Aeroservoelastic Modeling Of Aircraft And Design Of Gust Load Alleviation Controller

The gust encountered by an aircraft during flight will simultaneously excite the rigid-body motion and structural elastic vibration.The excited rigid-body motion not only disturbs the control of pilot,but also reduces the ride quality of passengers.Meanwhile,the induced elastic vibration increases the structural loads and reduces the fatigue life.Along with the development of aerospace technology,the aspect ratios of aircrafts are becoming higher and the wings are becoming more flexible,leading to smaller gap between the frequencies of the rigid-body modes and the elastic modes.As a result,the response characteristics of aircraft due to gust excitations are being more and more complicated.Therefore,it is of great importance to build up the aeroelastic model for flexible aircrafts and to design the control law for gust load alleviation.The research work of this paper is summarized as follows:1.During construction of the aeroelastic model which couples the aircraft's structural and unsteady aerodynamic properties,a crucial part is to build up the time-domain aerodynamic model.To facilitate control design at early stages of aircraft design,the unsteady aerodynamic model is usually constructed by frequency-domain panel method,which should be transformed to time domain by rational function approximation(RFA)technique.However,common RFA methods are either too computationally expensive or too lack of accuracy.In view of this dilemma,an improved minimum state(MS)method is proposed.The proposed method simplifies the iterative process in the MS method to a simple algebraic solving process,and improves the fitting precision by adding a nonlinear optimization stage in the algorithm.The numerical results show that the proposed method can maintain the overall fitting precision with increased computational efficiency,whilst effectively improving the fitting precision of the items related to the key structural mode.2.During the design process of gust load alleviation controller,the performance of a controller,which is designed on the basis of a single flight condition,cannot be sustained when flight conditions change.To solve this problem,the dependence of the time domain aeroelastic model on the Mach number and dynamic pressure is modeled by constructing a linear/nonlinear parameter varying model.A linear fractional transformation model is then built up for design of robust controller.Through two numerical examples,the proposed method is proved to have both satisfactory gust load alleviation performance and robust property.3.To further increase the design efficiency of the gust alleviation controller,another method is proposed to reach a better compromise between the robust stability of the control law and complexity of the design process.The new technique differs from the traditional Linear Quadratic Gaussian(LQG)method by the introduction of properly constructed fictitious high frequency noise.Furthermore,to accurately quantify the stability margins of the multi-input multi-output controller,a variable-structure ? analysis method is proposed.Using a general transport aircraft model,the improved design efficiency and robust performance of the improved LQG method are validated.4.To explicitly handle the constraints of the control surface deflection and to utilize the gust measurement information,an LQG based model predictive control(MPC)technique is proposed.To guarantee the nominal stability of the classical MPC controller,the prediction horizon is extended to infinity.Afterwards,the LQG controller is combined with the finite optimized control sequence in the online optimization problem to extend the control horizon to infinity.Through the preceding modification,the stability property and robust performance of the MPC controller are effectively improved.Furthermore,a strategy to deal with the control delay due to the online optimization is designed.The nominal performance and robust performance of the proposed MPC controller is verified by a numerical example.5.For high aspect ratio very flexible aircrafts,a complete aeroelastic model is constructed combing the geometric exact intrinsic beam theory and the unsteady strip theory.Based on the constructed model,the static aeroelastic properties of very flexible aircraft are studied first,including the divergence and control reversal.Then,the dynamic aeroelastic properties are studied,including the modal characteristics of the trimmed condition and gust response characteristics.For the given aircraft model,static output feedback(SOF)controller,LQG controller,and MPC controller are designed,respectively.It is found that the SOF controller performs slightly better than the LQG controller for uniform distributed gusts,while the LQG controller is much superior to the SOF controller for non-uniform gusts.On the other hand,the MPC controller always has the best gust load alleviation performance,either for the uniform or non-uniform gusts.