A ducted fan air-vehicle having a duct lip shape is described. The duct lip has an internal duct lip portion and an external duct lip portion, both of which are substantially curvature continuous around the circumference of the air duct. The duct has a plurality of cross-sections along its circumference and at least some of the plurality of cross-sections have a different shape. Further, each cross-section around the circumference of the duct has an a/b ratio and the a/b ratio for each cross-section is substantially the same.