Research On Helicopter Rotor Lead-lag Damper Nonlinear Dynamics Characteristics

The helicopter rotor lead-lag damper dynamic characteristic modeling has the important role and significance for helicopter rotor system dynamic analysis in all three major research fields that is structural loads, motion response and system stability. The difficult part of this work is the variety of dampers means different working principle and the nonlinearity of the stress-strain relationship, for example the elastomeric damper with the strongest nonlinearity and its dynamic characteristic is highly influenced by excited frequency、strain amplitude、temperature,also the nonlinear stiffness of the elastomeric material and the dual frequency excitation during helicopter forward flight which contains both first order lead-lag mode frequency and 1/rev frequency can’t be ignored. The existing model can’t cover all those problem above and the lacking of a unified form both has brought inconvenience to rotor system dynamic modeling. An research on helicopter lead-lag damper nonlinear dynamic characteristic modeling is presented in this dissertation, a few helicopter elastomeric lead-lag damper dynamic models is developed to catch all the influence factor has mentioned before, and one approach has extended on fluid-elastomeric damper and MRFE damper(Magnetorheological Fluid-Elastic lag damper) dynamic modeling, also the validation of those model is presented.Aiming on the elastomeric damper as the study topic because its strongest nonlinearity, based on the internal variables based constitutive model of elastomeric material and the influence of temperature is considered an modified constitutive model of elastomeric material is developed. By using this model and considering the structural characteristics of the embedded multi-layer elastomeric damper typically used for helicopter rotor system, an embedded multi-layer elastomeric damper model is developed, and the dynamic characteristic of the embedded elastomeric damper under high frequency excitation is studied. In order to reflect the nonlinear stiffness of the elastomeric material, and generalized the form of the elastomeric damper dynamic model, by adding the nonlinear spring model an internal variables constitutive model of elastomeric material with adjustable stiffness is developed, based on this model two elastomeric damper model characterization with different stiffness variation type with was completed and validated.An experimental study of elastomeric damper dynamic characteristic was made, by testing the embedded multi-layer elastomeric damper test piece and the tubular elastomeric damper test piece the hysteresis of stress-strain was collected, the data was used for theoretical model characterization and model validation. And stress-strain hysteresis with different stiffness variation type was also collected.Because the internal variables based model contains too many model parameters and it was inconvenient for extending on other types of lead-lag damper, a fractional derivative model of elastomeric damper is developed. The model is established by replacing the first derivative term of an integer calculus differential dynamic equation by an fractional derivative term and adding an variable order equation to gain the ability of catching the nonlinear stiffness of elastomeric material. And the problem of solving this equations is the cost of calculation increasing by the second power function of the length of the solving interval length, by using the Short-Memory Principle, the increasing relationship is decreased to linear function. The model is validated by numerical simulations and compare the result from prediction and testing under different excited frequency and strain amplitude and dual frequency condition.The approach of elastomeric dynamic characteristic modeling based on the fractional calculus theory is attempted to extend on fluid-elastomeric damper and MRFE damper. By adding the initial order variable function, the influence of current amplitude on MRFE damper dynamic characteristic is considerate. And the possibility of using fractional derivative elastomeric damper model in research on fluid-elastomeric damper and MRFE damper dynamic characteristic is validated.