Aerofoil having Shock Control The invention relates to an aerofoil having means to control shock waves occurring on the surfaces of an aircraft wing. It has particular but not exclusive application to aircraft flying at high subsonic speeds, but, more generally, to aircraft with wings or surfaces on which there are locally, high, streamwise, positive pressure gradients (for example, shock waves).

Various methods have been tried to control shock waves on aircraft. US Patent 5433404 describes a variable geometry aerofoil which can be used to eliminate shock waves. However there are considerable practical and economical disadvantages in trying to design a wing having variable geometry.

An alternative known method of shock control uses ventilated surfaces in the region of the shock on the upper surface of aerofoils, which act to encourage pressure equalisation upstream and downstream of where the shock occurs. These ventilation methods are generally of two types: those providing slots which are normal to the direction of the flow, with one placed upstream of the shock and another downstream of the shock; or alternatively by means of a perforated surface with holes drilled normal to the aerofoil surface.

These methods reduce wave drag by inducing compression waves associated with the injection of air into the flow upstream of the shock. However both of these systems are associated with viscous drag penalties. The main reasons for the increased viscous drag appear to be due to an increase in boundary-layer momentum thickness upstream of the shock as a result of normal injection of the air into the boundary layer and the increased skin friction drag just downstream of the shock, due to the high normal suction velocities in this region.

In addition, these methods reduce the pressure rise through the shock which, by itself, would alleviate, delay or prevent shock separation. However, this benefit is more than offset by the increase in boundary-layer momentum thickness upstream of the shock and the increase in skin friction downstream of it, both of which cause the boundary layer to lose energy downstream of the shock.

A possible way of reducing the influence of these effects is to incline the holes so that air issues into the boundary layer in the downstream direction in the region upstream of the shock and is removed from the boundary layer in the downstream direction downstream of the shock.

This ensures that there is both a streamwise component of momentum into the boundary layer, and that skin friction is reduced by lowering the normal component of suction. Such a solution is necessarily complicated and presumes that the shock position is fixed, which in general it will not be so.

It is an object of the invention to provide the benefits of reduced wave drag and shock pressure rise while minimising losses in benefits from increased boundary-layer thickness.

Accordingly the invention provides an aerofoil having one or more streamwise slots located on its upper surface.

In addition to providing a streamwise component of flow at the wing surface, streamwise slots introduce a divergence in the streamlines close to the surface in the boundary-layer flow which reduces the growth of the boundary layer downstream of the shock.

A further advantage of utilising slots is that relatively simple devices such as actuable covers may be used to expose slots as and when required.

The invention will be described by way of example only and with reference to the following figures of which: Figure 1 shows a plan view of a wing having a number of streamwise slots.

Figure 2 shows a cross sectional elevation along the aerofoil section of a preferred embodiment of slot according to the invention.

Figures 3 and 4 show results of aerodynamic tests on slotted and unslotted aerofoils.

Figure 5 a to c shows a further embodiment of the invention wherein the slots are located in the knuckle region of an aileron or flap of a wing.

Figure 1 shows a plan view of an aerofoil 1 having a number of streamwise slots 2 each extending from 50 % to 60% of chord. They are separated by a distance of 6% of chord length and have a width of 0.24% chord.

The position of the slots should generally be such that they straddle where the shock would occur.

Figure 2 shows a cross sectional elevation along the aerofoil section at upper surface 3 at the location of a preferred embodiment of slot according to the invention. The slot 4 has a dovetail shape forming acute angles a with the aerofoil upper surface at the downstream and upstream edges of the slot. These preferred acute angles assist recirculation of airflow from the downstream end of the slot to the upstream end of the slot. The slot includes a plenum 5 Figs 3 and 4 show results obtained from an aerofoil model of figure 1. The figures show data for both the datum aerofoil (without slots), 6, and the aerofoil with slots, 7,. for a Mach number of 0.7 and a Reynolds number of 19 x 106.

Fig 3 shows normal force coefficient CN against trailing edge pressure coefficient CPTE for slotted and unslotted aerofoils and shows that the slotted aerofoil 7 gives a higher maximum lift than for an unslotted aerofoil 6. If one takes the inflexion in the curve of the datum as an indication of maximum useable lift then the slots would provide an increase in maximum lift of at least 10%. This is a considerable improvement over a perforated design, with holes drilled normal to the surface, previously tested.

Fig 4 shows drag coefficient CD against normal force coefficient CN for slotted and unslotted aerofoils and that the slotted aerofoil 7 has a higher drag than the unslotted aerofoil 6 but at higher lift coefficients, above about 0.7, the slotted aerofoil has lower drag. This is approximately the same as for a perforated system tested before, although the reduction in the drag at drag rise is more pronounced for the slotted configuration.

Slotted-wings offer significant benefits for transonic manoeuvre ; if the aircraft were to be limited in design by the manoeuvre requirement, slots offer a significant reduction in wing area which would more than compensate for the increased drag coefficient due to the slots at lower lift coefficients. Alternatively, as noted before, the ability to open and close the slots would ensure that the drag penalty at lower lift is not an issue.

The slot configuration given above is an example only. The skilled artisan would recognise that variations in the length, number and width of the slots enable shock control would be included within the scope of the invention. Additionally actuable covers may be utilised so as to selectively expose the slots when required and to cover the slots in flight conditions when not required as mentioned in the previous paragraph.

Slots as described can also be used for the control of air flow in the region of knuckles of controls such as leading and trailing-edge flaps or ailerons. The flow in such regions is similar to that close to shock waves in that there is a region of pressure rise just downstream of the knuckle which can lead to flow separation. Figure 5 a and b show two side views of the aerofoil section 8 with a plain flap 9 showing the flap undeflected and deflected respectively.

When the flap is deflected slots 10 in the knuckle region 11 are exposed between points A and B. In this region the pressure will rise rapidly and a recirculation will set up between A and B causing significant streamwise blowing upstream of the pressure rise and suction just downstream of it. This leads to separation of the flow from the knuckle region being delayed to higher angles of incidence or lift resulting in higher maximum lift and larger control forces.

Preferably the slot porosity, that is to say the proportion of surface in the slot region which has slots, is between 4 and 10 %.