Aeroelastic Analysis Of Tiltrotor Aircraft In Conversion And Airplane Flight Modes

Tiltrotor aircraft has the advantages of vertical takeoff and landing,hovering and high speed crusing.The flight modes of tiltrotor aircraft include helicopter flight mode,conversion flight mode and turboprop airplane flight mode.Besides the dynamic problems occuring in helicopter and turboprop airplane,there are many severe vibration and aeroelastic stability problems occurring in tiltrotor aircraft during conversion flight and turboprop airplane flight modes.According to the characteristics in different flight modes,the aeroelastic model of tiltrotor aircraft has been established and the aeroelastic calculation and analysis were carried out in this thesis.In conversion flight mode,the continuously tilting rotor is in complex unsteady inflow and unsteady aerodynamic enviroment.In the thesis,the Peters-He unsteady inflow model and ONERA unsteady aerodynamic model,considering wake bending of rotor and reversed flow region were used in the aerodynamic model to simulate the inflow pattern in conversion flight,and the aerodynamic model was modified by compressibility and stall effects.The elastic blades and wing were modeled by using finite element method and moderate deflection beam theory,and the structural dynamic equations were derived based on the Hamilton's generalized principle,by considering the inertial coupling and rigid-flexible coupling.The aeroelastic coupling stability concerning the fuselage degrees of freedom and the full-span structure on aeroelastic coupling stability in turboprop airplane flight mode was studied.The full-span dynamic equations for analyzing aeroelastic stability in forward flight were obtained by assembling dynamic equations of the left and right rotor/ nacelle/wing systems at the center of gravity.Based on the developed nonlinear aeroelastic coupled dynamic equations,a MATLAB program was complied to conduct the model trimming and response calculation,and the nonlinear equations were linearized at trimmed position.The stability solution of the linearized dynamic equations was carried out by using Floquet theory,and the trim control inputs,the responses of each mode,aerodynamic forces and torque of each component,and the stability of dynamic system were calculated and analyzed in conversion flight and turboprop airplane flight modes.The main conclusions have been obtained by the modeling,calculation and analysis in this thesis:(1)The established half-span aeroelastic dynamic model of tiltrotor aircraft in conversion flight mode and full-span structural dynamic model in turboprop airplane flight mode can accurately reflect the aeroelastic characteristics and the motion mechanism of tiltrotor aircraft in both flight modes.(2)For the tilting rotor system,if the lift keeps constant,the collective pitch increases with the rotor tilting fowrad;and if the collective pitch is invariant,the lift of rotor decreases with the rotor tilting forward.The reversed flow region increases flapping modal damping and the stability of the system,and the effect becomes more obvious with the advance ratio grows.(3)For the half-span structure model of tiltrotor aircraft,the unsteady inflow has obvious influence on rotor lift and torque,and decreases the damping of each flapping mode.Compared with the isolated tilting rotor model,the influence of unsteady dynamic inflow on the remaining forces and moments of rotor except lift and torque is less obvious,due to the elastic deformation of wing.Since the component of forward velocity along the shaft direction becomes much greater than unsteady inflow while the rotor tilts forward and the advance ratio increases,the influence of unsteady inflow becomes insignificant.(4)In conversion flight mode,the greater the forward ratio is,the smaller the tilting angle is when the instability occurs,and the instability form of tiltrotor aircraft is the chordwise divergent motion of wing.In turboprop airplane mode,as the advance ratio reaches the critical instability speed,the instablility form of tiltrotor aircraft is the vertical divergent motion of wing.(5)The calculation results show that the fuselage degrees of freedom decrease the critical stability velocity of half-span structure for tiltrotor aircraft.Each motion mode of tiltrotor aircraft full-span structure has symmetric and anti-symmetric modal shapes,and the critical instability speed corresponding to each form is different.Some structural parameters,such as the stiffenss of wing,the position of nacelle and the nacelle length produce different influences on critical instability speed.The half-span aeroelastic stability model for tiltrotor aircraft in forward flight has been established by adopting ONERA unsteady aerodynamic model.The calculation results indicate the unsteady environment of the rotor in forward flight cannot be neglected.The innovative points of the research in this thesis are as follows:(1)The aeroelastic model of tiltrotor aircraft in conversion flight and turboprop airplane flight modes have been established,and aeroelasticity of tiltrotor aircraft in different flight modes are studied systematically and thoroughly.(2)The flight characteristics such as unsteady induced inflow,wake bending,unsteady aerodynamic forces,reversed flow effect and compressibility and stall were considered in the aeroelastic stability analysis for tiltrotor aircraft in conversion flight mode.(3)The study on aeroelastic stability of tiltrotor aircraft in turboprop airplane mode was conducted by calculating and drawing the motion trajectory of hub center,intuitively describing the motion images of rotor/nacelle/wing coupled system before and after the whirl flutter occurs.