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

[MetroMPG]

Any chance you could post the video that still came from - YouTube?

[PaleMelanesian]

From my years of driving the same car, no. In stock configuration that does not happen at all. Air flows all the way down on the sides and halfway down in the middle.

[christofoo]

Originally Posted by PaleMelanesian

From my years of driving the same car, no. In stock configuration that does not happen at all. Air flows all the way down on the sides and halfway down in the middle.

Hmm. Drove with the box off and the tufts still on this morning and saw the same flow pattern in the rear-view. EDIT: well... probably not as bad as with the box on....

EDIT: I have never seen the inverted rain flow prior to the 'Aero' Hitch Box.

Two seemingly contradictory data points.

[christofoo]

Originally Posted by MetroMPG

Any chance you could post the video that still came from - YouTube?

Let me revisit this tomorrow.

[California98Civic]

Originally Posted by christofoo

Hmm. Drove with the box off and the tufts still on this morning and saw the same flow pattern in the rear-view. EDIT: well... probably not as bad as with the box on....

First, thanks for posting even the troubles so that the rest of us can learn. Second, I think you might want more tufts on the car. And last, I can confirm that my 1998 Civic couple shows only a small recirculation bubble on the bottom, middle portion of the rear glass adjacent to the trunk lid. Post the video... it would be interesting too.

[NeilBlanchard]

Maybe a Kamm back over the window is a solution?

[euromodder]

Close the gap between the hitch box and the trunk, both horizontally and vertically, and you'll be fine.
The air is flowing well over the box by the looks of it.

[aerohead]

Originally Posted by christofoo

All of us have underestimated the potential for aerodynamic misbehavior in this design, although I think it will be fairly easy to repair by adding a baffle and controlling detachment lines.

Here is a preliminary shot around 35 MPH:

That's right, I've got air moving up the rear glass. If I'm not mistaken, the low pressure at the base of the windshield and trunk is actually pulling air up from the underside of the car.

It's unexpected, but something I knew was coming, since at 65 MPH last week I saw rain water fly up from behind the trunk and land in the middle of the rear glass. (And thought 'd&^#, what a spectacular goof'.) I even knew right from the first day I drove it to work that something was amiss, since I coast down a hill from my house and I couldn't tell any difference in coasting or P&G rhythm. Certainly the MPG numbers from our 2 long-hauls with the box in place have been lack-luster. I don't have good baselines to compare to, but I think they registered somewhere around 0% or -5% (loss).

(So, a lot of good I did myself meeting my fab deadline without any testing.)

Not shown, because the image quality was terrible after it got too dark, I also got a brief tuft test with a quick-and-dirty cardboard-and-duct-tape half Kammback over the rear windshield (the version that doesn't cover the trunk at all). Even after rewatching it twenty times to tease out the info, I have to call it preliminary. The tufts that are directly in the stream (top of the Kamm and top of the Aero Box) seem to look reasonably smooth, and I think the tufts on the rear windshield may indicate a reduction in the violence of the flow, but I'm still getting some flow from the bottom to top. The Kammback did not improve the turbulence along the sides that I noticed.

So I need to improve my camera boom and test in broad daylight, and add some more detachment garnishes on the sides, and maybe a baffle to stop the upward flow at the bumper, and see how far that gets me.

If you can make your way to the full-boat-tail trailer thread,page 25,#248 permalink,3rd image down, you'll see the effect gaps can make on a Clark-Y airfoil as a function of the gap,and also its position along the body.
I think this is what you're fighting.

[christofoo]

I'm surprised by how much time it takes to tuft test a design and remedies, not to mention how long it takes to process and upload the vids... about 6 hours for this, and the most immediate lesson is I need to find a smoother road and add some weight to my micro camera.

Cruising speed is 30-35 MPH.

Here's the flow over the stock Civic (not exactly stock, hitch reciever, passenger mirror delete, partiall grille block, and moon disks)

http://www.youtube.com/watch?v=qFKct3TIDzg

Now with the Aero Hitch Box:

http://www.youtube.com/watch?v=fvCEjG04i_k

Now for some attempts at cardboard remedies:

First a partial Kammback,

and a garnish to force detachment on the side. I shot it this way to emphasise the mismatch in curvature, which I don't think has shown up in my other pics.

http://www.youtube.com/watch?v=ZtcRu7cV3vk

Next a wheel skirt,

and a baffle at the bumper to stop vertical air flow.

http://www.youtube.com/watch?v=MJwGUnPOhYg

[California98Civic]

Thanks for the vids. I'm also involved in some cardboard aided design in my build thread. I have no special training. I just hang around this forum. And it looks to me like there is a larger than stock recirculation zone on the glass in clip #1 and that that zone grows very significantly in clips 2 and 3 with the kamm. I would think these all indicate down stream turbulence, but then the flow seems attached at the hitch box. Still couldn't that attached flow be just the air getting forced around the box, with turbulent wake above it and downstream of the hitch box?

Can you bring the hitch box closer to the rear bumper and close the gap, especially at the top? I bet their is a sort of bow wave at the front of the hitch box caused by the air rushing down from the roof and in from the sides down into that gap at the tail for the trunk.

EDIT: It occurs to me that if all you end up with is a cargo box that adds no big drag but does not reduce it, either, then you are still ahead of the roof mounted units.]