What is the basic wood used for aircraft wing spars?

• A. Sitka spruce
• B. Basswood
• C. Douglas fir
• D. Redwood

Answer: (A)

Wing spars must carry the main bending loads of the wing while staying light and stiff enough to resist fatigue. The wood chosen for spars should have a very high strength-to-weight ratio, excellent stiffness along the grain, and good grain uniformity so you can produce long, straight spars that hold up under repeated cycling.

Sitka spruce provides these characteristics exceptionally well. It has a superior strength-to-weight ratio and high stiffness, meaning it can carry more load per unit of weight and resist bending with less material. Its grain is typically straight and consistent, which helps in making strong, reliable spars and in laminating or joining pieces without weak points. These properties made Sitka spruce the standard choice for aircraft wing spars, especially in traditional wood aircraft.

Other woods like basswood are much softer and lighter, which makes them unsuitable for primary spars. Redwood is heavier and not as stiff or strong along the grain for spars purposes. Douglas fir is strong and stiff as well, but it is heavier and does not offer the same light-weight advantage as spruce. So, while some woods can perform adequately, Sitka spruce is the best overall fit for the critical role of wing spars due to its optimal balance of strength, stiffness, and light weight.