Ultra light weight and wind tunnel tested. NACA ducts bring air into a vehicle with minimal increase in drag. The purpose of a NACA duct is to increase the flow rate of air while not disturbing the boundary layer. When the cross-sectional flow area of the duct is increased, you decrease the static pressure and make the duct into a vacuum cleaner (without the drag effects of a standard scoop). The reason why the duct is narrow, then suddenly widens in a graceful arc is to increase the cross-sectional area slowly so that airflow doesn’t separate and cause turbulence and drag.