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Need help designing large model

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Old 12-15-2003, 09:08 PM
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N1EDM
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Default Need help designing large model

I'm trying to build a 1/3 scale Cosmic Wind (the Ballerina). At 1/3 scale, the wing would be 78" and 1000+ square inches of wing area.

However, the fuse will be about 8" across and about 60" long, which is quite large.

Is there any literature anywhere with tips and techniques for building such large fuse structures and still keeping them light? Building this thing like an LT-40 is going to make it waaaaaay too heavy, and using the construction techniques of a 3D plane would make this too flimsy, I think. I think I need to be somewhere in between these two methods.

I'm trying to build light, strong, and intelligently. The wing and empennage should present no problems, but the fuse could wind up weighing like a brick.

Is there any place or publication that you can direct me to?

Thanks,

Bob
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Old 12-15-2003, 11:03 PM
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Default RE: Need help designing large model

The key to a large container such as this is to build a lot of air into the design. Yeah, I know that sounds cute but that's how you need to think about it. At that size you may want to consider following the scale sheet aluminium methods fairly closely but instead of aluminium try using 1/32 plywood or 1/8 balsa for the skins. The close spaced scale former spacing should support the thin skin quite well. If you use balsa go for an overall outer coating of 2 oz cloth and epoxy for strength and local ding resistance.

Alternatley you could to a honeycomb type structure for the shell. Two layers of 4 oz glass cloth and epoxy with a 1/4 EPS foam core in betweeen the two layers would make a nice light shell that would be inherently stiff. Formers and stringers would still be required in key places to support the wing, firewall and tail areas from localized stress and crush loads but the shell would/should be pretty much self supporting for much of the rest of the fuselage. Localized hard points would be made of 1/4 hard balsa inset into the foam.

To build I would see a foam plug being carved and shpaed that was about 5/16 undersized. The plug would be from blue foam for a little more stiffness. At that point the white foam would be sliced as 1/4 inch thick flat panels and this would be bent around the form and fit to a nice but not critical fit. A bit of work with a heat gun used carefully will form the plastic foam to the curves. The form would then be slicked up with a layer or two of that strechy package wrap wound on tight and with few wrinkles and waxed with release agent. The inner layer of epoxy and glass would go on followed by the foam inner core. The whole lot could then be either vacumn bagged or wrapped with more stretchy wrap to clamp it firmly but not crushingly. After it cured the wrap is taken away and any areas that require local hard points are mortised by carving away the foam and fitting either hard END grain balsa or plywood. Then apply any local strengthening patches and the outer epoxy and glass layer and leave to cure. Cut away any nasty's while the resin is stiff but still green. After it cures sand to smooth but don't cut the glass fibers more than you need. Apply a layer of epoxy and microballoon mix as a coarse filler. Then sand for form and level surfaces and you can spot fill and prime.

Large but light for it's weight.
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Old 12-18-2003, 07:38 PM
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Default RE: Need help designing large model

Thanks for the tip, Bruce.

Unfortunately, I can't do the fiberglass thing in this house... too much odor to deal with, especially at a time when you can't open the doors to ventilate.

I was wondering if there were some books or articles out there on how to design large sized models with conventional techniques, or else I'll have to invent my own...

Thanks again,

Bob
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