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VGs increase drag. They are used to their most potential in aviation. Installed on the upper leading edge of the wings, and on tail surfaces ahead of control surfaces, they "re-energize the boundry layer", and postpone flow separation. This is done to increase control surface effectiveness at slow airspeed (when installed on the tail) and to decrease stall speed (when installed on the wing), NOT to reduce drag. This is seen by the reduction of cruise speed of aircraft equipped with VGs. Attached airflow does not necessarily mean less drag, especially if that attached airflow is at the cost of a bunch of swirling vortices. There ain't no free lunch.
My favorite is the truckers that install the big scoopy VGs on the very back edge of their trailers. What are they trying to get the airflow to stay in contact with? Their virtual boat tail? Ha!
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-Terry
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