Aeroelastic Limit Cycle Oscillations Mitigation Using Linear and Nonlinear Tuned Mass Dampers

Flutter is a destructive aeroelastic phenomenon occurring on flexible aeronau- tical structures because of an energy exchange between two or more of the system’s vibration modes and an airflow. In some rare cases, limit cycle oscil- lations (LCOs) related to flutter are observed because of nonlinearities, which might require aircraft redesign or flight envelope limitations. One way of sup- pressing such LCOs could be using Linear and Nonlinear Tuned Vibration Absorbers ((N)LTVAs), which are widely used in civil engineering but have to date received very little attention in the aerospace community. The objectives of this thesis are the understanding of nonlinear aeroelastic phenomena and the investigation and demonstration of the beneficial e ects of such absorbers for flutter and LCO suppression. Two nonlinear aeroelastic systems featuring smooth (continuously hardening) and non-smooth (freeplay) nonlinearities are investigated by means of mathematical models and wind tunnel experiments. An increase in flutter speed of up to 35% is observed on the former system, both in the wind tunnel and using the model, however, a very precise tuning of the absorber’s natural frequency is required. On the other hand, a negli- gible increase in LCO onset speed is observed on the latter system although a reduction in LCO amplitude of up to 60% is achieved in a given airspeed range, using a nonlinear absorber whose nonlinearity mimics that of the aeroe- lastic apparatus. The e ect of linear and nonlinear vibration absorbers on the shape of the limit cycle branches of aeroelastic systems is described in detail and it is shown that such devices can change the nature of bifurcations from supercritical to subcritical and vice versa and can even cause the appearance of isolated solution branches. Therefore, extreme care must be taken when designing and implementing LTVAs and NLTVAs, as their e ectiveness in in- creasing the linear flutter speed can be compromised by the change in the nature of the bifurcation. Furthermore, it is shown that a LTVA can not only delay classical flutter but also delay/suppress stall flutter