Scarf joint question

[harry curzon]

I am about to make the wing spars for the Charlesworth K-18, 4 metre. I need to scarf join the spruce spars, what is the recommended length of the joint? 5 x width? More?

[john greenfield]

Harry

A structural scarf joint should be 15 x the thickness of the pieces being joined.

AEB

[harry curzon]

Thanks John,
the spars are 10mm wide x 3 mm thick, so the joint should be 15 x 3?

[RobbieB]

.................and Harry, make sure they are perfect. Check both down their length and across the width with your straight edge and clamp really tight.

[John Vella]

Harry , what glue are you using on such a critical joint ?

[john greenfield]

Thanks John,
the spars are 10mm wide x 3 mm thick, so the joint should be 15 x 3?

Yes Harry. 45mm.

AEB

[harry curzon]

Thanks John

[VinceC]

This information from the old site

[John Vella]

Vince, the splice angle recommended from the table is 15 to 1 for ply and 20 to 1 for wood. This is what we were working on with K13s and K8s. What I can't understand is the diagram below ? Surely the joint is stronger in bending load than shear load ? Regards John.

[chris williams]

What about the scaling factor...? Surely, a 1/4 scale spar would only need 1/4 of those dimensions?