Structural Modeling And Attitude Control Of Very Flexible Aircraft

The very flexible aircraft(VFA) have the capability to perform a variety of missions with high-altitude long-endurance flight, and have received widely attention recently. The very flexible aircraft, featuring light wings with a high-aspect ratio, may undergo large deformations or aerodynamic stall under normal flight conditions, which has a significant impact on flight characteristics. It brings challenges for modeling and flight control design of aircraft. This dissertation focuses on the research of VFA, including system modeling, dynamic characteristics analysis, model reduction and attitude control.First, the dynamic equations are derived based on the principle of virtual work. Constant strain formulation is applied for nonlinear beam structural model. The unsteady aerodynamics are calculated by using the finite state aerodynamic theory. The structural dynamic equations, the aerodynamic model, and the nonlinear six degrees-of-freedom rigid-body equations are conbined together to construct the complete dynamic differential equations.Next, the motion mode of VFA is analyzed based upon the distribution regularity of the system’s eigenvalues. The dynamic characteristics of the flexible wings in the situation of active manipulation or atmosphere disturbance are analyzed by adopting a state-space Newmark method based on constant average velocity, and the lateral stability influenced by the coupling between those flexible modes is also discussed.Then, the model reduction method of VFA is studied. To facilitate the control design of the high-order model of VFA, balanced truncation, balanced residual, and optimal Hankel-norm approximation are applied respectively in model reduction. Simulation results show that for the research object in this dissertation, balanced residual is the most appropriate method that makes the reduced-order model approach to the original system.Finally, a model reference adaptive controller is designed for the VFA model with uncertainties based on the reduced-order model with an observer. A baseline LQR-PI controller is designed for optimal tracking control. Simulation results show that the controller based on the simplified model stabilizes the original system under nominal conditions; the adaptive controller guarantees tracking stability and improved transient response when the aircraft is subject to some uncertainties.