partial boat-tail

[MetroMPG]

(EDIT: preliminary results posted in thread below on Jul. 18.)

Come back, 95metro!

Originally Posted by 95metro

Well, there's a mockup of the adjusted spoiler/boattail. It only extends back the same distance as the rear bumper (but it could go maybe 6" over) and it shouldn't (knock on wood) impair vision through the mirror.

Spent about 45 minutes playing this afternoon. No, it's not tested yet, and was more of a design exercise than anything else for the moment. I may reinforce and test it if the right weather comes up in the next day or 3 (before it falls off/gets wet). Otherwise, it'll come off having served its purpose as a learning tool for how V 2.0 should look.

Constructive feedback welcome!







Above: what this crappy picture from my antique 1 MP camera intended to show was that Metro95 was right: it doesn't seriously obscure rear vision. The boat-tail cuts of 1/5 of the rear window view, and though you can't tell from this photo, the horizon on level ground is just below the "roof", so full visibility is retained of following traffic.

[Mighty Mira]

Be (very!) curious to see what mpg you get.

[zpiloto]

This boat tail for you 95metro. Metro you so inspired me with this post that I took the rest of my colorplast and put one on casper extending off the roofline. . I don't know what it will do for aero but went for a test drive on my test loop and came in with 38.2 which is the best every for that loop and this was at night with the lights on. I'll be out of town this week but I'll try to get some testing and some pics this weekend. I could tell from my route that it coasts better, but only testing will tell for sure.

[JanGeo]

use clear plexiglass . . .

[Ryland]

I would think about getting a hatch from a junk yard, and rebuilding fully, then dimple it.

[ZugyNA]

I can visualize a nice row of vgs right in front of where the top drops down at the rear window.

Then right above the taillights a sort of flat fin extending straight out maybe 1.5'.

And a row of vgs on the sides...much like the airtabs?

In theory...the top vgs would keep the flow more on the rear glass...while the fin would force a high pressure plume of air to the back....preventing the big disturbed vortexes from forming?

The side vgs in front of the rounded corners would help to fill in the low pressure also.

[MetroMPG]

My thoughts while making this:

- compound curves are hard to get right (I got the "roof" of the tail eventually, but not the sides right).

[MetroMPG]

zpiloto: glad to hear that lunacy is infectious. Hope your immunization is up-to-date for unsolicited public comments.

[zpiloto]

Originally Posted by MetroMPG

zpiloto: glad to hear that lunacy is infectious. Hope your immunization is up-to-date for unsolicited public comments.

I've never had too much unsolicited public response to the wheel skirts, but the cardboard partial boat tail drew a small crowd. A question and answer session ensued in which at one point my sanity was debated.

No hope. I don't do shots. I'm past the comment stage, everybody thinks I'm the mad scientist. When I meet someone new in the neighborhood it's always "you're the guy that rides his bike all the time and has weird stuff all over your car" Then when you start talking about the car you get this blank stare and then why are you doing that. But on a high note the commute to and from turned out some good numbers, 39.5 MPG, That's about 2 MPG better(man I wish I had time to test it) than normal since I had to stop EOCing. The transmission has started to complain. I think you'll see some big numbers once you're done since you have a good eye for detail and are very talented at these mods

[Gary Palmer]

I like the overall look of your shape. If you could live with the harder corners, you could build a prototype out of coroplast, possibly. If you wanted to do something out of fiberglass, you could get the rounded shape you are looking for.

[MetroMPG]

I'd prefer the rounded top-side transition if possible.

[MetroMPG]

Originally Posted by zpiloto

No hope. I don't do shots. I'm past the comment stage, everybody thinks I'm the mad scientist.

Hooha. Good story!

Have you posted details of your EOC transmission complaints? It may be worth adding to the appropriate thread.

[Gary Palmer]

MetroMPG: If you want to do that, I think the information you've looked at is taking a more complicated route than you need. I'll try to find the web site, but their is another way to do the shape with foam and fiberglass, where you just make the shape, like you want, put fiperglass over the outside, shave/carve the foam on the inside out, down to like a 1/4 inch and then fiberglass over the inside. It gives you the ability to make a one of, that is very strong and you can accomplish those shapes.

I'm off to the Chiropractor, but when I get back later today, I'll try to find the link and post it up.

[MetroMPG]

Yes please, Gary. I'd be interested in seeing that.

I'll check back in later. Enough GS for today. My productivity hurts.

[Gary Palmer]

MetroMPG: Here is the link to the website I was describing. This is for a homebuilt aircraft, but the process and technique is what I think might be what you are looking for. Look at the totally useless box!

http://visionaircraft.com/

[MetroMPG]

And here's another one - got some keywords off the aircraft link that helped find more sites:

One-Off Construction Using GRP/Urethane Foam Composite

[basjoos]

You can make rounded curves in coroplast by the careful use of a heat gun.

[MetroMPG]

Did a non-rigorous test - A-B only.

[zpiloto]

Originally Posted by MetroMPG

Did a non-rigorous test - A-B only.

Only 61 MPG You're killing your FE. Testing can hurt at times. I was hoping you would be over 5%. Are you going to make a version 2?

[krousdb]

I'm also surprised at the highway FE. I thought it would be 70ish.

[MetroMPG]

You're right zpiloto - testing can hurt.

More than testing has hurt... what with the vacuum leak, transmission swap & clutch cable adjustment, and now this boat tail testing, it all adds up to not a record tank. I'm struggling to just keep it above the current 90 day average.

But - the results are good news for you. They support the idea that your mod may have been successful, despite the non-test. You have pics yet?

I'm thinking this is worth tuft testing, refining and fabricating properly. I've also asked for feedback at MaxMPG.

[MetroMPG]

Originally Posted by krousdb

I'm also surprised at the highway FE. I thought it would be 70ish.

Consider ...

[MetroMPG]

Today I should have tested at 85 km./h, not 55 mph.

[JanGeo]

Wonder what a boattail on my xB would look like....I think the rear belly pan will be first then maybe the vortex generators.

[zpiloto]

Originally Posted by MetroMPG

You're right zpiloto - testing can hurt.

More than testing has hurt... what with the vacuum leak, transmission swap & clutch cable adjustment, and now this boat tail testing, it all adds up to not a record tank. I'm struggling to just keep it above the current 90 day average.

But - the results are good news for you. They support the idea that your mod may have been successful, despite the non-test. You have pics yet?

I'm thinking this is worth tuft testing, refining and fabricating properly. I've also asked for feedback at MaxMPG.

I was able to test late today without positive results. There was no significant change in FE. I was surprised because of the good FE on the commute without the EOC. (Need to start that thread). Proof positive for me that you have to do at least A-B testing for the butt dyno just don't cut it. The shape of the Mazda doesn't cry out for the boat tail like the fire flea though. I think you'll see even better results with version 3.
Since it really didn't hurt the FE I might try again with a longer extension, and see what effect that has, but I'm out of good peices of colorplast. You can come a long way down without obstructing the view through the rear window.

[ZugyNA]

Here's a pattern for easy to make vortex generators:

Use doublesided tape or silicone caulk to mount...can bend by hand the surface that mounts...to conform it to the car's surface to some extent.

If you have a roll of flashing material 2 1/2" wide....you can make a bunch of these by making one 30* cut...alternating the direction.

Narrow end faces forward.

Sharp point on vanes needs to be rounded.

Vanes can be made higher by making the mounting surface narrower...Mitsubishi says up to 1" high OK.

[The Toecutter]

That rear partial boat-tail that MetroMPG built is pretty much what I was trying to recommend to krousedb for his Del Sol.

I considered such a mod for my GT6, but it would make the rear hatch area inaccessable.

[ZugyNA]

Originally Posted by MetroMPG

- partial boattail gain: 1.38 mpg (US) / 2.3% above "stock"

I'd bet a nickel that some vgs 4" apart and 4" back from the break above the rear window...and some along the sides spaced 4" & 4" would do just as well. But only a nickel.

[MetroMPG]

I don't think VG's will work in my hatchback application.

My understanding of the Mitsubishi document (PDF) is that the VG's work in the scenario tested because they delay flow separation on an otherwise sub-optimally angled surface (in that case, the rear window of the EVO sedan, which slopes downward too sharply toward the trunk to retain attached flow from the roof).

Without that downstream surface for their vortices to interact with, they have no boundary layer to affect. Their overall result in that case may actually be to slightly increase drag (as is also acknowledged in the document).

Also note the Mitsu document estimates a Cd reduction of -0.006 through properly placed delta-wing VG's on their sedan. (-0.004 for the bump style VG's, also tested.)

I plugged in my car's numbers into the Aero & Rolling resistance calculator http://metrompg.com/tool-aero-rr.htm, and, assuming a Cd change of -0.006 could be achieved, it only improves fuel consumption @ 55 mph by 0.83 mpg (US), or 1.34%. So the cardboard hatch tail still comes out ahead, at 2.3%.

I'm not saying that VG's won't work in ALL hatchback applications, but the hatch angle would have to be much less steep than mine, where some flow is retained part-way down the hatch - closer instead to the angle of the Evolution's back window.

So, I'd say: save your nickel!

[DaX]

Originally Posted by MetroMPG

My thoughts while making this:

- compound curves are hard to get right (I got the "roof" of the tail eventually, but not the sides right).

I would say to try drawing the curves in CAD and let the computer figure out the intersections. If you use a sheet metal tool, you can then "flatten" the part and print full size to make a stencil. This is how I designed the pulse jet engine I plan on building someday.