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 stabiliser 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 they 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.
Sesquiplane or sesquiwing 
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 of the Great War. The later Waco Custom Cabin series proved to be a popular example in general aviation.
Tandem wing 
The tandem wing is an aircraft configuration with one wing or set of wings in front of the other (e.g. a wing in the nose and a wing in the tail). This arrangement is not usually considered a biplane, as the two wings do not overlap in plan view.
Advantages and disadvantages 
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 and 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.
Early designers considered both monoplane and biplane designs. However, the weakness of the materials and design techniques available required these designers to place great effort into making wings capable of withstanding the required loads. A biplane (having the characteristics of a box girder) can be made lighter for a given strength requirement, and was therefore a more common choice.
Most successful early aircraft were biplanes, in spite of considerable early experimentation with monoplanes, triplanes and even quadraplanes. During the period (~1914 to 1925) almost all aircraft were biplanes.
Early monoplanes and biplanes were often externally braced, having struts and/or bracing wires. These elements gave added strength without excess weight, but they did add unwanted aerodynamic drag. The long-term answer to the problem was a cantilever monoplane wing – having sufficient stiffness to dispense with external bracing. Such wings were already being developed by several designers, including Hugo Junkers, whose work during 1915 resulted in the pioneering Junkers J 1, the world's first practical all-metal aircraft of any type. Cantilever monoplane wings were becoming the norm for most applications by the early nineteen thirties; the era of the biplane was almost over.
Several air forces continued to use biplanes for primary training up till WWII and even beyond: the de Havilland Tiger Moth in the Royal Air Force, Stampe SV.4 in French and Belgian Air Forces, and the Boeing Stearman in the USAF.
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.
Famous biplanes include the Sopwith Camel, Antonov An-2, Beechcraft Staggerwing, Boeing Stearman, Bristol Bulldog, Curtiss JN-4, de Havilland Tiger Moth, Fairey Swordfish, Pitts Special and the Wright Flyer. The Stearman is particularly associated with stunt flying with wing-walkers. Famous sesquiplanes include the Nieuport 17 and Albatros D.III.
Ultralight aircraft 
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 are the Belgian-designed Aviasud Mistral, the German FK12 Comet, and the Lite Flyer Biplane.
In avian evolution 
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.
See also 
- Airplane Aerodynamics, Dommasch and Lomb, 1961 ed.
- Larry Mauro and Bill Lishman
- Lite Flyer Biplane
- Chatterjee S, Templin RJ (30 January 2007). "Biplane wing planform and flight performance of the feathered dinosaur Microraptor gui". Proc Natl Acad Sci USA 104 (5): 1576–80. Bibcode:2007PNAS..104.1576C. doi:10.1073/pnas.0609975104. PMC 1780066. PMID 17242354.
|Wikimedia Commons has media related to: Biplanes|
- Historical Collection of Biplane Pictures
- Jacqui Hayes: Bird wings evolved from biplane dinosaurs, Cosmos (magazine)
- Spicerweb.org, Octave Chanute biplane hang glider
- Wacoclassic.com, Waco Classic Aircraft, present day manufacture of sport biplanes
- Biplaneforum.com, sport biplane builder's website