Aeromechanical Stability Of Bearingless Rotor Helicopter With Different Blade Tip Shapes And Elastomeric Lag Damper

Bearingless rotor has been widely used in new-generation helicopters for its structural simplicity,good maintenance and enhanced control power.In bearingless rotor system,the flapping,lagging and torsional motions of blade are allowed by the deformation of the composite flexbeam at the blade root.The elastomeric lag dampers with nonlinear dynamic characteristics are installed in the rotor to improve the aeroelastic stability,which increase the coupling effects among the blade's motions and complicate the aeroelastic analysis of bearingless rotor.The research in the dissertation is focused on the aeromechanical stability of bearingless rotor helicopter.The aeroelastic model of bearingless rotor/fuselage coupled system with various blade tip shapes and elastomeric lag dampers and the transient analysis method have been presented.The influences of blade tip shapes on the aeromechanical stability of bearingless rotor/fuselage coupled system were investigated.An improved Particle Swarm Optimization(PSO)method has been proposed to identify the parameters of aeroelastic models with nonlinear hysteretic properties.The influences of the nonlinear elastomeric lag damper on the aeromechanical stability of bearingless rotor/fuselage coupled system were also studied by using the Anelastic Displacement Field(ADF)model of elastomeric damper.In this dissertation,the aeroelastic equations of bearingless rotor/fuselage coupled system were derived by using the Hamilton's principle.Based on the structural characteristic of bearingless rotor,the dynamic equations were discretized by using finite element method,and the elemental matrices were integrated to obtain the finite elemental equations of bearingless rotor/fuselage coupled system and to build the calculation model for the analysis of aeromechanical stability.In the modeling process,three special problems caused by the structural characteristics of bearingless rotor were studied.The transient analysis method for the aeromechanical stability of rotor/fuselage coupled system was presented to investigate the influences of nonlinear aeroelastic models on the stability.In this method,the Newmark-Newton-Raphson method was used to calculate the transient response,and the moving block method with Hanning window was used to identify the damping of regressing lag modal.The accuracy of the calculation model and the analysis method were validated by comparing the calculated results with test results.The aeroelastic model of blade tip shapes was proposed by considering the influences of tip sweep,droop and taper on the aeroelastic equations.The integration of blade tip element with blade element was achieved by using nonlinear transformation matrices to obtain the aeroelastic equations of bearingless rotor/fuselage coupled system with various blade tip shapes.The influences of blade forward-sweep,backward-sweep,anhedral,droop,combined transformation and taper on the aeromechanical stability of bearingless rotor/fuselage coupled system were investigated.An improved PSO algorithm for the parameter identification of nonlinear hysteretic aeroelastic model was proposed in this dissertation.A special function of fitness value was defined in the algorithm to prevent the deficiency of standard PSO algorithm,such as the premature convergence and the easy trap of local optimum,and to improve the convergent speed and the accuracy.The improved PSO algorithm was successfully applied to the parameters identification of the ADF model of nonlinear elastomeric lag damper and the Leishman-Beddoes(LB)dynamic stall model.The accuracy and superiority of the improved PSO algorithm were validated by comparing the identified results with the results calculated by using the standard PSO algorithm.Based on the structural characteristics of bearingless rotor,the ADF model of elastomeric lag damper was integrated into the bearingless rotor/fuselage coupled system to obtain the aeroelastic equations of bearingless rotor/elastomeric lag damper/fuselage coupled system.The effects of nonlinear elastomeric lag damper on the aeromechanical stability of bearingless rotor/fuselage coupled system were calculated by using the proposed transient analysis method.Besides,the transient responses of regressing lag modal,blade lag modal and fuselage roll modal under different situations and the influences of the structural parameters of elastomeric lag damper on the aeromechanical stability were also investigated.