Identification of Modal Parameters of Aircraft during Taxi

Kurzfassung

Modal parameters such as resonance frequencies, damping ratios, and mode shapes of aircraft prototypes are determined experimentally in a modal survey test, which is called a ground vibration test (GVT) in the aeronautics community. These modal parameters are used for example 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. More precisely, the Finite Element (FE) model validation process has to be shifted to earlier stages of aircraft development. So the timeline can be reduced required for the validation of the aircraft's FE model that is part of the flutter clearance process in aircraft certification. Hence, essential data for FE model validation have to be obtained in a different way, or respectively, from other vibration tests a long time before the performance of GVT. This paper focuses on the topic of multi-objective testing, which means that one test is performed to obtain enough data to achieve several objectives; i.e. it aims at multiple uses of the test setup and the test data. The most straightforward approach to multi-objective testing is to use those tests that have to be performed anyway on the complete aircraft within the certification process (e.g. take-off and landing or taxi tests) to address other goals as well (such tests are performed prior to GVT). The basic concept is to make use of these tests in such a way that the modal parameters of the aircraft can be identified from output-only vibration measurements acquired while the aircraft is driving on the taxiway. The current paper presents the results of a study performed on a test rig simulating the conditions of an aircraft during taxi. The test rig itself consists of the DLR laboratory test structure AIRMOD (AIRcraft MODel) equipped with landing gears, which is driving on a conveyor belt to simulate taxiing. Modal parameters are identified from vibration measurements using an operational modal analysis (OMA) procedure. Measurements are performed for different configurations of the conveyor belt. Finally, the identified modal parameters are compared with modal parameters stemming from a classical modal test.