Rear aero project - Civic notchback coupe

[brucepick]

And thanks to California8Civic!!

I'm not giving up YET. As hinted earlier, I plan to test this version on the highway with tougher tape holding it on. And if the results still aren't good, I can trim away stuff to see if I can improve it.

First, trim the fins so they don't exist at the tail end. Just a straight line from the current top forward corners, going down to the back to meet the spoiler rear edge.

If "no joy" from that, then I can remove every part of the side panels that is forward of the spoiler's front edge, and every part that is above the spoiler itself. So the spoiler would look like this from the rear:

_______
/______\

[Tango Charlie]

What if you brought the upper, most aft, ends of the side fins inwards, so that they "pinch in" over your trunk lid. *shrug*

[HighMPG]

^^ What he said, why not bring the ends of the sides inwards to taper at the aft end.
It might help the air merge together farther back causing less of a wake from the sides.
I can't wait to pick up the 95 civic HB/CX I just went to see for the second time today!
I took some garbage bags with me to cover up the broken pass window and engine because it has no hood, waiting on lien sale papers 92k miles clean title! =D
I was thinking of doing something similar to my 90 corolla coupe with some plexi-glass and heat gun once I get a proper shape.

[brucepick]

TangoCharlie,

You mean, to bend the fin tops down at the rear so then lay on the spoiler panel?

Or if you mean that the side panels can be angled, they are angled now. This photo shows the angle pretty well, but the panel hasn't been cut to shape yet. It's at about this same angle now:

[HighMPG]

Angle them inwards more towards the end of trunk? and cut the center panel into more of this( \_/ ) shape so the ends of the side panels curve inwards and lie down more or do your panels have to stay straight/flat?

[brucepick]

Yes, the side panels are closer together at the rear than they are up by the glass. I think that's what you're suggesting. Another way of saying it is, the spoiler panel is tapered - it's about 5" narrower at the back than it is up front where it meets the glass.

5" narrower doesn't sound like much, but that's with the spoiler angled slightly up at the front. If it were dead level, the measured taper would be more. Because I'm measuring the front width higher up, where the spoiler meets the side panels.

I got the rear narrowing rate, and also the side slope of the side panels, by matching and following the contour of the body. I'm just trusting that the engineers did a pretty good job. So far it looks like that's the case - my first attempt likely increased the drag.

[HighMPG]

Yea, so taper it more? That's what I was suggesting but what are your plans now? extension or different angle? The civic has a pretty nice rear end already and slope from the roof to rear glass to trunk.
I think you may have to build a kamma-back for the trunk which should help more because in my mind, that's where all the original turbulence is. When you made this style it shows you made the drag greater because the air leaving is trailing off a larger/higher edge, I just have these ideas since I have been looking at many different cars for good rear aero since it seems where most of the improvements go. Your car is already a kammaback up to the end of the trunk, I think kamma backs are left for the hatch backs or non fastback style cars. You could use Vortex Generators on the rear of the roof before the glass and tape up the gap between the trunk and glass to get the air to stick more.
Since I am getting a 95 Hatchback I would totally go Kamma back or tapering tail but for a coupe the trunk is the best spot to build it off of imo. you may see more improvements there but I would love to see a Civic Coupe Boat tail!

[Tango Charlie]

Originally Posted by brucepick

TangoCharlie,
... you mean that the side panels can be angled, they are angled now. This photo shows the angle pretty well, but the panel hasn't been cut to shape yet. It's at about this same angle now...

Yeah, that's what I was thinking. Only get progressively more aggressive toward the rear. Tuft testing would tell you if you go too far.
Just a suggestion.

Very interested to see further testing/results!

[California98Civic]

Originally Posted by brucepick

And thanks to California8Civic!!

First, trim the fins so they don't exist at the tail end. Just a straight line from the current top forward corners, going down to the back to meet the spoiler rear edge.

My bet, looking at the aerocivic is that this tail shape will be the winner even on our car, if any work well at all.

Originally Posted by brucepick

I got the rear narrowing rate, and also the side slope of the side panels, by matching and following the contour of the body. I'm just trusting that the engineers did a pretty good job. So far it looks like that's the case - my first attempt likely increased the drag.

Following their lead might suggest not having the fins at all. Seems possible that your fins are creating more turbulence, given how some of the air seems to travel down the back (my tuft images above). Some of the flow is spilling over and around and twisting, out back of the car, off those fins I suspect. In my tuft image, someone pointed out to me that you can see evidence of airflow sliding off the trunk lid at the sides, airflow that is probably spinning off the backend of the corners as a result. A kamm, a spoiler/kamm like this one, needs to eliminate that. I think the fins accomplish the opposite goal.

The kamm/spoiler wins if it reduces the turbulence off the c-pillar, and the subsequent eddies off the rear corners. Basjoos has the shape. Question is whether your spoiler version can accomplish it too. If you are keeping the fins, I like the suggestion of trimming them so they don't exist anylonger when you get to the end of the spoiler/kamm.

Just my untrained, hobbyist, opinion.

[brucepick]

Thanks, all.

Well - - -
It's not a Kamm back, at least not now. You could call it a spoiler, or a false, lifted trunk lid, or maybe a short Bonneville spoiler. The reason for the design is I want to be able to open the trunk and see out the back. If this basic design would give aero improvement, I have a scheme to make 2 cuts across the flat panel, that will allow the trunk lid to open. I'll spare you the details.

I'll try lowering the horizontal panel's trailing edge. I can try a flat panel over the whole assembly, making a Kamm that goes to where the horizontal panel currently ends - though I think that will kill the scheme to open the trunk. I would trim down the rear of the fins for that. But I will want to test that concept before I start cutting the fins away completely; the forward part of the fins are part of that Kamm design.

I can also reduce and then eliminate the fins. That would make it pretty much as if it were made with a taller trunk, with the trunk trailing edge at the template point.

Oh yes - I have another idea, completely different. A span of clear flexible plastic, the heavy duty stuff for temporary storm windows, stretched across the whole area from roof rear to the current spoiler rear edge. Maybe some bow strips going across to keep it from sagging in the wrong places. Detachable at the sides (velcro??) so you can lift the trunk. But as I said, that's a completely different concept.