TORSION BOX
The upper and lower surfaces and the spars can form a torsion box. This structure increases the
rigidity of the wing and resists the twisting and bending motion without increasing the mass of
material used in construction. In diagram 3.7A, a cut-away drawing of the outer section of a
737-300 wing, the torsion box structure is formed between the front and rear spars. On larger and
heavier aircraft, the torsion box encloses the front, main, and rear spars.
SPARS
Diagram 3.6 shows the location of spars in large aircraft wing structure. There are normally three
spars in a large aircraft:
Front or forward spar
Main spar
Rear or auxiliary spar
The front spar is designed to take the air-load acting to bend the wing backward. Where external
loads such as engines are mounted, the front spar shares some of the load.
The main spar is designed to be the main structural member of a conventional wing and it carries
the majority of the loads placed on the wing. The rear spar acts as a mounting structure for the
trailing edge flaps and ailerons. It also resists the upward-twisting action of the up-wash felt at the
trailing edge. The spars also have to resist the torsional effect created at the wing tip by the
movement of the aileron. When a wing flexes, the greatest motion is at the wing tip. However, the
greatest strain is at the wing root.