In afixed-wing aircraft, thespar is often the mainstructural member of the wing, runningspanwise at right angles (or thereabouts depending onwing sweep) to thefuselage. The spar carries flightloads and the weight of the wings while on the ground. Other structural and forming members such asribs may be attached to the spar or spars, withstressed skin construction also sharing the loads where it is used. There may be more than one spar in a wing or none at all. Where a single spar carries most of the force, it is known as the main spar.[1]
Spars are also used in other aircraftaerofoil surfaces such as thetailplane andfin and serve a similar function, although the loads transmitted may be different from those of a wing spar.
The wing spar provides the majority of the weight support and dynamic load integrity ofcantilevermonoplanes, 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 employingflying wires have much of the flight loads transmitted through the wires andinterplane struts enabling smaller section and thus lighter spars to be used at the cost of increasingdrag.
Some of the forces acting on a wing spar are:[2]
Many of these loads are reversed abruptly in flight with an aircraft such as theExtra 300 when performing extremeaerobatic manoeuvers; the spars of these aircraft are designed to safely withstand greatload factors.
Early aircraft used spars often carved from solidspruce orash. Several different wooden spar types have been used and experimented with such as spars that are box-section in form; and laminated spars laid up in ajig, and compression glued to retain the wingdihedral. Wooden spars are still being used in light aircraft such as theRobin 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 andfungal attack can have on the component; consequently regular inspections are often mandated to maintainairworthiness.[4]
Wood wing spars of multipiece construction usually consist of upper and lower members, calledspar caps, and vertical sheet wood members, known asshear webs or more simplywebs, that span the distance between the spar caps.
Even in modern times, "homebuilt replica aircraft" such as the replica Spitfires use laminated wooden spars. These spars are laminated usually from spruce or douglas fir (by clamping and glueing). A number of enthusiasts build "replica" Spitfires that will actually fly using a variety of engines relative to the size of the aircraft.[5]
A typical metal spar in ageneral aviation aircraft usually consists of a sheetaluminium spar web, with L- or T-shaped spar caps being welded orriveted 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 provideintegral fuel tanks.Fatigue of metal wing spars has been an identified causal factor in aviation accidents, especially in older aircraft as was the case withChalk's Ocean Airways Flight 101.[6]
The GermanJunkers J.I armoured fuselage ground-attacksesquiplane of 1917 used aHugo Junkers-designed multi-tube network of several tubular wing spars, placed just under the corrugatedduralumin wing covering and with each tubular spar connected to the adjacent one with a space frame of triangulated duralumin strips — usually in the manner of aWarren truss layout — 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 afterWorld War I by American aviation designerWilliam Stout for his 1920s-eraFord Trimotor airliner series, and by Russian aerospace designerAndrei Tupolev for such aircraft as hisTupolev ANT-2 of 1922, upwards in size to the then-giganticMaksim Gorki of 1934.
A design aspect of theSupermarine Spitfire wing that contributed greatly to its success was an innovative spar boom design, made up of five square concentric tubes that fitted into each other. Two of these booms were linked together by an alloy web, creating a lightweight and very strong main spar.[7]
A version of this spar construction method is also used in theBD-5, which was designed and constructed byJim 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 theVickers Wellington, ageodesic 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 usecarbon fibre andKevlar in their construction, ranging in size from largeairliners to smallhomebuilt aircraft. Of note are the developments made byScaled Composites and the Germanglider manufacturersSchempp-Hirth andSchleicher.[8] These companies initially employed solidfibreglass spars in their designs but now often use carbon fibre in their high performance gliders such as theASG 29. The increase in strength and reduction in weight compared to the earlier fibreglass-sparred aircraft allows a greater quantity ofwater ballast to be carried.[9]
Aircraft utilizing three or more spars are consideredmulti-spar aircraft. Using multiple spars allows for an equivalent overall strength of wing, but with multiple, smaller, spars, which in turn allow for a thinner wing or tail structure (at a cost of increased complexity and difficulty of packaging additional equipment such as fuel tanks, guns,aileron jacks, etc.). Although multi-spar wings have been used since at least the 1930s (for example, the World War 2-eraCurtiss P-40 had 3 spars per wing), they gained greater popularity when the increasing speed of jet fighters demanded thinner wings to reduce drag at high speeds. TheMach 2F-104 Starfighter used numerous slender spars to allow for a wing of unusually thin section; theF-16 Fighting Falcon uses a similar construction. Other aircraft like theF-4 Phantom,F-15 Eagle and others use 3 or more spars to give sufficient strength in a relatively thin wing, and thus qualify as multi-spar aircraft.[10]
False spars, like main spars, are load bearing structural members running spanwise but are not joined to the fuselage. Their most common purpose is to carry moving surfaces, principallyailerons.[11]