Rotorcraft Transmission Noise Path Model, Including Distributed Fluid Film Bearing Impedance Modeling

A computational approach for simulating the effects of rolling element and journal bearings on the vibration and sound transmission through gearboxes has been demonstrated. The approach, using ARL/Penn State s CHAMP methodology, uses Component Mode Synthesis of housing and shafting modes computed using Finite Element (FE) models to allow for rapid adjustment of bearing impedances in gearbox models. The approach has been demonstrated on NASA GRC s test gearbox with three different bearing configurations: in the first condition, traditional rolling element (ball and roller) bearings were installed, and in the second and third conditions, the traditional bearings were replaced with journal and wave bearings (wave bearings are journal bearings with a multi-lobed wave pattern on the bearing surface). A methodology for computing the stiffnesses and damping in journal and wave bearings has been presented, and demonstrated for the journal and wave bearings used in the NASA GRC test gearbox. The FE model of the gearbox, along with the rolling element bearing coupling impedances, was analyzed to compute dynamic transfer functions between forces applied to the meshing gears and accelerations on the gearbox housing, including several locations near the bearings. A Boundary Element (BE) acoustic model was used to compute the sound radiated by the gearbox. Measurements of the Gear Mesh Frequency (GMF) tones were made by NASA GRC at several operational speeds for the rolling element and journal bearing gearbox configurations. Both the measurements and the CHAMP numerical model indicate that the journal bearings reduce vibration and noise for the second harmonic of the gear meshing tones, but show no clear benefit to using journal bearings to reduce the amplitudes of the fundamental gear meshing tones. Also, the numerical model shows that the gearbox vibrations and radiated sound are similar for journal and wave bearing configurations

Quieting a Rib-Framed Honeycomb Core Sandwich Panel for a Rotorcraft Roof

A rotorcraft roof composite sandwich panel has been redesigned to optimize sound power transmission loss (TL) and minimize structure-borne sound for frequencies between 1 and 4 kHz where gear-meshing noise from the transmission has the most impact on speech intelligibility. The roof section, framed by a grid of ribs, was originally constructed of a single honeycomb core/composite face sheet sandwich panel. The original panel has acoustic coincidence frequencies near 600 Hz, leading to poor TL across the frequency range of 14 kHz. To quiet the panel, the cross section was split into two thinner sandwich subpanels separated by an air gap. The air gap was sized to target the fundamental mass-spring-mass resonance of the panel system to less than 500 Hz, well below the frequency range of interest. The panels were designed to withstand structural loading from normal rotorcraft operation, as well as "man-on-the-roof" static loads experienced during maintenance operations. Thin layers of viscoelastomer were included in the face sheet ply layups, increasing panel damping loss factors from about 0.01 to 0.05. TL measurements show the optimized panel provides 611 dB of acoustic TL improvement and 615 dB of structure-borne sound reduction at critical rotorcraft transmission tonal frequencies. Analytic panel TL theory simulates the measured performance within 3 dB over most frequencies. Detailed finite element/boundary element modeling simulates TL slightly more accurately, within 2 dB for frequencies up to 4 kHz, and also simulates structure-borne sound well, generally within 3 dB