Ground Vibration Test on an airplane model in wind tunnel

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As part of a research project carried out at the Department of Aerospace Sciences and Technologies of the Politecnico di Milano (, Vicoter performed the GVT (Ground Vibration Test) of an aeroelastic model for wind tunnel tests and the identification of the V-g diagram for the determination of the flutter speed.

The aircraft, which has a traditional layout, T-tail and a wingspan of approximately 2.5 m, was designed for the study of control laws for the active suppression of the flutter using ailerons. A peculiar feature is the presence of two pods at the wingtips containing the emergency system in case of flutter: in case of excessive accelerations, a mass is fired from the rear to the front position, changing the torsional inertia of the wing and the relative modes, killing the phenomenon.

Vicoter carried out two sets of modal tests for both positions of the masses in the pod. The former was performed on the entire aircraft, using a MIMO roving technique. 5 accelerometers were used as references and the structure was excited in 46 locations by means of an instrumented hammer. These tests made it possible to identify the vibrating modes of the entire model, mounted in the wind tunnel, minimizing the mass loading effect introduced by the sensors.

In the latter, 28 single-axis accelerometers were installed on the left half-wing. They were distributed in 7 stations along the span, 2 locations along the chord for each station, with reading directions orthogonal and parallel to the wing surface. These tests were used to identify the modal deformations with greater care, to identify the front-back flexural mode as well as to allow a more detailed investigation of the interaction between wing and pod. Also in this case, a MIMO excitation technique was used by means of an instrumented hammer

A post processing activity was also carried out on the data collected during the wind tunnel test campaign. Starting from the accelerations detected by sensors placed in the wing and acquired by the on-board system, the modes of the wing at different air speeds have been identified. From them, the V-f and V-g diagrams, necessary to determine the flutter speed, were plotted. The trends were then compared with the results obtained using alternative methods of identification developed internally by the Politecnico di Milano.