Coupled Pitch Links for Multiharmonic Isolation Using Fluidic Circuits

Abstract

Replacing stiff pitch links on rotorcraft with coupled fluidic devices has the potential to reduce the aerodynamic blade loads transmitted through the pitch links to the swashplate. Analytical models of two fluidic devices coupled with three different fluidic circuits are derived. These passive fluidlastic systems are tuned, by varying the fluid inertances and capacitances of each fluidic circuit, to reduce the transmitted pitch-link loads. The different circuit designs result in transmitted pitch-link loads reduction at up to three main rotor harmonics. The simulation results show loads reduction at the targeted out-ofphase and in-phase harmonics of up to 88% and 93%, respectively. Experimental validation of two of the fluidic circuits demonstrates loads reduction of up to 89% at the out-of-phase isolation frequencies and up to 81% at the in-phase isolation frequencies. NomenclatureA piston area, m 2 C a accumulator capacitance, m 3 /Pa C p pitch-link capacitance, m 3 /Pa c d elastomer damping, N · s/m c o baseline damping, N · s/m x o i mass displacement, i = 1, 2 m ρ fluid density, kg/m 3 rotor frequency, rad/s ω angular frequency of excitation, rad/s