An Analytical and Experimental Study of a Scaled Helicopter Tailcone

This paper presents results from an ongoing investigation to understand and quantify the limitations of traditional industry methods for vibration prediction in rotorcraft airframes. As part of this work a scaled model representative of a helicopter tailcone was constructed in order to provide validation data for comparison with finite-element vibration models. The experimental investigation of a scaled model, as opposed to flight hardware, was intended to eliminate some of the uncertainty associated with a manufactured airframe and to determine what errors are inherent to modeling stiffened-skin structures. First, the mode shapes and frequencies for the Scaled Rotorcraft Tailcone Model (SRTM) were obtained without the skin to verify the modeling of the ring and stringer substructure. The experimental results initially showed good correlation of the mode shapes when compared to ABAQUS finiteelement modal analysis results, but the modal frequencies were significantly overpredicted. These results demonstrated the necessity of careful modeling of the joints and bolted connections. The finite-element models were changed to include joints that more closely replicated the physical connections and experimentally measured material properties. This resulted in excellent correlation between the measured and predicted modal frequencies, with errors reduced to less than 2%. The skin was then bonded to the substructure, and mode shapes and frequencies were experimentally measured. Finite-element modal analysis was performed by using both a traditional low-fidelity model and by using a high-fidelity model with over one million degrees-of-freedom. Although the low-fidelity model captured the beam modes with reasonable accuracy, modes with localized deformation were poorly predicted. The high-fidelity model showed fair accuracy for all modes, but inaccuracy due to the modeling of the boundary conditions was still evident.