Experiments on transonic limit-cycle-flutter of a flexible swept wing

Abstract

The aeroelastic behavior of wing models is nonlinear particularly in the transonic speed range. The interaction between aerodynamic and structural forces can lead to the occurrence of Limit-Cycle Oscillations (LCOs). If in addition the wing model is flexible and backward swept, the kinematic coupling between bending and torsion makes the situation even more complex.In the research project “Aerostabil” such a wing was investigated, which was equipped with pressure transducers in three sections and accelerometers. The experiments were performed in the adaptive test section of the transonic wind tunnel TWG in Göttingen. Already Dietz et al. (2003) have reported about experimental details and preliminary results. Based on these data Bendiksen (2008) studied numerically LCO-flutter behavior using a very similar, theoretical model (G-wing) and Stickan et al. (2014) used the original data as a LCO flutter test case. The influence of flexibility on the steady aerodynamics of the wing was described in Schewe & Mai (2018). In this paper now the flutter experiments with the same flexible model were analyzed systematically in the transonic range 0.84 <Ma <0.89 and for six angles of attack from 1.46°to 2.7°. Maps of stability, LCO amplitudes and instantaneous pressure distributions are presented. It was found that unstable regions are islands, whose extent depends on the angle of attack. A LCO test case, already treated in the literature is examined in more detail. The analysis of the time functions showed that during LCO-flutter the motion induced aerodynamic sectional lift forces particularly in the outer wing are asymmetric and thus acting as amplitude limiter. The reason for the asymmetry lies in the shock/boundary layer interaction. The test case, containing the stages of built-up and the transition to the limit cycle provides an excellent opportunity for improving our knowledge about LCOs and for code validation purposes.