Reduction of skin-friction drag
Four main types of drag are found in aerodynamics – see Section 1.5.5 – namely: skin-friction drag, form drag, induced drag, and wave drag. The methods in use for
the reduction of each type of drag are discussed in turn in the sections that follow. A more detailed recent account of drag reduction is given by Gad-el-Hak.
In broad terms skin-friction drag* can be reduced in one of two ways. Either laminar flow can be maintained by postponing laminar-turbulent transition, this is the so-called laminar-flow technology, or ways are found to reduce the surface shear stress generated by the turbulent boundary layer. The maintenance of laminar-flow by prolonging a favourable or constant-pressure region over the wing surface is discussed briefly in Section 7.9. Active laminar-flow control requires the use of boundary-layer suction and this is described in Section 8.5.1. Another laminar-flow technique based on the use of compliant walls (artificial dolphin skin) is described in Section 8.5.2. Riblets are the main technique available for reducing turbulent skin-friction and their use is described in Section 8.5.3.