Wing Effect on Jet Noise Propagation

Jet aircraft with engines under the wings may produce higher flyover noise levels than similar aircraft with other engine mounting arrangements. To determine the cause of higher flyover noise of such aircraft, an experimental investigation was performed in an anechoic chamber. Basic experimental apparatus consisted of an ASME 15.24 cm (6 in.) diameter converging nozzle and a wing section which corresponded to the horizontal projection area of a portion of a wing of a typical jetliner. Results of this experiment indicate that the presence of the wing in the vicinity of the jet enhances the noise produced by the jet alone. This noise enhancement may be attributed to two sources. The boundary layer generated on the surface of the wing as the result of entrainment of the air into the region between the wing and the jet is believed to be responsible for the low-frequency noise enhancement. Reflection of jet noise incident on the wing surface contributes to enhancement of noise primarily at high frequency. The jet is found to have considerable effect on noise enhancement at high frequency where strong refraction effects on sound waves occur. The substantial enhancement of high-frequenc y noise measured in planes at oblique angles to the wing surface may require consideration in aircraft noise prediction and design. Based on static test results, it appears that the wing effect may increase the sideline noise levels of aircraft during takeoff.