Keep in mind that the air affected by both Aircraft and Cars is a Quantum Leap greater in volume than the air within 1-2 inches of the skin. I have seen pictures of aircraft flying over water and creating a disturbance on the surface 150 feet below....as in Directly Under the aircraft, not a wake. Even experienced pilots and people who design aircraft don't really understand how lift is generated, it is Vodoo stuff, anyone who tells you they know exactly how it works is full of their own hot air. The classic "Faster across the top low pressure theory" is crap. Every time a plane flies upside down in an air show (or anywhere else really) without immediately slamming into the ground dispels that myth. Current theory holds that a large volume of air is being directed downward by the wing, the opposite reaction to this downward thrusted air is upward thrusted airplane.
But don't take my word for it, read about it here.
Nasa saying "Traditional" explanation of how a wing works is Horse Hockey
Look around the NASA sight to help yourself understand what we don't really know better. This page, for example, says what I said lift is...maybe.
Nasa Tries to Explain Lift
I reason that Car aerodynamics are very similar in complexity and misconceptions.
There are huge volumes of air involved in both, and the "Air Tab" gimmicks may affect a small part of this air, and may show "Attached Flow" at the surface, but, you still need to think of the MUCH bigger picture and ask how it is affecting the air at 1 foot, 3 feet, and 20 feet away.......chances are, it ain't, and if it is, it is not likely to be helping you out. This is why "Form Drag" is what we try to minimize on a car, and "Skin Drag" is what we try to minimize on an airplane. There is no question that VG's help aircraft in some ways. The Form Drag of an aircraft better be minimized as much as possible.
Skin drag vs. Form drag along with ground effect are the reasons that you Can Not take much away from Aircraft Aero and apply it to Car Aero, you're wrestling 2 different demons in 2 different environments.
Skin drag on a car is fairly insignificant because of the speeds involved, and the surface area of a crappy shape is going to be equal to an ideal shape meaning the skin drag will be the same on both....so why even bother to talk about it, besides the air is by definition never in laminar flow on a car, it is already turbulent. Bottom line Skin drag is a non-issue.....for now. Maybe when we're all driving VW XL1 type cars we can revisit this.
Attached flow IS NOT the holy grail for achieving better Cd, so just because you can get some air blowing down your back window does not mean you have lowered your cars drag. It is much more complicated than that. Great aerodynamic shapes with low drag have attached flow, shapes with attached flow are not always low drag. You must understand this distinction to move past this blind fixation towards "Attached Flow".
As for VG's, they are silly 99% of the time on a car. In my opinion, they are pure.......
