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A tandem wing is a wing configuration in which a flying craft or animal has two or more sets of wings set one behind another. All the wings contribute to lift. The tandem wing is distinct from the biplane in which the wings are stacked one above another, or from the canard or "tail first" configuration where the forward surface is much smaller and does not contribute significantly to the overall lift. In aviation, tandem wings have long been experimented with, but few designs have ever been put into production. Tandem wings in nature occur only in insects and flying fish, although in the past there have been tandem wing flying reptiles.

A tandem wing configuration has two main wing planes, with one located forward and the other to the rear. The difference is greater than the wing chord, so there is a clear gap between them and the aircraft centre of gravity (CG) lies between the wings. Compared to the conventional layout, where the tailplane exerts little or no vertical force in cruising flight, both tandem wings contribute substantially to lift. The basic tandem configuration uses wings which are equal in size and in line with each other. Examples have flown successfully, such as the Peyret glider of 1922. However the rear wing is usually placed either above or below the fore wing, in order to avoid its turbulent wake. One wing is often made a little smaller than the other, according to the details of the design. Indeed, there are no clear dividing lines between the conventional vs. tandem, or the tandem vs. canard configurations. The high mounted fore wing and low mounted aft wing arrangement is also sometimes treated as an extreme staggered biplane and referred to as the Nénandovich biplane. Interference effects between the two wings can make a tandem layout less efficient in cruise than the equivalent conventional design, however examples such as the Scaled Composites Proteus are capable of exceptional efficiency. The tandem layout creates a "slot effect" in which the front wing deflects air downwards over the rear wing, reducing the angle of attack (AoA) of the rear. At high aircraft AoA, this causes the front wing to stall first, allowing safer flight at low speeds than the equivalent conventional layout. It also offers good STOL performance. The joined wing is also an example of a closed wing. The Ligeti Stratos is a rare example to have flown.

In a conventional layout, the moment arm of the outer section's lifting load is large, and this stresses the root section. However, in a tandem design each wing is smaller and the outer load is absent. This allows the wing structure to be lighter overall. In a conventional design, the fuselage is supported only in one place, with the fore and aft fuselage sections cantilevered out from it. This creates significant bending stresses. A tandem wing supports the same fuselage in two places, reducing the bending stresses. However the torsion stresses on the centre section between the wings are greater. Several pioneers had long made successful gliders. In 1905 John J. Montgomery flew a tandem monoplane glider, confirming that the aerodynamic principle was sound. Powered flight followed two years later. In 1906 Louis Blériot built his third aeroplane with tandem elliptical closed wings, later modifying it as his type IV with the fore wing converted into a conventional biplane. But it was not until the next year that his type VI, a wheeled tandem monoplane of broadly similar configuration to Langley's Aerodrome, became the first tandem wing aeroplane to fly. Between 1907 and 1911, the aerodynamics studies of Gustave Eiffel showed that the tandem layout was inherently less aerodynamically efficient that the more conventional. Overlapping with Eiffel's work, Stefan Driezewicki developed and wind tunnel tested an inherently stable tandem wing design. He then built and successfully flew a full sized example at the end of 1912. The tandem Lysander was not completed until 1941, when Penrose began test flights. Although it performed flawlessly – he wrote that "here was a military prototype that needed no alteration" – it was not ordered into production. George Miles saw the tandem Lysander at RAF Boscombe Down and realised its potential as a short span, short take off Naval fighter. The ensuing Miles M.35 Libellula test aircraft differed from the Delanne design in having wings of approximately equal span, but with the rear wing given a longer chord and swept back. Although the design was rejected, it flew well enough to prompt development of the larger M.39B, a subscale test aircraft for the proposed M.39 high speed bomber to meet Specification B.11/41. This time the fore wing was smaller and mounted low, while the swept rear wing was high mounted with twin engine nacelles slung beneath. Flying in 1943 it performed well, but the bomber requirement was subsequently cancelled.

Several orders of flying insects employ tandem wings, each with its characteristic anatomy and flight modes. Insects with tandem flapping wings include the Odonata (dragonflies and damselflies), Lepidoptera (butterflies and moths) and some Thysanoptera or Thrips. Odonata species typically have long, thin wings and can synchronise the flapping of fore and aft pairs in various different modes, allowing them to be both fast and highly manoeuvrable. By comparison the Lepidoptera have wider wings which are flapped in synchrony and may even overlap in flight, and are better suited to endurance flying. Thrips are smaller insects and the flying species have relatively stiff wings. Due to their small size, they generate lift via clap and fling flapping rather than the usual leading edge vortex generation of most insects. Many flying beetles, such as the ladybird, have forward wing cases which open out in flight but do not flap significantly. While on the ground they protect the delicate main, hind wings, while in the air they may be used to modify the aerodynamics of the flapping main wing.

Flying fish have enlarged pectoral fins and are capable of gliding flight, though not of true flapping flight. Some species, such as the band wing, also have sufficiently enlarged pelvic fins, further back along their bodies, to form a tandem layout.

Microraptor was a genus of tandem winged dinosaurs, possibly only a single species. It is known only from the fossil record, principally in China. Both fore and hind limbs were covered in flight feathers and it is thought to have been capable of true flapping flight as well as gliding. Its flight mode is not known.