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Old 03-04-2014, 03:48 PM   #1 (permalink)
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Civic VX Boattail

I may take the plunge... I drive 150 miles a day...

How does this blueprint look? I assume good. Before I begin thinking about fabrication (I teach auto shop, so this will be a fun class project) I just want to make sure that my theory is good.

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Old 03-04-2014, 06:12 PM   #2 (permalink)
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Woohoo! Boat tails are so awesome. What an excellent idea to involve the class.

First: The top looks great.

On the bottom, I wouldn't go so "fast". The reason: you're not dealing with clean flow on the bottom of the car as you are on the top, so the angle can't be as extreme. Bottom flow is already turbulent, so more prone to complete separation if you arc away from horizontal too aggressively.

I've tuft tested the bottom surfaces of both my Insight & Metro/Firefly tails, and observed attached flow on both by not exceeding ~13 degrees on the Honda and respecting a much more conservative 6.5 degrees on the Metro/Firefly. (Arguably the Honda has a much cleaner underbody design, which is why I went steeper with it.)
Another critical consideration to keep in mind: the "transitions" from the side to top/bottom surfaces. Use generous radii there, not hard corners, to minimize the chance of "tripping" the flow into vortices.
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Old 03-04-2014, 06:15 PM   #3 (permalink)
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lean burn loves boat tails

Postscript: adding a tail to the VX is extra-brilliant because it effectively expands the "lean burn window" inside which that magical Honda engine gives you astounding cruising fuel economy. You'll be able to hold lean burn at higher speeds, or under other higher load situations like accelerating or climbing.

The gains from a boat tail are greater on a lean-burn equipped car than a conventional one.
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Old 03-04-2014, 07:23 PM   #4 (permalink)
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Well if its all for the children's benefit, you should also have them design and build a full belly pan and rear wheel skirts
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Old 03-04-2014, 07:31 PM   #5 (permalink)
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Hi gijoe985,
I say its just fine as its damn near a copy of Basjoos's car.
And heres the proof your overlay on Basjoos.
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Old 03-04-2014, 07:32 PM   #6 (permalink)
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Well, we just did an ( overly aggressive) air dam today. As well as a simple warm air intake.

if we came up with a very efficient air dam and side skirt setup, how beneficial would the belly pan be?

Also, what do you guys favor for fabrication material? Abs seems popular. We have thin sheets of stainless. We could possibly fiberglass.
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Old 03-04-2014, 07:37 PM   #7 (permalink)
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Quote:
Originally Posted by WD40 View Post
Hi gijoe985,
I say its just fine as its damn near a copy of Basjoos's car.
And heres the proof your overlay on Basjoos.
With respect to the AeroCivic, I'm not convinced the bottom of that tail is optimal. And Mike will tell you he has never tuft tested it. :P

Obviously the tail is "good", but maybe it could be "better".

Quote:
Well if its all for the children's benefit...
Ha. Yes! Think of the children!
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Old 03-04-2014, 07:50 PM   #8 (permalink)
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I'll add, discussing the bottom of the boat tail, if we do aggressively block off air from going under the car, how will that affect the bottom of thr boat tail design? Though real world testing will be the final say, I'd like to get as good of a design on paper as ai can before we start building...

then there is the whole practicality factor. I hope to make it so that space is usable. We'll see...
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Old 03-04-2014, 07:59 PM   #9 (permalink)
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Even Fancier Metro - '14 Mitsubishi Mirage top spec
90 day: 70.75 mpg (US)

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Re: air dams....

You never see a super high-efficiency aerodynamic concept that has a giant air dam. (EV1? Nope. VW 1L cars? Nope. Etc..)

Air dams are band-aids to fix dirty underbodies. Why not smooth out the underbody instead, like AeroCivic, and then you know you're "feeding" relatively clean flow to the bottom of your tail?

I'm not sure what would happen if you put a giant air dam on a boat-tailed car. I never thought to test it out the combination.
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Honda mods: Ecomodding my $800 Honda Fit 5-speed beater
Mitsu mods: 70 MPG in my ecomodded, dirt cheap, 3-cylinder Mirage.
Ecodriving test: Manual vs. automatic transmission MPG showdown



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Old 03-04-2014, 08:10 PM   #10 (permalink)
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Looking forward to seeing how this develops also! Will be heading up to east Washington state in April, so would enjoy seeing the project first hand if you're in that area.

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