Camber

How can we alter the shape of the aerofoil section in an attempt to obtain better results? The main changes that we can make are in the curvature, or camber, of the centre line, i. e. the line equidistant from the upper and lower surfaces, and in the position of the maximum camber along the chord.

In symmetrical sections, some of which have been very successful, there is of course no camber of the centre line; other sections have centre line cambers of up to 4 per cent or more of the chord.

Generally speaking we get good all-round characteristics and a smooth stall when the maximum camber is situated about 40 per cent of the chord back. Aerofoils with the maximum camber well forward, say at 15 per cent to 20 per cent of the chord, may have low drag but are apt to have poor stall charac­teristics – a rather sudden breakaway of the airflow.

The other main features that can be varied are the maximum thickness, the variation of thickness along the chord and the position of maximum thickness – not necessarily the same as that of maximum camber. There is considerable variation of maximum thickness (Fig. 3.21) even in commonly used aerofoils, from very thin sections with about 6 per cent of the chord to thick sections of 18 per cent or more. Reasonably thick sections are best at low speed, and for pure weight carrying, thin ones for high speed. Remember that it is the thickness compared with the chord that matters, thus the Concorde with its large chord of nearly 30 metres achieved a remarkable thickness/chord ratio of 3 per cent.

The greater the camber of the centre line the more convex will be the upper surface, while the lower surface may be only slightly convex, flat or even slightly concave (though this is rare in modern types). Sometimes there is a

reflex curve of the centre line towards the trailing edge (Fig. 3.12); this tends to reduce the movement of the centre of pressure and makes for stability.