Improved finite element modeling of helicopter airframe connections

Previous work has described an experimental investigation aimed at identifying the physical mechanisms and modeling errors that are the culprits for the lack of correlation between experimental measurements and analytical predictions of the modal parameters of helicopter airframes. Large discrepancies were observed between measured and predicted natural frequencies that can be primarily assigned to the modeling 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 which stiffnesses are determined experimentally. In the proposed finite element model, the joint compliance is represented by beams with a circular cross-section which diameter is evaluated from the measured joint stiffnesses. The frequencies predicted by this proposed numerical model were compared with the measured frequencies for panels with various types of joints and improved correlation was found when compared with current industry modeling practices