control surface (elevator) mounted elsewhere - usually on the horizontal stabilizer. Conventional: "tailplane" surface at the rear of the aircraft, forming part of the tail or empennage. It did not become the convention for some years after the Wrights, with the Blériot VII of 1907 being the first successful example. Canard: "foreplane" surface at the front of the aircraft. Common in the pioneer years, but from the outbreak of World War I no production model appeared until the Saab Viggen in 1967. Tandem: two or more main wings, one behind the other. Both provide significant lift. An example is the Rutan Quickie. To provide longitudinal stability, the wings must differ in aerodynamic characteristics: typically the angle of incidence and/or the aerofoils chosen differ between the two wings. The wings on each side may meet at their tips to form a joined wing (see above). Three surface: both conventional tail and canard auxiliary surfaces. Modern examples include the Sukhoi Su-33, while pioneer examples include the Voisin-Farman I. Outboard tail: split in two, with each half mounted on a short boom just behind and outboard of a wing tip. It comprises outboard horizontal stabilisers (OHS) and may or may not include additional boom-mounted vertical stabilisers (fins). In this position, the tail surfaces interact constructively with the wingtip vortices to significantly reduce drag. Used for the Scaled Composites SpaceShipOne. Tailless: no separate surface, at front or rear. The lifting and stabilizing surfaces may be combined in a single plane, as on the Short SB.4 Sherpa whose whole wing tip sections acted as elevons. Alternatively the aerofoil profile may be modified to provide inherent stability, as on the Dunne D.5. Aircraft having a tailplane but no vertical tail fin have also been described as "tailless". Dihedral and anhedral Angling the wings up or down spanwise from root to tip can help to resolve various design issues, such as stability and control in flight. Dihedral: the tips are higher than the root as on the Santos-Dumont 14-bis, giving a shallow 'V' shape when seen from the front. Adds lateral stability. Anhedral or catahedral: the tips are lower than the root, as on the first Wright Flyer; the opposite of dihedral. Used to reduce stability where some other feature results in too much stability. Some biplanes have different degrees of dihedral/anhedral on different wings. The Sopwith Camel had a flat upper wing and dihedral on the