Low-frequency acoustic shielding by the silent aircraft airframe

The Silent Aircraft airframe has a flying-wing design with a large wing planform and a propulsion system embedded in the rear of the airframe with intake on the upper surface of the wing. In the present paper, boundary-element calculations are presented to evaluate acoustic shielding at low frequencies. Besides the three-dimensional geometry of the Silent Aircraft airframe, a few two-dimensional problems are considered that provide some physical insight into the shielding calculations. Mean-flow refraction effects due to forward-flight motion are accounted for by a simple time transformation that decouples the mean-flow and acoustic-field calculations. It is shown that a significant amount of shielding can be obtained in the shadow region where there is no direct line of sight between the source and observer.