DLR (Deutsches Zentrum für Luft- und Raumfahrt) is the German aerospaceresearch center and space agency. It has more than 4,500 employees, eight major sites, 30 institutes and an R&D budget of approximately Euro 350 million. As a competing market participant, the agency earns 35 percent of this amount with entrepreneurial activities: performing work on a diverse range of aerospace as well as non-aerospace projects – such as high-speed trains, automobile research, and within the energy sector.One of the DLR sites is located in Göttingen, where the Institute of Aeroelasticity is active in theoretical and experimental structural dynamics – with a traditional focus on the experimental part. Objects under investigation are mainly the load-carrying structures of aerospace constructions like aircraft, helicopters and satellites.
Theoretical work concentrates on the numerical analysis of the dynamic characteristics of large-scale structures using Finite Element methods. Complex numerical models are employed for the prediction of the vibration behavior and the preparation of modal identification tests. Finite Element models are then updated and validated with experimentally identified modal parameters.
Large Scale Ground Vibration Testing
On the experimental side the focus is the performance of modal identification and dynamic qualification tests. Modal identification tests are performed, in a specially designed test area, the so-called Ground Vibration Test Facility. With this facility, large-scale structures can be excited with forces of up to 2200N and the structural responses can be measured at 700 positions. The department collaborates with several European partners for the larger projects; these include ESTEC in the Netherlands for satellite testing and ONERA in France for the Airbus program.LMS CADA-X systems have been in use at1DLR since the early days. The first, a 64-channel LMS
SCADAS I system purchased in 1990, was soon upgraded to a 72-channel SCADAS II system. A second smaller system is used for component testing. In 1999 DLR purchased a 240-channel VXI system (soon to be upgraded to 384 channels) for the large-scale ground vibration testing of aircraft and satellites. The VXI was chosen for its compatibility with other VXI systems in use at the institute and with the DLR partners: it allows us to put together a very large system when necessary.
According to Dr. Michael Sinapius, scientist of the test group, a typical Airbus 340 GVT test can take between 3-6 weeks, depending upon the test configuration. With the upcoming Airbus 340-500 test which will be performed in cooperation with ONERA, around 500 accelerometers and a maximum of eight shakers will be used simultaneously, although up to 30 shakers can actually be installed to minimize the time for changing the exciter configuration. As can be expected, the eigenfrequencies are very low: between 1Hz and 15Hz there are around 60 modes. In the past smaller aircraft were suspended on rubber bungees, but these days airsprings are used for dynamic suspension.
Satellite Testing at Estec
Other tests at ESTEC were on the Polar Platform Envisat (PPF/ENVISAT-1), the biggest satellite ever to be developed by the European Space Agency. The PPF is due to be launched soon, and will be used to monitor ozone layer depletion over the earth’s poles. On the Structural and Thermal Model (STM) a modal identification test was performed in 1997. During this test the structure was equipped with 575 accelerometers. The modal identification was realized with DLR developed software and by using the Phase Resonance Method. A total of 89 modes were measured. The Flight Model (FM) of PPF/ENVISAT-1 was investigated in dynamic tests in summer 2000. Modal parameters were identified from two different types of test. Firstly, a simplified modal survey test with a single electrodynamic exciter was performed. Secondly, data of base excitation tests with ESTEC’s large vibration test facility HYDRA were used for modal identification. In both cases Frequency Response Functions were evaluated with LMS MODAL, and the results used to validate the Finite Element model.
Putting New Ideas into Practical Reality
“We try to integrate commercial tools with our own methods,” said Dr. Sinapius, “and the LMS User programming facility suits us perfectly. UPA helps us to integrate new ideas coming from our research teams into practical reality”. For example, DLR often makes use of a special excitation signal termed “multisine” to stimulate the structure. Multisine comprises a user-defined mix of sine tones with phasing of the sinewaves carefully tuned by the Schr2der algorithm to minimize the ‘crest-factor’ of the signal in the time domain. Not only does this reduce the stress on the structure during testing; the use of multisine also results in faster overall tests because the resulting high signal-noise ratios mean that no averaging is required.
There are four engineers actively using the LMS systems. Once the data is collected, Frequency Domain Parameter Identification (FDPI) techniques are used to estimate the eigenfrequencies and damping values. Around ninety minutes after the acquisition ends the test data will have been analyzed and validated. The data are then correlated with the FE model of the structure. Dr. Sinapius concluded, “DLR has a great experience in the testing of large aerospace structures and LMS technologies play a important role in our strategy.” DLR and ONERA intend to collaborate on the Ground Vibration Test of the Airbus 380 SuperJumbo just before its maiden flight in 2004.
