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Stalling
For most wing sections, the amount of lift generated is directly proportional to the angle of attack, for small angles; the graph of CL against angle of attack is a straight line, as shown in Fig. 1.17. However, as illustrated, a point is reached where the lift starts to fall off. This effect is known as stalling. The fall-off may occur quite sharply, as in Fig. 1.17 which shows the variation of lift coefficient with angle of attack for a wing with a moderately thick aerofoil section (15 per
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Reversed flow Fig. 1.18 Flow separation and stalling At large angles of attack, the flow fails to follow the contours of the section, and separates leaving a highly turbulent wake. When this happens there is a loss of lift and an increase in drag |
cent thickness to chord ratio). You can see that the stall occurs at an angle of attack of around 12°. A thin uncambered wing may stall even more sharply, and at an angle of attack of 10° or less. A sudden loss in lift can obviously have disastrous consequences, particularly if it happens without warning.
The stalling characteristics of an aircraft wing depend not only on the aerofoil section shape, but also on the wing geometry, since not all of the wing will stall at the same angle of attack.
Stalling occurs when the air flow fails to follow the contours of the aerofoil and becomes separated, as illustrated in Figs 1.18 and 1.19. The causes of this flow separation are dealt with in detail in Chapter 3.
Once the flow separates, the leading-edge suction and associated tangential force component are almost completely lost. Therefore, the resultant force due to pressure does act more or less at right angles to the surface, so there is a significant rearward drag component. The onset of stall is thus accompanied by an increase in drag. Unless the thrust is increased to compensate, the aircraft will slow down, further reducing the lifting ability of the wing.
After the stall has occurred, it may be necessary to reduce the angle of attack to well below the original stalling angle, before the lift is fully restored. An aircraft may lose a considerable amount of height in the process of recovering from a stall, and trying to prevent its unscheduled occurrence is a major concern of both pilots and aircraft designers. Later on, we shall describe some of the preventive measures and warning systems that may be employed.
