Surface Finish and Performance: The Antipathetic Problems of Aerodynamic Refinement and Practical Construction

IN order to appreciate how surface finish affects drag one must be familiar with the main characteristics of boundary layer flow. These have been described in great detail elsewhere, see, for example, Ref. 1, but a brief outline will not be out of place here. At the surface of a body moving through air there is a thin layer of air called the boundary layer in which the velocity relative to the body rapidly falls to zero as the surface of the body is approached. Because of the large velocity gradients across the boundary layer the viscous forces are appreciable there; outside the boundary layer the flow approximates closely to the ideal inviscid flow of classical hydrodynamics. The flow in the boundary layer beginning at the forward stagnation point is usually laminar for some distance, then after a transition region the flow becomes turbulent. The transition region is appreciable in extent at low Reynolds numbers and in turbulent airstreams, but at the Reynolds numbers usual in flight it is short enough to be referred to as a point. Wo now know enough about both laminar and turbulent types of flow over smooth surfaces and the associated frictional forces to be able to calculate with fair accuracy the profile and skin friction drags of a smooth aerofoil given its thickness, Reynolds number and the position of the transition points2. It is found that the skin friction in the laminar boundary layer is much smaller than the skin friction in the turbulent boundary layer; this is illustrated in Fig. 1, which shows the skin friction distribution on one side of a smooth flat plate at a Reynolds number of 107 and with the transition points at 05 c. and 25 c. Fig. 2 shows the variation of drag with the position of the transition point for the flat plate. Similar curves are obtained for aerofoils and bodies of revolution. It is evident that the further back transition occurs the less will be the drag. Fig. 3 illustrates a point that is worth emphasizing, namely, the relative importance of the drag change due to a given transition point movement increases with wing thickness and Reynolds number.