Ordered Airtabs today!

[ZugyNA]

Originally Posted by trebuchet03

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).

[ZugyNA]

Originally Posted by ZugyNA

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.