Aeroelastic flutter emulation based on ground vibration test

Abstract

The dissertation presents an improvement and application of the newly proposed technique for aeroelastic flutter test referred to as the emulated flutter test (EFT). The fundamental idea of EFT is that the aeroealstic closed-loop can be emulated by the proper set of sensors and actuators. The idea is based on the principle of the aeroelasticity in that the aerodynamic force is directly related to the structural deformation, and the distributed aerodynamic force can be reduced to the equivalent concentrated forces. The idea under consideration is a very attractive one to substitute traditional flutter test techniques since the required hardware devices (accelerometer and point-loading actuators) to realize this idea exactly coincides with the set-up for structural ground vibration test (GVT) hardware. Doublet hybrid method (DHM) is chosen as aerodynamic model to calculate the unsteady aerodynamic force in subsonic regime. The DHM aerodynamic force model is written in a matrix form, the aerodynamic influence coefficient (AIC) matrix, then the AIC matrix on the aerodynamic panel grids is reduced to relate deformation on the sensor points to the force on the actuator points by the surface spline matrix. The obtained aerodynamic equivalent force is applied to the target structure through the proper force controller. In this regard, the EFT module for the flutter emulation test consists of two main parts: aerodynamic equivalent force calculator and multi-input-multi-output (MIMO) force controller. These two main parts together with the target test structure completes an aeroelastic closed-loop. On the developed EFT module, validation strategies to examine the accuracy of emulated flutter were suggested

check the aerodynamic damping trend in pre-flutter airspeed, and compare the flutter mode shape. Together with the suggestion of these strict validation means, application of accelerometer (in addition to the displacement sensor) and compact type of cheaper actuator, direct drive linear actuator (DDLA) are important improvements achieved in this research compared to the previous works. Further, with guaranteed flutter emulation accuracy, the EFT module was applied for the experimental verification of flutter suppression effect. As in an earlier period of development, 2-dimensional thin plate structure was chosen as a target structure. Due to its simplicity, linear structure and aerodynamic model were sufficient to analyze its aeroelastic behavior. Flutter boundary and the mode shape from the EFT module were in a great agreement with the typical wind-tunnel flutter test result. In addition, the aerodynamic damping effect before reach to the flutter airspeed was as predicted in the analytical method, increased first and then decreased to induce a flutter. Either laser displacement sensors (LDS) or accelerometers were applied as a sensor to develop two different types of EFT module. Savitzky-Golay (SG) filter was utilized for the LDS signal differentiation whereas the Leaky integration filter (LIF) together with high-pass filter (HPF) was utilized for the accelerometer signal integration. Since the flexibility of the test structure makes signal drift issue of the integration more significant, EFT module based on LDS showed better performance in flutter emulation. Finally, the LDS based EFT module was applied to experimentally verify the effect of passive flutter suppression design. EFT modules for the structures before and after the modification was developed and then applied to examine the suppression effectiveness. This is one good example of a new technique’s application, taking advantage of the accuracy and simplicity of EFT module in flutter emulation.