Taxi Vibration Testing - An Alternative Method to Ground Vibration Testing of Large Aircraft

Abstract

Modal parameters such as eigenfrequencies, damping ratios, and mode shapes of aircraft prototypes are determined experimentally in a modal survey test, which is called ground vibration test (GVT) in the aeronautics community. These modal parameters are used e.g. to validate the numerical model of the aircraft for reliable predictions of flutter critical speeds. Since it is the aircraft manufacturer’s goal to make aircraft development more cost-effective, it is one of their aims to reduce the cost for a GVT. This can either be achieved by shortening the test time, or by increasing the amount of information gained from the GVT to make it more cost-efficient. In the past, improvements with respect to the test strategy, measurement hardware and method developments have significantly contributed to the reduction of testing time. However, a vast improvement with respect to test time reduction or cost-effectiveness will only be achieved if the aeroelastic certification process is taken into account. Currently, the validation of the Finite Element (FE) model is performed on the basis of experimental data which is obtained from the GVT shortly before the first flight. From a risk mitigation point of view, the FE model validation process should be addressed as early as possible in the aircraft development to avoid costly re-designs and excessive model validation tasks in the latest stages of aircraft development. To achieve this, essential data for FE model validation have to be obtained in a different way, or respectively, from other vibration tests long time before the performance of the GVT. DLR has invented the so-called Taxi Vibration Test (TVT) which can either be performed as an alternative to the GVT or as a supplement to conventional ground testing. The TVT utilizes output-only vibration measurements acquired while the aircraft is e.g. driving on uneven taxiway. It can therefore provide modal parameters of the aircraft for FE model validation to achieve permission for the first flight earlier. The current paper presents the results of a preliminary study performed on a commuter class aircraft (DO 228-101). The modal parameters obtained from TVT are compared to those stemming from a conventional GVT.