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Author Topic: Balsa tube for fuselage of Wright Stuff plane?  (Read 283 times)
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Little-Acorn
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« on: October 08, 2017, 01:29:32 PM »

We've had the usual problems with fuselages for Wright Stuff planes. They are so long (a 1/16" motor can be more than 16" to weigh 1.5 grams), that they get bendy when you wind them all the way up, putting maximum tension on them, They sometimes bend like archery bows. We've tried gluing an additional strip along the side, which helps, but sometimes it take two additional strips, and weight and complexity keep increasing.

One fellow made a fuselage out of two 1/16"x3/8" strips, side by side with 1/16"x3/8" spacers every 6 inches or so. And innovative idea, and he's had pretty good luck with it. I wonder a little about the drag of the extra surface area, plus the drag of the spacers, but it's hard to argue with success.

Has anybody ever tried getting a 1/32"x3" sheet, cutting a 1-1/4" strip out of it 20" long, soaking it in water, and rolling it into a 20"x3/8" tube? By wrapping it around a 3/8" spruce wood dowel? And putting the rubber motor INSIDE the tube?

Stress would be evenly distributed across the cross section, there would be practically NO forces trying to bend the tube the way the stick fuselages get bent.

I'm sure I'll have to wet it thoroughly, or else the balsa will crack a lot as I try to roll it around such a tight curve.

Problem is, I would like to glue the seam together while the wood is rolled up around the dowel. But the balsa is still wet at that time, and I haven't found a glue yet that works well on wet wood. I might have to wind thread around the balsa tube as it dries on the dowel, to keep it in place, without using any glue. Once the balsa is completely dry (overnight or longer), I'll remove the thread and slide out the dowel, and hopefully the balsa will stay in its rolled shape, without cracking or unrolling. Then gluing should be relatively easy.

Anybody ever tried this? Got any tips or comments or "Ain't no way that'll work" or "1/32 is too thin" or etc.?

Thanx all!
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frash
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« Reply #1 on: October 08, 2017, 01:42:04 PM »

You are on a good track for your students. A rolled tube is often used for other indoor rubber duration events.

One standard reference for rolling a tube is below.
 
Larger outdoor rubber models usually have the rubber motor inside. The indoor ones usually have the rubber motor outside the tube but  parallel below.

http://www.parmodels.com/Techniques_and_References/Rolled%20Balsa%20Motor%20Stick.pdf

Fred Rash
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Little-Acorn
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« Reply #2 on: October 08, 2017, 04:52:54 PM »

An EXCELLENT reference! Thank you Fred!

And they used a 1/4" dowel of aluminum tube as a form... wow
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leop
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« Reply #3 on: October 09, 2017, 02:04:28 PM »

A rolled tube is not necessarily any stiffer than a solid motor stick.  However, for a given weight, a rolled tube is stiffer than a solid one.  A tube rolled over a 1/4" mandrel will be much lighter than a 1/4" diameter solid balsa rod but the solid rod will be much stiffer.  In Wright Stuff, a rolled tube is neat but is definitely not needed at the high minimum weight.  A better solution to motor stick bend is do stiffen the motor stick (against the rubber motor tension) using a bracing post and wire.  Although tungsten wire was used in the past on the light weight planes (like F1D), many today use Kevlar or UHMW polyethylene (like SpiderWire fishing line) for the bracing "wire" or fiber.  One can take a look at pictures and plans of planes like F1D's to see bracing posts and wires.  As an added benefit, the fiber can be adjusted (such as with few of the individual Kevlar fibers in a bundle) to adjust the bending stiffness.

LeoP
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mkirda
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« Reply #4 on: October 12, 2017, 02:02:13 PM »

If you decide to go this route, choose first a mandrel size.
The use the following formula to cut the wood width:

Pi times ((Mandrel OD) + (two times thickness of balsa sheet used))

Not sure you can even perfectly straight A-grain 1/32" sheet around a 1/4" mandrel though, even if really light i.e. 4# sheet.
And putting it internal to even 3/8" ID invites a whole host of rubber knot rubbing issues.

External is best.
Bracing it as suggested is even better. You will need to have internal webs at the front and back to stop the crushing forces. Vertical grain 1/32" would be fine.

Regards.
Mike Kirda
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leop
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« Reply #5 on: October 12, 2017, 03:52:20 PM »

I suggested above that the solid motor stick be braced.  The picture shows a braced, solid motor stick on a plane I built recently  Please note that the scale is in centimeters.

LeoP
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Re: Balsa tube for fuselage of Wright Stuff plane?
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Hepcat
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« Reply #6 on: October 12, 2017, 04:33:22 PM »

As is usual with Wright or SO stuff it is difficult to know what the actual problem is and nearly impossible to find out the rules and regulations that have to be met.  In the case of a fuselage distorting under the loads of a rubber motor I am virtually certain that a built up fuselage with a square or triangular cross section and three or four longerons will be lighter and resist the loads of a rubber motor better than anything where the structure is asymetrically loaded.
John
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