Research On The Key Technologies Of Modeling And Control Of The Advanced Fighter At High Angle Of Attack

As the weapon of the national defense,the advanced fighter with super stealth capability,supersonic cruise capability,super maneuver capability and over the horizon combat capability is deeply favored by the military powers.The supermaneuverability of the advanced fighter is still the crucial factor to win the battle in future air combat,especially close combat,so the research on post stall maneuver is still the important issue at home and abroad.When the advanced fighter maneuvers at high angle of attack,the air flowing through the body changes from the attached flow to the vortex flow,then to the vortex breakdown and finally becomes the separated flow,and generates unsteady aerodynamic force with serious nonlinearity,coupling and hysteresis.How to establish accurate unsteady aerodynamic model when the aircraft flies at high angle of attack,and how to achieve the coordinated control of redundant surface and thrust vectoring nozzle with this kind of aerodynamic characteristics are all urgent problems to be solved in the study of the advanced fighter.In this paper,key problems such as the aerodynamic modeling and coordinated control of the advanced fighter with high angle of attack are analyzed and studied comprehensively and systematically based on the large oscillation wind tunnel experiment data of a scale advanced fighter model.Unsteady aerodynamic modeling under uniaxial coupled oscillation and biaxial coupled oscillation with large amplitude,the mathematical model and natural characteristics analysis of the aircraft,nonlinear control allocation method and the control method under unsteady aerodynamic with high angle of attack are completed.The research contents of this paper involve the cross disciplinary of aerodynamics and control in aviation field,which will lay a theoretical foundation for the actual flight control of advanced fighter at high angle of attack.Main innovations and work are as follows:(1)An improved fuzzy logic modeling method for unsteady aerodynamic is proposed under uniaxial oscillation with large amplitude.Based on the analysis of the wind tunnel experimental data,the angle of attack,the derivative of the angle of attack,the second derivative of the angle of attack,the angle of sideslip and the reduced frequency are transformed into fuzzy inputs by the quadratic membership function.And fuzzy outputs are the aerodynamic force and the aerodynamic moment coefficients calculated by fuzzy rules.The backtracking method and the Newton's iteration method are combined to identify the structure and parameters of fuzzy logic model in a unified framework,and the model precision is also high.(2)Unsteady aerodynamic modeling methods under biaxial coupled oscillation with large amplitude are developed.A cross validation-minimal sequence optimization method based on support vector regression(CV-SMO-SVR)is presented for the characteristics of complicated aerodynamic model and large data sample.Model input variables,output variables and kernel function are reasonably selected according to the analysis of the wind tunnel data.To improve the model predicting speed,an unsteady aerodynamic modeling method under biaxial coupled oscillation based on improved extreme learning machine(ELM)is designed and the convergence of the model is proved.Simulation results show high precision,rapid predicting speed and strong versatility of the unsteady aerodynamic modeling methods under biaxial coupled oscillation.(3)A nonlinear control allocation method based on the feedback of the position of surface is proposed for the coupling between surfaces and the nonlinearity of maneuvering coefficients of the aircraft at high angle of attack.Three-axis moment coefficients of quadratic term and coupling term with surface deflection in previous sampling interval are removed from the expected three-axis moment coefficients.And the nonlinear allocation problem is transformed into linear control allocation problem.The deflection angle of surfaces can be solved by the existing linear control allocation method to avoid huge computation burden and poor real-time performance of the existing nonlinear allocation method.The stability of this method is proved and the feasibility for practical engineering is analyzed.Simulations compared with other methods and MATLAB xPC Target real-time simulations demonstrate that this method has advantages of the high accuracy,great real-time performance as well as strong practicability.(4)Considering the control errors under unsteady aerodynamic,a design method for control law at high angle of attack based on the improved dynamic inversion method is presented.The time scale separation dynamic inversion control law is designed in terms of singular perturbation theory.For the control errors,the integral function is added into the fast loop to improve the control precision.And for the hysteresis caused by the unsteady aerodynamic,the lag correction is added into the slow loop to improve the phase lag.Simulations of post stall maneuvers of the aircraft at high angle of attack under the unsteady aerodynamic of uniaxial oscillation and biaxial oscillation with large amplitude are accomplished.And simulation results shows advantages of high control accuracy and absence of phase lag.(5)A backstepping control law design method based on radial basis function(RBF)compensation at high angle of attack under unsteady aerodynamic is proposed.The modeling errors caused by unsteady aerodynamic are treated as uncertainties in the angular rate loop and the air flow angle loop,and the RBF network is used to approximate these uncertainties.The control laws for angular rate loop and loop flow angle loop with uncertainties are designed by the backstepping method.The update rule of RBF network is given.The stability of the closed loop system with this control law is proved.Simulation results show that the designed control laws can eliminate the effects of unsteady aerodynamic and obtain better control accuracy.