Dynamic Gust Loads Analysis For Flexible Aircraft With Nonlinear Control Effects

The gust load alleviation system and the stability augment system are the main sources of non-linear response,and the nonlinearity has unpredictable effects on the performance and stability of the system due to the aeroelasticity and the complexity of servo control.The gust load alleviation system and stability augment system with a hydraulic actuator are established for a General Transport Aircraft(GTA),and the nonlinear influence on the two systems is studied.The main contents:Firstly,the structural model and the aerodynamic model of the aircraft are established respectively by the Finite Element Method and the Doublet Lattice Method,and the aeroelastic model of aircraft is obtained by spline interpolation.A hydraulic actuator model is established,the discrete gust model and continuous turbulence model are established according to CCAR25.341,and the Gust Load Alleviation System is designed based on PI control to complete the gust response calculation.Secondly,the Stability Augment System is designed based on the rigid body flight mechanics.The aeroelastic model is modified to obtain the rigid-elastic coupled dynamic model including rigid body flight mechanics,which neglected the influence of unsteady aerodynamic term.The rigid body dynamic model of the Stability Augment System is replaced by the rigid-elastic coupled dynamic model to complete the elastic influence on the gust response of the stabilizing system.Thirdly,the nonlinear sources of the system are analyzed according to the system architecture,and the nonlinear modeling method of the system is studied.The simulation model of the Gust Load Alleviation System and the Stability Augment System with nonlinear system are established.Finally,the influence of system nonlinearity on the Gust Load Alleviation System and the Stability Augment System is analyzed.The results show that: the saturation and the rate limit can suppress the order signal,lag the system phase,reduce the load alleviation effect and dynamic performance;the dead zone and hysteresis can reduce the tracking accuracy of the actuator,lead to the Limit Cycle Oscillation(LCO);the delay limit the function of the control system,and it's an important factor that affects the stability of the system;the friction in different systems has different properties and functions,the self-excited oscillation of friction can cause the system limit cycle oscillation,and the damping effect can enhance the stabilityof the system.It can provide the basis for nonlinear control design through the study of the nonlinear influence on aeroservoelastic system,and provide technical support for civil aircraft design,verification and airworthiness certification by extracting regular conclusions and airworthiness review.