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Old 01-21-2021, 10:17 AM   #1
Edge540
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Wing sheet joints


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Just looking for some opinions here. I built a Carden 40% Edge back in 2000 and still flying it. When I did the build thread, I used scarf joints on the wing sheeting and staggered the extra 6 inch joint from tip to root each sheet. I know this uses about 4 more 48 inch sheets than simply butt joining. The original plans show butt joint in the same spot in 6 inches from the tip on one side of the wing and 6 inches in from the root on the other.

Currently I'm building another from scratch and getting to that stage today. Once again thinking of alternating a scarf joint is the way to go.

Maybe I'm over engineering or missing something but just dont see how a butt joint contributes any strength.

I see Kam-aero design does the same butt joint 10 inches in on the wing tip.

Any input??
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Old 01-21-2021, 10:31 AM   #2
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Re: Wing sheet joints

Doug I think you are over engineering as 100s of Cardens built this way. Go extra strength visit some of TBrum Carden builds an inlay some carbon tw in wing cores before sheeting.

Once saw Cam McCausey demonstrate how strong a stock Carden wing was by placing tip up on a 12” high bleacher and root on the ground then stand on the wing. Took three hard jumps on the wing before it snapped and Cam was pushing 260 lbs back then.��
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Old 01-21-2021, 10:51 AM   #3
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You are likely correct. I have super lightened the foam cores and have CF reinforcement going in. Just in all my building I try to make the strongest joints possible and on this build, weight is important. My target is under 32lbs I dont see a down side in a scarf joint except some balsa waste over a butt joint. I do see some added value in strength. When Dennis did the balsa sheeting layout I wonder if he expected the butt joints to altered on each sheet or simply as drawn?
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Old 01-21-2021, 11:12 AM   #4
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Re: Wing sheet joints

I also use scarf joints for wing sheeting. Much stronger with little added weight. Use wood glue and the joint will disappear.
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Old 01-21-2021, 11:22 AM   #5
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Re: Wing sheet joints

Here is Carden's layout for clarification and here is my new lightened core
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Old 01-21-2021, 12:09 PM   #6
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Re: Wing sheet joints

Got a friend here that does a zig-zag lazer cut to join sheeting . That way not a lot of waste and the strength is proportional to the surface of the glue joint. This us in 2 inch and wider sheeting. As for narrower stuff , like sticks for spars , the scarfe works better.

Doug , presume that foam lightening is CNC routed ? Looks great . How do you glue the sheeting ?
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Old 01-21-2021, 12:50 PM   #7
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Re: Wing sheet joints

I would scarf joint myself because I find they are less likely to lift and show through the covering but even if I was forced to butt joint I see no reason to put all the but joints at one end in a line , that's asking to make the joint visible . I would put as many butt joints as I could to the tip while staggering . that picture makes no sense to me at all but that could just be my OCD talking LOL. cheers .
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Old 01-21-2021, 01:59 PM   #8
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Re: Wing sheet joints

I built a Carden Extra 260 and I butt joined the balsa as in the diagram. Wasn't happy in the end.
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Old 01-21-2021, 02:24 PM   #9
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Re: Wing sheet joints

Just finished sheeting a foam Vertigos wings. I did scarf joints but only two per wing as they are narrow. I put the joints at opposite end of the wings.
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Old 01-21-2021, 07:01 PM   #10
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Re: Wing sheet joints

Quote:
Originally Posted by Edge540 View Post
Here is Carden's layout for clarification and here is my new lightened core
Sorry to be a nerd but the wood joint you have in the second picture is not a scarf joint but a square edge to square edge joint at least some of the glue joints being diagonally across the face of several planks of wing sheeting - the diagonal cut presumably marginally avoids a glue edge falling on a larger percentage of the wing core where the lightening circles in the foam core are located.

For an image of a classic scarf joint - without any interlocking hooks - see the center image in the third row of this collection of scarf woodworking images:
https://www.google.ca/search?sxsrf=A...w=1024&bih=710


But on the larger issue: a square edge to square edge glue joint along the side edges of balsa sheeting is quite strong (but end grain to end grain glue ups are weak)

I don't think such diagonal glue joints across the face of several planks in your situation make any significant difference in the strength of joints/structure and are an "unnecessary" waste of balsa.

I've made fully balsa sheeted wings, stabilizers, etc. over a balsa rib and spar skeleton - using several balsa planks glued side edge to side edge without any issues.
Even more common are balsa sheeting of the aft fuselage where not all edge to edge glue ups fall on a fuselage stringer/longeron.
Yes, I will take advantage of any spars/longerons to have edge to edge joints to fall on such timbers and have that backing support but I wouldn't adding any batten support to such an edge to edge glue joint for that purpose alone.
The covering further strengthens such a joint.
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