Low-drag wing sections

We have already explained how surface friction arises from the shearing action in the boundary layer. Because a laminar layer produces less drag on a given area than a turbulent one of the same thickness, there is an advantage in main­taining a laminar boundary layer over as much of the surface of the aircraft as possible.

Low-drag wing sections

Early wing sections similar to that shown in Fig. 4.5(a) were derived by adding camber to the streamlined fairing shape, and were intended to minimise the boundary layer normal pressure (form) drag. The position of minimum pressure on the upper surface is usually near to the point of maximum thick­ness, which on these early shapes is about 1/4 to 1/3 of a chord back from the

6 – six series aerofoil

5 – favourable pressure gradient to 5/10 chord

1 – low drag for a CL range of +/- 1/10

2 – designed for low-drag operation at CL = 2/10 12 – thickness-to-chord ratio 12 per cent

leading edge. A laminar boundary layer would normally extend up to this point, but beyond it, the adverse pressure gradient (air flowing from a low pressure to a higher one) would provoke transition to turbulence. It was later realised that by moving the position of maximum thickness aft, it would be possible to maintain a favourable pressure gradient, and hence a low-drag laminar boundary layer, over a much larger proportion of the wing surface.

By the 1930s, advances in theoretical methods, using a technique known as conformal transformation, made it possible to design aerofoil sections for which the form of the velocity or pressure distribution could be specified. Several low-drag so-called ‘laminar’ sections were designed, the most well known being the NACA 6-series; an example of which is shown in Fig. 4.5(b). Sections from this family of shapes came into use during the Second World War, and the adoption of a 6-series aerofoil on the P-51 Mustang is probably one reason why that aircraft had such an excellent performance.

As we have shown in the previous chapter, however, transition from lam­inar to turbulent boundary layer flow also depends on the Reynolds number and the roughness. A favourable pressure gradient alone is not sufficient to ensure a laminar boundary layer. To help maintain laminar flow over the front portion of the aerofoil, the wing needs to be manufactured to a precise profile, with a high standard of surface finish. This led to a move away from the tradi­tional riveted form of construction, to the adoption of different methods, as outlined in Chapter 14.

Despite the care taken in manufacture, it is often difficult to maintain a good surface finish in normal operational conditions. A swarm of insects squashed on to the wing can significantly affect the range and cruising efficiency of an aircraft. Small dents must also be detected and filled.