IMPROVING MODAL PARAMETER PREDICTIONS FOR JOINTED AIRFRAME PANELS - PART II - IMPROVED NUMERICAL-MODEL

A companion paper has described an experimental investigation aimed at identifying the physical mechanisms and modelling errors that are the culprits for the lack of correlation between experimental measurement s and analytical predictions of the modal parameters of helicopter air frames. Large discrepancies were observed between measured and predict ed natural frequencies that can be primarily assigned to the modelling errors associated with current industry practices. The present paper focuses on the development of simple, improved numerical models based on a lumped parameter approach which accounts for joint flexibilities by a set of concentrated Linear and torsional springs whose stiffnesse s are determined experimentally. In the proposed finite element model, the joint compliance is represented by beams of a circular cross-sect ion whose diameter is evaluated from the measured joint stiffnesses. T he frequencies predicted by this proposed numerical model were compare d with the measured frequencies for panels with various types of joint s and improved correlation was found when compared with current indust ry modelling practices.