In aeronautics, bracing comprises additional structural members which stiffen the functional airframe to give it rigidity and strength under load. Bracing may be applied both internally and externally, and may take the form of strut, which act in compression or tension as the need arises, and/or wires, which act only in tension. In general, bracing allows a stronger, lighter structure than one which is unbraced, but external bracing in particular adds drag which slows down the aircraft and raises considerably more design issues than internal bracing. Another disadvantage of bracing wires is that they require routine checking and adjustment, or rigging, even when located internally. During the early years of aviation, bracing was a universal feature of all forms of aeroplane, including the monoplanes and biplanes which were then equally common. Today, bracing in the form of lift struts is still used for some light commercial designs where a high wing and light weight are more important than ultimate performance.
1 Design principle
1.1 Bracing methods 1.2 Bracing wires
1.3 Internal bracing 1.4 External bracing
2 History 3 Biplanes
3.1 Interplane struts 3.2 Bays 3.3 Interplane strut gallery 3.4 Cabane struts
4.1 Cabanes 4.2 Lift struts 4.3 Jury struts
5 See also 6 References
6.1 Notes 6.2 Bibliography
Design principle Bracing works by creating a triangulated truss structure which resists bending or twisting. By comparison, an unbraced cantilever structure bends easily unless it carries a lot of heavy reinforcement. Making the structure deeper allows it to be much lighter and stiffer. To reduce weight and air resistance, the structure may be made hollow, with bracing connecting the main parts of the airframe. For example, a high-wing monoplane may be given a diagonal lifting strut running from the bottom of the fuselage to a position far out towards the wingtip. This increases the effective depth of the wing root to the height of the fuselage, making it much stiffer for little increase in weight. Typically, the ends of bracing struts are joined to the main internal structural components such as a wing spar or a fuselage bulkhead, and bracing wires are attached close by. Bracing may be used to resist all the various forces which occur in an airframe, including lift, weight, drag and twisting or torsion. A strut is a bracing component stiff enough to resist these forces whether they place it under compression or tension. A wire is a bracing component able only to resist tension, going slack under compression, and consequently is nearly always used in conjunction with struts. Bracing methods
A square frame made of solid bars is not rigid but tends to bend at
the corners. Bracing it with an extra diagonal bar would be heavy. A
wire would be much lighter but would stop it collapsing only one way.
To hold it rigid, two cross-bracing wires are needed. This method of
cross-bracing can be seen clearly on early biplanes, where the wings
and interplane struts form a rectangle which is cross-braced by wires.
Another way of arranging a rigid structure is to make the cross pieces
solid enough to act in compression and then to connect their ends with
an outer diamond acting in tension. This method was once common on
monoplanes, where the wing and a central cabane or a pylon form the
cross members while wire bracing forms the outer diamond.
Most commonly found on biplane and other multiplane aircraft, wire
bracing was also common on early monoplanes.
Unlike struts, bracing wires always act in tension
The thickness and profile of a wire affect the drag it causes,
especially at higher speeds. Wires may be made of multi-stranded
cable, a single strand of piano wire, or aerofoil sectioned steel.
Bracing wires primarily divide into flying wires which hold the wings
down when flying and landing wires which hold the wings up when they
are not generating lift. (The wires connecting a basket or gondola to
a balloon are also called flying wires.) Thinner incidence wires are
sometimes run diagonally between fore and aft interplane struts to
stop the wing twisting and changing its angle of incidence to the
fuselage. In some pioneer aircraft, wing bracing wires were also
run diagonally fore and aft to prevent distortion under side loads
such as when turning. Besides the basic loads imposed by lift and
gravity, bracing wires must also carry powerful inertial loads
generated during manoeuvres, such as the increased load on the landing
wires at the moment of touchdown.
Bracing wires must be carefully rigged to maintain the correct length
and tension. In flight the wires tend to stretch under load and on
landing some may become slack. Regular rigging checks are required and
any necessary adjustments made before every flight. Rigging
adjustments may also be used to set and maintain wing dihedral and
angle of incidence, usually with the help of a clinometer and
plumb-bob. Individual wires are fitted with turnbuckles or threaded
end fittings so that they can be readily adjusted. Once set, the
adjuster is locked in place.
Internal bracing was most significant during the early days of
aeronautics when airframes were literally frames, at best covered in
doped fabric which had no strength of its own. Wire cross-bracing was
extensively used to stiffen such airframes, both in the fabric-covered
wings and in the fuselage, which was often left bare.
Routine rigging of the wires was needed to maintain structural
stiffness against bending and torsion. A particular problem for
internal wires is access in the cramped interior of the fuselage.
Often, providing sufficient internal bracing would make a design too
heavy, so in order to make the airframe both light and strong the
bracing is fitted externally. This was common in early aircraft due to
the limited engine power available and the need for light weight in
order to fly at all. As engine powers rose steadily through the 1920s
and 30s, much heavier airframes became practicable and most designers
abandoned external bracing in order to allow for increased speed.
Bracing, both internal and external, was extensively used in early
aircraft to support the lightweight airframes demanded by the low
engine powers and slow flying speeds then available. From the very
Nearly all biplane aircraft have their upper and lower planes connected by interplane struts, with the upper wing running across above the fuselage and connected to it by shorter cabane struts. These struts divide the wings into bays which are braced by diagonal wires. The flying wires run upwards and outwards from the lower wing, while the landing wires run downwards and outwards from the upper wing. The resulting combination of struts and wires is a rigid box girder-like structure independent of its fuselage mountings. Interplane struts interplane struts hold apart the wings of a biplane or multiplane, also helping to maintain the correct angle of incidence for the connected wing panels. Parallel struts: The most common configuration is for two struts to be placed in parallel, one behind the other. These struts will usually be braced by "incidence wires" running diagonally between them. These wires resist twisting of the wing which would affect its angle of incidence to the airflow. N-struts replace the incidence wires by a third strut running diagonally from the top of one strut to the bottom of the other in a pair. V-struts converge from separate attachment points on upper wing to a single point on the lower wing. They are often used for the sesquiplane wing, in which the lower wing has a considerably smaller chord than the upper wing. I-struts replaces the usual pair of struts by a single, thicker streamlined strut with its ends extended fore and aft along the wing. Bays
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The span of a wing between two sets of interplane or cabane struts is
called a bay. Wings are described by the number of bays on each side.
For example, a biplane with cabane struts and one set of interplane
struts on each side of the aircraft is a single-bay biplane.
For a small type such as a World War I scout like the Fokker D.VII,
one bay is usually enough. But for larger wings carrying greater
payloads, several bays may be used. The two-seat
Parallel struts on a Sopwith Camel
V-struts on a Nieuport 10
N-struts on a Boeing-Stearman Model 75
I-struts on a
Warren truss struts on a Fiat C.R.42
The World War I British
Cabane N-struts and torsion wires on a de Havilland Tiger Moth
Where an aircraft has a wing running clear above the main fuselage, the two components are often connected by cabane struts running up from the top of the fuselage or crew cabin to the wing centre section. Such a wing is usually also braced elsewhere, with the cabane struts forming part of the overall bracing scheme. Because cabane struts often carry engine thrust to the upper wing to overcome its drag, the loads along each diagonal between fore and aft struts are unequal and they are often formed as N-struts. They may also have cross-braced torsion wires to help stop the wing twisting. A few biplane designs, like the British 1917 Bristol Fighter two-seat fighter/escort, had its fuselage clear of the lower wing as well as the upper one, using ventral cabane struts to accomplish such a design feature.
Wire braced monoplane with wires from central mountings to wings, i.e. Fokker Eindecker
Early monoplanes relied entirely on external wire bracing, either directly to the fuselage or to kingposts above it and undercarriage struts below to resist the same forces of lift and gravity. Many later monoplanes, beginning in 1915, have used cantilever wings with their lift bracing within the wing to avoid the drag penalties of external wires and struts, Cabanes In many early wire-braced monoplanes, e.g. the Blériot XI and Fokker Eindecker (both wing warping designs), dorsal and sometimes ventral strut systems or cabanes were placed either above, or above and below the fuselage. This could be used both to provide some protection to the pilot if the craft overturned on the ground, and also for the attachment of landing wires which ran out in a slightly inclined vee to fore and aft points near the wing tips. In parasol wing monoplanes the wing passes above the fuselage and is joined to the fuselage by cabane struts, similarly to the upper wing of a biplane. On some types the cabane is replaced by a single thick, streamlined pylon. Lift struts
On a high-wing aircraft, a lift strut connects an outboard point on
the wing with a point lower on the fuselage to form a rigid triangular
structure. While in flight the strut acts in tension to carry wing
lift to the fuselage and hold the wing level, while when back on the
ground it acts in compression to hold the wing up.
For aircraft of moderate engine power and speed, lift struts represent
a compromise between the high drag of a fully cross-braced structure
and the high weight of a fully cantilevered wing. They are common on
high-wing types such as the
Consolidated PBY Catalina
Less commonly, some low-winged monoplanes like the
From early times these lift struts have been streamlined, often by
enclosing metal load bearing members in shaped casings. The Farman
F.190, for example, had its high wings joined to the lower fuselage by
parallel duralumin tubes enclosed in streamlined spruce fairings
Complex jury struts on a Fleet Canuck
A lift strut can be so long and thin that it bends too easily. Jury
struts are small subsidiary struts used to stiffen it.
Problems which jury struts prevent include resonant vibration and
buckling under compressive loads.
^ de Havilland
Crane, Dale: Dictionary of Aeronautical Terms, third edition, Aviation
Supplies & Academics, 1997. ISBN 1-56027-287-2
Halliwell, F.W. "Rigging: The Erection and Trueing-Up of Aeroplanes".
Flight, 23 January 1919. p. 107.
Kumar, B. An Illustrated Dictionary of Aviation. New York McGraw Hill,
2005. ISBN 0-07-139606-3
Steventon, H.W.B.; "Theoretical Considerations in the Design of Wing