operational Modal Analysis of Helicopter Flight Data

In this work the authors present a method for estimating the modal properties (natural frequencies, damping and mode shapes) of a helicopter fuselage using available flight data. The identification process is based on Operational Modal Analysis (OMA). The modal parameters obtained from OMA are subsequently used in conjunction with Ground Vibration Testing (GVT) results and the Finite Element Model (FEM) of the helicopter structure to extract scaled mode shapes. Since in-flight vibration testing typically employs a limited number of measuring points, significantly less than the number of points represented in the FEM, the authors propose a method of expanding the OMA identified mode shapes onto the FE model geometry, enabling engineers to regulate the influence of local displacement and achieve improved representation of the structural dynamics.