Patent US 5,433, 404 the describes a method of distorting the wing in the region of a shock wave to reduce drag. The underlying principle of this concept is to change the shape of the wing, either with a bump or a ramp, placed so that it produces compression waves upstream of the shock. These compression waves weaken the shock wave in the flow-field, thus reducing wave drag. Some limited benefit was also found with these devices in terms of improved buffet performance but their main purpose was to reduce drag. However the problem with these is that they cause greater drag at the cruise and lift conditions.

It is an object of the invention to provide an aerofoil having improved resistance to shock buffeting due to shock-induced separation whilst minimising any increase in drag.

It is an object of the invention to improve the buffet performance without causing increased drag at the cruise lift coefficient and Mach number.

The invention consists of an aerofoil incorporating a bump profile on the upper surface thereof such that the bump forms causes a discontinuous change in slope of the upper surface and wherein the leading edge of the bump member is located within plus or minus 2% of chord length of the point of maximum curvature of the upper surface.

The bump member is located such that the main part of the shock is positioned just upstream of the shape change and thus the shape change is applied just downstream of the shock of the un- modified aerofoil rather than either side of the shock as disclosed in the prior art.

The bump may be a fixed feature of the design and is positioned to provide improved buffet performance at Mach numbers at and above the cruise Mach number. Alternatively the idea might be applied as a retro-fit to enhance buffet performance and reduce the drag of an aircraft which requires a higher lift coefficient than that originally specified (as might happen if the weight of the aircraft increased to carry a greater load than originally projected).

By way of example, the invention will now be described with reference to the drawings of which: Figure 1 shows a schematic figure of a bump which forms part of the upper surface of an aerofoil.

Figure 2a and b show figure of a straight edged bump and curved bump respectively when incorporated on the upper surface of an aerofoil.

Figure 3 shows the effect of the bump on the lift of the aerofoil.

Figure 4 shows the response of the pressure measured just upstream of the trailing edge to changes in lift coefficient.

Figure 5 shows the influence of the bump on the drag of the aerofoil Figure 1 shows a preferred embodiment of a bump member 1 which is attached or attachable to the upper surface of the aerofoil. The bump member is positioned on the aerofoil upper surface such that it extends from 55% chord to 70% chord. The bump member has three substantially straight sections 2,3, and 4. The bump member is positioned such that the first section 2 extends from 55% to 60% chord, the second section 3 of substantially zero gradient which extends from 60 to 65% chord, and a third section 4 of downwardly inclined slope extending to 70% chord. The slope of angle of the bump member at its leading edge 0 is 4. 4°.

The location of the bump member leading edge is very important if it is to reduce buffeting and should be positioned on the upper surface of the aerofoil just downstream of where shock occurs.

This is usually within 1 % around the point of maximum curvature, usually downstream of the leading edge at about 10% chord.

The invention however may comprise of various types of bump members. Such embodiments include curved bump members or bumps which may even comprise an arc of a circle. The main consideration is that there is discontinuous change in slope on the upper surface of the aerofoil provided by the bump member at its leading edge Figures 2a and b show a straight edged bump 5 (as described above) and curved bump 6 respectively when incorporated on the upper surface of an aerofoil 7.

Additionally the invention extends to aerofoils having integral bump members as well as to bump members which may be retro-fitted. The bump member may also be selectable as disclosed in the US patent 5433404.

The length of the bump member can vary from 15 to 25 %. The maximum amplitude of the bump member should be limited to between 0.05 tan 3° to 0.05 tan 6° of wing chord length.

The effect of the ramp at the upstream face is to incline the shock with the result that the strength of the shock (measured by the pressure rise through it) is reduced. This reduction in strength is proportional to the angle 6.

The further consequence is that the separation of the boundary layer at the foot of the shock is delayed to higher angles of incidence, which delays shock-induced separation and buffet.

Figure 3 shows normal force coefficient CN (a measure of lift coefficient) against angle of incidence a at a Mach number of 0.69 and a Reynolds number of 19 x 106, for an aerofoil with (8) and without (9) the bump respectively. The bump allows an increase in maximum lift of about 2%.

Figure 4 shows the trailing-edge pressure coefficient CPTE against normal force coefficient CN on an aerofoil with (8) and without (9) the bump respectively and indicates that the bump increases the lift coefficient; which there is a rapid decrease in trailing edge pressure equivalent to about 3%.

Figure 5 shows drag coefficient CD against normal force coefficient CN on an aerofoil with (8) and without (9) the bump respectively the influence of the bump on the drag of the aerofoil. It indicates that the shape change reduces the drag by a significant amount for normal force coefficients at the cruise value (0.55 approximately) and above. At lower lift coefficients the bump increases the drag; however since the aerofoil would not normally operate at such low lift this is not a significant disadvantage.