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Old 11-13-2007, 06:01 PM   #91
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I had five on the roof...didn't notice that much but wasn't doing any highway travel...added 3 to each rear side and subjectively the Jeep felt more stable (and I am lifted). I got my best mileage of 30+ on the highway for a distance of 226 miles. Some of the gain could have been from the "HyperTank" competition but I'll take it.
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Old 11-17-2007, 05:08 PM   #92
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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?
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Old 11-17-2007, 05:31 PM   #93
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Quote:
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).
Matt, I have yet to come across any study that shows VG's effectiveness at bridging a physical gap (such as wheel openings, etc.) of filling the low pressure zone of a wake.

It's not that they create vortices to "jump" - it's that they put faster moving flow into a boundary layer that is fizzling out as the boundary layer gets thicker and slows down over a length of a body. By moving some higher energy (faster) flow into a unenthused BL, you can decrease the effects separation.

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The summary of this is that air tabs can hurt! if your car is already quite slippery
The slippery conditions necessary are streamlined conditions (most drag comes as a result of pressure drag - not the wake) - of which, most cars are not. Even so, really long streamlined bodies can benefit too

Most cars, unfortunately, are bluff bodies - where most losses are a result of wake formation. So it's quite beneficial to slightly increase pressure drag for a reduction in wake.

And as always, the devils in the details of application. They work when applied slightly in front of separation caused by low energy boundary layer. As far as decreasing wake effects due to a physical void (wheel arch, end of car, etc.) - it's an open book to be written by the person that does the analysis and scrutinized by everyone else

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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?
Not ripping The conclusions you drew from your assumptions weren't too far in the outfield But the initial assumptions were a little off


The interesting thing about the EVO VG's is that they are very different to the air tab design... and very different to other designs The air tab design is almost like a spoon scoop whereas the EVO's is a delta V (a shape with a whole of engineering background).
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Old 11-18-2007, 01:56 AM   #94
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trebuchet03 -

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The interesting thing about the EVO VG's is that they are very different to the air tab design... and very different to other designs The air tab design is almost like a spoon scoop whereas the EVO's is a delta V (a shape with a whole of engineering background).
They are a scoop, but aren't they also forming the opposite shape when used in unison to the other airtabs? If two of them have a 4 inch centerline application, isn't there a complimentary shape that could be effecting aerodynamics also?

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Old 11-18-2007, 05:58 AM   #95
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It's not that they create vortices to "jump" - it's that they put faster moving flow into a boundary layer that is fizzling out as the boundary layer gets thicker and slows down over a length of a body. By moving some higher energy (faster) flow into a unenthused BL, you can decrease the effects separation.
I think they do cause the air flow to "jump" some openings....i.e....not get into gaps such as wheel wells where more turbulent flow is created.

In theory what they do at the rear of a car/truck is to reduce the "amount" of low pressure area formed at the back.

If the shape is "blunt" then large "cells" of roiling low pressure air are formed that affect stability and cause tire wear. Semi trucks.

With a more streamlined shape like a car probably not so much of this...but still some low pressure area that might be "filled" some. The idea here would be to design a vg that would not only create a vortex or a some kind of high pressure stream, but would also deflect the flow a few degrees toward the centerline of the vehicle..reducing the area of low pressure that is formed? I think the airtabs attempt to do this.

Saw a school bus with a large wind deflector at the top in back...not sure if it was for keeping the back free of dust or for mpg. (both?)
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Old 11-18-2007, 06:10 AM   #96
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In Summary: If you have an already aerodynamic vehicle then I think air tabs are more likely to do harm than good.
I would call the two pickups that have shown "results" with the airtabs rather high on the aero scale...though the shapes are pretty complex and not ideally aero for sure...at least no uneeded sharp corners?
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Old 11-18-2007, 09:55 AM   #97
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Sounds like you all need to get a fan and a cardboard box and mount a scale on the box to measure air drag forces then add air tabs to it and see if drag is reduced. You can also run the yarn air flow telltails on the box and see the effects right in front of you.
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Old 11-18-2007, 11:39 AM   #98
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Have 11 AirTabs on my Jeep Liberty...and the taillights are also shaped like an AirTab from the side (by accident I am sure). I did not do a test with only the AirTabs as the change...so I can not give any empirical data. I will say that the rear window does stay a bit cleaner in the rain, the Jeep feels a bit more stable at highway speeds and in turbulent air and fuel mileage did go up but other things probably contributed to that also. I think with a SUV type shape, you do get results that are most likley measurable because there is such a large area to improve upon.

The good thing is that the AirTabs are cheap to buy and/or similiar vortex tabs are easy enuff to make that there is no real harm. I think the trick is not to go overboard and create some kind of "Armadillo Skin" on a vehicle. Put them in the areas where the air leaves the vehicle in the rear...most likely benefit.
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Old 11-19-2007, 07:12 AM   #99
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Sounds like you all need to get a fan and a cardboard box and mount a scale on the box to measure air drag forces then add air tabs to it and see if drag is reduced. You can also run the yarn air flow telltails on the box and see the effects right in front of you.
I live about 75 miles from where the Wright Brothers original air tunnel is located...maybe I could borrow it?
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Old 12-08-2007, 12:02 PM   #100
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Good information on this thread.

I'm new. Thanks for having me.

As a racer, I have a couple things I think about. One, I'd like to go faster, and, two, I'd like to do it cheaper. My travel costs are kind of high with a shuttle bus and an enclosed trailer.

I will agree, yeah, if the aerodynamics are good already, it's going to be hard to get dramatic improvements. But if you've got a brick, well, you might be able to polish the piece up and get something for a gain.

I teach six schools were I have to drive 650 miles one way to do it. So, if I could improve my FE from 11 to 12 MPG at $3.50 a gallon of diesel, I'll have got the cost of some of the Airtabs back, depending upon how many I use. If there's some stability to be gained, I'd like that too.

Looking forward to hearing more. I won't be making any changes myself until later toward spring when it gets warm out here in Wisconsin.
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