Most advanced helicopter rotors are typically fitted with lag dampers, such as elastomeric or hybrid fluid-elastomeric (FE) lag dampers, which have lower parts counts, are lighter in weight, easier to maintain, and more reliable than conventional hydraulic dampers. However, the damping and stiffness properties of elastomeric and fluid elastomeric lag dampers are non-linear functions of lag/rev frequency, dynamic lag amplitude, and operating temperature. It has been shown that elastomeric damping and stiffness levels diminish markedly as amplitude of damper motion increases. Further, passive dampers tend to present severe damping losses as damper operating temperature increases either due to in-service self-heating or hot atmospheric conditions. Magnetorheological (MR) dampers have also been considered for application to helicopter rotor lag dampers to mitigate amplitude and frequency dependent damping behaviors. MR dampers present a controllable damping with little or no stiffness. Conventional MR dampers are similar in configuration to linear stroke hydraulic type dampers, which are heavier, occupy a larger space envelope, and are unidirectional. Hydraulic type dampers require dynamic seal to prevent leakage, and consequently, frequent inspections and maintenance are necessary to ensure the reliability of these dampers. Thus, to evaluate the potential of combining the simplicity and reliability of FE and smart MR technologies in augmenting helicopter lag mode stability, an adaptive magnetorheological fluid-elastomeric (MRFE) lag damper is developed in this thesis as a retrofit to an actual fluid-elastomeric (FE) lag damper. Consistent with the loading condition of a helicopter rotor system, single frequency (lag/rev) and dual frequency (lag/rev at 1/rev) sinusoidal loading were applied to the MRFE damper at varying temperature conditions. The complex modulus method was employed to linearly characterize and compare the performance of the MRFE damper with the baseline FE damper performance. Based on experimental measurements, it is shown in the research that at all test temperatures, a significant damping control range, extending beyond the baseline FE damper, can be provided by the MRFE damper with the application of varying magnetic fields. This controllable damping range can be programmed to potentially provide the required damping augmentation as a function of different flight conditions. The added benefits of employing smart MR fluids in MRFE lag dampers are to produce adequate damping at critical flight conditions while concurrently reducing periodic hub loads at other flight conditions and to compensate damping losses associated with temperature.

The other main objective of the present research is to develop and formulate a comprehensive analytical model that can accurately describe the non-linear hysteretic behavior that is demonstrated by the MRFE lag damper. Thus, a hydromechanical model, which can delineate the physical flow motion of the system and accurately describe the non-linear hysteretic behavior of the MRFE damper is proposed. The hydromechanical model explored in this study is a design-based model which describes the damper system with a series of lumped hydraulic, mechanical and magnetorheological components. The model employs physical parameters such as inertia, damping, yield force and compliances that are dependent on damper geometry and material properties of components and which can potentially be approximated a priori. Further, temperature variation will mainly cause material properties to change. Once model parameters have been established, the model is shown to simulate accurately the measured hysteretic force-displacement history under single and dual frequency excitations, and varying temperatures.