The buffet boundary

In designing a wing it is necessary to consider not only one specific design point in the cruise, but off design conditions as well. We have already discussed this with reference to low speed requirements at landing or take-off, and the air­craft must also be able to accelerate safely through the entire speed range to the cruising condition. The idea of an aircraft having a single ‘design point’ is in itself somewhat misleading. A typical transport aircraft will have to operate over a number of different routes and hence ranges. It will also be required to carry a variety of different payloads. A military aircraft (Fig. 9.8) may, for example, also be required to carry a variety of underwing ‘stores’ such as mis­siles, bombs or fuel tanks for extended range. It must be able to perform these

tasks safely and inadvertent excursions from the cruise condition, such as gusts or a reasonable degree of pilot error, must not put it in danger.

One of the problems with transonic aircraft is that the speed margin for safe operation is fairly small. For example the difference between the stalling speed and cruising speed for a large airliner (Fig. 9.9) may be as little as 60 m/s. This may seem surprisingly small, but it must be remembered that we are talking about the aircraft in the ‘clean’, flaps up, condition. Because the aircraft usu­ally cruises at a considerable altitude the density is low, which means that the stalling speed will be increased (Chapter 2). Furthermore, because the temper­ature of the air is also low, the speed of sound will be reduced, and the speed at which the cruise Mach number occurs will be lower than we might expect (Chapter 5).

If we inadvertently impose an extra load on the wing, say due to a gust, or allow the Mach number to rise, we encounter another factor which limits the speed range in the upwards direction, not because of lack of power and excess­ive drag, but because of a potentially dangerous ‘buffeting’ effect.

We have seen that one of the design features of a supercritical aerofoil is that the supersonic flow over the top surface is recompressed by, at worst, a relat­ively weak shock wave. Provided that this shock wave is not too far back on the section, it will not produce any extensive separation of the boundary layer, although a small separation bubble may form. If the intersection point is nearer

Air speed

Fig. 9.11 Stall and buffet boundaries

As the aircraft flies higher its speed range gets smaller as it is squeezed between the two boundaries

the trailing edge, however, extensive separation may occur (Fig. 9.10). This can lead to unsteady flow in which the shock wave moves rapidly backwards and forwards over the section – clearly not a desirable state of affairs. The loading on the section then fluctuates significantly and buffet is said to occur.

Thus, as well as the need for good characteristics at the design point, it is necessary to ensure that there is adequate margin before buffet occurs, and that its onset will not be sudden and catastrophic. Usually the presence of a weak shock wave at a point in front of, or at least not too far downstream of the maximum thickness point on the aerofoil, is helpful in giving reasonably good buffet behaviour.

An idea of the restricted operating range of a typical transonic aerofoil is given by Fig. 9.11 where both stall and buffet boundaries are shown.

Buffet behaviour can be improved by devices other than section design. One way of doing this on the three-dimensional wing is to introduce a series of

bodies starting near the point of maximum thickness and extending beyond the trailing edge (Fig. 9.12). These are colloquially known as Kuchemann carrots or Whitcomb bumps after the two people who first, independently, suggested their use. The local flow fields produced by these bodies break up the shock wave when it moves towards the trailing edge, thus improving the buffet behaviour.