Insight 1 Boattail-An Experimental Approach
 

With all the talk of first gen Insight boattails, which interest me considerabley, the engineer in me begins to emerge. Specifically, I'm beginning to stategize as to the most cost effective and flexible way to get the data on performance. I want a practical boattail, but I don't want to invest a fortune on something which will be proven by the data to be ineffective. Rolling it over mentally, I think I see an approach which will gather the relevant data and get me pointed in the right direction - leveraging on the work that RobertSmall and botsapper and 3-wheeler have already invested. (BTW, my Hucho arrives any day now!) Since I have extra vehicles, and a garage, I do not have to drive the Insight during construction or modification. My approach goes like this.

1. Test the baseline car for Cd and fuel economy under know and stable conditions, as best as driving technique and conditions allow.

2. Using corregated cardboard, add a rear underbelly, AND build a "first effort" boattail, largely after the MetroMPG model of ribs, cardboard and garbage bag covering.

3. Test using yarn tufts and a webcam. Make any required modifications.

4. Test for fuel economy and Cd.

5. Once the "prototype" is firmly established and tested, then, and only then, move on to a more permanent construction method, such as fiberglas over foam.

6. Finish the project with a coat of professionally applied matching paint.

How does this sound strategy wise?

Your build plan seems reasonable. Do your least expensive beta testing with cardboard mock ups. Any revisions could be made easily & quickly. Your final shape could be done confidently and with proper materials & better fabrication techniques. Documentation & records are eagerly expected. We support your efforts and will follow your build progress here or on your own build site. Good luck.

I like it too.

Keep us informed on your progress...

Jim.

Jim,

I'm glad to see your enthusiasm, and I strongly encourage you to put it into action. And yes, if you can build a prototype much more easily than a well-finished final product, by all means do so.

Just a few comments on some of your steps.

2. If your boat-tail is as small and light as I think it will be, you may only need one or two ribs.
3. I'd love to see video of Insight tuft testing!
5. Your prototype will be cruder, and more angular, than a foam and fiberglass version, and the final version will be aerodynamically better. So if it works on the cardboard mockup, it's definitely going to work on the fiberglass version.
6. A professional paint job would be a nice touch. On Sunday, I painted a few panels on my Insight, and the good news is that they look fine from three feet away.

Keep us posted.

A "3 foot paint job" is pretty good for a non-professional!

Subscribed, and watching this thread with interest.

First one to record tuft testing of a 1st gen. Insight boat tail prototype wins! :P

I would advise against the garbage bag coverings, I tried it on my mirror deflector, and it made a ton of noise, flapped in the wind, and pulled air in somehow making it into more of a balloon. If you are worried about the cardboard getting wet, remember you plan on testing in good conditions, and don't have to drive it on days that are poor. Also, you can look into the used campaign sign plastic if you want to go an intermediate route. I would suggest you do coast down testing to get the Cd and Crr of your car to begin with. Then when you make changes, you can repeat the testing to see the difference. I have done coast down testing a couple times now, and I have found it is more post processing than anything, and there are a lot of us around the forum that can help with getting you started if you need. I think you will do well to just continue the lines of the Insight, since it is already super sleek. I once saw a picture of an insight that had a custom built trailer behind it that was super long with a fin. I think it's Cd got down to around 0.12, which would be pretty amazing. If you get a back of car bicycle rack, you can do your testing easy without ever putting duct tape on your car (unlike many of us). Best of luck!

wyatt,

I think one has to use a layer of contact cement to hold the gargage bag material in contact with the base material, but maybe MetroMPG will confirm.

I guess I never tried to glue it down, I just figured that it was temporary, and thus not worth spending too much time protecting.

Try 3M Blue 72 spray adhesive. Great tack, low soak & repositionable. Works w/ all your material choices.

I've used garbage bags on several big cardboard prototypes. Never had problems, and I didn't do anything fancy beside making sure it was taut and well stuck at the edges with good, sticky duct tape.

Now that you have all that good experimental data, are you building something smother and more permanent?

Cardboard can be spray painted to seal it for temporary water tightness just like wood. two coats. let the first one dry overnight would be best.

You could also roll it with an oil based paint for an even better water repellant finish but then I'm not sure how recycle-able that would be.

I'm at work of my approach, as time and other commitments permit. In order to use small sections of cardboard and not compromise the complex curves too much. I'll be gluing the cardboard down to a lightweight wooden framework and taping over the joints, so paint won't be needed for that. As a final step, I intend to cement black trash bag material onto the boattail, as MetroMPG did.

as time permits
 

Your project will never keep up with our curiosity! The Insight is an amazing starting point to begin from and it will be really something to know what someone can actually squeeze out of one of these 21st-Century wunderkind.
Wish you a lot of mild, low-humidity days to work on it.The 'sauna' has already begun in Texas.:thumbup:

Eventually, yup. :turtle:

Jim,

I thought of an experiment I wish I had time to run before building my boat-tail. I'd like to know the maximum angle for mantaining attached flow across the underside of the tail. If I could get away with 9° instead of 7°, it would save a few inches of overall length, which would be great. Or if I can only get away with 4°, but I need 7° to meet my departure angle requirements, I'll make the first half 4° and the second half 11°.

Tuft testing at 30mph would determine that angle. Aerohead assures us Re* is achieved by that speed, so if flow is attached at 30mph, it will be attached at 80mph.

Attach a sheet of cardboard (with camera underneath) or plexiglas (with camera peering out through the hatch) to the stock rear diffuser, on a duct tape hinge. Attach 20 strands of yarn to its underside. Support the rear of the cardboard or plexi with a string, tied to the hatch. Adjust the length of the string with a clamp or something, to modulate the angle of the board (measured with protactor and plumb bob). Start at 0°, and test at various angles.

For extra credit, use a smoothly curved piece of cardboard or plexi.

Accuracy would improve if a cardboard boattail was already in place.



Did I just spend more time describing an experiment than it would take to execute it?

piano hinge
 

Robert,I saw a trick General Motors used on their Aero 2000 concept car at Epcot Center.
Their diffuser was hinged at the attach point to the car,and once they were at cruise speed away from urban hazards,the panel lowered to the desired orientation for best airflow.
Piano hinge would do the job.A cable and pulley system could operate it,with no more than something like an emergency brake handle,with release for manual control,locking,and release.
Best of both worlds!

Phil: Pop Sci would like to chat with you
 

Sorry to go off topic here, but I just wanted to post in this thread for Phil in case he's not checking PM's...

---

Any interest in talking to a Popular Science writer about DIY aero modding? :)

He's doing a piece for the July issue, and has been reading the aerodynamics forum. I've talked to him already - seems genuinely interested in the subject matter.

He asked about you specifically, owing to the amount of good info you post. So I told him I'd check if you were interested.

If so, send me email - darin@ecomodder.com - or a private message and I'll give you his contact info.

Darin

----

We now return you to your regularly scheduled aeromodding topic...

.

PM
 

Darin,I just now caught your PM and gave you a shout back.Thanks!,Phil.