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Author Topic: Balsa tube for fuselage of Wright Stuff plane?  (Read 804 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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lincoln
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« Reply #7 on: January 16, 2018, 03:07:39 AM »

Hepcat:
The really long duration indoor models almost always use rolled tubes with some kind of bracing and maybe some boron along the tube. I understand, however, that boron can be pretty nasty to work with. I think part of it is that you can wind externally and not break the model if you break the rubber.
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Leop:
A tube of 1/32" balsa rolled over a 1/4" mandrel will have an OD of 5/16". Or it would if it didn't crack while rolling. Stiffness goes as the 4th power of the diameter. .25^4 = .00391     .3125^4=.00954   The ratio between the two is (.00954-.00391)/.00391 = 1.43.  However, the tube uses only 56 percent as much wood. More realistically, .015" balsa CAN be rolled over such a mandrel. In that case, you'd have 25 percent as much wood, and only 56 percent as much stiffness. But you can easily make up for this by using stiffer wood, which is generally heavier.

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Little-Acorn:
The double strip with spacers technique is giving up some torsional stiffness.That's not ALWAYS a bad thing. If there's a bit of twist between the wing posts, then a model will have extra wash in to compensate for the extra torque from the rubber at the beginning. But that may be tricky.

Depending on how much the glue joint weighed, a box might give most of the same advantages as a tube. The glue would have to be sparingly applied, only just enough to do the job.

If foam is legal, a Styrofoam stick of circular or rectangular shape could have strips of wood glued to it. These strips needn't be balsa, as long as they're small enough not to weigh very much. By Styrofoam, I mean extruded polystyrene foam, not expanded bead foam. Foamular would also work, and I'm sure there are other acceptable brands. Maybe meat tray foam. These foams are attacked by the solvents in Duco or Ambroid, so you need to use another kind of glue. Foam Tac, Elmers, epoxy and I'm sure there are a bunch of others. If you get a sheet of unidirectional, cured carbon fiber, you can cut off narrow bits the same way your do balsa, but watch out for the splinters. At least you can see them, unlike fiberglass or, I'm told, boron. But even wood or bamboo strips would work ok. The foam doesn't have to be much larger than the original motor stick to be much stiffer when the reinforcing strips are applied.

I don't know how it is where you live, but where I live it's easy to find dried reeds, usually from swampy areas, that are just as stiff as indoor quality balsa of the same weight. I haven't built anything with them, but I did a test. If this is legal, and you  have them around, it may be the easy way to go.

Another approach that wouldn't be all THAT much work is to get the lightest balsa you can find that's of good quality. If you cut a stick that weighs the same as the sort of stick you've been using, it will be larger and therefore stiffer. You probably should increase both dimensions, especially if you're using a relatively thin rectangle already. You can also test your existing balsa. Some of it will be stiffer for the same weight, or just as stiff and a bit lighter. I've seen variations upwards of 30 percent in stiffness for balsa of about the same density.

You can find more info about this sort of thing in old issues of INAV, and you could also look up beam theory.

old issues of INAV:
https://indoornewsandviews.com/downloads/

measuring balsa stiffness easily:
http://www.indoorfreeflight.com/balsa.htm
Keep in mind that the "stiffness coefficient" for a spar or motor stick is different from the stiffness coefficient for a prop blade or other sheet material. The latter favors even lighter, somewhat thicker balsa if it's usable. I once made a pair of EZB prop blades from .011" thick balsa that was around 3.5 lb density! They were good blades, too. The danger with measuring balsa stiffness is that you'll find a few pieces that are far better than the rest and you'll be tempted to throw that other stuff away! Don't. If it's light, you can make low aspect ratio catapult glider wings with it. Or if not, jigs and fixtures, etc.

http://www.gryffinaero.com/models/ffpages/tips/euler/euler.html

Rolled motor sticks:
https://web.archive.org/web/20140410085732/http://www.indoorduration.com:80/INAVMotorstickConstruction.htm
note that the current version of the site seems to be messed up. But it seems that in the archives, there may be a lot of good articles.
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