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As an integral part of NASA's goal to reduce aircraft noise, efforts are under way to develop the technology to measure and attenuate airframe noise generated by various components of transport aircraft. Noise caused by approach-deployed landing gear on modern transport aircraft is comparable to that from other airframe noise sources (excluding the engines). Total airframe noise is comparable to engine noise on approach. To understand and alleviate the problem, Ames Research Center personnel, in collaboration with Langley Research Center personnel, recently completed the first wind-tunnel study of a 26%-scale model of a Boeing 777 landing gear (fig. 1).
Using a 70-element phased microphone array developed at Ames, detailed noise maps and spectra were obtained from the 26%-scale landing-gear model in the Ames 7- by 10-Foot Wind Tunnel. Using a novel microphone assembly recessed behind a Kevlar cover flush with the wind-tunnel wall, flow noise about the array microphone was minimized, allowing landing-gear noise sources as small as 6 millimeters to be identified (23 millimeters full scale). A typical phased microphone array image of the measured noise sources is shown in figure 2. Over 50 configurations were studied, starting with a baseline six-wheel bogey, which included complex stereolithic plastic struts, hoses, wire harness, brakes, gear door, etc., to model the flight article as accurately as possible. Yaw and truck angles were varied. Various combinations of parts were removed from the baseline to rank-order noise sources. In addition, a full-faired bogey, and a simple tube and wheel mockup were evaluated. Preliminary data analysis indicates that the faired landing gear generated considerably less noise than the unmodified gear and, though full fairings may not be commercially practical, the data represent a probable lower limit of landing gear noise. The simplified tube and wheel geometry also generated less noise than the unmodified gear. Noise directivity for both flyover and sideline were obtained by mounting the landing gear model from the test section floor, ceiling, and wall. Different noise sources were important in different directions. Because drag is an important benefit of landing gear, drag data from the wind-tunnel balance will be correlated with acoustic data. Wake flow velocities were mapped to aid efforts at Langley Research Center to model landing-gear wakes and potential interactions with high-lift systems.
The 26%-scale landing-gear model was designed to attach to the 26%-scale STAR wing, scheduled for testing in the Ames 40- by 80-Foot Wind Tunnel by the end of FY01. In collaboration with Boeing, Langley Research Center, and others, Ames researchers will measure airframe flyover noise and surface wing pressures with and without the landing gear deployed in simulated approach conditions. Various noise control devices will be evaluated. The data will be collected using the Ames phased microphone array.
Point of Contact: P. Soderman/C. Horne
(650) 604-6675/(650) 604-4571
psoderman@mail.arc.nasa.gov
chorne@mail.arc.nasa.gov
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Fig. 1. Scale model of Boeing 777 landing gear in the Ames 7- by 10-Foot Wind Tunnel.
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Fig. 2. Phased microphone array image of noise sources.
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