Load reduction in lead-lag dampers by speed-scheduled aperture and modulated control of a by-pass 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 by-pass 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 by-pass 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, while 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 by-pass augmented damper is conducted by using validated coupled physics-based mathematical models of the A109E 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 drop at values that range between 30-60%, depending on the flight condition, of the loads generated by the standard passive device for the speed-scheduled aperture, and at values around 20-25% for the higher harmonic modulated law.