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Building Tips and technical articles. => Building techniques => Topic started by: Dwayne Donnelly on April 05, 2020, 06:47:40 AM
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I've been plugging away on my latest when someone mentioned I should not have cut up my foam wing with no additional reinforcement, so how about it, did I screw up? And if so what's the fix, I was thinking about using some carbon strapping along the trailing edge sheeting and where the spars would go in a traditional wing, top and bottom.
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This worked, but as noted, it has spars. The spars also help give some joint surface to the cap strip/sheeting interface. Also, spars help to give a hard point for bellcrank mounts.
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Nice looking ship Fred, I don't have spars so for some peace of mind I'm adding 5/16 carbon straps top and bottom. fingers crossed...lol
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I'm not a structure expert, but you might save the carbon until some others weigh in. I don't think that will solve the problem.
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I would not expect a long life for that wing. I see problems with it and it looks to be too late to try and fix it. Others might disagree but that is my opinion based on experience with building foam wings for quite some time.
Next time, just sheet the entire wing. Use Bob Hunt's method for applying epoxy. You will use only the minimum amount of adhesive that way. It will be easier, faster, stronger, and you will have a straight, stiff wing that will be no heavier (maybe even lighter) than open bay structure covered with polyspan or some other covering and finished with dope or other finish.
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The foam that has been cut away between the "ribs" is not a big deal. The foam is not providing much, if anything, anyway.
You MIGHT be able to salvage the wing by adding some center sheeting. However, by cutting out the foam closer to the center section, there does not appear to be enough left to add some center section sheeting, cut at an angle, so that in plan view, the center sheeting is a trapezoid shape, longer side at the forward end. Right now, your center sheeting will have a stress riser right at the outer edge of the sheeting. I do not think the carbon strip will help much.
I have built partially sheeted foam wings, covered with polyspan and dope, and they hold up. But, only on smaller planes, such as Nobler size. I managed to get away with it on something a bit larger (575 sq. in.) but, in the end, fully sheeted is the way to go, no matter what I build now.
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I'm going to hang it up for a bit, if push comes to shove I'll just carefully remove the cap strips and sheet it.
Thanks guys. y1