Ground Vibration Testing and Finite Element Model Updating of Very Flexible Aircraft

Abstract

As aircraft wings become more flexible as a consequence of searching for more fuel efficient and higher performance solutions, structural nonlinearities become more apparent. Geometric nonlinearities make the structure’s modal parameters a function of the deformed shape and, therefore, of the loading condition. Modal characterization of very flexible structures is challenging due to these nonlinearities and the very low natural frequencies (the fundamental mode is typically below 1 Hz). Therefore, testing practices that are well established and mature for traditional, relatively rigid aircraft need to be re-examined.

Ground vibration testing consists of conducting a series of experimental tests on an aircraft to identify its modal parameters (natural frequencies, mode shapes, and damping ratios). These parameters are then used as a reference for updating the aircraft’s finite element model such that it is dynamically representative of the actual vehicle. This work proposes examining the conventional ground vibration testing and finite element model updating processes for very flexible aircraft and identifying the impact of flexibility and geometric nonlinearities on the process. The modified ground vibration testing and finite element model updating methodology are then applied to and verified on numerical models of very flexible test cases. Results are provided to showcase the applicability of the model updating process on the test cases, along with a discussion of the limitations and additional considerations required to enhance the process.

As part of ensuring compatibility with current/conventional processes for finite element model updating, the finite element model updating technique developed in this work was demonstrated using experimental data from a 3D printed swept wing that does not exhibit large deformations (i.e., is not very flexible). Tests were conducted on the structure to characterize its as-built behavior and then its numerical model was updated using the model updating methodology developed in this work to better match the experimental results. The model updating process was modified to account for uncertainties coming from the 3D printing process as well as to consider the effects of nonlinear boundary conditions, which yield a different set of challenges than the effects of flexibility, and highlight some of the limitations of the process.

By properly characterizing the suspension or support system in the structure’s numerical model, experimental results from various deformed conditions allow for recovering the structure's true modal parameters once the influence of the suspension is modeled, tuned, and removed from the finite element model. This ground vibration testing and finite element model updating methodology was demonstrated experimentally on a very flexible beam and validated on a very flexible aircraft. Results from the finite element model updating process indicate that the new methodology proposed and verified as part of this work improves upon the established finite element model updating technologies when considering very flexible structures undergoing loads causing large displacements.