Discussion
Determine the correctness or otherwise of the following statements, [a] and [r]:
[a] A closed-section box beam configuration is used in aircraft wings.
[r] Closed-section box beam configuration is capable of resisting torsional loads.
Answer: A
Both [a] and [r] are true and [r] is the correct reason for [a].
A wing carries large torsional loads from the aerodynamic pitching moment, from the offset of the lift from the shear centre, and from control-surface loads. A closed section, such as the box formed by the front and rear spars with the upper and lower skins, carries torque through a constant shear flow around the closed cell (the Bredt-Batho relation), which makes it very stiff and strong in torsion. Open sections of the same material are far weaker in twist.
So the box beam is used in wings precisely because it resists torsion, and [r] explains [a]. That rules out the options saying either statement is false, and the option that accepts both but denies the link.
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