CORRELATION OF PREDICTED VIBRATIONS AND TEST DATA FOR A WIND TUNNEL HELICOPTER MODEL

This paper presents the results of a limited correlation study which was conducted by applying a vibration analysis to a model helicopter rotor fuselage system. The vibration analysis uses a substructure method. The analysis was developed to serve as a predictive and design tool. Features of the analysis used in this application are an aeroelastic rotor, rotor induced variable inflow, a modal model of an elastic fuselage, and higher harmonic control applied to the rotor system to reduce fuselage vibrations. Results were compared with fuselage vibrations measured on a flexible tail boom and blade moment data for a wind tunnel model employing a one-sixth scale UH-60A rotor. The rotor was dynamically scaled and was operated at full scale tip Mach number. The test program was conducted in the NASA Langley Transonic Dynamics Tunnel. It was found that the analysis was able to predict the proper trends of variations of vibrations with advance ratio. The proper trends in vibrations were predicted with application of optimal values of higher harmonic control. Absolute values of predicted vibrations were sensitive to airframe modal characteristics and were not uniformly consistent with measured values. The study should be viewed as a first attempt to validate and understand a helicopter vibration analysis. A definite conclusion on the absolute predictive ability of the analysis is not made because of significant uncertainties concerning the accuracy of the modal attributes of the fuselage and rotor.