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A biplane is a fixed-wing aircraft with two main wings stacked one above the other. The first aircraft to fly, the 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 over a monoplane, it produces more drag than a similar unbraced or cantilever monoplane wing. Improved structural techniques and materials and the quest for greater speed made the biplane configuration obsolete for most purposes by the late 1930s.
The tandem wing design differs in that one of the two wings is placed forward and the other aft, such that no horizontal stabilizer is necessary.
In a biplane aircraft, two wings are placed one above the other. Both provide part of the lift, although they are not able to produce twice as much lift as a single wing of similar size and shape because the upper and the lower are working on nearly the same portion of the atmosphere and thus interfere with each other's behaviour. For example, in a wing of aspect ratio 6, and a wing separation distance of one chord length, the biplane configuration will only produce about 20 percent more lift than a single wing of the same planform.
In the biplane configuration, the lower wing is usually attached to the fuselage, while the upper wing is raised above the fuselage with an arrangement of cabane struts, although other arrangements have been used. Either or both of the main wings can support ailerons, while flaps are more usually positioned on the lower wing. Bracing is nearly always added between the upper and lower wings, in the form of wires (tension members) and/or slender interplane struts positioned symmetrically on either side of the fuselage.
The space enclosed by a set of struts is called a bay, hence a biplane with one set of such struts on each side is said to be a single-bay biplane. Two bay biplanes, with one set of struts closer to the fuselage and another closer to the wing tips like the Avro 504 of World War I, were also common during the biplane's heyday and some larger biplanes such as the Handley Page Hyderabad had three or more bays.
Variations on the biplane concept include the sesquiplane, where one wing (usually the lower) is significantly smaller than the other either in span, chord or both. The name means "one-and-a-half wings." The arrangement reduces interference drag between the wings whilst retaining some of the biplane's structural advantage. The 1920s Pander E is an example of an aircraft with a lower wing of exactly half the span and nearly one quarter (23%) of the area of the upper one. Some designs keep the upper and lower spans nearly equal for structural optimization, whilst reducing the lower chord, allowing near vertical interplane struts; probably the best known examples are the Nieuport military aircraft — from the Nieuport 10 through to the Nieuport 27, all designed by Gustave Delage during the Great War. The later Waco Custom Cabin series proved to be a popular example in general aviation.
Aircraft built with two main wings (or three in a triplane) can usually lift up to 20 percent more than can a similarly sized monoplane of similar wingspan. A biplane will therefore typically have a shorter wingspan than the equivalent monoplane, which tends to afford greater maneuverability.
The struts and wire bracing of a typical biplane form a box girder. Particularly when divided into bays, this permits a very light but strong and rigid wing structure. This allows a biplane to fly with very little power, and in the early days of aviation most fixed-wing aircraft (including the very first, the Wright Flyer) were biplanes.
On the other hand there are many disadvantages to the configuration. Each wing negatively interferes with the aerodynamics of the other, requiring greater overall surface area to produce the same lift as the equivalent monoplane. A biplane typically also produces more drag than a monoplane, especially as speed increases.
Biplanes were originally designed with the wings positioned directly one above the other. Moving one wing forward relative to the other is called positive stagger or, more often, simply stagger. It can help increase lift and reduce drag, by reducing the aerodynamic interference effects between the two wings. Many biplanes have such staggered wings. A common example from the 1930s is the layout found for the Waco Standard Cabin series.
It is also possible to place the lower wing's leading edge ahead of the upper wing, giving negative stagger. This is usually done in a given design for practical engineering reasons. Examples of negative stagger include the Airco DH.5, Sopwith Dolphin and Beechcraft Staggerwing. However forward stagger is more common because it improves both downward visibility and ease of cockpit access for open cockpit biplanes.
Staggering the wings may distort the box girder effect of the wing and reduce the structural benefits of the biplane layout.
Before any successful powered flight, most aeroplane designs envisaged monoplanes. The weakness of the materials and design techniques available made it difficult to design wings which were both light and strong enough to fly. Many designs used external bracing struts and wires. The Cody kite, which comprised a box kite with wings attached to its upper surfaces, took a different approach. By 1896 Octave Chanute was flying a biplane hang glider and concluded that the externally braced biplane offered better prospects for powered flight than the monoplane. The Wright Flyer biplane of 1903 became one of the first successful aeroplanes.
Throughout the pioneer years, both biplanes and monoplanes were common, but by the outbreak of the First World War biplanes had started to gain favour due to their better manoeuvrability and inherent strength, as exemplified by the Sopwith Camel. During the period from 1914 to 1925 almost all aircraft were biplanes. Sesquiplane types included the French Nieuport 17 and German Albatros D.III.
However as the available engine-power increased so did speeds, and at higher speeds the bracing - of both biplanes and monoplanes - caused increasing drag. In order to fly faster, it would be necessary to do away with the external bracing to create a clean "cantilever" wing.
Early cantilever designs were either too weak or too heavy. The Fokker V.4 prototype of 1917 (identified by some as the V.3) was an unusual cantilever triplane but suffered excessive flexing of the wings, while the Junkers J 1 of 1915 was heavy and had a poor rate of climb. Up until the 1930s biplanes such as the Bristol Bulldog and Hawker Fury remained commonplace. But by the 1930s engine power had risen to the point where the fast cantilever monoplane took over and the slower biplane all but died out.
Several air forces continued to use biplanes in specialist roles such as primary training or shipboard operation up until WWII and even beyond. Trainers include the de Havilland Tiger Moth in the Royal Air Force, Stampe SV.4 in the French and Belgian Air Forces, and the Boeing Stearman in the USAF. The British Fleet Air Arm still flew the Gloster Gladiator and Fairey Swordfish from its aircraft carriers at the start of the war.
The Stearman is also particularly associated with stunt flying such as wing-walkers.
Modern biplane designs still exist in specialist niche roles such as aerobatics and agricultural aircraft. The Pitts Special dominated aerobatics for many years and is still in production, while the WACO Classic YMF is a reproduction of the original Waco design.
The vast majority of biplane designs have been fitted with reciprocating engines of comparatively low power; exceptions include the Antonov An-3 and WSK-Mielec M-15 Belphegor, fitted with turboprop and turbofan engines respectively. Some older biplane designs, such as the Grumman Ag Cat and the aforementioned An-2 (in the form of the An-3) are available in upgraded versions with turboprop engines.
Although most ultralights are monoplanes, the low speeds and simple construction involved have inspired a small number of biplane ultralights, such as Larry Mauro's Easy Riser. Mauro also made a version powered with solar cells driving an electric motor called the Solar Riser. Mauro's Easy Riser was used by the man who became known as "Father Goose," Bill Lishman. Other biplane ultralights include the Belgian-designed Aviasud Mistral, the German FK12 Comet, and the Lite Flyer Biplane.
It has been suggested the feathered dinosaur Microraptor glided, and perhaps even flew, on four wings, which were held in a biplane-like arrangement. This was made possible by the presence of flight feathers on both the forelimbs and hindlimbs of Microraptor, and it has been suggested the earliest flying ancestors of birds may have possessed this morphology, with the monoplane arrangement of modern birds evolving later.
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