Abstract: A turbo fan engine nacelle houses an engine which forces engine exhaust through a primary nozzle and a fan nozzle positioned along a common central axis. The primary nozzle is bordered interiorly by a primary nozzle plug and bordered exteriorly by a primary nozzle sleeve. The fan nozzle is bordered interiorly by a core cowl and bordered exteriorly by a fan nozzle sleeve. The primary nozzle plug includes a generally cone shaped outer surface which is concave when viewed in side elevation. The concave outer surface has a radius of curvature (Rc). The core cowl has a distal end which is radially spaced from the outer surface of the primary plug by a distance (&Dgr;H) in a direction perpendicular to the central axis. The primary nozzle plug and core cowl define a ratio Rc/&Dgr;H between approximately 14 and 18. The distal end of the core cowl has a core cowl radius Rp with respect to the central axis.