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

The ground vibration test (GVT) is an important phase in a new aircraft development program, or the structural modification of a certified aircraft, to experimentally determine the structural vibrational modes of the aircraft and their modal parameters. These modal parameters are used to validate and correlate the dynamic finite element model of the aircraft to predict potential structural instabilities (such as flutter), assessing the significance of modifications to research vehicles by comparing the modal data before and after the modification and helping to resolve in-flight anomalies. Due to the high cost and the extensive preparations of such tests, a new method of vibration testing called the taxi vibration test (TVT) rooted in operational modal analysis (OMA) was recently proposed and investigated as an alternative method to conventional GVT. In this investigation, an experimental setup was constructed to further investigate the applicability of the TVT to flexible airframes encountered in fixed-wing autonomous aerial systems with oleo-pneumatic shock absorber landing gears in a tricycle configuration. The influence of the taxiing speed and the landing gear–shock absorber damping setting on the outcome of the TVT is also investigated. The taxiing speed was found to strongly influence the success of the test with an optimal taxiing speed existing for the assembled airframe that allows for the best TVT outcome. The shock absorber damping setting was found to increase the level of the airframe excitation during the TVT; however, it did not impact its outcome as compared to the taxiing speed. Certain modes were not identified during the TVT tests, which was attributed to the way the assembled aircraft is secured to the moving belt and to the lack of sufficient excitation through taxiing. The experimental vibrational modes were successfully matched against the modes obtained from the normal modal analysis of an uncorrelated flexible multibody dynamics model. Further investigations are suggested before the TVT method can be deemed suitable for all classes of fixed-wing aerial systems.