NACA duct

[2][3] Prior submerged inlet experiments showed poor pressure recovery due to the slow-moving boundary layer entering the inlet.

The NACA 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.

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 greater 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.

[4][5] Sports cars featuring prominent NACA ducts include the Ferrari F40, the Lamborghini Countach, the 1996–2002 Dodge Viper, the 1971–1973 Ford Mustang, the 1973 Pontiac GTO, the 1979 Porsche 924 Turbo, the Maserati Biturbo, the Nissan S130, and the Porsche 911 GT2.