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Composite wing spar

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Old 01-28-2004, 04:23 PM
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LancelowDowell
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Default Composite wing spar

Well, this is my attempts at lightening my Ohio R/C 28% Sukohi. This is the start of my new wing. What you're seeing here is one of 2 pieces that will form the top and bottom of a box spar. The pieces here are 1/8"x1"x88" spruce, laminated with carbon fiber cloth, and West Systems epoxy to hold it all together. I don't have a vacuum bag setup, so I just did one side at a time and used lots of weights (read old collage text books). So far they've come out pretty good. I did the wing design in compufoil, and sent the dxf file off to laser lizard to be laser cut. From everything that I've been able to find, people have been getting these birds in around 20 lbs. I'm hoping to cut as much weight as possible. For an engine, I'll have to wait to see what the weight will be, but I'm leaning twords a zdz or da 50. The first image below is of the original wing that I'll be replacing. The original builder of the plane enlarged the rudder and elevator as well, so I'm hoping that when I'm done this thing will be a 3d beast
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Old 01-29-2004, 11:45 PM
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Default RE: Composite wing spar

Got my wing ribs in tonight, so got things put together a bit more. Unfortunatly, my wing jig is just short of the 86" I need... so I left off the end 2 ribs on each side for now till i get my leading edge and top spar on. more pics!
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Old 01-30-2004, 12:46 AM
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Darrinc
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Default RE: Composite wing spar

Why did you reinforce spruce with carbon???? You just added weight since the spruce is overkill anyway. You use uni carbon to reinforce balsa (with thin CA), which comes out about 3-4 times lighter than spruce. Next time use 0.007 uni carbon instead of bidirectional. The bidirectional is heavy and weak when used for your purpose which is just strength in one direction. Every time there is a cross thread there is a kink which weakens the structure.

There are few applications in models for epoxy.

You did use contest balsa for the ribs, sheeting and on the fuse.

A 28% should weigh around 15lbs.
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Old 01-30-2004, 09:28 AM
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Default RE: Composite wing spar

Well, the spruce is only 1/8". For the loads I'm going to be putting on this plane, that alone wouldn't have been enough, hence the carbon fiber. From my guesses, 5 g's would be a normal load for some moves, at worse case the airplane will weigh 20 lbs, so I need to be able to support 100 lbs roughly. For spruce alone, I would need a pretty hefty spar, which adds weight. That is the whole problem with the current wing. By cutting down the amount of spruce and adding cf, I just chopped a load of weight off. Whether a balsa/cf spar could support the weight of the plane the same as spruce... I dunno. I talked with several of the more experienced builders in my club and they all recommended going with a thinner spruce spar layered in cf. I guess if I get bored one day, I'll build up a second wing and use a balsa/cf spar to see if it makes any difference. My other biggest concern with the wing would be having it twist on me. I was hoping that using the bi-directional cf it would help reduce the twist at the spar, and it worked too. Whether it will help to keep the entire wing from twisting, I'll have to wait and find out.

But as for epoxy, how else are you going to do it? CA becomes brittle after time, I didn't want to spend the money nor risk my plane because of a bad ca bond. Epoxy, when done correctly, will hardly add any weight, and will give a stronger bond then ca can. We've had a few local guys come in to our club meetings and do vacuum bag wings that are fiberglass reinforced use the west system epoxy, and the result is incredible. Very light wings that are incredibly stiff. I also have a few guys in my club that build their planes with nothing but epoxy, and they are as light, if not lighter, then the guys using ca. It's all in how you use it....
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Old 01-30-2004, 05:46 PM
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Default RE: Composite wing spar

Well, the spruce is only 1/8". For the loads I'm going to be putting on this plane, that alone wouldn't have been enough, hence the carbon fiber.
The thing is that, if you are going to use CF to seve weight, then save weight. Once you have the CF for strength, why worry about the wood? You could have laminated unidirectional CF to a piece of 1/32 balsa and it would have served the same purpose. And the uni would have been much lighter for the strength since the fibers could have run tip-to-tip. What weight of cloth did you use. Looks very heavy for the application. Have you weighed that spar cap to compare it to the stock piece?

As for laminating bi-directional on the bias for anti-twist, that's inefficient as well. You want to use the wing shape for twisting stiffness by sheeting the area in front of the spar as a D-box. Your original picture shows this.
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