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Building Tips and technical articles. => Building techniques => Topic started by: Motorman on December 04, 2015, 03:33:15 PM
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One of the first things I have to do on this Thunder Grazer build is laminate a strip of .010" carbon fiber laminate between 2 balsa sticks that are 3/32"x3/16". This will be the spar. I figure I can get them together fast enough by wetting my finger and thumb with epoxy and wiping it on the CF then apply the balsa to both sides.
How do you squeeze it together and keep everything from sliding around and keep it straight? I have a flat building surface and another flat surface to put on top for weight but what about side to side?
Thanks,
MM
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I would look through my metal stock and see if I had anything 3/16" thick or thicker; lay that on a bench underneath the waxed paper I'm using to keep things from sticking, and then lay the pieces up against that. I'd brace everything with the brass weights I have laying around for such tasks.
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Be careful of splinters from the carbon. About the first thing I learned about carbon-epoxy composite is not to run your fingers along a piece. Use an acid brush. That will keep the epoxy off your fingers, which is also a virtue.
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How do you squeeze it together and keep everything from sliding around and keep it straight? I have a flat building surface and another flat surface to put on top for weight but what about side to side?
Assuming you are sandwiching a piece of .010" x 3/16" carbon between two pieces of 3/32" x 3/16", how about routing a slot in a board about 5/32" deep and an RCH greater than 3/16" wide, then putting Saran Wrap in the slot, then putting the sandwich in the slot? Put weights on the part than sticks out. A more wasteful way, which is how I usually do something like this, is to vacuum bag a slab 3" wide or so, then rip pieces from it.
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HOWARD! LL~ LL~ LL~
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HOWARD! LL~ LL~ LL~
I thought it sounded pretty sensible. Once every few years you make up a sheet of spar material, then when you use it up you do it again. Use unidirectional tow for the CF and you could probably strip the spars of with an X-acto knife and some work (and a new blade for each cut).
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I have to strip the CF anyway. I'm using the 1/2" Dave Brown stuff.
I'm wondering how necessary this really is. I've seen the same kind of construction done with 1/4" square spars.
MM
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I have to strip the CF anyway. I'm using the 1/2" Dave Brown stuff.
I'm wondering how necessary this really is. I've seen the same kind of construction done with 1/4" square spars.
You need to build it the way it shows. I don't think you fully appreciate the loads involved, and the remarkable wear-and-tear these airplane undergo. To be honest with you, although obviously David's has stayed together, I would beef it up even more.
Brett
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I'm with Howard. The spars I have laminated are much-o easier-o to make oversize and then cut to what I want. The slot and press method is also what worked the best for me.
ps
the advice about splinters of CF is very real and you don't want that crap in your body.
K
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I'm with Brett. My own Thunder Gazer used 1/4" square SPRUCE spars, along with a generous 0.6mm ply doubler out to at least 10" each side. Stress is pretty fierce at wing roots!
Laminating spars isn't worth the effort. Besides, CF in the middle isn't the most effective way to strengthen. The carbon should be on the outside. (stress in the center is zero)
Floyd
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Laminating spars isn't worth the effort. Besides, CF in the middle isn't the most effective way to strengthen. The carbon should be on the outside. (stress in the center is zero)
If the spar ran down the center of the wing, yes. But these spars are out at the edges where they're mostly stressed in tension, so no. You lose a tiny bit of wing rigidity by putting the CF down the middle of the spar, but you gain a lot of ease of construction, so all in all it sounds sensible to me.
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I'm with Brett. My own Thunder Gazer used 1/4" square SPRUCE spars, along with a generous 0.6mm ply doubler out to at least 10" each side. Stress is pretty fierce at wing roots!
Laminating spars isn't worth the effort. Besides, CF in the middle isn't the most effective way to strengthen. The carbon should be on the outside. (stress in the center is zero)
Floyd
In this case, the carbon is in tension/compression and arranged "vertically" between two pieces of balsa. It would have zero stress if it was cantilevered but not in this case.
I also somewhat questioned the design when it was first conceived (sitting at lunch at Harry O'Shortal's in Napa), but with one exception it has proven adequate, even for hard-core practice and competition.
Brett
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Brett's spot on. Carbon's greatest benefit is in compression. Kevlar's greatest is in tension. Often the two are utilized in a mixed weave for those reasons. Kevlar thread is great for wrapping pushrod ends, carbon not as much.
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Brett's spot on. Carbon's greatest benefit is in compression. Kevlar's greatest is in tension. Often the two are utilized in a mixed weave for those reasons. Kevlar thread is great for wrapping pushrod ends, carbon not as much.
So we should use kevlar/carbon tow instead of just carbon?
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Actually, yes, if it was available in a prepreg condition as carbon already is, for spars it would be superior to just carbon. Applied horizontally to the spars the two materials would compliment each other as they hand off positive and negative loads such as during the transitions from inside to outside maneuvers. Maybe we could get Howard to make us some ;).
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I thought only lazy people use prepreg.
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You toast your acrylonitrile fibers, too? I assure you that laminating and cooking your own prepreg will not violate the BOM. Using store-bought precooked sheets might. It's a gray area.
Stunt plane spar caps have an equal amount of tension and compression (right, Mr. Stress Man?). The yellow stuff is useless in compression. If you have enough carbon to take the compression load, it will take the tension load, too.
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I thought only lazy people use prepreg.
No. Prepreg is preferred in the aerospace industry for waste reduction and reduced process controls required. The biggest drawback is limited shelf-life even when refrigerated/frozen.