One possible method for reducing aircraft flyover noise is to site the engines so that the wing vortex can refract sound away from the ground. A series of experiments was carried out in the RAE 24-ft wind tunnel using a model of the HP 115 slender delta research aircraft, which produced a strong leading-edge vortex when set at incidence. The engine noise was simulated by a Hartmann whistle mounted above the engine intake. The results are compared with a theoretical prediction based on ray theory and a simplified representation of the wing vortex structure. I. Introduction T HE reduction of aircraft flyover noise and of the nuisance to communities living near major airports is a subject of continuing importance. Although considerable benefits have already been achieved from imporvements in engine design and acoustic treatment, further substantial noise reductions below the lowest levels of current aircraft may only be achieved with an acceptable economic penalty if, in addition, new design and operational features that can affect the amount of noise reaching the ground may be efficiently utilized. Among design features, application of airframe shielding techniques has been shown to be effective for simple localized sources, although for realistic engine noise sources prediction of shielding under flight conditions with reasonable accuracy still presents some difficulties. Another possibility, which is the subject of an idealized study in this paper, is to use the vortex system trailing from the wing to refract the engine noise away from the ground to give noise reduction benefits. The results presented in this paper are complementary to those of Jeffery and Holbeche.' In both cases, experiments were made to measure the noise alleviation due to shielding by the airframe and refraction by the airflow for the Handley Page 115 slender delta research aircraft (Fig. 1) fitted with an intense noise source on top of the engine nacelle. Originally built to provide information on the low-speed handling qualities of a slender-winged aircraft, it subsequently lent itself to acoustic studies through having a relatively simple planform shape amenable to diffraction calculations and a well-defined leading-edge vortex structure which simplified calculation of the effects of vortex refraction. In the previous work, emphasis was placed on measuring the noise in the shadow region under the aircraft, where there was a substantial noise reduction due to shielding by the delta wing. Measurements were made in flight and at model scale in a wind tunnel and compared with shielding levels predicted by a simple theory based on Cooke's2 asymptotic approximation to the exact solution for sound diffraction by a half-plane. The accuracy of this approximation under ideal conditions was shown to be good from comparisons with measured sound diffraction around a screen with a straight sharp edge mounted between the walls of an anechoic chamber.