Ordered Airtabs today!

[GasSavers_mattW]

Ok here is my theory on why they sometimes work and sometimes don't. This is purely speculation, based on the Evo paper, people's air-tabs test results and the air tab website but i think it fits the data. The air tabs form a little vortex which acts like a bridge for the air immediately after it so imagine a small (probably a foot long?) bridge on the car that allows the air to jump over places where it would become more tripped up (wheel wells, gaps on trucks, bad windscreen designs and rear windows on 3 box cars). While the air tabs themselves actually INCREASE drag (increasing effective frontal area and adding energy to the air) they are a good way to avoid areas which would have increased the drag by more anyway. The summary of this is that air tabs can hurt! if your car is already quite slippery there is nothing for the vortexes to jump over- all they will do is increase drag. This is especially true on smooth sides and roofs. I am assuming this is why they are marketed specifically at trucks- they have big gaps that need jumping because they aren't designed all that well. On the other side of the coin if you have a relatively box-like car/truck with areas of turbulent flow or harsh angles for air to bounce off then using air tabs placed ahead of these points in areas of attached flow would allow the air to avoid the problem areas thus increasing you cars slipperyness.

The reason I think they work on the back of the car (i.e. the evo) is that the vortex stirs up the air which makes it easier to suck down so it can reattach to the car. I would be surprised if using them right at the back of a hatchback would make a difference because the air that is being sucked (vortex) is already turbulent- off the back of the car that would more or less the same as the wake was anyway. It would be possible that in certain situations the air tab vortex would reduce the size of the wake but it could also just as easily increase it. And I don't think we as hobbyists have the resources to test this accurately enough for it to be worth the risk. On the body of the car wool tuft testing would be a must to make sure the air tabs are doing their job (both before and after) but you can't do wool tuft testing behind the car. I am not saying that air tabs won't decrease your wake, but I am saying that unless they are done well they will possibly increase your wake.

In Summary: If you have an already aerodynamic vehicle then I think air tabs are more likely to do harm than good. If you insist on using them then I would try in front of wheel wells and under the car in front of anything that sticks out (unless you have wheel fairings/ a belly pan). If you are driving a car that has sticky out bits and lots of turbulence then air tabs would be a very good investment, but make sure if you are going to do it then do it well. Learn how to do air tuft testing, work out where there is turbulence and put the air tabs ahead of where it trips so they are still in the attached flow bit. Remember if you just stick them on willy nilly you are risk increasing drag.

It might help to think of air tabs as directing visitors past the dirty rooms in your house so they won't be unimpressed. If the rooms are already clean (aerodynamically) then you might as well show them in (no air tabs) and let be impressed by its cleanliness. Avoiding clean rooms would probably give them a worse impression of the house (more drag).

Feel free to rip this theory apart, I'm only trying to be helpful and like I said its purely speculation. What do you think?