Active control of Flutter and LCOs for a nonlinear aeroelastic underactuated wing

Abstract

This dissertation proposes a robust solution for the design and stability of nonlinear aeroelastic airfoils of fixed-wing drones, dealing with the different aeroelastic instabilities that may take place during flight, such as flutter and limit cycle oscillations (LCOs). Therefore, a two-dimensional nonlinear aeroelastic airfoil is firstly modeled. Where the aerodynamic lift and moment are expressed based on the Wagner’s function for unsteady aerodynamics. The dynamic model describes plunge and pitch motions of the aircraft wing section with trailing- and leading-edge control surfaces. After that, some robust control plants are designed and applied on the built model, in order to eliminate flutter and LCOs. These plants are based essentially on the use of sliding mode control (SMC) which is characterized by the design of a switching function to bring the system’s state-trajectory to a sliding surface, and force it to stay in vicinity of this surface, converging then towards stability position. SMC is combined with a fuzzy logic controller to suppress any eventual appearance of chattering phenomenon that may occur because of the switching feature of the SMC. Finally, a high gain observer is added of the combination to estimate some system’s states using some other known ones, allowing to minimize the sensors’ number and to deal with the system’s complexity due to unsteady aerodynamics and the system’s structural and aerodynamic nonlinearities. The obtained simulation results has been exposed and discussed, proving the controllers’ efficiency in stabilizing the system, limiting vibrations and delaying flutter appearance, giving then a powerful and economic tool to have well-stabilized chattering-free wings with high performances despite nonlinearities and unsteady aerodynamic loads.