Computational Investigation of a Flexible Airframe Taxiing Over an Uneven Runway for Aircraft Vibration Testing

Ground vibration testing (GVT) is an important phase of the development, or the structural modification of an aircraft program. The modes of vibration and their associated parameters extracted from the GVT are used to modify the structural model of the aircraft to make more reliable dynamics predictions to satisfy certification authorities. Due to the high cost and the extensive preparations for such tests, a new method of vibration testing called taxi vibration testing (TVT) rooted in operational modal analysis (OMA) was recently proposed and investigated by the German Institute for Aerospace Research (DLR) as alternative to conventional GVT. In this investigation, a computational framework based on fully coupled flexible multibody dynamics for TVT is presented to further investigate the applicability of the TVT to flexible airframes. The time domain decomposition (TDD) method for OMA was used to postprocess the response of the airframe during a TVT. The framework was then used to examine the impact of the taxiing speed, shock absorber damping coefficient, and bump geometry on the outcome of the computational TVT. It was found that higher taxiing speed does not necessarily mean a better quality TVT, and one must find the optimal speed using the computational framework presented herein. A higher shock absorber damping coefficient was found to increase the amplitude of the response during the TVT without significantly impacting the extracted modes and their frequencies. Also, the quality of the TVT was found to be inversely proportional to the curvature of the bump cross section. The proposed TVT computational framework is validated against the normal modal analysis technique and certain experimental data.