Research On Flight Mechanics And Flight Control Technology Of Boom Aerial Refueling System

Aerial refueling transfers aviation fuel from one aircraft to another,which enhances the flight efficiency of the receiver.Two methods for aerial refueling are the probe-and-drogue refueling and the boom-and-receptacle refueling.The latter is easier in contacting with receiver and faster in fuel transferring than the former.Nowadays,we are developing large transport aircraft.It is an opportunity to develop large dedicated tanker and boom refueling technology,which will greatly enhance our refueling capability.Boom refueling system is composed of tanker aircraft,flying boom and receiver aircraft.They are connected by universal joint or cardan joint.It is a typical multi-rigid-body system with complex and variable structures,and high degrees of freedom.Meanwhile,it works in complex circumstance.These issues make it difficult to analyze the system and design the controller.In China,most researches focus on boom's dynamic modeling and controller designing in ideal condition.Problems such as aerodynamic,modes of boom attitude motion,characteristics of system multi-rigid-body motion,have not been researched sufficiently.Problems such as automatic load alleviation,controller designing for manually controlled boom,automatically control throughout refueling process,have not been researched either.These problems are researched in the thesis.Firstly,boom's aerodynamic model is formulated and mode characteristics of boom motion are analyzed.Components of the boom are regarded as wings,so that its aerodynamic force can be calculated by subsonic aerodynamic estimation method.Aerodynamic characteristics are analyzed by simulation.Then,a high accuracy mathematical model in compact form is built to describe the aerodynamic of boom according to simulation results.Dynamic model is built for boom attitude motion in ideal condition that the tanker is in straight level flight and the extension boom is static.Theoretical analysis is conducted to reveal the great differences between boom and conventional aircraft.Then,characteristics of boom motion,such as low damping ratio,dynamic coupling and parameter perturbation,are analyzed by simulation.Secondly,Kane method is employed to derive the multi-rigid-body dynamic model of the system,and characteristics of the system motion are analyzed.The models of system in uncoupled mode and coupled mode are both derived,so that the whole refueling process can be researched in simulation.Meanwhile,effects of tanker's wake vortex on receiver are taken into consideration to achieve higher fidelity.After automatic pilots of tanker and receiver are designed,a typical simulation circumstance is built to analyze system's multi-rigid-body motion characteristics.In uncoupled mode,the analysis focuses on how tanker's maneuvering and boom's extension affect boom's attitude motion.In coupled mode,the analysis focuses on how the nozzle loads come out and what induces them.Thirdly,requirements of control system for the boom operated manually are proposed and the designing method is researched.The controller consists of two parts: stability augmentation controller and automatic load alleviation controller.The former is designed to enhance boom's handling quality in uncoupled mode.During designing,requirements such as enhancing handling quality,relieving elastic mode disturbances,lower controller sensitivity during precisely tracking but higher during rapidly operating are taken into consideration synthetically.A new method is proposed,that the latter controller should be designed on the base of the former one in order to reduce the complexity of controller.During designing,feedback signals of attitude angles and nozzle loads are used together to achieve the best load alleviation ability.Meanwhile,the joystick is designed to be driven by the controller to relieve the bad influence of control law switching.In last part,an idea is presented considering the tendency of less manual operation but more automation in today's aerial refueling,that the boom is operated automatically throughout refueling process.Then,designing of the automatic control system is researched.In uncoupled mode,the control law is designed to make the boom precisely tracking the moving receptacle under strong uncertainty and disturbance circumstances.In coupled mode,the control law is designed to minimize the nozzle loads.A method,combining feedback linearization,disturbance observer and backstepping adaptive control,is employed to design the tracking control law.For nozzle loads minimizing,a control law using boom's attitude feedback and nozzle load feedback together is designed.The refueling process is divided into four phases,standby phase,docking phase,fuel transferring phase and departing phase.After control laws for each phase are designed and rules for phase conversion are formulated,the integrated controller is obtained.Finally,effectiveness of the controller during the whole refueling process is verified in simulation.