Correlation of Finite-Element Structural Dynamic Analysis with Measured Free Vibration Characteristics for a Full-Scale Helicopter Fuselage

Both the Sikorsky Finite-Element Airframe Vibration Analysis Program (FRAN/Vibration Analysis) and the NASA Structural Analysis Program (NASTRAN) have been correlated with data taken in full-scale vibration tests of a modified CH-53A helicopter. With these programs the frequencies of fundamental fuselage bending and transmission modes can be predicted to an average accuracy of three percent with corresponding accuracy in system mode shapes. The correlation achieved with each program provides the material for a discussion of modeling techniques developed for general application to finite-element dynamic analyses of helicopter airframes. Included are the selection of static and dynamic degrees of freedom, cockpit structural modeling, and the extent of flexible-frame modeling in the transmission support region and in the vicinity of large cut-outs. The sensitivity of predicted results to these modeling assumptions is discussed.