Perforated fairings for landing gear noise control

Abstract

Landing gears of commercial aircraft make an important contribution to total aircraft noise in the approach configuration. Using fairings to shield components from high speed impingement reduces noise. Furthermore, perforating these fairings has been confirmed by flight tests to further enable noise reduction. Following a more fundamental study on the application of perforated fairings, a study has been performed to investigate and optimize the benefits of bleeding air through landing gear fairings. By means of wind tunnel tests, an aerodynamic and acoustic survey has been performed on a simplified generic main landing gear to explore the influence of (perforated) fairings on the lower part of the gear.
The results show that for this specific case, the application of impermeable fairings reduces noise in the mid- and high frequency range by shielding sharp edged components from high velocity impingement. However, below 1 kHz the noise is shown to increase significantly. Application of the perforations is shown to diminish this low frequency increase whilst maintaining the reduction in the mid- and high frequency range. The aerodynamic and acoustic measurements point in the direction of the separated flow of the fairings interacting with the downstream gear components responsible for the low frequency noise increase. Bleeding of the air through the fairings reduces the large scale turbulence in the proximity of these components and hence diminishes the low frequency increase. A sharp noise increase is present in the ground view direction at the 500 Hz centred tertsband for the solid fairing, which is confirmed to originate from the articulation link fairing. The flow around the wheels remains largely unaltered by application of the fairings. Furthermore the application of fairings is shown to suppress vortex shedding from the lower part of the main leg.
A study on the effectiveness of perforation location reveals that the stagnation area perforations are responsible for most of the low frequency noise decrease. Both articulation link and undertray fairing contribute to the reduction. Exposing the perforate outside the stagnation area induces perforate self-noise, most significantly on the sides of the lower articulation link fairing. Agreeing with previous studies, the phenomenon scales with the local component of the shearing flow velocity and orifice diameter.

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