The methods and algorithms used in experimental modal analysis are used in Ground Vibration Tests (GVTs) to identify modal parameters in vibration measurement data. Assuming that fluid mechanical environmental parameters only change slowly, these methods and algorithms can also be used to monitor the stability of aeroelastic systems, for example, during flight vibration tests or wind tunnel tests. As it is not possible to artificially induce vibrations using forces that are independent from the system during flight vibration tests, and it is only partially possible to induce this during wind tunnel tests, aeroelastic systems based on the output-only modal analysis method are drawn on to enable stability monitoring; something the Institute excels in due to the development of the taxi vibration test.
The development of methods for output-only modal analyses, which provide a robust tool for monitoring the aeroelastic stability of wind tunnel tests, is underway as part of the DLR project ALLEGRA. The aim of the project is to develop a reliable method for predicting critical, aeroelastic conditions in wind tunnel tests and flight vibration tests. Above all else, this requires the use of algorithms in real time, as well as the ability to improve damping assessments. One of the particular challenges in this is the use of physical models based on extrapolation in order to identify damping measurements, which can be used to determine flutter-critical speeds in advance. In order to develop and test robust software for the real-time monitoring of aeroelastic systems, the infrastructure at the facility for ground vibration tests is being expanded to include a real-time control system, which is able to simulate, in near real-time, the behaviour of aeroelastic systems in response to any stimuli. In this way, the software environment that has been developed can be qualified for the purpose of monitoring stability, in addition to wind tunnel and flight vibration tests. The methods developed thus far for monitoring aeroelastic stability have been used in flight vibration tests on the research aircraft HALO in an external load configuration with PMS Carrier, as well as during aeroelastic wind tunnel tests as part of the DLR project ALLEGRA.