This paper focuses on the design of a device for reducing main landing gear noise, since the landing gear had been identified as one of the main noise sources in a flyover measurement of airframe noise produced by JAXA's Jet Flying Test Bed 'Hisho'. Based on a simple concept using a porous plate, the noise reduction device described in this paper was evaluated by a flight demonstration using a modified 'Hisho'. To understand the practical feasibility of the device, its geometry, strength, functionality, safety, and maintainability were investigated by several methods, such as computational simulations, wind tunnel tests, and taxiing tests. The first demonstration flight was conducted in September 2016, and the sound pressure level (SPL) around main landing gears was reduced by about 2 dB in the 1.25-5 kHz band acoustic map. The power-averaged SPLs at emission angles of 30, 60 and 120 degrees show a significant noise reduction of greater than 1 dB.
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