Oh, no! Another scarfing question!

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Aerowerx

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Dec 1, 2011
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Marion, Ohio
My project requires scarfing some thin (3/32 inch) ply for the fuselage skins. Length will be about 8 feet for the inner skin and 16 feet for the outer skin.

Do the joints have to (A)be glued first flat on a table, and then attached to the fuselage as one piece, or can the individual sections be (B)glued on one at a time?

And, is there a preference to the orientation of the scarf joints:

1)___//___//___//___ or 2) ___//___\\___//___\\___ or 3) ___\\___\\___\\___

Front of the plane is to the left in this diagram.

The advantage of (A) is that it would be easier to align the scarf joints themselves, but the disadvantage would be having to mix up a bunch of epoxy all at once and get a 16 foot piece of floppy plywood all aligned on the fuselage before the epoxy work time runs out. (I have been using T-88, but maybe I should invest in some West System 205/109---longer working time?)

The advantage of (B) is that only a little epoxy would have to be mixed at a time and it is easier to align a 4 foot piece of ply than a 16 foot. The disadvantage is in aligning the scarf joints themselves.

As far as 1), 2), or 3), 1) would be used for attaching individual pieces starting at the tail, and 3) for individual pieces starting at the nose. I really don't like 2), but included it for completeness.
 

Dan Thomas

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Sep 17, 2008
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I've done it flat on the table so the whole length of the joint can be clamped. Doing it in place is messy and difficult and should be reserved for patches.

Dan
 

Abraham Leket

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Sep 18, 2012
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IsraeI
Hi-
Scarfing 16 feet of 3/32"ply all at once is taken from boat construction. In an aircraft- the side panels have just the fraction of the load that we see in a boat and you can safely do it in sections. The main load is well taken care of by the longerons and upright/diagonals.
In aluminim aircraft the sides are riveted small section at a time-and wood you know is much stronger.
 

rheuschele

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Jan 12, 2010
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533
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Chicago Il. USA.
I did it on the fuselage, in parts. But I decided not to rely on the scarf alone. Behind the scarf I added 1mm ply bandage style 6" on either side of the joint just for piece of mind.
Ron
 

Dan Thomas

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Sep 17, 2008
Messages
7,562
I did it on the fuselage, in parts. But I decided not to rely on the scarf alone. Behind the scarf I added 1mm ply bandage style 6" on either side of the joint just for piece of mind.
Ron

I made test scarfs. One-inch-wide strips cut out of the scarfed plywood, with the joint across the middle of the strip. I clamped them in a hydraulic press that had a pull function, and measured the force taken to pull the strip apart. I did about eight of these, and none of them failed in the scarf joint. It was 1.5mm Baltic Birch aircraft plywood, and it took around 1500 pounds of force to break the strips. After that I didn't worry about my scarf joints; they're as strong as the rest of the splywood.

Dan
 
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