Nearly all biplane aircraft have their upper and lower planes connected by interplane struts which divide the wings into bays braced by diagonal wires. The wires that run outward from the upper to the lower planes resist the distortion on the bay under gravity and are termed landing wires and those from the lower plane outwards to the upper (lift or flying wires) resist bay distortion under the aerodynamic lifting force. The resulting combination of struts and wires is a quite rigid, box girder-like structure independent of its fuselage mountings.
In contrast, 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 have used cantilever wings, with their lift bracing within the wing to avoid the drag penalties of external wires and struts but significant numbers have had, and continue to have, one or more external braces called lift struts for each wing. These are rigid struts rather than wires and so can resist compressive loads as well as extensions.
The placement of these wing struts struts and the forces acting on them depends chiefly on the wing configuration. They are most common in high-wing aircraft like the Cessna 152 or parasol wing aircraft like the Consolidated PBY Catalina, running from the wing underside to the lower fuselage longeron or equivalent structure. In flight, these lift struts are in tension from the lift forces. In level flight they convey the weight of the aircraft to the wings. On the ground they are under compression, supporting the wing against gravity. Less commonly, and chiefly in the past, 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.
Sometimes each wing has just a single lift strut, as on the Cessna 152, but they often come in pairs, sometimes parallel as on the Catalina, sometimes splayed or as V-form pairs (e.g. Auster Autocrat) joined to the fuselage at a single point. Many more complicated arrangements have been used, often with two primary lift struts augmented by auxiliary interconnections known as jury struts between each other or to the wing or the fuselage. Each pair of the inverted V struts of the Pawnee, for example, is assisted by a pair of vertical support struts.
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 and the Westland Lysander used extruded I section beams of light alloy, onto which were screwed a fore and aft pair of duralumin fairings. Later aircraft have had streamlined struts formed directly from shaped metal, like the extruded light alloy struts of the Auster AOP.9,  or from composites, for example the carbon fibre lift struts of the Remos GX eLITE. Designers have adopted different methods of improving the aerodynamics of the strut-wing and strut-body, using 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; other designs have an extended, faired foot, for example the Skyeton K-10 Swift.
Lift struts have a clear primary structural role but are sometimes also used to position devices like pitot heads and wind driven generators in a region of clear airflow, following the similar use of interplane struts in biplanes.
Lift struts are no longer used by large or fast aircraft in current production because of the drag penalties, but remain common on small (2/4 seat) high wing light aircraft in the ULS[disambiguation needed] and LSA categories. Not all current strut braced aircraft are small: the Pilatus Porter STOL aircraft, which first flew in 1960 and seats up to 10 passengers, remains in production and the de Havilland Twin Otter, a 19-seater has recently (2008) returned to production.
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