Ground Vibration Test (GVT)
Experimental structural dynamics involves the development of the entire infrastructure of the Ground Vibration Test facility. This concerns both the development of the hardware (sensors, vibration exciters, data acquisition systems, cable routing, signal conditioning etc.) and software (database systems, identification processes, the visualisation of results, etc.).
One of the main tasks of experimental structural dynamics is conducting Ground Vibration Tests. The aim of these is to experimentally determine an equivalent structural dynamic model for each of the structures under investigation. To achieve this, experimental modal analysis (EMA) is used to identify the modal parameters from the vibration measurement data. The results provide information on natural frequencies, damping measurements, natural vibration patterns and generalised masses in the frequency range being tested, which can then be used to help describe the behaviour of the structure being tested in a mathematical manner. These modal parameters are used, for example, in subsequent aeroelastic analyses as a structural model, or as a basis for validating corresponding FE models.
Nowadays, ground vibration tests (GVT) on large aircraft are primarily carried out using phase separation techniques (as opposed to the phase resonance method used exclusively two decades ago). The change to phase separation techniques made it possible to carry out GVTs on a large aircraft with a high modal density in an acceptable amount of time. Together with this change came new challenges for structural dynamic methods. Due to the large quantity of modal parameters, which can identify different excitation configurations from individual measurement series, one of the most important reasons for carrying out GVTs is to provide the client with a range of the highest quality representative modal parameters, which can be used as final test results. The DLR Correlation Tool was developed for this very purpose. On the one hand, the software comprises a SQL database, which saves the modal data for all excitation configurations alongside their specific characteristics. On the other hand, the Correlation Tool is equipped with a graphical user interface, which enables any modal data in the SQL database to be accessed very quickly. The data can be used for visualisation purposes, or be used to correlate findings with other data. The results stored in the database can be used together with various quality indicators for the purpose of carrying out interpretations and evaluations. Taking into account each of the quality indicators, the correlation of all modal data sets of various excitation configurations can be used to create a complete modal model of the tested structure that can, in turn, be made available to the client once testing is complete.
Taxi Vibration Test (TVT)
Experimental structural dynamics does not only involve the improvement of individual positions in the GVT process chain. In the reporting period, the development of the taxi vibration test (TVT) saw the development of an alternative method to conventional test methods. As opposed to GVT, this new method uses natural vibration exciters, such as those experienced, for example, when driving on uneven road surfaces. The vibrations present in the aircraft structure can, therefore, be recorded by a corresponding sensor that has been installed, which can then be fed back to a data acquisition system usually installed in the cabin. Algorithms used in Output Only Modal Analysis can be used to help identify the desired modal data of the structure from the measured response vibrations.
The main idea for developing the TVT came about in 2006, and was, at the time, tested on the AirMod (Aircraft Model) laboratory structure. For the purpose of this test, the structure was fitted with a landing gear and tested on a conveyor belt, which had purposefully been made uneven. In 2007, the method was then tested on a Dornier Do-228 at the DLR site in Braunschweig. In order to spark interest among aircraft manufacturers, the DLR's research aircraft, the A320 ATRA, underwent a TVT in 2009. A conventional GVT was also carried out so that an objective reference could be used when assessing the results of the TVT. The test campaign was carried out in Manching at the German Armed Forces' technological services station number 61 in close collaboration between the Airbus Flight Test Department and DLR Flight Operations in Braunschweig. Using resources from the FTEG joint project of the aeronautical research programme (LuFo IV), the TVT was carried out on a A340-600 as part of a research GVT in 2011. This test was conducted as part of a joint venture between Airbus Operations in Germany and France (Bremen and Toulouse, respectively), as well as ONERA (Paris), partners of the Airbus Institute at the ground vibration test site. The stages of development for the TVT can be seen in the image to the bottom right, according to the year.