Mechanics of Fluids

338 Boundary layers, wakes and other shear layers
Fig. 8.19 Polar diagram for
aerofoil.
Fig. 8.20 Flap increasing
lift coefficient.
Under normal conditions the lift L for high-speed aircraft may be obtained
with only moderate values of CL since L = 1
2ϱu2∞ ACL where A = plan area
and u∞ is large. To achieve low landing and take-off speeds, however, a
means of temporarily increasing CL is required, even if CD is also increased
thereby. For this purpose flaps at the trailing edge are employed (Fig. 8.20).
With the flap lowered, the effective curvature of the wing, and hence the lift,
is increased. Many different designs of flap have been proposed. To further
augment the lifting capability of a wing, slats located at the leading edge are
also sometimes used.
8.9 BOUNDARY LAYER CONTROL
As we have seen, the drag on a body depends greatly on whether the boundary
layer is laminar or turbulent, and on the position at which separation
occurs. The reduction of drag is of the greatest importance in aircraft design.
Much effort has been devoted both to reducing skin friction by delaying the
transition from laminar to turbulent flow in the boundary layer, and also
to delaying separation. Much may be done, by careful shaping of the body,
to avoid small radii of curvature, particularly at its downstream end, but