Helicopter blade lag damping using embedded fluid elastic inertial dampers

The potential for embedded fluid elastic inertial dampers to provide damping adequate to ensure the aeromechanical stability of the rotor blade lag mode is investigated. An embedded fluid elastic inertial damper is a small, single degree of freedom system that consists of a mass, rigidly connected to a fluid vessel, on an elastomeric spring located in the rotor blade cavity. The damper is tuned to a specific problem frequency and oscillates out-of-phase with the rotor blade resulting in an inertial moment about the lag hinge. This attenuates the lag motion of the rotor blade. Damper mass motion forces fluid through the inner chamber of the fluid vessel, creating a force that reduces the effective dynamic stiffness of the device. As a result, fluid elastic inertial dampers have a high static stiffness, which enables them to resist significant centrifugal forces, and a low dynamic stiffness, which permits a low damper tuning frequency, necessary for the aeromechanical stability of the rotor system. A parametric study was conducted using a rigid blade and embedded damper model. The study revealed that an embedded fluid elastic inertial damper is capable of producing adequate rotor blade lag damping within a desirable frequency band to ensure the aeromechanical stability of the system. For a representative application, embedded fluid elastic dampers are capable of producing approximately 1 % - 4% damping using approximately 5% - 10% of the blade mass. The study confirmed that using an appropriately configured fluid elastic element created a damper that is stiff enough to resist the high centrifugal loads, while maintaining the desired tuning frequency for rotor blade lag damping. Furthermore, the results of the study yielded design guidelines for rotor blade lag damping with embedded fluid elastic inertial dampers.