Relaxed Static Stability Analysis Of Flying Wing Aircraft And Research On Its Fault-tolerant Flight Control Methods

Flying wing aircraft is a kind of tailless aircraft with blended wing body configuration.Because of the advantage of high lift-drag ratio,low radar cross section?RCS?and better economy,it has a wide developing prospect in the fields of military and civilian.However the tailess design combined with short fuselage structure makes the pitch arm shorter,resulting in lower longitudinal control efficiency,especially at low speed.For another problem,faults are inevitable to any control system and the fact that flying wing aircraft often performs high altitude,long endurance and enemy-defense area missions undoubtedly increases its probability of faults.Consequently,study on flight control system of high reliability for flying wing aircraft is also an urgent problem to be solved.This paper deeply focuses on the problem encountered in design and control of flying wing aircraft.Firstly,the nonlinear mathematical model of flying wing aircraft is established.Then the relaxed static stability?RSS?technology is applied to the flying wing aircraft,which improves the control efficiency and lays the foundation of the following study on flight control system?FCS?.Secondly,for the poor flight qualities after RSS,a robust eigenstructure assignment?EA?method is studied to improve the system dynamics.Nextly,for the actuator failures of flying wing aircraft with system uncertainties,two fault-tolerant control?FTC?approaches are developed: the robust H₂/H_? FTC and the satisfactory FTC.Finally,a control allocation algorithm based on fixed-point iteration is studied for the shortcomings of passive fault-tolerant control.The main contents and innovation points are as follows:1.For the problem of lower longitudinal control efficiency in flying wing aircraft design,moving backward the CG?center of gravity?is proposed to improve the control efficiency after summarizing the RSS method.Considering the inherently insufficient directional stability of flying wing aircraft,not only the impact of the CG shift backward on longitudinal aerodynamic characteristics is studied,but the changes of lateral aerodynamic characteristics are also analyzed.Finally on the basis of experiences and analysis of trimming results,the static margin of RSS is presented.2.To improve the poor qualities of flying wing aircraft after RSS,a robust eigenstructure assignment method applied in the design of FCS is proposed.In this method the parameterized representation of the feedback controller is derived utilizing Sylvester equation,which provides all the designing degrees of freedom.By introducing a choosing matrix,the EA is converted into mathematical optimization problems,which facilitates the controller design.For the weak robustness of EA,a robust index is proposed and the robustness of the control system is improved by optimizing this index.For the multi-objective optimization problem of the controller design above,a multi-objective adaptive chaos differential evolution algorithm,combining the differential evolution and chaos theory,is developed.3.For the practical demand for highly reliable FCS,a robust H₂/H_? FTC method is developed.This method allows for the system uncertainties and can meet the H₂ and H_?performance requirements at normal and fault operation.To reduce the conservative of the controller,the different Lyapunov variables are adopted under each fault conditions and for the resulting non-convex optimization an iterative LMI algorithm is presented.For the conservation problem that exists in the iterative algorithm,the additional variables to decouple the Lyapunov variable and controller gain are introduced and a strong constraint condition guaranteeing robust stability of the system is obtained.Then an improved iterative algorithm is proposed to reduce the impact of the strong constraint condition on the control system.4.A satisfactory FTC of robust H? control with regional poles constraints is studied.This method allows for the system uncertainties and only its spectrum norm bounded is needed,which has a better feasibility.A fault reference matrix is introduced to the actuator fault model so that the controller parameters have nothing to do with the specific fault,which simplifies the controller caculation.To further improve the performance of the control system,a satisfactory FTC based on extended LMI is proposed.In this algorithm the disc pole constraint,H? and guaranteed cost control constraints are organically fused and the extended LMI technology is used to decouple the Lyapunov variable and controller gain to avoid unnecessary constraints of the Lyapunov variable,which reduces the controller conservation and thus improves the performance of the control system.5.The control allocation technology,an important method of active FTC,can reconfigure the control law online without changes of the control structure when faults occur.It also can coordinate multiple control surfaces to achieve multi-target allocation.Therefore,a control allocation algorithm based on fixed-point iteration is proposed.In this algorithm the control allocation problem is converted to the caculation of the fixed-point equation and the constraints of the actuator saturation are considered.Since the equation is internal mapping and compressible at 2-norm,the uniqueness and global convergence of the solution are guaranteed,which greatly facilitates the design of the control system.Finally,in order to further meet the requirements of quickness for FCS,an improved acceleration algorithm is given to speed up the convergence of iterations.