Which joint is used to splice a solid or rectangular wood spar?

• A. Butt joint
• B. Scarf joint
• C. Lap joint
• D. Dovetail joint

Answer: (B)

When you're joining a solid or rectangular wood spar, the aim is to keep the spar long enough and with a smooth, continuous shape so it carries bending loads evenly. A scarf joint does this by beveling and overlapping the ends of the two spar pieces. The angled cuts create a long glue surface, which lets loads transfer gradually from one piece to the other and preserves the spar’s stiffness and aerodynamic profile. Because the ends are joined over a length, the joint behaves much like a single piece rather than a weak point at a butt, and it avoids creating a thick step that would disturb airflow.

The other joints don’t fit as well: a butt joint offers little surface area and introduces a weak spot where the pieces meet; a lap joint adds extra thickness at the joint, which can disrupt the wing’s shape and load path; a dovetail joint is very strong but impractical for long spars due to complexity and added weight.