Taxi vibration testing: A new and time efficient procedure for the identification of modal parameters on aircrafts

Kurzfassung

In order for aircraft prototypes to perform first flight, a flutter clearance to guarantee aeroelastic stability has a fundamental role in the required certification process. The Ground Vibration Test (GVT) is the standard means to determine the dynamic characteristics of the structure which are subsequently used to update numerical models for the flutter clearance. The GVT must be performed at a time critical phase of an aircraft’s development and has therefore been significantly improved and optimized over the last 20 years. In order to achieve further reduction of testing time a completely new philosophy for testing was needed. This resulted in the Taxi Vibration Test (TVT) which uses advanced methods to process output-only data during aircraft taxi to identify modal parameters. The method was developed at DLR Göttingen and has gone through a process of maturation including model and full scale investigations. In this work the final maturation of the method as a viable alternative for efficient certification is demonstrated on data from the A340-600 Research GVT performed in cooperation with Airbus and ONERA in the year 2011. A comprehensive set of modal parameters were identified using the Stochastic Subspace Identification (SSI) method. The influence of the measurement duration on the uncertainty of the modal parameters was investigated statistically. In a TVT, the landing gears are involved in the vibration. These are known to be non-linear, mainly because of the slip-stick type of friction non-linearity in the shock absorbers. Therefore, the importance of the relationship between the taxi speed, which is related to the unknown excitation force, and the modal parameters was identified. The modal acceleration spectra were used as a proxy to quantify this non-linearity in order to provide a meaning comparison between GVT and TVT. Trends of decreasing frequency and increasing damping with increasing speed were observed, which is in agreement with friction type non-linearity. The influence of the landing gear to increase damping and reduce frequency of specific modes was also identified and discussed. The comparison between GVT and TVT confirm this force dependency of the modal parameters. The modal parameters identified from SSI show significantly improved damping estimates and trends, increasing the confidence in the TVT method as a viable alternative for optimal aircraft certification.