A Study On Flight Envelope Protection Strategy For A Large Civil Aircraft Using Nonlinear Control Methods

Developing large civil aircraft is one of the most important research programs in China.Supported by the Chinese Civil Aircraft Development Plan(No.MJZ-2011-S-06,and No.MJZ-2015-S-080),this thesis focuses on the critical problem in flight control system design and studies the flight envelope protection strategy for a large civil aircraft using nonlinear control methods.The main work includes four aspects: nonlinear dynamics modeling of a large civil aircraft,backstepping flight control law design,fast robust backstopping control law design,and flight envelope protection strategy study.Specific contents of each part are listed as follows.1.The 6-DOF rigid-body nonlinear dynamics is studied for a large civil aircraft B747-100/200.First,equations in wind-axes and flightpath-axes are derived from rigid-body general equation of motion.Then,the expressions of gravational,aerodynamic,propulsive and wind forces and moments are given.The aerodynamic model is established by using aerodynamic coefficients and derivatives to calculate the aerodynamic forces and moments.Last,the dynamics models including the one for simulation and the one in matrix form for controller design are summarized.The simplified longitudinal dynamics model is decoupled from the original nonlinear model.The open-loop characteristics are analyzed using MATLAB/Simulink.2.The command tracking control law using the backstepping method is designed for a nominal large civil aircraft model.First,definitions and theorems of Lyapunov stability are introduced,which are fundamentals of nonlinear control theory.The general form of backsstepping design is given.Combined with an explicit longitudinal aircraft model,the effect of control parameters on the performance of the closed-loop system is studied.Then pitch angle and angle of attack command tracking controllers are designed for the longitudinal single input single ouput(SISO)aircraft model using the backstepping method.Last,a vectorial backstepping attitude command tracking control law is designed for the 6-DOF multiple input multiple ouput(MIMO)aircraft model.Simulation results show that the command tracking performance is excellent when the model is nominal and there is performance degradation when the model is uncertain or a fault occurs.3.Based on fast terminal sliding mode,a class of fast robust backstepping control method is studied to deal with the control problem of an uncertain large civil aircraft model.First,related stability theory and knowledge of radial basis function neutral network(RBFNN)are introduced.Then a fast robust adaptive backstepping(FRAB)controller is proposed using fast terminal sliding mode and RBFNN.The advantage of the proposed algorithm is that the tracking error has fast convergence rate,high accuracy and better performances in overshoot,settling time,and steady error compared with conventional backstepping methods.To make the estimation of composed disturbance more accurate,a super-twisting sliding mode observer based fast robust backstepping method is designed.Simulation results show that the controller inherits the advantages of FRAB and estimates the disturbances fast and accurately.Last,a sensor based backstepping control law is designed.The method,independent of model,does not need to estimate disturbance and has strong robustness.4.The flight envelope protection problem formulation is described and different flight envelope protection strategies are discussed.First,the performance of a classic AOA limiter is analyzed.Secondly,predictive control is used to design a controller that can directly protect the envelope.Last,using the FRAB command tracking controller,two protection schemes including command limiting and control mode switching are proposed to implement the flight envelope protection task in this study.Control mode switching is easy to realize but difficult to analyze the stability.Stability can be guaranteed by command limiting design which is based on model.Both schemes with fault-tolerant ability are better than traditional limiter for the nonlinear aircraft system with faults in structure.Furthermore,simulation results of the Boeing 747 airplane model validate the effectiveness of the developed algorithm for flight envelope protection with model uncertainties,disturbances,and partial loss structural faults.In general,the thesis solves the robust command tracking problem of a large civil aircraft by using nonlinear backstepping method.The fault-tolerant flight envelope problem is solved using the FRAB controller with command limiting and switching strategies.