Hybrid Finite Element Analysis of a Rotorcraft

The Hybrid FEA method is based on combining conventional Finite Element Analysis (FEA) with Energy Finite Element Analysis (EFEA) for mid-frequency computations. The difficulty in using conventional FEA at higher frequencies originates from requiring a very large number of elements in order to capture the flexible wavelength of the panel members which are present in a structure. In the Hybrid FEA the conventional FEA model is modified by de-activating the bending behavior of the flexible panels in the FEA computations and introducing instead a large number of dynamic impedance elements for representing the omitted bending behavior. The excitation is considered to be applied on the conventional FEA model and the vibration analysis is conducted. The power flow through the dynamic impedance elements is computed and applied as excitation to the EFEA model of the flexible panels. The EFEA analysis computes the vibration of the flexible panels. Gearbox excitation is an important contributor to the vibration induced in the fuselage of a helicopter. The frequency of excitation is typically in the 500Hz - 3,000Hz range. The excitation is applied at the highly rigid foundation structure and transmitted to the flexible panels of the fuselage through the frame structure. In order to demonstrate the validity of the Hybrid FEA and its utility for analyzing rotorcraft structures, a production rotorcraft was analyzed using the Hybrid FEA method. Excitation was applied separately through a shaker at two gearbox mount locations in the 250Hz - 3,000Hz frequency range. For each excitation the vibration at approximately thirty frame locations and at approximately ten panels was measured and compared successfully to the simulation results.