Annular Wing
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A closed wing is a
wing A wing is a type of fin that produces lift while moving through air or some other fluid. Accordingly, wings have streamlined cross-sections that are subject to aerodynamic forces and act as airfoils. A wing's aerodynamic efficiency is expres ...
that effectively has two main planes which merge at their ends so that there are no conventional
wing tip A wing tip (or wingtip) is the part of the wing that is most distant from the fuselage of a fixed-wing aircraft. Because the wing tip shape influences the size and drag of the wingtip vortices, tip design has produced a diversity of sha ...
s. Closed wing designs include the annular wing (commonly known as the cylindrical or ring wing), the joined wing, the box wing and spiroid tip devices. Like many
wingtip device Wingtip devices are intended to improve the efficiency of fixed-wing aircraft by reducing drag. Although there are several types of wing tip devices which function in different manners, their intended effect is always to reduce an aircraft' ...
s, the closed wing aims to reduce the wasteful effects associated with
wingtip vortices Wingtip vortices are circular patterns of rotating air left behind a wing as it generates lift.Clancy, L.J., ''Aerodynamics'', section 5.14 One wingtip vortex trails from the tip of each wing. Wingtip vortices are sometimes named ''trailing'' ...
which occur at the tips of conventional wings. Although the closed wing has no unique claim on such benefits, many closed wing designs do offer structural advantages over a conventional
cantilever A cantilever is a rigid structural element that extends horizontally and is supported at only one end. Typically it extends from a flat vertical surface such as a wall, to which it must be firmly attached. Like other structural elements, a canti ...
monoplane A monoplane is a fixed-wing aircraft configuration with a single mainplane, in contrast to a biplane or other types of multiplanes, which have multiple planes. A monoplane has inherently the highest efficiency and lowest drag of any wing confi ...
.


Characteristics

Wingtip vortices Wingtip vortices are circular patterns of rotating air left behind a wing as it generates lift.Clancy, L.J., ''Aerodynamics'', section 5.14 One wingtip vortex trails from the tip of each wing. Wingtip vortices are sometimes named ''trailing'' ...
form a major component of
wake turbulence Wake turbulence is a disturbance in the atmosphere that forms behind an aircraft as it passes through the air. It includes variety of elements, the most significant of which are wingtip vortices and jetwash. Jetwash refers to the rapidly moving g ...
and are associated with
induced drag In aerodynamics, lift-induced drag, induced drag, vortex drag, or sometimes drag due to lift, is an aerodynamic drag force that occurs whenever a moving object redirects the airflow coming at it. This drag force occurs in airplanes due to wings or ...
, which is a significant contributor to total drag in most regimes. A closed wing avoids the need for wingtips and thus might be expected to reduce wingtip drag effects. In addition to potential structural advantages over open cantilevered wings, closed wing surfaces have some unique aerodynamic properties: * For a lifting system constrained to fit within a rectangular box of fixed horizontal (spanwise) and vertical dimensions as viewed in the freestream flow direction, the configuration that provides the absolute minimum
induced drag In aerodynamics, lift-induced drag, induced drag, vortex drag, or sometimes drag due to lift, is an aerodynamic drag force that occurs whenever a moving object redirects the airflow coming at it. This drag force occurs in airplanes due to wings or ...
for a given total vertical
lift Lift or LIFT may refer to: Physical devices * Elevator, or lift, a device used for raising and lowering people or goods ** Paternoster lift, a type of lift using a continuous chain of cars which do not stop ** Patient lift, or Hoyer lift, mobile ...
is a closed system, i.e. a rectangular box wing with lifting surfaces fully occupying all four boundaries of the allowed rectangular area. However, the induced-drag performance of the ideal closed box wing can be approached very closely by open configurations such as the C-wing discussed below. * For any lifting system (or portion of a lifting system) that forms a closed loop as viewed in the freestream flow direction, the optimum lift (or circulation) distribution that yields the minimum induced drag for a given total vertical lift is not unique, but is defined only to within a constant on the closed-loop portion. This is because, regardless of what the circulation distribution is to start with, a constant circulation can be added to the closed-loop portion without changing the total lift of the system or the induced drag. This is the key to explaining how the C-wing produces nearly the same induced-drag reduction as the corresponding fully closed system, as discussed below. The upshot is that although closed systems can produce large induced-drag reductions relative to a conventional planar wing, there is no significant aerodynamic advantage that uniquely accrues to their being closed rather than open.


Configurations

Various types of closed wing have been described: * Box wing * Rhomboidal wing * Flat annular wing * Concentric wing and fuselage


History


Pioneer years

An early example of the closed wing was on the
Blériot III The Blériot III was an early French aeroplane built by pioneer aviators Louis Blériot and Gabriel Voisin. It was later modified and renamed the Blériot IV, but both versions failed to fly. Design and development The Blériot III was radically ...
aircraft, built in 1906 by
Louis Blériot Louis Charles Joseph Blériot ( , also , ; 1 July 1872 – 1 August 1936) was a French aviator, inventor, and engineer. He developed the first practical headlamp for cars and established a profitable business manufacturing them, using much of th ...
and
Gabriel Voisin Gabriel Voisin (5 February 1880 – 25 December 1973) was a French aviation pioneer and the creator of Europe's first manned, engine-powered, heavier-than-air aircraft capable of a sustained (1 km), circular, controlled flight, which was made ...
. The lifting surfaces comprised two annular wings mounted in tandem. The later
Blériot IV Blériot may refer to: * Louis Blériot, a French aviation pioneer * Blériot Aéronautique, an aircraft manufacturer founded by Louis Blériot * Blériot-Whippet, a car * Bleriot (moonlet), a propeller moonlet in Saturn's A Ring * 11248 Blériot ...
replaced the forward annular wing with a biplane and added a canard foreplane to make it a
three-surface aircraft A three-surface aircraft or sometimes three-lifting-surface aircraft has a foreplane, a central wing and a tailplane. The central wing surface always provides lift and is usually the largest, while the functions of the fore and aft planes may ...
. It was able to leave the ground in small hops before being damaged beyond repair. Based on the work of G.J.A. Kitchen, Cedric Lee and G. Tilghman Richards built and flew several annular-wing aeroplanes in which the fore and aft segments were on the same level. The first was a biplane. It was followed by a series of monoplanes, the last of the line remaining in use until 1914.


World War II

In 1944, the
German German(s) may refer to: * Germany (of or related to) **Germania (historical use) * Germans, citizens of Germany, people of German ancestry, or native speakers of the German language ** For citizens of Germany, see also German nationality law **Ger ...
designer
Ernst Heinkel Dr. Ernst Heinkel (24 January 1888 – 30 January 1958) was a German aircraft designer, manufacturer, ''Wehrwirtschaftsführer'' in Nazi Germany, and member of the Nazi party. His company Heinkel Flugzeugwerke produced the Heinkel He 178, th ...
began working on an annular-wing
VTOL A vertical take-off and landing (VTOL) aircraft is one that can take off and land vertically without relying on a runway. This classification can include a variety of types of aircraft including helicopters as well as thrust-vectoring fixed-win ...
multirole single-seater called the '' Lerche'', but the project was soon abandoned.


Postwar

During the 1950s, the French company
SNECMA Safran Aircraft Engines, previously Snecma (''Société nationale d'études et de construction de moteurs d'aviation'') or Snecma Moteurs, is a French aerospace engine manufacturer headquartered in Courcouronnes and a subsidiary of Safran. It ...
developed the Coléoptère, a single-person
VTOL A vertical take-off and landing (VTOL) aircraft is one that can take off and land vertically without relying on a runway. This classification can include a variety of types of aircraft including helicopters as well as thrust-vectoring fixed-win ...
annular wing aircraft. The aircraft proved dangerously unstable despite the development and testing of several prototypes, and the design was abandoned. Later proposals for closed-wing designs included the
Convair Convair, previously Consolidated Vultee, was an American aircraft manufacturing company that later expanded into rockets and spacecraft. The company was formed in 1943 by the merger of Consolidated Aircraft and Vultee Aircraft. In 1953, it ...
Model 49 Advanced Aerial Fire Support System (AAFSS) and the 1980s Lockheed "Flying Bog Seat" concept. Dr. Julian Wolkovitch continued to develop the idea in the 1980s, claiming it was an efficient structural arrangement in which the horizontal tail provided structural support for the wing as well as acting as a stabilizing surface. The Spiroid
winglet Wingtip devices are intended to improve the efficiency of fixed-wing aircraft by reducing drag. Although there are several types of wing tip devices which function in different manners, their intended effect is always to reduce an aircraft' ...
, a design currently under development by
Aviation Partners Aviation Partners Inc. (API) is a Seattle-based private corporation that specializes in performance-enhancing winglet systems. The corporation was founded in 1991 and is owned by The Washington Companies. History API was founded in 1991 by Joe Cl ...
, is a closed wing surface mounted at the end of a conventional wing. The company announced that the winglets fitted to a
Gulfstream II The Gulfstream II (G-II) is an American twin engine business jet designed and built by Grumman and then in succession, Grumman American and finally Gulfstream American. Its Grumman model number is G-1159 and its US military designation is C-11A. ...
reduced fuel consumption in the cruise phase by over 10%. The Finnish company FlyNano flew a prototype of a closed wing
ultralight aircraft Ultralight aviation (called microlight aviation in some countries) is the flying of lightweight, 1- or 2-seat fixed-wing aircraft. Some countries differentiate between weight-shift control and conventional three-axis control aircraft with ailer ...
, the
FlyNano Nano The FlyNano Nano is a Finnish electric single seat seaplane, designed by Aki Suokas and produced by FlyNano of Lahti. It was introduced at AERO Friedrichshafen in 2011 and the prototype ''Proto'' version first flew on 11 June 2012. When it was av ...
on . An aircraft was also designed and constructed with a closed wing in
Belarus Belarus,, , ; alternatively and formerly known as Byelorussia (from Russian ). officially the Republic of Belarus,; rus, Республика Беларусь, Respublika Belarus. is a landlocked country in Eastern Europe. It is bordered by R ...
. Miscellaneous modern examples include: * Stanford study * Lockheed Ring-wing Closed wings remain mostly confined to the realms of studies and conceptual designs, as the engineering challenges of developing a strong, self-supporting closed wing for use in the large airliners which would benefit most from increases in efficiency have yet to be overcome. The closed wing is also used in water, for
surfboard A surfboard is a narrow plank used in surfing. Surfboards are relatively light, but are strong enough to support an individual standing on them while riding an ocean wave. They were invented in ancient Hawaii, where they were known as ''papa he'e ...
fins of the type also known as the
tunnel fin A tunnel fin is a type of surfboard fin used on surfboards, especially heavy longboards and longboard guns. The weight and length of these boards make it easier to control the fore and aft angle of the tunnel. As the name suggests, it is shap ...
.


Lockheed Martin Environmentally Responsible Aviation Project

During 2011, the Environmentally Responsible Aviation Project at
NASA The National Aeronautics and Space Administration (NASA ) is an independent agency of the US federal government responsible for the civil space program, aeronautics research, and space research. NASA was established in 1958, succeeding t ...
's Aeronautics Research Mission Directorate invited study proposals towards meeting NASA's goal of reducing future aircraft fuel consumption by 50% compared to 1998. Lockheed Martin proposed a box wing design along with other advanced technologies.


Prandtl Box Wing

In 1924, the German aerodynamicist
Ludwig Prandtl Ludwig Prandtl (4 February 1875 – 15 August 1953) was a German fluid dynamicist, physicist and aerospace scientist. He was a pioneer in the development of rigorous systematic mathematical analyses which he used for underlying the science of ...
suggested that a box wing, under certain conditions, might provide the minimum induced drag for a given lift and wingspan. In his design, two offset horizontal wings have vertical wings connecting their tips and shaped to provide a linear distribution of side forces. The configuration is said to offer improved efficiency for a range of aircraft. In the 1980s, the Ligeti Stratos used this approach. The name "PrandtlPlane" was coined in the 1990s in research by Aldo Frediani ''et al.'' of the
University of Pisa The University of Pisa ( it, Università di Pisa, UniPi), officially founded in 1343, is one of the oldest universities in Europe. History The Origins The University of Pisa was officially founded in 1343, although various scholars place ...
. It is currently also used in some
ultralight aircraft Ultralight aviation (called microlight aviation in some countries) is the flying of lightweight, 1- or 2-seat fixed-wing aircraft. Some countries differentiate between weight-shift control and conventional three-axis control aircraft with ailer ...
, IDINTOS (IDrovolante INnovativo TOScano) is a research project, co-funded by the regional government of Tuscany (Italy) in 2011 in order to design and manufacture an amphibious ultralight PrandtlPlane. The research project has been carried out by a consortium of Tuscan public and private partners, led by the Aerospace Section of the Civil and Industrial Engineering Department of Pisa University, and has resulted in the manufacturing of a 2-seater VLA prototype. The configuration is also claimed to be theoretically efficient for wide-body jet airliners. The largest commercial airliner, the
Airbus A380 The Airbus A380 is a large wide-body airliner that was developed and produced by Airbus. It is the world's largest passenger airliner and only full-length double-deck jet airliner. Airbus studies started in 1988, and the project was annou ...
, must make efficiency trade-offs to keep the wingspan below the 80-meter limit at most airports, but a closed wing with optimal wingspan could be shorter than that of conventional designs, potentially allowing even larger aircraft to use the current infrastructure.


C-wing

The C-wing is a theoretical configuration in which much of the upper centre section of a box wing is removed, creating a wing that folds up and over at the tips but does not rejoin in the centre. A C-wing can achieve very nearly the same induced-drag performance as a corresponding box wing, as shown by the calculations illustrated below. Each of the first three rows in the illustration shows a different C-wing configuration as it is taken through a sequence of theoretical induced-drag calculations in which the wingtips are brought closer together, culminating in the limiting case on the right, where the gap has been taken to zero and the configuration has become a closed box wing (referred to as the "Quasi-closed C-wing" because the calculations were carried out in the limit as the gap went to zero). The parameter ε is the optimal aerodynamic efficiency ratio and represents the ratio between the aerodynamic efficiency of a given non-planar wing and the corresponding efficiency of a reference classical cantilevered wing with the same wing span and total lift. Both efficiencies are evaluated for their respective optimal lift distributions. Values of ε greater than 1 indicate lower induced drag than that of a classical cantilevered wing for which ε = 1. Note that all of the C-wing configurations have ε greater than 1 and that there is little difference (no difference to the two decimal places shown in two of the cases) between a configuration with a substantial gap (the second entry in each row) and the corresponding closed configuration (the third entry in each row). This is because the optimum lift loading calculated for the quasi-closed cases is very small over the upper centre section, and that part of the wing can be removed with little change in lift or drag. The lift distributions shown here for the quasi-closed cases look different from those typically shown for box wings in the classical literature (see Durand, figure 81, for example). The classical solution in Durand was obtained by a conformal-mapping analysis that happened to be formulated in a way that led to equal upward loadings on the horizontal panels of the box. But the optimum lift distribution is not unique. A constant inward loading (corresponding to a particular constant circulation) can be added to a classical loading like that shown by Durand to obtain a loading like those in the quasi-closed cases below. The two methods of analysis give different-looking versions of the optimum loading that are not fundamentally different. Except for small differences due to the numerical method used for the quasi-closed cases, the two kinds of loading are in principle just shifted versions of each other.


References


External links

* * * {{Authority control Aircraft wing design Wing configurations