BIPLANE, a form of aero plane (q.v.) in which the support ing surface is arranged in two planes one above the other. This is the most widely adopted ar rangement, its only important competitor being the monoplane (q.v.). The chief advantages of a biplane are compactness and light weight of structure. Almost invariably the planes are braced together by struts and wires. The effective depth of the girder thus formed is the vertical distance between the planes, which is much greater than the depth of the wing itself. This enables the structure to be made light and stiff.
The resistance of the inter-plane bracing is appreciable, but for a given area and span of wing the biplane uses less power than a monoplane in developing a given lift, and on the balance there is little to choose between them on this account. The biplane is inevitably rather more complex and maintenance costs tend to be correspondingly greater, but design is steadily tending to the elimination of details requiring adjustment, and the biplane will probably remain the predominating type. A typical biplane is shown in the figure and further examples will be found in the articles referred to above.