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May 16, 2019, 09:28 AM
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Carbon fiber wing spar problem

Not sure the best place to post this but I built a 39" EPP 3D model and used a full wingspan flat carbon ribbon about a half inch wide glue upright which I thought would provide great strength and prevent the wing from bending. In the past Ive used much narrower strips with no problem. After a couple of flights I noticed one wing was bending and when I looked at it closely, the spar had broken about 4-5 inches out from the fuse. I added a short piece of the same material in the groove and reglued everything. After drying, I was testing the wing strength and damned if the other wing didnt break at the exact same distance from the fuse. I fixed it the same way and everything seems strong now.

Is it possible that I used a carbon strip that was too wide and this made it weaker and maybe prone to twist and break on an almost 40" wing? My Multiplex Parkmaster is heavier and has a flat spar as well but its not nearly as wide. Ive never had a problem with it but elapor is much stiffer than EPP so it wouldnt need as much reinforcement.
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May 16, 2019, 09:51 AM
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Sounds to me like you already understand the likely cause - torsion due to weakness of wing material, causing the vertical strip to torque.
A wider strip should have handled compression and tension better than a narrower strip - so assuming both strips were made of the same quality CF, the only thing left is torsion.

Of course, the solution is a CF tube, preferably of square cross-section.
May 16, 2019, 09:59 AM
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Is square stronger than round? I have a round tube from a 32" wing that's been "decommissioned" after a few too many impacts with the ground. If I have any more problems, I will dig out the strip and replace with a tube. I just never thought about a carbon strip being too wide and causing me problems
May 16, 2019, 10:12 AM
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Round of 6 mm diameter or more will be alright for a 40 inch span.
Also, like I said, the wider width of the strip was not likely the issue - rather, it was just likely that this plane's wing torqued more than the other plane's wing.
May 16, 2019, 10:14 AM
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Thanks for the info

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