Looks like the small spoiler keeps the TOP air flow going straight back and then the low pressure area at back is filled by air from the SIDES?

If you look at the Airtabs applications...that might be how they work...causing a straight sheet of air...a clean break?

I was assuming that they caused the boundary layer to follow around the curve into the lower pressure area behind the car.

If you look at how the Airtabs are made...look like they lift a stream of air in the center and force it out away from the surface plane to some extent? Between the Airtabs seems like there would be a lower pressure area that might supply air to the center??

Whereas the vgs I made induce trailing vortexes. By placing them 4" apart...the 2 vortexes from ajoining vgs counter-rotate and shouldn't interfere with each other.

My ZX has a small spoiler at back...so maybe the top vgs help to keep the air more down on it...while the side vgs help air getting around the back...or maybe hinder it?

I think that the jury on this is going to be out until we get enough people testing these things in a rigorous manner.

Spoilers
 

So it would probably be beneficial to extend the spoiler down the sides of the hatch also...

Here is a pic of the kind of vgs I use on the side...that are supposed to be similar in effect to the airtabs.

Far as I know...the air going into the center gets concentrated at a higher pressure than "ambient"....and the air going between them is possibly at a lower pressure than "ambient"?

Might be that the lower pressure air...fills in the large low pressure area behind the car?

These should probably be placed in a series 1" or so apart.

Here is the pattern used...found on the web...I used alum flashing material.

You know, I'm starting to think that it wouldn't be that difficult to make an effective lip spoiler. I had a look at my girlfriend's mother's Mercedes E300 Turbodiesel (I think it's a '99) and that has one built right into the trunk lid. It's literally just a vertical lip formed into the top of the trunk, right at the rear edge. As those sources point out it seems like it's just a matter of optimizing size and angle. I think that the tougher part would be making it look good, and then reliably measuring its effects without a wind tunnel.

I agree: a lip spoiler is an easy item to make, and the theory behind its purpose is solid (& logical, which makes it even nicer). As for angle, I think the safest bet would be to simply extend the plane of the trunk lid. As for size, I'm less certain, but the article mentioned 20 mm.

You can probably guess where I place looks on the scale of things :)

But you hit the nail on the head about reliably measuring effects.

Logically, a lip extension should improve aero. But the change will likely be small enough that on-road testing - whether coastdown based, or speed-controlled & averaged bi-dir. runs - may show nothing conclusive; the effect will likely be lost in the normal static of variability.

It's something I've been thinking about lately, since there are a number of other known aero tweaks I'd like to try, but whose individual effects (if properly executed) are known to be small. How to quantify...

Detail of the integrated roof extension spoiler on my rear hatch (Firefly #1) that ensures clean flow separation via a sharp angle and small radius:

https://images6.theimagehosting.com/a...-detail.th.jpg


https://images6.theimagehosting.com/a...h-det-2.th.jpg

Above: note the hatch sides also have an integrated extension, to about 2/3 of the way down.

Why they stopped there is a question. The remaining of side-to-rear transitions are horribly large radii that would likely also benefit just as much from sharper transitions & cleaner flow separation.

Sounds fair. The other car I looked at this weekend was my girlfriend's sister's '98 BWM 328 coupe. (What can I say, their father is German-born.) Anyway, that also appears to have a built-in extension that I can only assume is designed to do the same thing. The horizontal part of the trunk lid is simply extended ~20-30mm beyond the vertical part of the trunk lid below, tapering backward (err...forward?) to meet it. It makes a lot of sense now that I know what they were trying to accomplish.

If you look at the oil flow plots...looks like the gradual curves at the sides allow the air to flow into the rear area to increase pressure?

The spoiler they mention is about 3/4" high and at a 20* angle to the downsloping surface.

As far as testing various vgs and ideas....if ??????? can test and find a 1.5% gain with a V6 (?) from a row of rooftop vgs...what's the issue with testing....except for the time involved?

A lighter 4 cylinder would probably show increased effects from reduced drag.

Spoiler
 

Spent four hours traveling for work today. It's amazing the number of newer cars with an aero device at the rear of the roofline! Magnum, Pacifica, Caravan, Matrix, Trailblazer, Beemer and Lexus Suvs etc. Must be there for a reason....