Every aircraft vibrates in flight and during take-off and landing. It is important that critical vibration events are avoided. The Ground Vibration Test (GVT) is a key element to enable a new aircraft to receive its certificate of airworthiness. Taxi Vibration Tests (TVTs) are a less complex further development of the GVT, during which the aircraft travels along a runway. The German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) successfully completed these tests on its new Falcon 2000LX 'In-flight Systems and Technology Airborne Research' (ISTAR) aircraft in June 2021. They were performed at the DLR facility in Braunschweig.
Researchers from the DLR Institute of Aeroelasticity used the measurement campaigns to collect data for building the aircraft's digital twin. At the same time, they experimented with new methods to improve the efficiency and precision of vibration testing.
Both the GVT and TVT procedures focus mainly on recording vibration frequencies and modes such as wing bending and twisting. If an aircraft is incorrectly designed, in-flight superposition of these natural modes of vibration can trigger flutter phenomena that could produce unexpected vibrations and even lead to structural failure.
More than 200 sensors provide information
Researchers conducting the tests installed acceleration sensors on all the main components of the aircraft, such as the wings, tail unit, fuselage and control surfaces. "More than 200 acceleration sensors recorded even the tiniest movements," explains Julian Sinske from the DLR Institute of Aeroelasticity in Göttingen. "The purpose of the tests was to produce a detailed analysis of the base aircraft's vibration patterns and in doing so make our computer model of ISTAR entirely consistent with reality." Researchers will now be able to use the computer model to simulate subsequent modifications such as attachments for future research campaigns before they are installed and therefore determine their influence on the aircraft's vibration behaviour at an early stage.
New test methods for aeronautics
During aircraft development, the GVT is performed shortly before the maiden flight. That is why saving time is an important factor. The researchers also used the tests to investigate innovative methods for AI-based, automated analysis of measurement results. In future, the AI methods could help to speed up analysis of raw data, but first need to be validated in real trials.
The researchers also closely examined innovative analysis methods to determine non-linearities in the test structure. They want to improve the efficiency of the tests and enable better predictions of in-flight vibration behaviour. Aircraft simulation models are built using assumptions such as linear-elastic material behaviour. This means that doubling the load would also double the deformation. But in reality, this is frequently not the case. Researchers must investigate any such effects observed during the tests and use the data acquired to develop a mathematical model for it. Doing so is significantly more laborious than standard test procedures.
In addition, the scientists tested a smartphone app that they have developed. It can be used to check the signals from the acceleration sensors on the screen of a mobile phone during installation.
First stationary, then in motion
In addition to the GVT, the researchers also conducted a TVT on ISTAR. "The TVT involves using a tug to tow the aircraft along the runway and taxiways, in this case at the Braunschweig Research Airport," Sinske explains. "During the process, minor irregularities in the surface stimulate vibrations in the aircraft as it moves along." The purpose of the TVT was to check the ISTAR flight test instruments, which comprise 62 acceleration sensors and 40 strain sensors used as a means of obtaining reference values for ISTAR at the start of its DLR service life. The effort involved in a TVT is considerably lower than for a GVT. For example, much of the measurement equipment that would be needed for a GVT is not required. Using the unevenness in the taxiway to excite the aircraft provides the researchers with important information regarding the dynamic properties. They can, for example, record oscillation amplitudes of the wings and identify dynamic behaviours of the structure. In future, the researchers can use the results of the TVT to review how minor changes affect the aircraft structure by repeating the tests in an experimental setting, which will reduce the effort involved.