High-Lift Devices

High-lift devices are small aerofoil-like elements that are fitted at the trailing edge of the wing as a flap and/or at the LE as a slat (Figures 3.20a and b). In typical cruise conditions, the flaps and slats are retracted within the contour of the aerofoil. Flaps and slats can be used independently or in combination. At low speed, they are deflected about a hinge line, rendering the aerofoil more curved as if it had more camber. A typical flow field around the flaps and slats is shown in Figure 3.20. The entrainment effect through the gap between the wing and the flap allows flow to remain attached in order to provide the best possible lift.

Considerable lift enhancement can be obtained by incorporating high-lift devices at the expense of additional drag and weight. Figure 3.21 lists the experi­mental values of the incremental lift coefficients of the Clark Y aerofoil. These val­ues are representative of other types of NACA aerofoils and may be used if actual data are not available.

Higher-performance, high-lift devices are complex in construction and there­fore heavier and more expensive. Selection of the type is based on cost-versus – performance trade-off studies – in practice, past experience is helpful in making selections.