Spar (aviation)
In a fixed-wing aircraft
Fixed-wing aircraft
A fixed-wing aircraft is an aircraft capable of flight using wings that generate lift due to the vehicle's forward airspeed. Fixed-wing aircraft are distinct from rotary-wing aircraft in which wings rotate about a fixed mast and ornithopters in which lift is generated by flapping wings.A powered...

, the spar is often the main structural member of the wing, running spanwise
The wingspan of an airplane or a bird, is the distance from one wingtip to the other wingtip. For example, the Boeing 777 has a wingspan of about ; and a Wandering Albatross caught in 1965 had a wingspan of , the official record for a living bird.The term wingspan, more technically extent, is...

 at right angles (or thereabouts depending on wing sweep
Swept wing
A swept wing is a wing planform favored for high subsonic jet speeds first investigated by Germany during the Second World War. Since the introduction of the MiG-15 and North American F-86 which demonstrated a decisive superiority over the slower first generation of straight-wing jet fighters...

) to the fuselage
The fuselage is an aircraft's main body section that holds crew and passengers or cargo. In single-engine aircraft it will usually contain an engine, although in some amphibious aircraft the single engine is mounted on a pylon attached to the fuselage which in turn is used as a floating hull...

. The spar carries flight loads and the weight of the wings whilst on the ground. Other structural and forming members such as rib
Rib (aircraft)
In an aircraft, ribs are forming elements of the structure of a wing, especially in traditional construction.By analogy with the anatomical definition of "rib", the ribs attach to the main spar, and by being repeated at frequent intervals, form a skeletal shape for the wing...

s may be attached to the spar or spars, with stressed skin
Stressed skin
In mechanical engineering, stressed skin is a type of rigid construction, intermediate between monocoque and a rigid frame with a non-loaded covering:...

 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.

Spars are also used in other aircraft aerofoil
An airfoil or aerofoil is the shape of a wing or blade or sail as seen in cross-section....

 surfaces such as the tailplane
A tailplane, also known as horizontal stabilizer , is a small lifting surface located on the tail behind the main lifting surfaces of a fixed-wing aircraft as well as other non-fixed wing aircraft such as helicopters and gyroplanes...

 and fin
Vertical stabilizer
The vertical stabilizers, vertical stabilisers, or fins, of aircraft, missiles or bombs are typically found on the aft end of the fuselage or body, and are intended to reduce aerodynamic side slip. It is analogical to a skeg on boats and ships.On aircraft, vertical stabilizers generally point upwards...

 and serve a similar function, although the loads transmitted may be different to those of a wing spar.

Spar loads

The wing spar provides the majority of the weight support and dynamic load integrity of cantilever
A cantilever is a beam anchored at only one end. The beam carries the load to the support where it is resisted by moment and shear stress. Cantilever construction allows for overhanging structures without external bracing. Cantilevers can also be constructed with trusses or slabs.This is in...

A monoplane is a fixed-wing aircraft with one main set of wing surfaces, in contrast to a biplane or triplane. Since the late 1930s it has been the most common form for a fixed wing aircraft.-Types of monoplane:...

s, 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. Biplane
A biplane is a fixed-wing aircraft with two superimposed main wings. The Wright brothers' Wright Flyer used a biplane design, as did most aircraft in the early years of aviation. While a biplane wing structure has a structural advantage, it produces more drag than a similar monoplane wing...

s employing flying wires
Flying wires
The flying wires of an aircraft work in conjunction with other wing components such as spars and interplane struts to transmit flight loads. Most commonly used on biplane aircraft they are also used on monoplanes and triplanes.-Purpose:...

 have much of the flight loads transmitted through the wires and interplane strut
Interplane strut
An interplane strut is an aircraft airframe component designed to transmit lift and landing loads between wing panels on biplanes and other aircraft with multi-wing designs. They also maintain the correct angle of incidence for the connected wing panels and are often braced with wires...

s enabling smaller section and thus lighter spars to be used.


Some of the forces acting on a wing spar are:
  • Upward bending loads resulting from the wing lift
    Lift (force)
    A fluid flowing past the surface of a body exerts a surface force on it. Lift is the component of this force that is perpendicular to the oncoming flow direction. It contrasts with the drag force, which is the component of the surface force parallel to the flow direction...

     force that supports the fuselage in flight. These forces are often offset by carrying fuel in the wings or employing wing-tip -mounted fuel tanks; the Cessna 310
    Cessna 310
    The Cessna 310 is an American six-seat, low-wing, twin-engined monoplane that was produced by Cessna between 1954 and 1980. It was the first twin-engined aircraft that Cessna put into production after World War II.-Development:...

     is an example of this design feature.
  • Downward bending loads whilst stationary on the ground due to the weight of the structure, fuel carried in the wings, and wing-mounted engines if used.
  • Drag loads dependent on airspeed
    Airspeed is the speed of an aircraft relative to the air. Among the common conventions for qualifying airspeed are: indicated airspeed , calibrated airspeed , true airspeed , equivalent airspeed and density airspeed....

     and inertia
    Inertia is the resistance of any physical object to a change in its state of motion or rest, or the tendency of an object to resist any change in its motion. It is proportional to an object's mass. The principle of inertia is one of the fundamental principles of classical physics which are used to...

  • Rolling inertia
    Moment of inertia
    In classical mechanics, moment of inertia, also called mass moment of inertia, rotational inertia, polar moment of inertia of mass, or the angular mass, is a measure of an object's resistance to changes to its rotation. It is the inertia of a rotating body with respect to its rotation...

  • Chordwise
    Chord (aircraft)
    In aeronautics, chord refers to the imaginary straight line joining the trailing edge and the center of curvature of the leading edge of the cross-section of an airfoil...

     twisting loads due to aerodynamic
    Aerodynamics is a branch of dynamics concerned with studying the motion of air, particularly when it interacts with a moving object. Aerodynamics is a subfield of fluid dynamics and gas dynamics, with much theory shared between them. Aerodynamics is often used synonymously with gas dynamics, with...

     effects at high airspeeds often associated with washout
    Washout (aviation)
    Washout refers to a feature of wing design to deliberately reduce the lift distribution across the span of the wing of an aircraft. The wing is designed so that angle of incidence is higher at the wing roots and decreases across the span, becoming lowest at the wing tip...

    , and the use of aileron
    Ailerons are hinged flight control surfaces attached to the trailing edge of the wing of a fixed-wing aircraft. The ailerons are used to control the aircraft in roll, which results in a change in heading due to the tilting of the lift vector...

    s resulting in control reversal
    Control reversal
    Control reversal is an adverse effect on the controllability of aircraft. The flight controls reverse themselves in a way that is not intuitive, so pilots may not be aware of the situation and therefore provide the wrong inputs; in order to roll to the left, for instance, they have to push the...

    . Further twisting loads are induced by changes of thrust
    Thrust is a reaction force described quantitatively by Newton's second and third laws. When a system expels or accelerates mass in one direction the accelerated mass will cause a force of equal magnitude but opposite direction on that system....

     settings to underwing-mounted engines.

Many of these loads are reversed abrubtly in flight with an aircraft such as the Extra 300 when performing extreme aerobatic
Aerobatics is the practice of flying maneuvers involving aircraft attitudes that are not used in normal flight. Aerobatics are performed in airplanes and gliders for training, recreation, entertainment and sport...

 manoeuvers; the spars of these aircraft are designed to safely withstand great load factors.

Wooden construction

Early aircraft used spars often carved from solid Spruce
A spruce is a tree of the genus Picea , a genus of about 35 species of coniferous evergreen trees in the Family Pinaceae, found in the northern temperate and boreal regions of the earth. Spruces are large trees, from tall when mature, and can be distinguished by their whorled branches and conical...

 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
Jig (tool)
In metalworking and woodworking, a jig is a type of tool used to control the location and/or motion of another tool. A jig's primary purpose is to provide repeatability, accuracy, and interchangeability in the manufacturing of products. A jig is often confused with a fixture; a fixture holds the...

, and compression glued to retain the wing dihedral. Wooden spars are still being used in light aircraft such as the Robin DR400
Robin DR400
|- References :*Exavia Ltd - article "A DR400 Buyers' Guide"* The Illustrated Encyclopedia of Aircraft , 1985, Orbis Publishing, Page 2799...

 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
Dry rot
Dry rot refers to a type of wood decay caused by certain types of fungi, also known as True Dry Rot, that digests parts of the wood which give the wood strength and stiffness...

 can have on the component; consequently regular inspections are often mandated to maintain airworthiness
Airworthiness is a term used to describe whether an aircraft has been certified as suitable for safe flight. Certification is initially conferred by a Certificate of Airworthiness from a National Airworthiness Authority, and is maintained by performing required maintenance actions by a licensed...


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.

Metal spars

A typical metal spar in a general aviation
General aviation
General aviation is one of the two categories of civil aviation. It refers to all flights other than military and scheduled airline and regular cargo flights, both private and commercial. General aviation flights range from gliders and powered parachutes to large, non-scheduled cargo jet flights...

 aircraft usually consists of a sheet aluminium
Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....

 spar web, with "L" or "T" -shaped spar caps being welded or rivet
A rivet is a permanent mechanical fastener. Before being installed a rivet consists of a smooth cylindrical shaft with a head on one end. The end opposite the head is called the buck-tail. On installation the rivet is placed in a punched or pre-drilled hole, and the tail is upset, or bucked A rivet...

ed 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
Fatigue (material)
'In materials science, fatigue is the progressive and localized structural damage that occurs when a material is subjected to cyclic loading. The nominal maximum stress values are less than the ultimate tensile stress limit, and may be below the yield stress limit of the material.Fatigue occurs...

 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
Chalk's Ocean Airways flight 101
Chalk's Ocean Airways Flight 101 was an aircraft crash that occurred off Miami Beach, Florida, in the United States on December 19, 2005. All 20 passengers and crew on board the 1947 Grumman G-73T Turbine Mallard died in the crash, which was attributed to metal fatigue on the starboard wing...


Tubular metal spars

The German Junkers J.I
Junkers J.I
-See also:-References:NotesBibliography* Flight 18 March 1920* Grey, C. G. Jane's All the World's Aircraft 1919. London: Putnam, 1919.* Grosz, P.M. Junkers J.I, Windsock Datafile 39. Hertfordshire, UK: Albatros Productions Ltd., 1993. ISBN 0-948414-49-9.* Taylor, Michael J. H. Jane's Encyclopedia...

 armoured fuselage ground-attack sesquiplane of 1917 used a Hugo Junkers
Hugo Junkers
Hugo Junkers was an innovative German engineer, as his many patents in varied areas show...

 -designed multi-tube network of several tubular wing spars, placed just under the corrugated duralumin
Duralumin is the trade name of one of the earliest types of age-hardenable aluminium alloys. The main alloying constituents are copper, manganese, and magnesium. A commonly used modern equivalent of this alloy type is AA2024, which contains 4.4% copper, 1.5% magnesium, 0.6% manganese and 93.5%...

 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 by American aviation designer William Stout
William Bushnell Stout
William Bushnell Stout was an inventor, designer whose work in automotive and aviation fields was notable. Stout designed an aircraft that eventually became the Ford Trimotor and was an executive at the Ford Motor Company.-Early years:William Bushnell Stout was born March 16, 1880 in Quincy,...

 for his 1920s-era Ford Trimotor
Ford Trimotor
The Ford Trimotor was an American three-engined transport plane that was first produced in 1925 by the companies of Henry Ford and that continued to be produced until June 7, 1933. Throughout its time in production, a total of 199 Ford Trimotors were produced...

 airliner series, and by Russian aerospace designer Andrei Tupolev
Andrei Tupolev
Andrei Nikolayevich Tupolev was a pioneering Soviet aircraft designer.During his career, he designed and oversaw the design of more than 100 types of aircraft, some of which set 78 world records...

 for such aircraft as his Tupolev ANT-2
Tupolev ANT-2
-References:Duffy, Paul and Andrei Kankdalov. Tupolev The Man and His aircraft. Warrendale, PA: Society of Automotive Engineers....

 of 1922, upwards in size to the then-gigantic Maxim Gorki
Tupolev ANT-20
-See also:*Dornier Do X-External links:* * Russian: Babelfish rough English translation * * Babelfish rough English translation from Russian History of Aviation, Publ Young Guards...

 of 1934.

A design aspect of the Supermarine Spitfire
Supermarine Spitfire
The Supermarine Spitfire is a British single-seat fighter aircraft that was used by the Royal Air Force and many other Allied countries throughout the Second World War. The Spitfire continued to be used as a front line fighter and in secondary roles into the 1950s...

 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
The undercarriage or landing gear in aviation, is the structure that supports an aircraft on the ground and allows it to taxi, takeoff and land...

 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.

A version of this spar construction method is also used in the BD-5 which was designed and constructed by Jim Bede
Jim Bede
James R. "Jim" Bede is an aircraft designer, who is often credited with the creation of the modern kitplane market. He has designed well over a dozen aircraft since the 1960s, but a string of business failures have kept most of these designs out of widespread use. -Bede Aviation:Bede was raised in...

 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.

Geodesic construction

In aircraft such as the Vickers Wellington
Vickers Wellington
The Vickers Wellington was a British twin-engine, long range medium bomber designed in the mid-1930s at Brooklands in Weybridge, Surrey, by Vickers-Armstrongs' Chief Designer, R. K. Pierson. It was widely used as a night bomber in the early years of the Second World War, before being displaced as a...

, 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.

Composite construction

Many modern aircraft use carbon fibre
Carbon fiber
Carbon fiber, alternatively graphite fiber, carbon graphite or CF, is a material consisting of fibers about 5–10 μm in diameter and composed mostly of carbon atoms. The carbon atoms are bonded together in crystals that are more or less aligned parallel to the long axis of the fiber...

 and Kevlar
Kevlar is the registered trademark for a para-aramid synthetic fiber, related to other aramids such as Nomex and Technora. Developed at DuPont in 1965, this high strength material was first commercially used in the early 1970s as a replacement for steel in racing tires...

 in their construction, ranging in size from large airliner
An airliner is a large fixed-wing aircraft for transporting passengers and cargo. Such aircraft are operated by airlines. Although the definition of an airliner can vary from country to country, an airliner is typically defined as an aircraft intended for carrying multiple passengers in commercial...

s to small homebuilt aircraft
Homebuilt aircraft
Also known as amateur-built aircraft or kit planes, homebuilt aircraft are constructed by persons for whom this is not a professional activity. These aircraft may be constructed from "scratch," from plans, or from assembly kits.-Overview:...

. Of note are the developments made by Scaled Composites
Scaled Composites
Scaled Composites is an aerospace company founded by Burt Rutan and currently owned by Northrop Grumman that is located at the Mojave Spaceport, Mojave, California, United States...

 and the German glider
Glider (sailplane)
A glider or sailplane is a type of glider aircraft used in the sport of gliding. Some gliders, known as motor gliders are used for gliding and soaring as well, but have engines which can, in some cases, be used for take-off or for extending a flight...

 manufacturers Schempp-Hirth
Schempp-Hirth Flugzeugbau GmbH is a glider manufacturer based in Kirchheim unter Teck, Germany.-History:Martin Schempp founded his own company in Göppingen in 1935, with the assistance of Wolf Hirth. The company was initially called "Sportflugzeugbau Göppingen Martin Schempp"...

 and Schleicher
Alexander Schleicher
Alexander Schleicher was a German pioneer of sailplane design. The company that he founded and which bears his name - Alexander Schleicher GmbH & Co - is today one of the world’s leading sailplane manufacturers....

. These companies initially employed solid fibreglass
Glass-reinforced plastic
Fiberglass , is a fiber reinforced polymer made of a plastic matrix reinforced by fine fibers of glass. It is also known as GFK ....

 spars in their designs but now often use carbon fibre in their high performance gliders such as the ASG 29
Schleicher 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.

External links

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