.Here's a drawing of the outer lip of the NACA curved-divergent submerged duct from RM # A 7130 . BTW, if you plan on putting a "liquid drain duct" on your plane for those really long trips after the morning coffee, have the drain extend out into the freestream about an inch to keep the liquid from wetting the adjacent surfaces. If an airfoil shape is placed around, say, a 1/4" drain tube, drag will be minimal, and flow discharge will actually produce negative drag! ref. Ardograph #264, para.6.3.4, drains.