| Summary: | Lead-lag dampers are present in most rotors to provide the required level of damping in all flight conditions. These dampers are a critical component of the rotor system, but they also represent a major source of maintenance cost. In present rotor systems, both hydraulic and elastomeric lead-lag dampers have been used. Hydraulic dampers are complex mechanical components that require hydraulic fluids and have high associated maintenance costs. Elastomeric
dampers are conceptually simpler and provide a ``dry" rotor, but are rather costly. Furthermore, their damping characteristics can degrade with time without showing external signs of failure. Hence, the dampers must be replaced on a regular basis. A semi-active friction based lead-lag damper is proposed as a replacement for hydraulic and elastomeric dampers. Damping is provided by optimized energy dissipation due to frictional forces in semi-active joints. An actuator in the joint modulates the normal force that controls
energy dissipation at the frictional interfaces, resulting in large hysteretic loops.
Various selective damping strategies are developed and tested for a simple system containing two different frequency modes in its
response, one of which needs to be damped out. The system reflects the situation encountered in rotor response where 1P excitation is present along with the potentially unstable regressive lag motion. Simulation of the system response is obtained to compare their effectiveness. Next, a control law governing the actuation in the
lag damper is designed to generate the desired level of damping for performing adaptive selective damping of individual blade lag
motion. Further, conceptual design of a piezoelectric friction based lag damper for a full-scale rotor is presented and various factors
affecting size, design and maintenance cost, damping capacity, and power requirements of the damper are discussed. The selective semi-active damping strategy is then studied in the context of
classical ground resonance problem. In view of the inherent nonlinearity in the system due to friction phenomena, multiblade transformation from rotating frame to nonrotating frame is not
useful. Stability analysis of the system is performed in the rotating frame to gain an understanding of the dynamic characteristics of rotor system with attached semi-active friction
based lag dampers. This investigation is extended to the ground resonance stability analysis of a comprehensive UH-60 model within the framework of finite element based multibody dynamics
formulations. Simulations are conducted to study the performance of several integrated lag dampers ranging from passive to semi-active ones with varying levels of selectivity. Stability analysis is performed for a nominal range of rotor speeds using Prony's method.
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