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Old 06-25-2013, 05:24 PM   #141 (permalink)
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sides

Quote:
Originally Posted by christofoo View Post
Extended the Kammback (rear windshield cover) to the rear bumper, also I added a baffle to stop flow from the side, (also I found a less bumpy road).



So this configuration is:
  • Full Kammback
  • Rear wheel skirt
  • Baffles between the car and the box, both horizontal and vertical.
  • Aero Hitch Box
  • EDIT: also I put the passenger side mirror back on, since I'm blocking 90% of the rear-view with the cardboard

Results:

I think that looks a lot better. The full Kammback always seemed like the solution that just had to work.

Video, cruising speed ranges a little from 30-40 MPH:


Well, I mean it still isn't perfect. I still need to find a form that fixes the turbulence on the side of the Aero Hitch Box, if possible.

But I think my next task is Kammback fab. (And duct tape gum removal... any pointers?)

EDIT: I forgot to mention; while I was scouting the road for this test this morning, I saw the sweetest full aero shell on a recumbent bike (velomobile). I wish I'd had time to snap a picture, but it was headed the other way. I think it was going at least 30 MPH on a very slight uphill. I wished I could have stopped the guy and asked if he made it himself, and if it was for a competition or not. I was so jealous.
It's the gap which is affecting the side flow.Over at the full-boat-tail trailer thread is a pictorial drag table depicting a Clark-Y airfoil of differing gap configurations and the drag as a function of position or magnitude.
The baffles will help create locked-vortices around which the outer flow will skip across.
As long as air can pass right through it's impossible for the air to be directed.

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Old 06-25-2013, 06:20 PM   #142 (permalink)
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(And duct tape gum removal... any pointers?)


It's the Great Circle Of Life.
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Old 06-25-2013, 09:15 PM   #143 (permalink)
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It's the Great Circle Of Life.
Where do you go after "No problem!"?

The Army version is:

Does it move?

Yes: Salute it!

No: Polish it!
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Old 06-26-2013, 11:58 AM   #144 (permalink)
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I'm a sucker for flow charts.

(... but don't forget, this is the wife's car. )

...

When I looked at this I thought:
Only 0.02? So a well-designed gap in an air-foil can be ignored for vehicle design purposes.

That doesn't seem like the right analysis?

This was always one of my favorite posts, BTW. gap vs drag coefficient
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Old 06-26-2013, 02:18 PM   #145 (permalink)
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(... but don't forget, this is the wife's car.
Maybe I should have been more verbose? WD-40 will melt the residual adhesive. It won't affect the paint, though it might take wax off.

I took a closer look at the airfoil chart. If the vehicle is ~4ft tall, the chart applies to a gap of 1.2ft. But the 6:1 fineness ratio doesn't scale out on my screen. Is the airfoil is in ground effect? The call-out for H implies that.

Thanks for pointing to gap vs drag coefficient. New stuff there. The post had 20 Thanks but I wasn't one of them so I added myself.
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Old 06-26-2013, 02:34 PM   #146 (permalink)
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Quote:
Originally Posted by freebeard View Post
Thanks for pointing to gap vs drag coefficient. New stuff there. The post had 20 Thanks but I wasn't one of them so I added myself.
Me too, Christafoo.

This image seems to be even closer to your inspiration than the one you posted above. How cool.

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Old 06-26-2013, 05:29 PM   #147 (permalink)
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Quote:
Originally Posted by freebeard View Post
Maybe I should have been more verbose? WD-40 will melt the residual adhesive. It won't affect the paint, though it might take wax off.
...
Oh, I see. I thought your implication was more like 'duct tape, oh yeah!'

Somewhat related; I actually use WD-40 for Water Displacement, right here on this project. (The hitch stinger and receiver can't take a lasting coating of anything, but they still deserve rust protection because water can get in there and hang out for a while.)

Quote:
Originally Posted by California98Civic View Post
Me too, Christafoo.

This image seems to be even closer to your inspiration than the one you posted above. How cool.
...
Aerohead has referred to the Clark Y airfoil chart a couple times on this thread. I'm still trying to figure out what I'm supposed to learn from the airfoil chart. Maybe he'll chime in and clarify.
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Old 06-26-2013, 07:00 PM   #148 (permalink)
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I'm baffled.

What I take away is that the tow vehicle should be concave, centered around the hitch point, and the towed vehicle should be convex, centered around the hitch point. Then the need for diverter baffles is minimized.

Your hitch mounted box would be a case at one limit.

Last edited by freebeard; 06-28-2013 at 07:53 PM.. Reason: concave≠convex
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Old 06-26-2013, 07:05 PM   #149 (permalink)
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Quote:
Originally Posted by christofoo View Post
Aerohead has referred to the Clark Y airfoil chart a couple times on this thread. I'm still trying to figure out what I'm supposed to learn from the airfoil chart. Maybe he'll chime in and clarify.
I will bet that his point in raising the airfoil is what you see in the last three trailer images I just shared from the post you linked. If you look at the last three trailer designs and imagine the airfoil overlaid upon them, the gap is more or less where it is in the split airfoil that produces the lowest drag. And these are the trailer set-ups that produce the lowest drag estimates. So, as aerohead said, you are on the right track with the baffles and kamm and such.
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Old 06-28-2013, 02:01 PM   #150 (permalink)
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Road-worthy gap filler

We're headed out on a road trip today, looking forward to seeing some gap-filled MPG.

Mostly this is coroplast, some aluminum ribs to give it curvature where required. Bought a 1ftx3ft cut of neoprene rubber at a local industrial hardware store. Used that to make some custom gaskets facing the car (not perfect, though they happen to look good in the resolution presented here, but a few ~0.5" gaps in places).



Sorry, you can just barely see it here, but there is an aluminum sheet to shape and protect the coroplast above the exhaust.

To facilitate articulation (when the box hinges up), the underside is restrained with clips.


It also tucks under the hitch receiver.


When the box lifts the under-panels can pull away from the car but don't snag on anything when they get pushed back in.

One 'gasket' is actually a strip that velcros to the top of the bumper.


I need to paint the coroplast to prevent UV degradation. One issue is that the coroplast deforms under sunlight if it's painted dark. This was noted by kach22i (http://ecomodder.com/forum/showthrea...tml#post375743), I've also noticed the problem on my black coro Corolla wheel skirts. The gray paint already on the box is also too dark (pretty sure). So I need to either rebuild the structure in a better material, or paint the whole box white.

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