Bracing (aeronautics)
In aeronautics, bracing comprises additional structural members which stiffen the functional airframe to give it rigidity and strength under load.
During the early years of aviation, bracing was a universal feature of all forms of aeroplanes, including the monoplanes and biplanes, which were then equally common.
To reduce weight and air resistance, the structure may be made hollow, with bracing connecting the main parts of the airframe.
Bracing may be used to resist all the various forces which occur in an airframe, including lift, weight, drag and twisting or torsion.
[1] In some pioneer aircraft, wing bracing wires were also run diagonally fore and aft to prevent distortion under side loads such as when turning.
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.
[3] 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.
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.
The SPAD S.XIII fighter, while appearing to be a two-bay biplane, has only one bay, but has the midpoints of the rigging braced with additional struts; however, these are not structurally contiguous from top to bottom wing.
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.
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.
Less commonly, some low-winged monoplanes like the Piper Pawnee have had lift struts mounted above the wing, acting in compression in flight and in tension on the ground.
[6] 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[7] and the Westland Lysander used extruded I section beams of light alloy, onto which were screwed a fore and aft pair of duralumin fairings.
[10] Designers have adopted different methods of improving the aerodynamics of the strut-wing and strut-body connections, using similar approaches to those used in interplane struts.
Sometimes the streamlining is tapered away close to the wing, as on the Farman F.190;[7] other designs have an extended, faired foot, for example the Skyeton K-10 Swift.
[11] Lift struts are sometimes combined with other functions, for example helping to support the engines as on the Westland IV or the undercarriage as on the Scottish Aviation Twin Pioneer.
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.
Most designs employed streamlined struts made either from spruce or ash wood, selected for its strength and light weight.
The German professor Hugo Junkers was seriously interested in doing away with drag-inducing struts and rigging around the start of World War I, and by mid-1915 his firm had designed the Junkers J 1 all-metal "technology demonstrator" monoplane, possessing no external bracing for its thick-airfoil cantilever wing design, which could fly at just over 160 km/h with an inline-six piston engine of just 120 horsepower.
By the end of World War I, engine powers and airspeeds had risen enough that the drag caused by bracing wires on a typical biplane was significantly affecting performance, while the heavier but sleeker strut-braced parasol monoplane was becoming practicable.
A turbojet-powered HD.45 was unsuccessfully proposed to compete with the Sud Aviation Caravelle, maybe due to the high-speed turbojet mismatched to a slower airframe.
