Load Reduction in Lead-Lag Dampers by Speed-Scheduled Aperture and Modulated Control of a Bypass Valve

Existing hydraulic dampers are passive devices, and their damping characteristics cannot be adapted to the varying damping needs of each different flight condition of a rotorcraft. Therefore, dampers and their interfaces to the rotor system are constantly subjected to large damping forces, which contribute to their damage and wear. In this work, we consider the possible use of a bypass valve as a means to reduce loads while still providing the required amount of damping for safe flight. Two different policies for the use of the bypass are considered: In the first and simplest of the two, the valve is opened of a constant amount, which varies only as a function of the vehicle flight speed, whereas in the second the valve aperture is modulated as a function of blade azimuth using a higher harmonic feedback law. The investigation on the load-reduction capabilities of the bypass augmented damper is conducted by using validated mathematical models of the A109E Power helicopter and of its damper. It is found that both policies are capable of delivering the required level of damping at substantially reduced loads, which for the speed-scheduled aperture drop at values that range between 30% and 60%, depending on the flight condition, of the loads generated by the standard passive device, and for the higher harmonic modulated law at values around 20%–25%.