Insight 1 Boattail-An Experimental Approach

[jime57]

Originally Posted by MetroMPG

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?

[orange4boy]

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.

[jime57]

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.

[aerohead]

Originally Posted by jimepting

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.

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.

[MetroMPG]

Originally Posted by jimepting

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

Eventually, yup.

[RobertSmalls]

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?

[aerohead]

Originally Posted by RobertSmalls

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?

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!

[MetroMPG]

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...

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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

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

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[aerohead]

Originally Posted by MetroMPG

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...

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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

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

.

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