Acoustically Tailored Composite Rotorcraft Fuselage Panels
PENNSYLVANIA STATE UNIV STATE COLLEGE APPLIED RESEARCH LAB
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A rotorcraft roof 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 corecomposite facesheet panel. The original panel has coincidence frequencies near 700 Hz, leading to poor TL across the frequency range of 1 to 4 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 double panel system to less than 500 Hz. 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 VHB 9469 viscoelastomer from 3M were also included in the facesheet ply layups, increasing panel damping loss factors from about 0.01 to 0.05. Measurements in the NASA SALT facility show the optimized panel provides 6-11 dB of acoustic transmission loss improvement, and 6-15 dB of structure-borne sound reduction at critical rotorcraft transmission tonal frequencies. Analytic panel TL theory simulates the measured performance quite well. Detailed finite elementboundary element modeling of the baseline panel simulates TL slightly more accurately, and also simulates structure-borne sound well.
- Structural Engineering and Building Technology