Coupled Fluidic Vibration Isolators for Rotorcraft Pitch Link Loads Reduction

Abstract

Replacing rigid pitch links on rotorcraft with coupled fluidic devices has the potential to reduce the aerodynamic blade loads transmitted from the blade root to the swashplate. An analytical model of two coupled fluidic isolators is derived and experimentally validated for even and odd harmonic pitch link loads. The system consists of two elastomeric pumpers with fluid chambers that are coupled by an inertia track. This passive fluidic device can be tuned to reduce the transmitted force at a particular odd harmonic of the rotor speed by tailoring the fluid inertance in the inertia track. Benchtop experimental results agree with theory, demonstrating a reduction in odd harmonic pitch link loads of up to 90% compared to the system without fluid. The coupled fluidic isolators also significantly reduce transmitted loads relative to a rigid pitch link over a wide frequency range. Simulation of a UH-60 Blackhawk retrofit example shows potential for targeted odd harmonic excitation loads reduction up to 94% for multiple frequencies without affecting the even harmonic excitation response.

Coupled Pitch Links for Multiharmonic Isolation Using Fluidic Circuits

“…A design procedure to reduce transmitted forces at one out-of-phase harmonic and one in-phase harmonic may be deduced from Eqs. (14) and (15). First, select an elastomer stiffness that provides sufficient 1/rev load transmission and an acceptable torsional natural frequency.…”

“…The required piston area may now be calculated from Eq. (14). If the calculated piston area is too large, the fluid track length may be increased, or the fluid track diameter may be reduced.…”

“…2) has two pitch links that are coupled by a horizontal fluid-filled tube of diameter d and length 2L, which gives loads reduction at an odd harmonic (Ref. 14). The fluid in the horizontal tube has density ρ and lumped inertance 2I , where…”