EXTERNAL FORM OF AEROPLANES or AIRPLANES.
is a striking lack of variation in the appearance as seen in plan of all modern aeroplanes. The main wings have a large span (fig. 7 a) in relation to their chord, the average ratio of span to chord being about seven. This is a direct consequence of the fact that the resistance due to the generation of lift varies inversely as the square of the span (§2). In some aeroplanes the chord diminishes towards the tip of each wing (fig. 7 b) and oc casionally the wings, although of uniform chord, are "swept back" (fig. 7 c). Neither of these modifications affects the aerodynamic efficiency appreciably.
Near the tip of each wing is an aileron or wing flap hinged about axes I-1 (fig. 7 a) and interconnected so that an upward movement of one is accompanied by a downward movement of the other. They are operated by a sideways movement of the pilot's control column, and are the means by which the wings are kept level in straight flight, or banked for a turn, as may be required. (See also AILERON.) Tail Plane.—At a distance of some three chords behind the leading edge of the wings is the tail plane, in area about one-ninth that of the wings. In steady flight its function is to balance or trim the aeroplane, i.e., to ensure that the resultant of all the air forces on the aeroplane passes through the centre of gravity. (See AERODYNAMICS.) It is generally in two parts. The front part, some 6o% of the whole, is normally fixed, though means may be provided by which the pilot can alter its angle of incidence by rotating it about an axis such as 2-2 (fig. 7 a), thus enabling the aeroplane to be trimmed for flying at various speeds. This opera tion is analogous to trimming the sails of a ship so that it carries no weather or lee helm. In a modern aeroplane a movable tail plane is generally considered indispensable, unless trimming planes such as Flettner controls (described later) are used in which case the stabilizer need not be adjustable.
The rear part of the tail, or elevator, is hinged on an axis such as 3-3 (fig. 7 a), and connected to the control column in such a way that a forward movement of the pilot's hand depresses the trailing edge of the elevator and causes the machine to dive, and vice versa. The elevator is the primary control organ of an aero plane, since by it the pilot alters the angle of incidence of the wings and hence in conjunction with the engine power controls the rate of climb or descent and the speed. (Fig. 9 of article AERO NAUTICS shows control organs, and the forms which they take in more modern machines will be seen in the articles, AERIAL NAVI