The NACA duct or NACA scoop is a common form of low-drag intake design, originally developed by the National Advisory Committee for Aeronautics in 1945. When properly implemented, it allows air to be drawn into an internal duct, often for cooling purposes, with a minimal disturbance to the flow. The design was originally called a "submerged inlet," since it consists of a shallow ramp with curved walls recessed into the exposed surface of a streamlined body, such as an aircraft. It is especially favoured in racing car design.
Prior submerged inlet experiments showed poor pressure recovery due to the slow-moving boundary layer entering the intake. This design is believed to work because the combination of the gentle ramp angle and the curvature profile of the walls creates counter-rotating vortices which deflect the boundary layer away from the intake and draws in the faster moving air, while avoiding the form drag and flow separation that can occur with protruding inlet designs. This type of flush inlet generally cannot achieve the larger ram pressures and flow volumes of an external design, and so is rarely used for the jet engine intake application for which it was originally designed. However it is common for engine and ventilation intakes.
- Frick, Charles W.; Davis, Wallace F.; Randall, Lauros M.; and Mossman, Emmet A.: An Experimental Investigation of NACA Submerged Duct Entrances NACA ACR 5I20, 1945.