Project: making a permanent Metro Kammback extension

[MetroMPG]

I started work on fabrication tonight (of the "good", permanent version).

I'm planning to glass directly over the upper cardboard panel because it's exactly the right shape already.

I removed the two "side" panels though - they were less than great (corrugated stuff doesn't do compound curves nicely). I recessed the panels about an inch or so to serve as a backing, and will fill the area with Great Stuff expanding foam, then shape it and glass it with the top section.


(The cavity that will be filled with foam. The tin foil is the "release agent" from the hatchback surface.)

I have it on good authority the foam is fairly easy to work. I filled one of the sides about an hour ago - hopefully it cures hard, because even though it's tack free, it seems pretty squishy.

I would say this, so far: based on the amount of time I think it'll take to do foam/fiberglass construction, I think if you know your plastic and can get a proper sheet of ABS (or whatever is the right type to heat form into compound curves) , that would be the way to go.

Once the upper surface is glassed, I'll remove it from the car and add foam to the center section & glass the entire bottom, to make a sandwich.

[MetroMPG]

Originally Posted by .Cd

Both you and I have a rounded tail end to our cars.
I'd guess that they are rounded for a reason, but as we know, a sharp edge is usually better than a rounded end.

I'm not sure you can get any useful info from doing a tuft test comparison of the two styles. A sharp cutoff of a proper taper will simply show the tufts streaming straight back before the cut. A radiused cut off will show some turbulence after the separation point on the curve.

I never did get a good answer to "which is better" after that long thread at GS. It's something I'd like to talk to a pro aerodynamicist about if I ever meet one.

[Cd]

Why not fire off an e-mail to one of the guys at the A-2 windtunnel ?
They seem like nice folks, and while they wouldn't want to share all of their secrets, they could also stand to gain some new business by advertising on this site.

Dave@aerodynwindtunnel.com
Office: 704.799.1001
Fax: 704.799.1112

Wouldn't it be cool if we could invite them to this site and ask their advice ?

( Since you head this site, you have a lot more 'pull' than i do. )


In the mean time, I'll try to call the Texas Tech windtunnel guys. I spoke with one of the guys a while back and he was cool about it, but they are really too busy to answer questions from any of us.

[Cd]

Originally Posted by MetroMPG

Any raised edge that's not parallel to the flow will trip the air. Angled raised edges will spawn vortices.

.

My mistake. I was thinking of the fences on supersonic jets.
The MIG 15 had fences on its wings.

[MetroMPG]

Well, the foam did cure stiffer than when I last checked last night, but it's still somewhat spongy. I was a little disappointed. Not ideal, but workable. Also, the expansion caused the foam to lift the "leading" edges where it meets the hatchback (the "C" pillar area).

I was able to carve/shape the contours I wanted using a bread knife for the roughing in, a hand held hacksaw blade used as a rasp for finer shaping, and some coarse sandpaper on a sanding block. But it's going to take a lot of "finishing" work with body filler & spot compound to get it smooth & looking nice after the glass is laid over top.

I'll do the other side this evening, and post up some pics. I'll lay up the first layer of fiberglass tomorrow.

[MetroMPG]

If this turns out well, I was thinking I could make a mold and produce copies for people, but shipping would be a problem! Not really practical.

[akcapeco]

freight shipping on something like that might not be too bad

[aerohead]

Originally Posted by dcb

So what is the verdict on open back or closed back?

Also, what is your time frame? I'm wondering if I'll have time to finish the auto CDA computing on the guino first. No biggie, but that is exactly the intended usage for it.

dcb,your's is a very good question and while I won't have the definitive answer,here's how I understand it.Both open and closed boat-tails, when properly configured,allow for a reduction in cross-sectional area of the car's body without flow separation.As the air is decelerating down the slope of the boatail(top and sides),it is regaining pressure.Following the "ideal" teardrop path,every 10% of additional tail length is good for about a 10% reduction in area,10% increase in pressure,and 10% decrease in drag,either open or closed.( It's not a linear relationship because of the curved wall of the ideal teardrop,versus a straight-walled cone).According to Ford,if there is a "back" to the tail,air separating at the truncation(chop-off), will round the corner of the back,and slam into the vertical surface,providing a minute degree of thrust.Chrysler calls it "bursting".I have no quanta to tell you the difference in performance between the two,however,if you want to split hairs,it looks like the closed boat-tail crawls away with the victory cup.

[aerohead]

Darin,it's great what you did with the image.It's exactly where I was going! My teardrop is long,although it's what's trotted out alot in fluids books.The 2.5:1 is supposed to be the absolute minimum for form drag and skin friction.The tail is 1.78 times the diameter of the drop.From the photos you can see smoke separating early on the Kamm K-Car and even the Insight.Why GM did the Impact/EV-1 the way they did may have been a styling concession and to improve rear vision.In plan-view,the EV-1 is beautiful,with body taper beginning midway on the doors.

[aerohead]

I think you're good to go with your red line!