Laminar flow aerofoils

The attainment of really high speeds, speeds approaching and exceeding that at which sound travels in air, has caused new problems in the design and in the flying of aeroplanes. Not the least of these problems is the shape of the aero­foil section.

Speed is a comparative quantity and the term ‘high speed’ is often used rather vaguely; in fact, the problem changes considerably at the various stages of high speed. In general, we may say that we have so far been considering aerofoil sections that are suitable for speeds up to 400 or 500 km/h (say 220 to 270 knots) – and we must remember that although these speeds have now been far exceeded they can hardly be considered as dawdling. Furthermore all aeroplanes, however fast they may fly, must pass through this important region. At the other end of the scale are speeds near and above the so-called ‘sound barrier’, shall we say from 800 km/h (430 knots) up to – well, what you will! The problems of such speeds will be dealt with in later chapters. Notice that there is a gap, from about 500 to 800 km/h (say 270 to 430 knots), and this gap has certain problems of its own; among other things, it is in this region that the so-called laminar flow aerofoil sections have proved of most value.

The significance of the boundary layer was explained in Chapter 2. Research on the subject led to the introduction of the laminar flow or low drag aerofoil, so designed as to maintain laminar flow over as much of the surface as possible. By painting the wings with special chemicals the effect of turbu­lent flow in the boundary layer can be detected and so the transition point, where the flow changes from laminar to turbulent, can actually be found both on models and in full-scale flight. Experiments on these lines have led to the conclusion that the transition point commonly occurs where the airflow over the surface begins to slow down, in other words at or slightly behind the point of maximum suction. So long as the velocity of airflow over the surface is increasing the flow in the boundary layer remains laminar, so it is necessary to maintain the increase over as much of the surface as possible. The aerofoil that was evolved as a result of these researches (Fig. 3.22) is thin, the leading edge is more pointed than in the older conventional shape, the section is nearly sym­metrical and, most important of all, the point of maximum camber (of the centre line) is much farther back than usual, sometimes as much as 50 per cent of the chord back.

The pressure distribution over these aerofoils is more even, and the airflow is speeded up very gradually from the leading edge to the point of maximum camber.

Fig 3.22 Laminar flow aerofoil section

There are, of course, snags – and quite a lot of them. It is one thing to design an aerofoil section that has the desirable characteristics at a small angle of attack, but what happens when the angle of attack is increased? As one would expect, the transition point moves rapidly forward! It has been found possible, however, to design some sections in which the low drag is maintained over a reasonable range of angles. Other difficulties are that the behaviour of these aerofoils near the stall is inferior to the conventional aerofoil and the value of CLmax is low, so stalling speeds are high. Also, the thin wing is contrary to one of the characteristics we sought in the ideal aerofoil.

But by far the most serious problem has been that wings of this shape are very sensitive to slight changes of contour such as are within the tolerances usually allowed in manufacture. The slightest waviness of the surface, or even dust, or flies, or raindrops that may alight on the surface, especially near the leading edge and, worst of all, the formation of ice – any one of these may be sufficient to cause the transition point to move right up to the position where the irregularity first occurs, thus causing all the boundary layer to become tur­bulent and the drag due to skin friction to be even greater than on the conventional aerofoil. This is a very serious matter, and led to the tightening up of manufacturing and maintenance tolerances.

With swept wings, the flow along the leading edge towards the tip usually causes transition to occur very near the leading edge and nullifying the effect of any laminar flow section. Because of scale effect, this may not happen on a wind tunnel model, making testing all the more difficult.

Another and more drastic method of controlling the boundary layer is to provide a source of suction, with the object of ‘sucking the boundary layer away’ before it goes turbulent.

This has the advantage that a much thicker wing section can be used (Fig. 3.23). The practical difficulty is in the power and weight involved in providing a suitable source of suction. Taminar boundary layers separate from the surface more easily than turbulent layers and suction may also be applied just before the point of separation to prevent this happening. Both suction and blowing (Fig. 3.24, overleaf) may also be used to prevent the separation of the turbulent boundary layer on an ordinary aerofoil.

Fig 3.24 Control of boundary layer by pressure (schematic drawing)