| Till now,researches on the ducted fan aircraft mainly focus on the flight control and application of its vertical flight mode near hovering condition.This dissertation concentrates on the level flight and flight mode transition control of a novel ducted fan configuration.As a newly emerged vertical take-off and landing(VTOL)aircraft,the ducted fan tail sitter VTOL is a special type of tail sitter aircraft,capable of multiple flight modes including vertical flight,level flight and transitional flight.Due to the introduction of thrust-vectored system comprising ducted fan and control vanes,the ducted fan tail sitter VTOL has stronger control capability in lower speed and high angle of attack conditions,resulting in better post-stall performance.However,this also brings challenge to the autonomous flight control of this type of aircraft,including: 1)The range of system operation points and the flight envelope are larger.2)During a full envelope flight,the aerodynamic characteristics of the aircraft system is extremely complex with high nonlinearity and coupling effect which has not been fully studied yet such that conventional model-based control techniques could hardly achieve satisfactory performance.Till now,studies on the level flight and flight mode transition control of ducted fan tail sitter VTOL can rarely be found in open publications,nor do successfully practical flight demonstrations.This dissertation concentrates on the flight mode transition control of ducted fan tail sitter VTOL,including aerodynamic modeling and identification during level flight and flight mode transition,full envelope flight control,and trajectory planning of the transition process.The main contributions of this dissertation are summarized as follows:(1)In this dissertation,we have deeply studied the aerodynamic characteristics of the ducted fan tail sitter VTOL during level flight and flight mode transition.The aerodynamic forces and moments as well as the coupling effect with driven outputs of the ducted fan comprehensive dynamic/driven system is analyzed.Particularly,in terms of the novel ducted fan tail sitter VTOL configured with pushed ducted fan and wing,we propose a compact numerical method to model the complex aerodynamic interaction between the ducted fan and the wing which is difficult to formulate analytically.Finally,we establish a complete nonlinear dynamic model for the aircraft system.(2)In this dissertation,we have proposed a full envelop flight control scheme in order to stabilize the aircraft at all admissible flight regimes.To estimate and compensate the unmeasurable external moments,which are regarded as unmodeled dynamics,we propose two different full envelope attitude control strategy based on nonlinear model referenced adaptive control and incremental nonlinear dynamic inversion,respectively.In the meanwhile,we propose a full envelop velocity control strategy using acceleration model.In order to decouple the desired thrust and attitude from a given velocity trajectory,a geometric method is proposed based on the acceleration measurements from onboard inertial measurement units.Furthermore,we propose a control effectiveness enhancement algorithm to tackle the control vane saturation problem,which is effective in maintain the stability of the closed-loop system.Also,under the proposed full envelope flight control scheme,a sufficient condition in guaranteeing the ultimately uniformly boundedness of the system states is derived considering modeling uncertainties and external disturbances.(3)In this dissertation,in terms of analysis and trajectory planning for the flight mode transition process of the ducted fan tail sitter VTOL,we have introduced the concept of transition corridor which characterizes the dynamic boundary of performing flight maneuvering during the transitional flight.Focusing on a special transition process in which the transitional flight is conducted at fixed altitude,we show the computational results and corresponding analysis of the transition corridor of a small ducted fan tail sitter UAV.Besides,we have proposed a corridorbased transition trajectory planning approach,which reduces the complex trajectory planning problem into a lower order nonlinear programming.(4)In this dissertation,simulations and practical flight tests are conducted on a selfdeveloped ducted fan tail sitter UAV.Through successfully flight demonstrations of the challenging level-path-flight-mode-transition,the effectiveness of the proposed flight mode transition control approaches is verified. |
