A NACA duct, also sometimes called a NACA scoop or NACA inlet, is a common form of low-drag air inlet design, originally developed by the U.S. National Advisory Committee for Aeronautics (NACA), the precursor to NASA, in 1945.
Prior submerged inlet experiments showed poor pressure recovery due to the slow-moving boundary layer entering the inlet. 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 inlet and draws in the faster moving air, while avoiding the form drag and flow separation that can occur with protruding scoop designs.
When properly implemented, a NACA duct allows air to flow 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.
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, such as the North American YF-93 and Short SB.4 Sherpa. It is, however, common for piston engine and ventilation intakes.
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Practical experience of fabricating simple NACA ducts for ventilation of a HPV