Rear aero project - Civic notchback coupe

[brucepick]

The 'vette looks like more of a taper than the template, both vertically and horizontally.
Volvo concept car might be "on template", definitely doesn't look like as much of a taper as the 'vette.
I expected the Phil Knox taper to be close to the template - - -
but it does not meet the outer perimeter of the cab. My guess is that there must be lots of eddies and turbulence at the cab corners.


I might put on a decal "Civic Elize MPG by BP"

Maybe model mine after this 2008 Lotus Elise:


Or the 2011 version:

[aerohead]

Originally Posted by brucepick

My notes above indicate I installed a 1/8" x 1" aluminum bar stock longitudinally. I only had the one piece handy so I did the other stiffeners in plywood, which is far stiffer than the coroplast.

I didn't mention it in the text, but there is also a 1/8" diam steel rod slipped into the coroplast groove. You can see its shadow going across the photo, and at the left end it's just about obstructed by the trunk's left rear corner. Its been there since the start of the project and allowed the 2.5" flexing I described two posts earlier.



However since it's still vibrating some, it would need more reinforcement to eliminate the vibration. All those stiffeners would need to be riveted in a permanent setup, not duct taped. And the point-contact nature of the rivet fasteners would not give as thorough a contact bond as the duct tape does, so the materials would flex and separate. That is why I think I'm better off with a stiffer base material.

Sorry - no angled folds in the center panel. I want to leave it as clean as possible on the top side to help the aero flow patterns. I don't really like the way coroplast folds, especially if you try folding diagonally across the flutes.

Yes, a tuft test would be good to do. Any ecomodders in southeastern New England, if you're interested we can get together for that. Especially if you have a pickup truck to shoot pics from.

Beef her up best you can.She's really going to try and explode with the air crashing down on her.

[brucepick]

Originally Posted by aerohead

Beef her up best you can.She's really going to try and explode with the air crashing down on her.

Thanks. Yes I will beef it up.

I think the air impact is mostly around the middle. Based on: Before I added the stiffeners, it was bowing down in the mid section and up at the rear - but I suspect the up-at-rear might have been a kind of lever action, with the panel pivoting half way between the low and high points.

Does the bowing tell you anything about whether I put the horizontal trailing edge at the template line as I hoped to?? Or merely proves your point that it's subject to a lot of pressure?

[ChazInMT]

Bruce buddy, I can't help but wonder why you would go flat on the back panel? I know you prolly want to see out the back, but in my mind, the Honda is pretty darn good to begin with. If you want to improve on what Honda has done you are going to have to sacrifice something and in this case, it is rear visibility.

I don't see how a flat back is going to smoothly transition the air down the back of the car. Also my intuition is telling me your sides being higher than the flat back are creating a vortex generation situation... as in the big bad wing tip type, where you have vastly different pressure areas in contact with each other....sucking the life out of your mod, and, the FUEL from your TANK!!!

but I digress.

I think you either need to shorten the sides so they don't stick up above the flat tail, or, to optimize the mod, put the top of the kamm back at the top of the rear window and bring it down to a inch or 2 higher than it is now at the back and keep your sides the same as they are.

Sorry to be so lazy here, but I'm just going to post what I want to do with my 5th gen 04 sedan, pretty close to the same idea. I think you'll still have some good views out the back under the rear edge of the Kamm.




You're doing great to just be doing something, keep it going.

Charlie

[brucepick]

Thanks Chaz.
Do you have other pics of your kamm back? Did it help FE?

The concept I'm trying to execute is actually the same as in the pickup truck where som can have better aero with the tailgate up than with it down. The pickup bed sets up a swirl pattern, probably an area with increased pressure. And the airflow over the cab skips over the bed and then over the tailgate, with a resulting smoother air flow than with the tailgate down.

So I'm hoping that the air flow off my roof skims over whatever turbulence sets up between the side panels. And then exits off the back edge of the spoiler, almost as if I'd instead installed a template-shaped "kamm back" that starts around the roof edge and ends up about 6" above the rear bumper.

The side panels hopefully do two things:
1) Guide the air flow from the side windows area slightly inwards towards the center line, so it exits with a smaller cross section.
2) Keep some air between the side panels so it will have some pressure so the roof airflow will skim over the spoiler's rear edge.

Don't forget - the air can't really follow the dropping roof line and the even steeper dropping rear glass. Yes there are tuft photos showing mostly nicely lined up tufts but I'm convinced there's a big vacuum being sucked behind the rear glass, because it's so much steeper than the template line.

Yes it's a challenge to build this thing so it works as intended. Especially so because you can't see the air. Real tuft testing requires probably two friends in a truck to get the photos. In other words, I think tuft testing is easier said than done.

[ChazInMT]

Bruce, the Kamm on my car is only a photoshop creation, keep meaning to build it, but....

This is why I respect what you're doing by getting out there and doing something.

Keep in mind a pickup drags a parachute/tailgate around, freakin anything can improve it.

Honda has a fairly aero stock car in your 97 Civic, I think the genius in what they do is to shape the c-pillar area and rear glass angles in such a way as to minimize the vortex generation here. By putting the design you have on there, it doesn't follow the template and leaves some odd areas where the relatively high pressure area on the sides of your design are right next to the very low pressure rear window suck area. This is no different than a wingtip on an aircraft and creates a horizontal tornado dragging behind the rear sides of your mod.

Do what you like, I just suggest you fill the back in and stick with the template in what you do.

[brucepick]

Originally Posted by aerohead

Beef her up best you can.She's really going to try and explode with the air crashing down on her.

Today I took aerohead's advice - not the first time I did that, I should point out - and installed four new stiffener bars, all 1" x 1/8" aluminum bar stock. Two of them are improved replacements for wood or too-short-metal ones I had installed earlier, the other two are new to the scheme.

This seems to have fixed the vibrations and flexing. Hopefully tomorrow I'll get to do a real road trip, about 150 miles in all, on the interstate.

I want to debug the finned design as much as possible before giving up on it. Stiffening was the first step. I think tapering it in horizontally will be next; it barely narrows at all from front to rear right now.

If stiffening and tapering don't give good results then I can cut the fins off - but once they're gone, they're gone. I need to fix the flaws in execution before I give up on the design.

[brucepick]

V. 2 of the spoiler was the three stiffeners added.
V. 3 is with four aluminum bar stock stiffeners; two of those are upgrades of ones that were there in v. 2. The two new stiffeners are longitudinal, placed between the center panel's center stiffener and the outer edges (sorry, no photo handy). I can say that the stiffeners resulted in a pretty stable panel that vibrated only about 1/8" up/down at the trailing edge at highway speeds. Also there was tape joining the leading edge to the car's rear glass. Otherwise the configuration was the same as in v. 2.

Did some coastdown testing this morning. Unfortunately the results are not good. Apparently I still haven't improved on what the Honda engineers built into the 6th Gen. Civic Coupe. I tested between two consistent points on the interstate, not the same two points as on the first coastdown test. The "Northbound" runs had a decent tailwind of about 15 mph so the speed loss there was less than on the southbound runs, which had a headwind. Admittedly, this was only an A-B test and not A-B-A. And, there was only one southbound run of each type. However the results are pretty consistent between runs so I think it's reasonable to draw a conclusion from what I recorded.

Northbound test runs, with spoiler:
START - - - END - - DELTA (MPH)
64.9 - - - - 52.8 - - 12.1
64.0 - - - - 52.0 - - 12.0

Northbound test runs, without spoiler:
START - - - END - - DELTA (MPH)
66.7 - - - - 57.8 - - 08.9
64.3 - - - - 54.1 - - 10.2


Southbound test run, with spoiler:
START - - - END - - DELTA (MPH)
59.6 - - - - 41.0 - - 18.6

Southbound test run, without spoiler:
START - - - END - - DELTA (MPH)
61.5 - - - - 43.5 - - 18.0

So this project will have to go back to the drawing board. I've already mentioned some possible options.

1) taper the side panels inward towards the rear
2) eliminate the "fins"; the part of side panel that's above the center panel
3) take a completely different approach that would be closer to a Kamm back, with a panel starting above the rear glass and going down to a trailing edge similar to this version's trailing edge - but likely narrower at the rear than this one.

[Cd]

Any plans to do a tuft test ?

[brucepick]

Originally Posted by Cd

Any plans to do a tuft test ?

Yeah, I'm thinking of getting a clamp + tripod screw to mount the camera on a coat hook, or a tripod screw with suction cup to stick it to the rear glass. Shucks, maybe all I need is some duct tape to hold it to the glass???

However I think I need to follow #1 + #2 as above in sequence. Because tapering the sides COULD improve it with the fins still there, and doing it without the fins will need the sides tapered anyway. If I can manage to set up the camera in a good place I could shoot tuft tests for both those configurations.