wake reduction
 

first off, 94 civic 4 door sedan. Im wondering if i stand to gain mpg without sacrificing handling/safety by attaching a 6-8inch long piece of coroplast & diy vg's on my trailing edge in an attempt to reduce the amount of disturbed air/wake following the vehicle. Fastening it with out making any permanent marks on the car is a different story, but that will be tackled at a later time.

i tried to illustrate with rough drawing of what i have in mind. I would be attaching the leading edge across the entire trunk door equal height as seamless as possible angled down towards the ground. I'm uncertain of the angle, but trial and error tuft testing is my plan to see what the steepest angle i can get without airflow separation. from leading edge to trailing edge 6-8 inches as to avoid license plate/tail lights obstruction. Just prior to the leading edge, add small VGs to help prevent premature separation.

my thoughts on the benefits:
reduce drag caused by "suction" on the rear of the car
i already have enough spare coroplast so it would be nice to find a use for it.

concerns:
increased rear end lift resulting in decreased handling/braking

on a hatchback, what is the advantage of a cam back creating earlier airflow separation rather than an attempt to keep it attached. It just seems like backwards thinking to me by forcing a larger wake.

feel free to dumb things down for me, i understand some of the basic concepts but I'm still learning.

http://www.youtube.com/watch?v=H1kooyIPzc0
http://www.youtube.com/watch?v=Fz9Xcv-xMXg&NR=1
Vortaflow + Torbjörn Gustavsson

That video looks like the back end of the airflow is being sucked downward, you can see because the smoke dissipates into the floor(mesh?).

You're more likely to get good results by putting a small lip going straight back from the trunk lid, or up at a slight angle.

Like Honda did on the Civic Hybrid:
http://l.images.easyautosales.com/20...874533-885.jpg

Here's one active discussion about why your original idea is looking at things the wrong way.

http://ecomodder.com/forum/showthrea...why-12732.html

Ah! Here it is:

http://ecomodder.com/forum/showthrea...rt-c-9287.html

Length-to-height ratio of 2.5 for the back end. That calculates to about 24 degrees. (update: wrong - see below)

If you measure the angle from the rooftop to the trunk trailing edge of your car, it's likely a little steeper than that. You want it flatter, so make a lip that extends out the back a short distance.

Edit: Wrong information! Sorry!
That's 24 degrees between the top angle and bottom angle, but only 12 from horizontal to either one. 12 degrees is what you want at the back of your car.

Now I wait until someone who actually knows what they're talking about comes and corrects me. ;)

template
 

laack,please go to Search Ecomodder and key in 'Aerodynamic Streamlining Template Part-C and look at the second image posted there.Then consider scaling a side-view image of the Civic to fit in under the template as it is specified for use.
The curve will demonstrate a reasonable path the air would follow without separation.
You could fashion a foil for the back of the roof and also a upswept spoiler or straight back foil which pushes the Civic's body form closer to that of the template curve.
I believe that the airflow on top of your trunklid is not 'clean' enough to allow VGs to operate.
The upswept spoiler will force air to re-attach and it should lower the base pressure behind the Civic for an overall drag reduction.
You can see from the template that the air likes long,gently tapering aft-bodies.If you'll stay close to this form you'll see an improvement at the pump.
P.S. If your bound and determined to play around with VGs,you'll want them on the roof ahead of the drop off for the backlight.Check out the Mitsubishi R&D for their Lancer.For these things to work,they must be carefully scaled and placed.

rough calculations
 

I ran some quick numbers just to see what my options might be.

• if i try to match the template, I would need to extend my tail end about 19 inches. I'm sure someone around here would rip that off to tighten up their parallel park job.. cant do it.
• If i extend it just 10" it would be 12 degrees slope, as previously recommended, from trailing edge to leading edge of rear windshield.
• the glass slope down is 22 degrees
• slope from leading edge of glass to trailing end of trunk is 14 degrees

disclaimer- this assumes that all my measurements and calculations were accurate. inconsistent measurements forced me to take an average of height and length from top of glass to point equal to top of trunk then from that point to the end of the trunk.

random thoughts/options at this point- I remember an older thread on here posting VG results including a youtube link. promising tuft testing but disappointing results. no change in mpg. rough experiment but not enticing enough for me to waste my time on that alone.
extend a piece coroplast back 11 inches-I'm not fully convinced that that will actually reduce my wake or smooth the transition of separated airflow at all. This plus VGs. out of curiosities sake i may do it just for the tuft test a couple of coast down tests.
attach an up-swept spoiler- in terms of drag reduction it seems counter productive but i do understand that it makes the air's path a little longer & gentler. I'm not willing to rule it out yet, but is there any data associated with how much improvement it can yield.

Any other thoughts?

later plans are grille block and rear wheel covers. rust is an issue on the rear wheel covers though so i'm still trying to figure out how to attach it w/o drilling into visible portions of the bumper. I also want to hook up an led to the rad. fan so i know when its on.

I have a 95 civic that I put a straight piece on the trunk lid that extended back 4 inches. I didn't see any improvement over the course of several tanks and ended up removing it. it will be interesting to see if an angled piece will show any improvement.

Mercedes 190E 2.5 EVO II
 

Laack,GOOGLE the Mercedes above in the title box and see what they did.

Hi All,

Those are quite interesting videos! That vortex generator just drives the air into the floor dramatically. You know they gotta be generating allot of drag to do that.

The issue with such techniques is tuning them to get the air to just deflect lightly, and curve away from the roadway and go parallel with it. Rather than as was seen in the video. And do that with a minimum of vortex generator. Otherwise the drag reduced by eliminating the tail tumble of the air, will just be overcome by the drag of the vortex generator.

Streamlining is a much more reliable method for those without the windtunel and force gauges needed to tune up vortex generators to the specific vehicle application.

I found some downloads on that web page. In his own report from 2004 he reports no VG's resulted in less drage (40 degree boattail).

http://www.vortaflow.com/consulting/...unt%20body.pdf

And that he was going to investigate smaller VG's.

Yes. If you read the guy's paper on his website, he admits that the drag is actually greater with the vortex generators than without them.

Vortex generators are far more effective in reducing rear lift, unless you have them along the sides of a large, square rear end, like a semi.

I see they extended the decklid with the giant wing...but are you saying the wing helped with drag reduction?

wing
 

Tygen,what caught my eye,was both the fairing at the top of the backlight which softens the curvature off the roof allowing 'clean' air to strike the wing,both above and below,and the boot extension,which runs straight back as far as the end of the bumper.
This is kind of a permutation of the Merkur XR4Ti bi-wing spoiler,where they look to manage the separated flow both high and low.
If the wing was just symmetrical,with no induced lift,It could serve as a re-attachment point and work to
'sculpt' the wake a bit.
I found an article on the 1988 Holden Commodore VL SS Group ASV by Tom Walkinshaw who created 18-aero add-ons for the car,dropping Cd from 0.43,to 0.32.
He extended a rear spoiler as far as the bumper and raised it about 7.5-inches ( 187.5 mm ) at the trailing edge.He also extended the C-pillars back to it to create the 'birdbath on the boot' also used on the Mustang GT.
The extreme spoiler on Mike Murillo's Pro-Stock '93 Mustang notchback,is as long as the trunklid itself,projecting straight back with zero upsweep.
And for protection to pedestrians,you just put a couple of SIMPSON drag chutes under it,sticking out a little bit.
If that doesn't 'man-it:rolleyes:-up' enough,then I don't know what will!