All the NACA ducts I've seen (admittedly, in cars and airplanes) operate at very shallow angles of attack, which is to say that the incident airflow is nearly parallel to the surface in which the ducts are placed. The idea was to induct air through the surface (usually for cooling) without disturbing the relatively laminar flow along the surface, and thus increasing drag. Except for the very thin boundary layer, this is not true of a windshield. So the whole idea of a NACA duct becomes a hole in the windshield, I'd expect. Probably better to enlarge the vent at the base of the windshield, so the fairing directs the air up the back side of the windshield. The 'NACA ducts' shown are vents from the high pressure side of the windshield to the low-pressure side, and are unlikely to generate much flow up the backside, as the sharp edges will separate the airflow from the back surface. Of course, I could be wrong. Anyone got yarn-tuft pictures at speed? Of course, you could add deflectors behind the 'ducts' to attach the incoming air to the back surface.
Last edited by Jimding; 08-20-2009 at 08:34 AM.