Tandem wing

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QAC Quickie Q2 in flight

A tandem wing aircraft usually involves two full-sized wings, both of which are full airfoils. Sometimes an aircraft of this configuration can look like a variation on the biplane, but is in fact very different. The forward wing is often technically a canard, fitted with elevators, but both forward and aft wings provide lift. In the case of the QAC Quickie the design has no horizontal stabilizer, as all pitch control comes from the forward wing.

The difference between a tandem wing and a biplane has to do with the horizontal proximity of one wing to the other. In a biplane, the wings are horizontally close, so that the center of lift (CL) of each wing is in close proximity to each other (measurured longitudinally). Because of their proximity, there is little difference between a biplane and a monoplane in the relationship between wing CL and the aircraft's center of gravity (CG). In a tandem wing design, however, the CLs are spread far apart longitudinally. The practical effect is to increase the stability of an aircraft. In simple terms, a monoplane and biplane configuration can be compared to a long board balanced on a single saw horse; if it is not balanced carefully, the board will tip forward or aft. In such an aircraft, the CG envelope (distance between the forward CG limit and aft CG limit) is very small; loading of the aircraft outside that limit will result in the aircraft becoming uncontrollable. On the other hand, a tandem wing aircraft can be compared to the board being supported by two saw horses, one at each end; the result is increased stability, and a more tolerant CG envelope.