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Old 11-06-2007, 11:55 AM   #81
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Quote:
Originally Posted by theclencher View Post

I've seen em and sat in one. It appears to me that, among other things, the weight and balance of the thing is way wrong, and accident reports seem to back that up.
I agree... I've sat in a Pizza Butt version...

Quote:
I've said before: I've got a box of Airtabs myself. I CAN say conclusively they do NOT save any fuel sitting in the basement

I decided they were worth investigating more last year, and had intended to to test them to the best of my abilities in the real world this summer ... but got pre-occupied by the ForkenSwift.
Something tells me that leaving them in the basement to work on the forkenswift yielded better FE
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Bike Miles (Begin Aug. 20 - '07): ~433.2 miles

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Old 11-06-2007, 11:59 AM   #82
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I rolled my own...or maybe bent them:

Decided that I couldn't be left behind in the vg race...so I made some out of ~.011" alum flashing material. A better material might be alum siding which tends to be ~ .022" thick. This stuff can be cut with regular scissors...but don't use your mommy's sewing scissors.

My analysis of the airtab:

The airtabs is around 5" long, but the "effective" area is only 3" and is angled at 30*. There is a straight "intro" area and an "exit" area that are likely shaped to "condition" the air flow to avoid any extra disruption of the air flow over the tab. The "exit" area probably helps to cause the vortexes to pull towards the centerline of the vehicle slightly? At any rate the complex shape probably makes them more efficient (cleaner vortexes with less drag) than a simple vane type vg.

I made two versions of vg...a smaller one that is just two straight vanes both angled at 20*...and a larger more complex one with the same 20* angle.

With the smaller vg the surface area of each vane is maybe 30 % greater than the airtab, but they are angled at 20* rather than 30*. The NASA research shows that too sharp an angle can cause the vortexes to break up with a straight vane type vg. With this vg it is easier to find a relatively flat place to install them.

The larger vg has a different design and is made to try to use streamlined shapes to cause less disruption of the air flow. The cross section of each vane is maybe 30% more than the smaller vg, but are also angled and not vertical.

The larger vg could be used on a van or on the hood of most vehicles. (pay no attention to those people in the other lane that are laughing about something)

* paste url into address bar and hit ENTER??

the vgs:

http://www.nonags.org/members/nijqk/vg-a.jpg


http://www.nonags.org/members/nijqk/vg-b.jpg


the patterns (large squares are centimeters):

http://www.nonags.org/members/nijqk/vg-patterns2.jpg


Can you get away with it?

Last time I had some on the side of a car...I had two cruisers...probably with their video going came up behind me...I was just about to hit the afterburner and flare switches...when they veered off.

This material...espec the .011" stuff is pretty flimsy...I sanded the edges, painted them, and used about 3 dabs of silicone caulk to stick them on...along with a level to get them straight. They should be placed less than 10" from the back edge and spaced maybe 4" to 5" apart on center.

They probably shouldn't project out much farther than the greatest width of your vehicle.

Cost?

Depending on whether you have the materials laying around...$0 to maybe $10. Your social standing might have to be defended. Buy a ray gun.
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Old 11-07-2007, 02:56 PM   #83
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@ZugyNA, none of your pictures work.
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Old 11-07-2007, 03:20 PM   #84
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I think I might spring for it sometime next year when money improves, but what I question is... Does one somehow drive better after installing these, and is this the actual factor that contributes to the increase?

Because on my 3/4 tons, on my '86 removing the tailgate increased mpg, but on my '95 it did not... The 86 has since been sold, but that was my first truck I removed the tailgate on, and then and now I wonder, did it make me drive better since I was already hoping I would get better mpg, and if so, is this what actually did it?

One figures by the time I got to fooling with the '95 it didn't affect my actual driving anymore, but there is a marked difference between the styling of the two (the 95 is far curvier than the 86 was). Oddly enough, the '86 got way better mpg than the 95 does, even thou the later model has a bigger engine it's also MPI (vs. carb'ed).

It can't hurt, the way I see things is I'm breaking up the flow of air before the rear of the car has a chance to suck it in. What I question is, why not let the air flow as it should in the first place, doesn't the airtab itself create just that amount of drag it in turn saves?

It strikes me like one of those vortex impeller tornado things one supposedly sticks in the intake to 'atomize' fuel or what have you...

Stranger still, this Audi race car uses a similar type of flaring near the fenders...
Although it appears to simply be for down force:
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Old 11-07-2007, 03:33 PM   #85
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Zugy, what criteria did you use to pick location?

Don't take this the wrong way - just offering my advice based on research I've done

The best way to figure out the location is with CFD and a wind tunnel. Find the separation point and place whatever flow control method you're going to use slightly before that point.

A not so perfect, but acceptable method is to do a rolling tunnel wool tuft test. Basically tape a whole bunch of strings to your car (a bright easy to see color) and take a bunch of pictures while at cruising speed. Watch for tufts pointing in a direction not parallel to air flow. Then place your flow control just in front of that....

As for the height -- the height should be only slightly thicker than the boundary layer. Remember that the goal is to bring a little bit of higher energy flow into a weakened boundary layer (weak because over a distance, it slows down and becomes thicker until separation).

The trickiest part of all of that.... the separation point changes with air speed.... So you have to pick a compromise...

Quote:
Does one somehow drive better after installing these, and is this the actual factor that contributes to the increase?
The concept itself is sound... but the devil is in the details of application Mitsubishi has a technical paper showing a reduction in cD by 6 points, many aircraft wings have them (not necessarily this product, but something of the same idea). Even power lines etc. use flow control (but for a very different reason ).
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Old 11-08-2007, 03:00 AM   #86
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Quote:
Originally Posted by boofighter View Post
@ZugyNA, none of your pictures work.
* right click and click view pic....or paste url into address bar and hit ENTER??
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Old 11-08-2007, 03:26 AM   #87
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Quote:
Originally Posted by trebuchet03 View Post
Zugy, what criteria did you use to pick location?

Don't take this the wrong way - just offering my advice based on research I've done

The best way to figure out the location is with CFD and a wind tunnel. Find the separation point and place whatever flow control method you're going to use slightly before that point.

A not so perfect, but acceptable method is to do a rolling tunnel wool tuft test. Basically tape a whole bunch of strings to your car (a bright easy to see color) and take a bunch of pictures while at cruising speed. Watch for tufts pointing in a direction not parallel to air flow. Then place your flow control just in front of that....

As for the height -- the height should be only slightly thicker than the boundary layer. Remember that the goal is to bring a little bit of higher energy flow into a weakened boundary layer (weak because over a distance, it slows down and becomes thicker until separation).

The trickiest part of all of that.... the separation point changes with air speed.... So you have to pick a compromise...

The concept itself is sound... but the devil is in the details of application Mitsubishi has a technical paper showing a reduction in cD by 6 points, many aircraft wings have them (not necessarily this product, but something of the same idea). Even power lines etc. use flow control (but for a very different reason ).
I'm using a combination of the NASA research and the airtab instructions.

These are supposed to be "clones" of the airtabs so I use the less than 10" away from the back edge (close as possible) and 4-5" spacing.

The NASA research was based on a 1" height for the larger vgs. Since with a car you will have a good bit of roughness to the surface in most cases...I go with the 1" high vgs to get above the disruptions.

The vortexes deteriorate over a short distance...thus the use of the 1" high vgs....they last longer.

Where I place them is based on...is there a flat surface?...is this place likely to do what I want...as in create a strong vortexed flow to either get the air past a wheel opening or to attempt to create the same type flow near the back in order to avoid the creation of the large disruptive vortexes?

If you hold your hand on the door surface when driving you can get a feel for the wind dynamics as you drive...there is usually a dead space where the flow has separated...with fast air above it. Put your hand behind the mirror and get an idea in miniature of the disrupted flow behind your car.

I think it's fairly easy to visualize what the flow is doing. Flow direction might surprise you some if tested though?

If you have a surface angled into the air flow you have higher pressure.

If you have a surface parallel with the airflow it might start to separate especially if there are variations in the surface.

If you have a surface angled away from the airflow...you have separation and lower pressures.

Look at the picture of the apple?

When you have separation...your vehicle is then dragging the large vortexes of disrupted air along with it...because they are at a lower pressure and tend to stick to the car.

I've got 22 small vgs on my car and hope to see maybe 1-2 mpg gain...but maybe not. The side of this car is fairly rough...with the windows set in some....not to mention the mirrors. Also can't put vgs on the roof due to a rack that is on the car. The placement decisions were fairly easy...at the back they need to be close to the trailing edge. In front of the wheel openings I needed a flat surface and a high pressure area (good flow).
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Old 11-08-2007, 10:03 PM   #88
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http://forums.tdiclub.com/showthread...=147718&page=4
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Old 11-09-2007, 03:50 AM   #89
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Quote:
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Interesting discussion, but when I hear too many scientific terms and formulas mentioned as to WHY something won't work...it impresses me about as much as another RIDICULE FEST. A lot of the talk is really just people trying to impress themselves and others with how clever they are...and they are doing a disservice to the truth...in order to inflate their own egos.

Some of the technical links and info are good sources though.

The difference between myself and others on this forum is that I am actively looking for ways to increase mpg (as a hobby) and too many others are just talking about it.

Bottom line is you have to TEST an idea vs real world mpg. I can understand most of the scientific mumbo jumbo...and I use the research as a guide.

As I find techniques that work....I will eventually put together a vehicle as a "system" that uses the various techniques.
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Old 11-13-2007, 05:41 PM   #90
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Originally Posted by ZugyNA View Post
I've got 22 small vgs on my car and hope to see maybe 1-2 mpg gain...but maybe not. The side of this car is fairly rough...with the windows set in some....not to mention the mirrors. Also can't put vgs on the roof due to a rack that is on the car. The placement decisions were fairly easy...at the back they need to be close to the trailing edge. In front of the wheel openings I needed a flat surface and a high pressure area (good flow).
Might have gained a little mileage...but sure didn't lose any. Hard to say...only 1 tank and using winter fuel. Car seemed more stable in a crosswind. Will be testing another aero idea next tank.

Did buy some airtabs for 1/2 price on Ebay.
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