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In an aircraft, the spar is the main structural member of the wing, running lengthways across the span of the wing, at right angles (or thereabouts) to the fuselage. The spar carries all of the forces of both lift, and the weight of the wings on the ground. Other structural and forming members such as ribs may be attached to the spar or spars. There may be more than one spar in a wing, though in general one carries the majority of the forces on it, and is called the main spar.

A typical aircraft spar in general aviation consists of a sheet of aluminium, with a "L" bracket or "T" bracket welded or riveted to the top and bottom of the sheet to prevent buckling under loads when applied. The general purpose of the average spar is to provide strength to the wing and work in conjunction with the wing ribs and skin, to provide strength. Today's aircraft are approaching a Carbon fiber/Kevlar construction, and as they do so, wing spars are changing. For example, in today’s composite construction, the skin of the aircraft wing carries the majority of the load, and subsequent spacing of internal ribs prevent or assist the skin in carrying the loads of the airframe as inflicted.

Several different spar types have been used and experimented with such as wood spars Corby Starlet which are either Box-spars, or laminated spars which are laid up in a jig, and compression glued to retain the wing incidence. Another example of a Spar material, is the tubular aluminium spar used in the BD-5 which was designed and constructed by Jim Bede in the early 1970's. The spar in the BD-5 and subsequent BD projects, was primarily aluminium tube, of approximately 2 inches in diameter, and joined at the wing root with a much thicker internal diameter aluminium tube to provide the wing structural integrity.

The wing spar provides the majority of the weight support and dynamic load integrity of the average aircraft, coupled with the strength of the wing box itself. Together, these two items collectively provide the wing rigidity needed to enable the aircraft to fly.

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