Project: ECOfamily Civic and its Aero Hitch Box / Boat Tail

[christofoo]

Originally Posted by MetroMPG

Woohoo!

After seeing the latest pictures, I'm a little concerned about this transition:

Hopefully it's not too abrupt, going from horizontal on top to the "template" angle. (If it is, do you have the ability to make a bigger radius/smoother transition there?)

Me too.

If the horizontal surface was just a deck lid it would have an abrupt lip, but if it were an aero-fish-scale (from the roof-line to the template) it should be rounded. I wasn't sure which to do in planning, now I'm thinking I guessed wrong in both cases.

It might work fine the way it is if I do a rear-window treatment (full or half? ...) to get separation right along the template and keep the air flow away from the trunk.

When I tuft test, I'm expecting to see the roof-to-rear-windshield radius pull the airflow down, a little turbulence at the base of the windshield followed by a lot of horizontal air-flow along the trunk line to the lid transition, where it will fall apart at the sudden ~19° drop.

The alternate solution, if I wanted just an Aero Box with no rear windshield treatment, should have been a very gradual curve from the horizontal trunk line back to the template - with a radius similar to the roof-to-rear-windshield. (This is what I think now.)

EDIT: what I think I'm trying to say - case 1 no detachment before trunk: need a large radius to move horizontal airstream back to the template - also I should arguably head to 12° instead of the template. Case 2 detachment somewhere on the template: need a small radius (~4") to keep reattachment stable. (I'm not really sure what I've got here, a 4" radius from 0 to 19° doesn't look like much visually.)

If I have to make a big change to that transition, the lid will get reborn in aluminum with snipped-out-fiberglassed-forward-corners, just like the rest of the box. (Hands down my favorite method between the two, so much so that I don't want to bother editing my somewhat bungled glass-over-foam version.) The lid would need compound edges but they would be well within the limits of tuck shrinking. That would set me back to the tail end of phase 2, basically, most likely needing to edit the upper lip of the box, but both items ought to go pretty fast. Cross that bridge when I get there though...

EDIT: Between the two methods, I'd expect the rear-window-treatment to work the best aerodynamically, because it is the closest to ideal profile in the final product, whereas relying on a large curve to bend the flow off the trunk results in not one but two regions of non-ideal flow. The big question of course is how big the split is and thus which is favored in cost-benefit. (If only I had CFD...) My guess is that the split is significant, considering the prevalence of Kammbacks in the eco (hybrid) market, also the more extreme Kammback-like sedans (Elantra, Cruze) ever since Congress 2007 wrote the CAFE hike.

[Xist]

How would you attach the Kammback? I do not know of any good solutions for that.

[christofoo]

Originally Posted by Xist

How would you attach the Kammback? I do not know of any good solutions for that.

My thought was a sandwich of aluminum lip, weatherstrip foam tape, exterior caulk, glass (or auto body for the lower section of the full Kamm, in my head the half-Kamm might all attach to the glass). I'd double check, but I'm assuming exterior caulk is as reversible against polished surfaces as its indoor counterpart. Accidental forces will be distributed, but deliberate focused forces can debond either the tape or the caulk, made possible by the flexibility of the tape and the aluminum sheet Kamm.

[PaleMelanesian]

Originally Posted by christofoo

When I tuft test, I'm expecting to see the roof-to-rear-windshield radius pull the airflow down, a little turbulence at the base of the windshield followed by a lot of horizontal air-flow along the trunk line to the lid transition, where it will fall apart at the sudden ~19° drop.

I can confirm that there is some slight separation at the middle bottom of the rear window, but it reattaches before the end of the trunk.

[MetroMPG]

I wish I'd paid more attention to your initial planning diagram or I would have mentioned that transition sooner. I'll admit until it became "real" I didn't really look closely at it.

If you're willing to build a Kammback structure on the back of the car, I think that'll net your best drag reduction.

If not, I think adding a radius to the transition on the box lid would work.

But before fretting too much about all this, see what the tufts say! Maybe we'll be pleasantly surprised to find it's working as-is.

[slownugly]

doesnt brucepick have a kammback on his hx? ask him for some advise on how strong his is. its the same generation honda just a 2 door instead of a sedan. maybe he would have good tips on how to make a strong one.

[Xist]

Originally Posted by slownugly

doesnt brucepick have a kammback on his hx? ask him for some advise on how strong his is. its the same generation honda just a 2 door instead of a sedan. maybe he would have good tips on how to make a strong one.

He built a short one and tuft-tested it and tried a long one and removed it. I do not know what improvement he saw on the short one.

[California98Civic]

Originally Posted by christofoo

When I tuft test, I'm expecting to see the roof-to-rear-windshield radius pull the airflow down, a little turbulence at the base of the windshield followed by a lot of horizontal air-flow along the trunk line to the lid transition, where it will fall apart at the sudden ~19° drop.

Agree with this prediction and Palemelanesian's affirmative. Here is my pic of tufts pasted to the body of my civic on a damp morning after a ride reaching sustained 45-50 mph speeds: http://ecomodder.com/forum/member-ca...sting-tail.jpg

[PaleMelanesian]

2nd opinions are always welcome!

[tony_2018]

So more cargo room in the back, won't that raise the front a little ( or a lot )? If that happens have you looked into ways to keep the car leveled as if there was no extra cargo in the back?