BC12 main spar

[taylorukuk]

Has anyone ever come across a BC12 main spar that has been spliced vertically along its whole length?

[Bernard]

yes, my BC12-D

[Hank Jarrett]

You mean divided root to tip along the length, right? Not spliced with a vert. cut!

[3Dreaming]

You mean divided root to tip along the length, right? Not spliced with a vert. cut!

What's wrong with a vertical cut? Isn't a scarf slice made with a vertical cut?

[Hank Jarrett]

Sorry, I am an engineer and can get pretty literal. A vertical cut would be perpendicular to the edges, a scarf is sloping along the length. I good scarf is not easy to make and a mistake or mis-match on a spar is something I wouldn't want to risk. If I scarfed a spar I would proof load it before using it. Of course a scarf on the tip isn't a big deal. One at the root would be way too risky for me. How far inboard you will accept a scarf repair depends on how risk averse you are.

[Scott]

My aileron spars were built up using three strips of different sizes. The factory maximized the use of wood supply by "laminating" in this way.

[N96337]

One of my spars had 14 pieces in it. It was signed by 2 guys that worked for Tcraft June 12, 1946. My friend Ken (who worked there then) knew them both. He said that Taylorcraft was so broke, that everyone was encouraged to pick up scraps and such and bring them in to be scarfed together. The only problem with it is that on mine, the glue joints were failing.

[taylorukuk]

Yes divided root to tip along the full length, we have one with a splice in as well - seems from the feedback that this is all fine - cheers

[Hank Jarrett]

The guys who did splices every day were good at it and had the tools to get the angles and glue gaps right. That and the fact that they have been in service for decades should give you plenty of confidence. Those guys could do a splice in their sleep better than most of us. As for a laminated spar there were tests done years ago that showed a laminated spar was actually STRONGER than a one piece. If there were flaws that weren't caught in the parent wood they wouldn't be aligned in a laminated spar which would give even more confidence. All I would ever worry about in a laminated or spliced spar, done correctly, would be deterioration of the glue.

Hank

[Scott]

Yes divided root to tip along the full length, we have one with a splice in as well - seems from the feedback that this is all fine - cheers

I wouldn't make that conclusion. As mentioned the glue is a problem, even if the wood is sound. All original glue joints have exceeded their best before dates. If you have the wing open i would replace the spar. It's only a matter of time before the glue joints fail.

[Mark Bowden]

+1 to Scott's comments about glue, the joint can go bad.

A span-wise splice between the top and bottom of the spar isn't loaded much. The top is in compression and bottom in tension, in the middle there is basically no stress, tension cancels compression.
This is a shear joint, adhesives are great for shear and if bonded right, stronger than the base material. The load is parallel to the joint.

If you cut the spar vertically this must spliced because span-wise loads (tip going to root attachment) have to be carried across the joint. That means the load in the fibers must jump across the joint. Stress loads HATE to change directions or jump across materials.

This is why an butt edge joint is bad, its just glue in tension. Instant failure.

To span-wise splice a spar it is defined in the regulations. But you must make a tapered joint that has reinforcements on each side of the scarf joint. The scarf has a very shallow taper or ramp and must fit very tightly. The outer splice plates cover the scarf cut or splice. It of course must be prepped and bonded properly.

Doing it in this manner helps the loads go from the tip to root in shear and not have to make sharp changes in direction. While load must transfer across the joint the scarf has 12-15 times more surface area than the spar itself. This lowers the stress on the glue. The load also goes across the splices in pure shear further reducing the load on the scarf glue.

[TC747paul]

My L2M trailing spar has two 3/8" laminated vertically (equaling 3/4" x height) from from tip to root. The front spar has a +/-2". top horizontally laminated from the root to the tip. Both are original to the plane.