A variable hydraulic damper for vibration reduction in helicopter blades

This study investigates the potential aeronautical application of structural control technology developed in earthquake engineering to reduce the vibration of helicopter blades. The major objective is to evaluate and to apply the semi-active control method. Reducing vibrations for helicopter blades is quite similar to the approach of earthquake protective systems. We need not regulate the displacement of blades to a certain value, or to force the blade vibration to track given time histories. Instead, we only want to limit the vibration level within an acceptable range.; Conventional blade vibration reduction by adding passive damping is ineffective. In this study, the concept of semi-active control is developed and successfully applied. The control philosophy is, first, to change the system stiffness so as to avoid resonance, to reduce input energy, and to lower the blade's displacement. The damping is applied to further reduce the response, to dissipate the imposed energy, and to minimize the structural damage. The semi-active control system originally developed for earthquake engineering application is modified to reduce the blade vibration.; As the essential element of semi-active control technology, a variable hydraulic damper is designed. Simulations are carried out to develop a mechanical model for the variable hydraulic damper. In the simulation, several nonlinearities are considered such as Karnopp friction model and cubic stiffness model. The superior performance by a variable damper is quantitatively observed from damper component tests, damper fatigue tests, and blade dynamic tests. The simulation results correlate well with experiments in both the force-displacement relation and the force-velocity relation.; Based on the mechanical model, a Kelvin-Voigt type of analytical model is developed, which is used in the finite element analysis of the blade with the variable damper. It is shown that the analytical model predicts the behavior of the damper measured in experiments.; Successful application of the seismic control technology to vibration control of helicopter blades has provided beneficial feed-backs to seismic response control. Several such feedbacks have been identified through the studies on the simplified model. The optimal damping exists and switching control states can achieve optimal structural responses.