[Tommy]

vector1,
First, it would be wrong to assume that anything about fluid flow could be reduced to a simple rule of thumb. Second, there are several types of boundary layers, and the forces associated with them vary (i.e., drag, lift or down-force). In general, a smooth shape that allows air to stay very close to the surface of the object allows what is called laminar flow and produces the least drag and most lift/down force. Any obstruction or sharp turn in the shape of the object is likely to cause separation of the air flow from the object, destroying the laminar flow and increasing the drag. .... If the air scoop is placed in an area where the air flow has already separated and is mostly flowing above the scoop, the scoop will not add much to the drag. But the body shape that caused the separation will. Said another way, if the scoop is on a body that doesn't have a good aerodynamic shape, it won't hurt much. But if it is on a sleek, aero body, it will.