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.
Design principles 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 Nenadović 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. Tandem wings have also been used on ground-effect vehicles, where the front wing is used to direct air downwards beneath the rear wing to create a lifting air cushion.
Stability, control and trim In a tandem wing the lift forces on the two wings are separated longitudinally, allowing them to act together to achieve stability, control and trim. The mechanisms of stability and control for a tandem wing are similar to those for the tail-first or canard layout; the distinction is mainly in the relative size of the forward surface. However, the larger trim forces available compared to a smaller tailplane or foreplane mean that a tandem design can offer a greater range of trim conditions, and hence of centre of gravity (CG) location than other layouts, which can offer a practical solution where weight loadings and distributions may vary during operations. However a wide CG range leads to other problems, including a compatible undercarriage layout and safe stalling characteristics.
Joined wing The joined wing is a tandem-wing layout in which the front wing sweeps back and/or the rear wing sweeps forwards such that they join at or near the tips to form a continuous surface in a hollow diamond or triangle shape. The joined wing is also an example of a closed wing. The Ligeti Stratos is a rare example to have flown.
Structural design 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. Because it is more compact, the tandem-wing structure is stiffer overall, meaning that less allowance needs to be made for bending, and a smaller safety margin in stress levels is possible, allowing yet further weight and cost reduction.
History
Pioneers
The tandem wing configuration predates successful crewed flight. As far back as the fifteenth century, Tito Livio Burattini experimented with a tandem-wing model. Four sets of wings in tandem variously provided lift and propulsion, and Burraitni's cat became the first aeronaut to fly in a tandem design. Having also flown simpler fore-and-aft tandem models of up to 14 feet (4.3 m) in span, in 1903 Samuel Pierpont Langley built a full-size tandem-wing monoplane, the Aerodrome, and launched it from the roof of a houseboat. It failed to fly. After his death the Smithsonian Institution sought to prove that he had flown in the weeks before the Wright brothers, and employed successful planemaker Glenn Curtiss to secretly modify the aerodrome until it could fly, as "proof" that it had flown in 1903. Curtiss added floats and made other improvements, enabling it to undertake short hops as a true waterplane in 1914. The ruse was eventually exposed, yet the Smithsonian still sought to claim that the 1902 version had been "capable of flight". It would be many more years before they recanted.
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