Editing Biplane (section)

==Characteristics==
[[File:Hang Glider 1920s.jpg|thumb|1920s biplane hang glider]]
In a biplane aircraft, two wings are placed one above the other. Each provides 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. In a biplane configuration with no stagger from the upper wing to the lower wing, the [[lift coefficient]] is reduced by 10 to 15 percent compared to that of a monoplane using the same airfoil and [[aspect ratio (wing)|aspect ratio]].<ref name=Norton>F. H. Norton, The effect of staggering a biplane, NACA TN-70, Table, 1 p.3 1921</ref>
[[File:Gloster Gladiator 1.jpg|thumb|The [[Gloster Gladiator]], a [[World War II]] [[Fighter aircraft|fighter]] biplane]]

The lower wing is usually attached to the [[fuselage]], while the upper wing is raised above the fuselage with an arrangement of [[cabane strut]]s, although other arrangements have been used. Either or both of the main wings can support [[aileron]]s, 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 [[interplane strut]]s positioned symmetrically on either side of the fuselage and bracing wires to keep the structure from flexing, where the wings are not themselves [[cantilever]] structures.

=== Advantages and disadvantages ===
[[File:Antonov an2 ha-mkf arp.jpg|thumb|Soviet [[Antonov An-2]] biplane from the 1940s]]
The primary advantage of the biplane over a [[monoplane]] is its ability to combine greater stiffness with lower weight. Stiffness requires structural depth and where early monoplanes had to have this provided with external bracing, the biplane naturally has a deep structure and is therefore easier to make both light and strong. Rigging wires on non-cantilevered monoplanes are at a much sharper angle, thus providing less tension to ensure stiffness of the outer wing. On a biplane, since the angles are closer to the ideal of being in direct line with the forces being opposed, the overall structure can then be made stiffer. Because of the reduced stiffness, wire braced monoplanes often had multiple sets of flying and landing wires where a biplane could easily be built with one bay, with one set of landing and flying wires. The extra drag from the wires was not enough to offset the aerodynamic disadvantages from having two airfoils interfering with each other however. Strut braced monoplanes were tried but none of them were successful, not least due to the drag from the number of struts used.{{facts|date=August 2021}}

The structural forces acting on the spars of a biplane wing tend to be lower as they are divided between four spars rather than two, so the wing can use less material to obtain the same overall strength and is therefore lighter. A given area of wing also tends to be shorter, reducing bending moments on the spars, which then allow them to be more lightly built as well.<ref>Berriman, 1913, p.26</ref> The biplane does however need extra struts to maintain the gap between the wings, which add both weight and drag.

The low power supplied by the engines available in the first years of aviation limited aeroplanes to fairly low speeds. This required an even lower [[Stall (fluid dynamics)|stalling]] speed, which in turn required a low [[wing loading]], combining both large wing area with light weight. Obtaining a large enough wing area without the wings being long, and thus dangerously flexible was more readily accomplished with a biplane.{{facts|date=August 2021}}

The smaller biplane wing allows greater [[Aerobatics|maneuverability]]. Following World War I, this helped extend the era of the biplane and, despite the performance disadvantages, most [[fighter aircraft]] were biplanes as late as the mid-1930s. Specialist sports [[aerobatics|aerobatic]] biplanes are still made in small numbers.{{facts|date=August 2021}}

Biplanes suffer aerodynamic interference between the two planes when the high pressure air under the top wing and the low pressure air above the lower wing cancel each other out.{{dubious|date=August 2021}} This means that a biplane does not in practice obtain twice the lift of the similarly-sized monoplane. The farther apart the wings are spaced the less the interference, but the spacing struts must be longer, and the gap must be extremely large to reduce it appreciably.

As engine power and speeds rose late in [[World War I]], thick cantilever wings with inherently lower drag and higher wing loading became practical, which in turn made monoplanes more attractive as it helped solve the structural problems associated with monoplanes, but offered little improvement for biplanes.{{facts|date=August 2021}}

=== Stagger ===
[[File:Fleet 16B (C-FPFF).jpg|thumb|Wing stagger on a [[Fleet Finch]] primary trainer]]
The default design for a biplane has the wings positioned directly one above the other. Moving the upper wing forward relative to the lower one is called [[Stagger (aviation)|positive stagger]] or, more often, simply stagger. It can increase lift and reduce drag by reducing the aerodynamic interference effects between the two wings by a small degree, but more often was used to improve access to the cockpit. Many biplanes have staggered wings. Common examples include the [[de Havilland Tiger Moth]], [[Bücker Bü 131|Bücker Bü 131 Jungmann]] and [[Travel Air 2000]].

Alternatively, the lower wing can instead be moved ahead of the upper wing, giving negative stagger, and similar benefits. This is usually done in a given design for structural reasons, or to improve visibility. Examples of negative stagger include the [[Sopwith Dolphin]], [[Breguet 14]] and [[Beechcraft Staggerwing]].<ref name="Sport Aviation">{{cite journal |journal=Sport Aviation |date=January 1961 |title=The Beechcraft Biplanes}}</ref><ref name="Cooksley p34">Cooksley 1991, p. 34.</ref> However, positive (forward) stagger is much more common.

=== Bays ===
The space enclosed by a set of [[interplane strut]]s is called a ''bay'' (much as [[Bay (architecture)|the architectural form]] is used), hence a biplane or [[triplane]] with one set of such struts connecting the wings on each side of the aircraft is a ''single-bay biplane''. This provided sufficient strength for smaller aircraft such as the [[First World War]]-era [[Fokker D.VII]] fighter and the [[Second World War]] [[de Havilland Tiger Moth]] basic trainer.<ref name = "jackson 3 4">Jackson 1966, pp. 3–4.</ref>

The larger two-seat [[Curtiss JN-4]] Jenny is a ''two bay biplane'', the extra bay being necessary as overlong bays are prone to flexing and can fail. 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.<ref name="profile p6-7">Andrews 1965, pp. 6–7.</ref> The [[Sopwith 1½ Strutter]] has a W shape cabane, however as it does not connect the wings to each other, it does not add to the number of bays.<ref name="bomber-40">Lake 2002, p. 40.</ref>

Large transport and [[Bomber|bombing]] biplanes often needed still more bays to provide sufficient strength. These are often referred to as ''multi-bay biplanes''. A small number of biplanes, such as the [[Zeppelin-Lindau D.I]] have no interplane struts and are referred to as being ''strutless''.<ref name=G0>Grosz 1998, p. 0.</ref>
<gallery>
File:Zeppelin-Lindau (Do) D.I.jpg|[[Zeppelin-Lindau D.I]] strutless biplane
File:Nieuport 23 C.1 (colour).jpg|[[Nieuport 23]] single-bay sesquiplane
File:SPAD S.XIII Front.jpg|[[SPAD S.XIII]] single-bay biplane with auxiliary struts
File:Fearless Freddie, stuntman cph.3b18313.jpg|[[Curtiss JN-4]] two-bay biplane
File:Handley Page V-1500.jpg|[[Handley Page V/1500]] four-bay or multi-bay biplane
</gallery>

=== Rigging ===
{{main|Bracing (aeronautics)#Rigging}}
Because most biplanes do not have [[cantilever]] structures, they require rigging wires to maintain their rigidity. Early aircraft used simple wire (either braided or plain), however during the First World War, the British [[Royal Aircraft Factory]] developed airfoil section wire named RAFwire in an effort to both increase the strength and reduce the drag. 
Four types of wires are used in the biplane wing structure. Drag wires inside the wings prevent the wings from being folded back against the fuselage, running inside a wing bay from the forward inboard corner to the rear outboard corner.<ref name=GunstonP210>Gunston, 2004, p.210</ref> Anti-drag wires prevent the wings from moving forward when the aircraft stops and run the opposite direction to the drag wires.<ref name=GunstonP51>Gunston, 2004, p.51</ref> Both of these are usually hidden within the wings, and if the structure is sufficiently stiff otherwise, may be omitted in some designs. Indeed many early aircraft relied on the fabric covering of the wing to provide this rigidity, until higher speeds and forces made this inadequate. 
Externally, lift wires prevent the wings from folding up, and run from the underside of the outer wing to the lower wing root.<ref name=GunstonP382>Gunston, 2004, p.382</ref> Conversely, landing wires prevent the wings from sagging, and resist the forces when an aircraft is landing, and run from the upper wing centre section to outboard on the lower wings.<ref name=GunstonP375>Gunston, 2004, p.375</ref> Additional drag and anti-drag wires may be used to brace the cabane struts which connect the fuselage to the wings, and interplane struts, which connect the upper and lower wings together.

===Sesquiplane===  <!-- linked from other pages-->
[[File:Early Nieuport 17 climbing (cropped).jpg|thumb|The lower wing of the [[Nieuport 17]] has smaller chord, but similar span, than the upper wing]]
The ''sesquiplane'' is a type of biplane where one wing (usually the lower) is significantly smaller than the other.<ref name=Wraff74>Wragg 1974, p. 54.</ref><ref name=Gunston>Gunston 2009, p. 606.</ref> The word, from Latin, means "one-and-a-half wings". The arrangement can reduce [[Drag (physics)#Aerodynamics|drag]] and weight while retaining the biplane's structural advantages. The lower wing may have a significantly shorter span, or a reduced [[chord (aircraft)|chord]].<ref name=Wraff74/>

Examples include the series of [[Nieuport]] military aircraft—from the [[Nieuport 10]] through to the [[Nieuport 27]] which formed the backbone of the Allied air forces between 1915 and 1917.<ref>Chassard 2018, p. 1.</ref> The performance of the Nieuport sesquiplanes was so impressive that the {{lang|de|[[Idflieg]]}} (the German Inspectorate of flying troops) requested their aircraft manufacturers to produce copies, an effort which was aided by several captured aircraft and detailed drawings; one of the most famed copies was the [[Siemens-Schuckert D.I]].<ref>Andrews 1966, pp. 3, 7.</ref> The [[Albatros D.III]] and [[Albatros D.V|D.V]], which had also copied the general layout from Nieuport, similarly provided the backbone of the German forces during the First World War.<ref name = "And 78"/> The Albatros sesquiplanes were widely acclaimed by their aircrews for their maneuverability and high rate of climb.<ref name="VanWyngarden 2007, p. 19">VanWyngarden 2007, p. 19.</ref>

During [[interwar period]], the sesquiplane configuration continued to be popular, with numerous types such as the [[Nieuport-Delage NiD 42]]/[[Nieuport-Delage NiD 52|52]]/[[Nieuport-Delage NiD 62|62]] series, [[Fokker C.V]]d & e, and [[Potez 25]], all serving across a large number of air forces. In the general aviation sector, aircraft such as the [[Waco Custom Cabin series]] proved to be relatively popular.<ref>[http://registry.faa.gov/aircraftinquiry/acftinqSQL.asp?striptxt=&mfrtxt=Waco FAA Registry Search for Waco] {{webarchive|url=https://web.archive.org/web/20120217034911/http://registry.faa.gov/aircraftinquiry/acftinqSQL.asp?striptxt=&mfrtxt=Waco |date=17 February 2012 }} accessed 12 June 2009.</ref> The [[Saro Windhover]] was a sesquiplane with the upper wing smaller than the lower, which was a much rarer configuration than the reverse.<ref name="London">{{harvnb|London|1988|pp=125–130|id=London1988}}</ref> The [[Pfalz D.III]] also featured a somewhat unusual sesquiplane arrangement, possessing a more substantial lower wing with two spars that eliminated the flutter problems encountered by single-spar sesquiplanes.<ref name = "And 78">Andrews 1966, pp. 7–8.</ref>