Aerodynamic/Propulsive Coupling Characteristics And Control Methods During Stage Transition Of A Turbine-Based Combined Cycle Engine

Turbine-based combined(TBCC)engines have advantages of no carried-oxidant required,horizontal take-off ability,and high safety,so they are promising candidates for engines of military-civilian applications such as two-stage orbital reusable carriers and hypersonic aircraft and have drawn a lot of attention in recent years.A TBCC engine is generally composed of a turbine engine and a ram/scramjet engine.The process of transiting the main thrust provider from one engine to the other is so-called stage transition.During that process,two stages work at the same time,so there are many adjustable variables involved and nonlinearity is significant.How to coordinate those variables to make sure the engine maintain a constant total thrust is the main problem of current TBCC control research.However,in addition to nonlinearity of TBCC engines that mentioned above,couplings between aircraft and engine are key factors that affect engine performance.Therefore,TBCC mode transition control with aerodynamic/ propulsive considered has more guiding significance for applications.The control research work usually starts with the controlled-object model in order to understand its characteristics,but the current model for TBCC stage transition still has certain shortcomings in the description of the windmilling operation characteristics and the aerodynamic/propulsive coupling relationship.The windmilling operation is a necessary stage of the turbine stage during TBCC stage transition.However,most of existing research on TBCC transition control ignore or simplify the process.Therefore,based on the previous research on modeling the subsonic turbine engine and combined with mature supersonic inlet modeling technique,a modelling method that can describe characteristics of the windmilling process is given in this paper.Static simulation of free windmilling operation,dynamic simulation of turbojet-to-scramjet and scramjet-to-turbojet transitions are made respectively.It is found that inlet unstart would happen and the inlet could be restarted successfully or not.The effect of inlet unstart on engine performance is found out,so are factors that depended wheter inlet could be restarted successfully or not,and solutions are given.This model that covers the windmilling process lays the foundation for characterization of nonlinear characteristics.The aerodynamic/propulsive couplings exist due to highly integrated design of the air-breathing engine and the airframe naturally and are difficult to decouple.Therefore,the TBCC stage transition to be controlled is more practical.However,there are still few reports about the aerodynamic/propulsive coupling model required for the research of transition control.Based on the predecessor’s modeling methods on the integrated flight/propulsion aircraft powered by ramjets and combined with the construction ideas of the TBCC engine model,this paper constructs an analytical mathematical aerodynamic/propulsive integrated model that can describe the coupling relationship between aerodynamics and propulsion systems during TBCC transition.Dynamic characteristics analysis shows that engine control variables and aircraft control variables have significant cross-coupling characteristics,so it is difficult to perform decoupling processing.Moreover,the model is gradually simplified on the basis of the established complex mathematical model,and finally a control-oriented aerodynamic/propulsive coupled model is obtained,which provides a research platform for the design of aerodynamic/propulsive coupled controller.Based on the platform established above,simulation of the TBCC stage transition based on the traditional control of the TBCC engine from the perspective of engine itself is firstly carried out and it is proved that TBCC mode transition control with aerodynamic/ propulsive couplings considered is necessary.Based on those analysis,the transition control objective is changed from controlling the total thrust to controlling the system to track the flight trajectory,then the control problem can be abstracted into a nonlinear output tracking problem.The feedback linearization combined with the linear quadratic controller method is then used to design the aerodynamic/propulsive integrated output tracking controller.Results show that this control method can realize tracking control tasks of the aerodynamic/propulsive coupled system to the expected trajectory during TBCC stage transition.Transition thrust pinch means the problem when the two stages of a TBCC engine both operate at operation points that are so far away from their design points that the total net thrust is not enough,and the problem is usually solved by redesigning the propulsion system.After the research object of TBCC stage transition control has been extended from the engine itself to the aerodynamic/propulsive coupled system,the transition thrust pinch problem now has possible alternatives other than the propulsion system design.Therefore,the thrust pinch problem is abstracted into an optimal control problem in this paper,and the problem is then solved by designing the optimal stage transition trajectory.Results show that the aircraft manage to make up the lack of net thrust during TBCC transition with the aid of the gravity component and smaller drag during the climb and dive,and the overload and heat flux are acceptable.Finally,controller proposed above is then used to track the optimized trajectory during TBCC stage transition.Results show that it can achieve the goal to track the optimized trajectory even when disturbance exists during the TBCC stage transition to solve the transition thrust pinch problem,so a solution to the transition thrust pinch problem is proposed through the aerodynamic/propulsive coupled control.