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| Main spar of a de Havilland DH.60 Moth |
In a fixed-wing aircraft, the spar is often the main structural member of the wing, running spanwise at right angles (or thereabouts depending on wing sweep) to the fuselage. The spar carries flight loads and the weight of the wings whilst on the ground. Other structural and forming members such as ribs may be attached to the spar or spars, with stressed skin construction also sharing the loads where it is used. There may be more than one spar in a wing or none at all. However, where a single spar carries the majority of the forces on it, it is known as the main spar.[1]
Spars are also used in other aircraft aerofoil surfaces such as the tailplane and fin and serve a similar function, although the loads transmitted may be different to those of a wing spar.
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The wing spar provides the majority of the weight support and dynamic load integrity of cantilever monoplanes, often coupled with the strength of the wing 'D' box itself. Together, these two structural components collectively provide the wing rigidity needed to enable the aircraft to fly safely. Biplanes employing flying wires have much of the flight loads transmitted through the wires and interplane struts enabling smaller section and thus lighter spars to be used.
Some of the forces acting on a wing spar are:[2]
Many of these loads are reversed abrubtly in flight with an aircraft such as the Extra 300 when performing extreme aerobatic manoeuvers; the spars of these aircraft are designed to safely withstand great load factors.
Early aircraft used spars often carved from solid Spruce or Ash. Several different wooden spar types have been used and experimented with such as spars which are either box-section in form; or laminated spars which are laid up in a jig, and compression glued to retain the wing dihedral. Wooden spars are still being used in light aircraft such as the Robin DR400 and its relatives. A disadvantage of the wooden spar is the deteriorating effect that atmospheric conditions, both dry and wet, and biological threats such as wood-boring insect infestation and fungal attack can have on the component; consequently regular inspections are often mandated to maintain airworthiness.[4]
Wood wing spars of multipiece construction usually consist of upper and lower members, called spar caps, and vertical sheet wood members, known as shear webs or more simply webs, that span the distance between the spar caps.
A typical metal spar in a general aviation aircraft usually consists of a sheet aluminium spar web, with "L" or "T" -shaped spar caps being welded or riveted to the top and bottom of the sheet to prevent buckling under applied loads. Larger aircraft using this method of spar construction may have the spar caps sealed to provide integral fuel tanks. Fatigue of metal wing spars has been an identified causal factor in aviation accidents, especially in older aircraft as was the case with Chalk's Ocean Airways Flight 101.[5]
The German Junkers J.I armoured fuselage ground-attack sesquiplane of 1917 used a Hugo Junkers -designed multi-tube network of several tubular wing spars, placed just under the corrugated duralumin wing covering and with each tubular spar connected to the adjacent one with a space frame of triangulated duralumin strips riveted onto the spars, resulting in a substantial increase in structural strength at a time when most other aircraft designs were built almost completely with wood-structure wings. The Junkers all-metal corrugated-covered wing / multiple tubular wing spar design format was emulated after World War I by American aviation designer William Stout for his 1920s-era Ford Trimotor airliner series, and by Russian aerospace designer Andrei Tupolev for such aircraft as his Tupolev ANT-2 of 1922, upwards in size to the then-gigantic Maxim Gorki of 1934.
A design aspect of the Supermarine Spitfire wing that contributed greatly to its success was an innovative spar boom design, made up of five square concentric tubes which fitted into each other. Two of these booms were linked together by an alloy web, creating a lightweight and very strong main spar. The undercarriage legs were attached to pivot points built into the inner, rear of the main spar and retracted outwards and slightly backwards into wells in the non- load-carrying wing structure. The narrow undercarriage track of this aircraft was considered to be an acceptable compromise as it allowed the landing impact loads to be transmitted to the strongest parts of the wing structure.[6]
A version of this spar construction method is also used in the BD-5 which was designed and constructed by Jim Bede in the early 1970s. The spar used in the BD-5 and subsequent BD projects was primarily aluminium tube of approximately 2 inches (5.1 cm) in diameter, and joined at the wing root with a much larger internal diameter aluminium tube to provide the wing structural integrity.
In aircraft such as the Vickers Wellington, a geodesic wing spar structure was employed which had the advantages of being lightweight and able to withstand heavy battle damage with only partial loss of strength.
Many modern aircraft use carbon fibre and Kevlar in their construction, ranging in size from large airliners to small homebuilt aircraft. Of note are the developments made by Scaled Composites and the German glider manufacturers Schempp-Hirth and Schleicher.[7] These companies initially employed solid fibreglass spars in their designs but now often use carbon fibre in their high performance gliders such as the ASG 29. The increase in strength and reduction in weight compared to the earlier fibreglass-sparred aircraft allows a greater quantity of water ballast to be carried.[8]
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