Leading-edge devices

In addition to trailing-edge flaps, a variety of leading-edge devices may be used. The simplest of these is the leading-edge flap, which is used to increase the cam­ber of the leading edge. Leading-edge flaps can take the form of a simple hinged section, as in Fig. 3.13(f), but a more effective arrangement is to slide or fold the flap forward to increase the wing area, as shown in Fig. 3.13(g). Leading – edge flaps may be seen deployed in Fig. 3.15.

We can utilise the principle of boundary layer control by introducing a gap or slot in the leading edge, as in Fig. 3.13(g). Like the slotted flap, the air gap allows a fresh boundary layer to develop behind the slot, which helps to prevent leading-edge separation. The slot may be formed by moving the lead­ing edge forward, in which case, the articulated portion is known as a slat. Leading-edge devices are particularly useful for thin aerofoil sections where leading-edge separation is likely to occur.

Simple unflapped aerofoils normally generate a maximum lift coefficient value of less than 2, but as long ago as 1921, Sir Frederick Handley Page and

G. V. Lachmann managed to achieve lift coefficient values as high as 3.9 using multiple element aerofoils. The patented Handley Page leading-edge slat was a feature of several aircraft built by the Handley Page company.

There are many variations in the mechanisms used for leading-edge devices. In some cases, the slat is held in by spring tension, and extends automatically at high angle of attack by the action of leading-edge suction.