Control Augmentation Research For A Flying Wing Demonstration Aircraft Based On The Free Flight Tests Using Sub-scaled Model In Wind Tunnel

A significant improvement of radar stealth,aerodynamic and load efficiency makes the flying wing configuration one of the most promising candidates for next-generation fighter,bomber and civil aircraft,which has gained much research interest of the aviation industry.Compared with the conventional aircraft configuration,the flying wing aircraft cancels the vertical and horizontal tail while highly blending the wing and body.As a result,it is always associated with more serious coupling of the aerodynamics,control,dynamics and structure,etc.To validate and evaluate the aerodynamic,control and flight characteristics with an integrated approach in the early design process for remarkably improved efficiency of research time and cost,it is urgent to develop an innovative technique for modeling,analysis and test in the wind tunnel.In this paper,the research is focused on validation and evaluation of the control augmentation approach for a flying wing demonstrator,which is modeled,analyzed and tested based on the wind tunnel flight tests.The main contents are as following:(1)Research on modeling of the flying wing flight dynamics in the wind tunnel.According to the dynamic similarity criterion,the physical parameters of the sub-scaled flying wing demonstrator are given,the composition and principle of the wind tunnel free flight system are introduced,and the hypothesis and coordinates related to the modeling of the flying wind demonstrator in the wind tunnel are also presented;the nonlinear kinematic and dynamic equations for the 6-DOF and 3-DOF free flights are derived based on the Newton-Euler’s laws,which are further linearized and decoupled at the trimmed equilibrium;the aerodynamic model is constructed with the static & dynamic wind tunnel test data,and the aerodynamic characteristics are analyzed;the engine,surface actuators and sensors are modeled with experimental calibrated data.(2)Analysis of the flying wing demonstrator’s global stability and controllability based on the method of bifurcation.The main issues that the flying wing demonstrator may encounter in the aspect of handling and stability performance are highlighted by comparing its mass,inertia and aerodynamic characteristics with the conventional aircraft;the equilibrium branches along with the associated modal characteristics are calculated using the method constrained bifurcation analysis for various combination of the longitudinal margin of static stability,directional control setup and degrees-of-freedom in the flight tests;According to the bifurcation curves,the reasonable range of the longitudinal margin of static stability is selected,the only effective measure to achieve ideal directional handling and stability performance is discovered to be the control augmentation system,and the feasibility of simulating the flying wing’s flight dynamics and validating the designed control laws with the 3-DOF free flight test setup is also validated.(3)Research on the approach of attitude stabilization based on the nonlinear dynamic inversion method composed with H∞.The longitudinal and lateral/directional nonlinear equations of the flying wing demonstrator are simplified and decomposed to the summarization of linear state dynamics,measurable nonlinear state dynamics,affine nonlinear control effectiveness and nonlinear errors;the method of nonlinear dynamic inversion is then employed to linearize the affine nonlinear control effectiveness,and convert the initial nonlinear system to be a linear one,which is compatible with the framework of H∞ controller syntheses;the designed attitude controller is adequate to stabilize the flying wing demonstrator and track the references well without significant response to the motion couplings and parameter disturbances.(4)Research on the approach of attitude stabilization based on the nonlinear dynamic inversion method composed with disturbance observer.The modeling error of the affine nonlinear equations of the flying wing demonstrator and the external disturbances including the gust and turbulence are taken into account as the lumped disturbances,which could be estimated by the nonlinear disturbance observer and compensated by the nonlinear dynamic inversion compensator to decrease the system error of feedback linearization and improve the closed-loop performance of disturbance rejection;the resulting nonlinear disturbance observer based dynamic inversion controller demonstrates good reference tracking,response decoupling and disturbance rejection in the attitude control simulations,and succeeds to keep the risks of Autoland at an acceptable level.(5)Free flight validation and evaluation of the flying wing demonstrator in the wind tunnel.The technique of integrated aerodynamics/control/flight validation and evaluation for the flying wing layout in the wind tunnel is developed.The 3-DOF free flight tests are carried out with an unpowered sub-scale model,which is stable and controllable before stall while being stabilized with the control augmentation system.Thus,the effectiveness and robustness of the implemented flight control laws are verified.The 6-DOF free flight tests are performed with a powered sub-scale model,which is held for 1g level flight at different airspeeds.Hence,the adopted flight control laws are further validated.During the free flight tests,the pilots are responsible to handle the aircraft model and then rating the closed-loop performance qualitatively according to the modified Cooper-Harper criterion.Besides,the closed-loop performance is evaluated quantitatively by applying the method of low-order-equivalent-system to the flight test scenarios excited via the standard reference inputs.In this thesis,an innovative approach for aerodynamic/control/flight integrated validation and evaluation for the flying wing layout is developed based on the free flight test technique in the wind tunnel.The proposed approach is intended to validate and evaluate the innovative directional effectors,the aircraft’s global stability &controllability and the flight control schemes at the early stage of flying wing design.It is expected that the time and economic costs will be largely saved by introducing this practical and prospective research approach to the industrial projects.