CivicTail project: ginormous aerodynamic tail on '07 Civic sedan

[MetroMPG]

Spring arrives this weekend, by the look of the forecast. Expect ABA testing in the next week or so.

[CigaR007]

I have also been waiting for warmer weather.. It will be the much-needed incentive to start working on the stealth aero mods for the CR-Z.

Looking forward to your A-B-A results !

[MetroMPG]

Near perfect conditions yesterday (mild, dry, light to no wind), so I got out to do testing on my usual flat test route, with no other traffic to interfere.

And I have to say: I am super disappointed with the numbers! I've never seen a tail have such a small impact:

2% improvement (1 mpg US) at 90 km/h / 56 mph.

I'll post the full details later on.

[Piwoslaw]

Originally Posted by MetroMPG

And I have to say: I am super disappointed with the numbers! I've never seen a tail have such a small impact:

2% improvement (1 mpg US) at 90 km/h / 56 mph.

Wow! That's like almost nothing! Within testing noise

I assume this was only the small tail, that the Kamm was on in the no-tail run? This would show that the Kamm is "doing most of the work",

[aerohead]

Originally Posted by MetroMPG

Near perfect conditions yesterday (mild, dry, light to no wind), so I got out to do testing on my usual flat test route.

And I have to say: I am super disappointed with the numbers! I've never seen a tail have such a small impact:

2% improvement (1 mpg US) at 90 km/h / 56 mph.

I'll post the full details later on.

We'll have to except the numbers for what they are.It is what it is.
All the time,energy,and trouble to do the testing will be valuable just the same.
Sir James Dyson said he never learned a thing from his successes.It was the 'failures' which were the most valuable.
Carl Breer's team, Dr.Gino Sovran,and Hucho have all warned about limited results from drag reduction if the engine could not be maintained at a constant load to maintain BSFC.
I don't know that this is what's going on,but since it's been emphasized in the literature,it's something obvious that we must consider.
If the load is reduced,you're at even less part-load.The throttle body may be nearly shut as,unlike a Honda carbureted CIVIC,with three throttle bodies,the '07 is taking all air in through a single barn door.Barely cracked open.(horrible volumetric efficiency).
Without another gear to load the engine back up,the BSFC may have fallen off its island of peak efficiency.


Darin,we're indebted to you for going through the whole process and all on your own nickel.Lets see what we can figure out.
Since the US dumped the 55-mph speed limit,the CIVIC may be calibrated to run at higher posted speeds.If we had a baseline at 75-mph,we might see a completely different trend.
Thanks for all you do!

[Insight for life]

hang in there! Just remember "wax on wax off"... not sure how that applies here, but just know that I am trying to encourage you.

[ennored]

At 56 MPH?

You may have taken your aero drag from low to really low. Add in some BSFC map contours and you may have explained your results.

I vote to try 80 MPH. (Or something like that.)

[MetroMPG]

Originally Posted by ennored

I vote to try 80 MPH. (Or something like that.)

I agree it's good to test aero at higher speeds. Unfortunately, I'm already over the limit on my test route at 90 kph (56 mph ... limit is 50 mph / 80 kph). I could get away with ~65-70 mph on the freeway, but traffic is much busier, and it's not as flat.

Also, I like to test mods at the speeds I typically drive.

Originally Posted by Piwoslaw

Wow! That's like almost nothing! Within testing noise

It's slightly better than that. The noise (standard deviation) in each set (4 runs, 2 each way) was just under 0.4 mpg. So a 1 mpg difference looks legit, unfortunately.

I assume this was only the small tail, that the Kamm was on in the no-tail run? This would show that the Kamm is "doing most of the work",

I actually also compared Kamm alone (no tail) vs. bare car, and the Kammback appears to have made no difference. 49.2 mpg with it on, 49.2 mpg with it off, 8 runs total (each way).

I'm going to post the details & numbers in just a minute.

[MetroMPG]

Originally Posted by aerohead

...Hucho have all warned about limited results from drag reduction if the engine could not be maintained at a constant load to maintain BSFC.

That could well be it.

And maybe it's possible I just haven't done the "drag reduction" part of the equation properly, and the tail isn't optimal.

Mulling over the disappointing results, when I look back at the amount of turbulence that tuft testing revealed on the sides of the tail (see my long Civic build thread for details, starting at #226) I wonder if most of the side of the tail is just sitting in turbulence & separation, contributing little to drag reduction.

Yesterday I was ready to call it a day and consign this experiment to the recycling bin. But I've had a change of heart: the tail will get some surgery to relax the side angles, plus a smallish spoiler on the top surface. And I'll re-test to see if it makes any difference.

I'll also post back with details (numbers, graph) of the first test later today.

[aerohead]

*if the old metric held true,then we'd look for a 5% mpg benefit for each 10% drag reduction at 55-mph according to GM Laboratories.But that's assuming a constant BSFC which is hard to know.Gino Sovran warned that we could lose up to 40% of the streamlining benefit without proper gear-matching.
*if there was separation along the sides,the low pressure of the separation line would be transferred to the wake,lowering the base pressure and increasing the pressure drag.
*I've got some of that happening on the T-100 and the only way I can fix it is to make a deployable diffuser,like GM's Epcot car, which can drop down to 2.5-degrees up-slope,only on the open road.(cable/pulley/hand-brake mechanism).You can see how 'nervous' the flow is under there in the video.
This would be a 'safe' contour for the sides.A lot of air is shunted over the roof on the CIVIC,leaving a more feeble flow on the sides which cannot tolerate much of a pressure increase (plan-taper) without triggering separation.

Here,Fachsenfeld has used a very conservative contour for the sides of his K-form car

All the 'Kamm' research was done with very gentle plan-taper

The Cd 0.12 MG EX 181 of 1957 used this gentle teardrop taper as well