Starting Platform. A runway to enable an aeroplane to leave the ground. Skin Friction. Resistance offered by planes or wings. Slip. The difference between the distance actually travelled by a propeller and that measured by the pitch. Soaring Flight. A gliding movement without apparent effort. Surface. The extent of planes measured on one side only. Sustaining Surface. Extent of wings or planes which lend support to an aeroplane. T Tail. The plane or planes, both horizontal and vertical, carried behind the main planes. Tandem. An arrangement of two planes one behind the other. Thrust. The power exerted by the propeller of an aeroplane. Tension. The power exerted by twisted strands of rubber in unwinding. Tractor. A propeller placed before the main plane. Triplane. An aeroplane with three main planes one above another. U Up Wind. Against the wind. W Wake. The churned or disturbed air in the track of a moving aeroplane. Wash. The movement of the air radiating from the sides of an aeroplane in flight. Wings. Planes or supporting surfaces, commonly a pair of planes extending out from the central body.