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Wing Geometry

An airfoil or (aerofoil), such as the wing of an aircraft, produces a resultant force acting upwards, i.e., transversely to the direction of flight. The forces involved are dependent on the speed of the aircraft (the dynamic pressure increases proportionally to the square of the speed) and more particularly also on the geometric features of the wing – i.e., its-cross sectional shape and its shape in plan.

The term drag in a general wording clarify a resistant force acting in a direction opposite to the direction of motion and parallel to the relative airstream. The wing of an aircraft flying at less than half the speed of sound encounters, in addition to surface-friction drag and pressure drag, a resistance called induced drag, which is the part of the drag associated with the development of lift and is proportional to the square of the lift coefficient.

This coefficient represents the relative lift of a particular airfoil. The induced resistance can be kept down to a low value by providing a large ratio of wingspan to mean chord – i.e., the ratio of the length of the wing to its average width (Fig.1). This ratio is especially important in aircraft whose wings are required to have a high lift coefficient and therefore develop a large lift e.g., gliders.

The sweep denotes the slant of the wing in relation to a line perpendicular to the longitudinal axis of the aircraft. It influences the behavior of the flow conditions in the so-called boundary layer immediately adjacent to the wing surface. In the case of a swept-wing aircraft with sweepback of the wing (Fig.2), the streamline flow first separates from the surface of the wing in the region of the wing tips when the angle of attack is very large.

Since the ailerons are located in that part of the wing they are liable to become ineffective under such conditions, so that any minor disturbance may cause stalling and uncontrolled rolling motion. On the other hand, with a forward sweep of the wing (Fig.3), separation of the flow starts nearer the wing root, so that the ailerons continue to be effective and the aircraft remains under control.

In the case of an aircraft flying at more than half the speed of sound supersonic speeds will occur in certain parts of the streamline flow around the wing, which are associated with shock waves that give rise to a considerable increase in drag. If the wing is given a sweep, the critical speed at which this objectionable effect occurs is shifted to a higher value. For this reason all aircraft designed to operate in the speed range between mach 0.5 (half the speed of sound) and mach 1.0 (the speed of sound) have swept wings.

For supersonic speeds, i.e., exceeding mach 1.0, the optimum wing shape in plan is different from the optimum shape for subsonic speeds. The ratio of span to mean chord is now of less importance; on other hand, a more pronounced sweep is desirable. These considerations led to the development of the delta-wing aircraft (Fig.4). Such aircraft develop a large lift only at high angles of attack as compared with straight-wing aircraft. This is especially important in connection with takeoff and landing. Landing speeds are very high.

At supersonic speeds above mach 1.5 the nonswept short-span wing (Fig.5) is, in terms of drag, more favorable than any other shape. However, though important, this is not the only consideration that governs the geometry of aircraft wings. In modern military aircraft the variable-geometry wing has been introduced in order to obtain relatively favorable conditions for takeoff and landing (long wingspan, position ‘a’ in Fig.6) and for super-sonic flight (position b). The changeover is affected by swinging the wings back when the aircraft is in flight (swing-wing aircraft).