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Scarfing and laminating wing spars

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Jeff R

Member
Joined
Nov 2, 2006
Messages
18
Another request for opinions....

The center spars for my F12A are a bit under 17' long. I know that laminating thin sections of spurce together to build up to the required thickness is acceptable, even encouraged for reasons that have been talked about elsewhere on this board.

I have not heard opinions about splicing shorter pieces together to make up the required length for the laminations. Specifically, since the longest lumber than can be shipped by UPS is 8' (and I wish to avoid expensive trucking fees and added costs to get 17' long spruce - yikes), is there really any problem in scarfing together three or four shorter pieces for each lamination layer? Of course, the scarf joints would have to be offset, such as the following 4-lamination spar diagram shows, where each - is a foot of spar (or so) and each / is a scaft joint.

--/------/-------/--
----/-------/------
------/--------/---
---/-------/-------

Is there any ideal placement of the splices? That is, is it better to center them with the fuselage, such as:

--/-------/-------/--
------/------/------
--/-------/-------/--
------/------/------

Would it be better to use longer lumber, such as 12', as follows, to reduce the number of splices?

-----/------------
-------/----------
---------/--------
------------/-----

The process to make the spar would be to cut strips of spruce maybe 3/8" thick. The wood would be inspected, scarfed at 1:12 or so, cleaned and glued straight to make the required length plus some spare. After curing, each lamination layer would be run through a planer to make the surfaces flat and parallel, then the laminations would be stacked and glued together. After the stack dries, the completed spar would be planed and edged to the required final dimensions.

Either T88 or possibly resorcinol would be used. I am using T88 for the ribs, but wonder if resorcinol might be better for the spars.

Thanks,
Jeff
 
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