GB2072603A - Aircraft wings - Google Patents

Abstract

An aerofoil body, for example a "delta" wing aircraft, in which strip- form devices (16,17) project rearwards from the trailing edge (4) of the wing and serve to improve the lift-to-drag ratio of the body, in some conditions of flight at least, by modifying the formation of the trailing edge vortex to allow the axes of the primary vortex sheets to lie closer to the fore-and-aft centreline of the body than would otherwise be the case. When the strips are basically flat in shape, the strips may project straight backwards from the trailing edge or may be angled (Figures 6,7, not shown) in various ways. The use of twisted strips (26) is also described, (Figure 8, not shown). <IMAGE>

Description

SPECIFICATION Improvements in or relating to aerofoil bodies This invention relates to aerofoil bodies. It relates in particular to aircraft wings of the kind presenting leading edges that are strongly swept relative to the direction of motion of the craft through the surrounding air. An extreme and relatively familiar version of such a wing is the so-called "delta" wing, the plan shape of which approximates to an isosceles triangle. Other known versions of such a wing have curved leading edges, and the wing tips need not be pointed.

In the accompanying drawings Figure 1 is a diagrammatic perspective view of such a wing, and Figure 2 illustrates the spanwise lift distribution of such a wing diagrammatically. In Figure 1 a "delta" wing 1 presents leading edges 2, 3 and a trailing edge 4. Reference 5 indicates the fore-and-aft centreline of the wing in flight, and arrow 6 the "free stream" direction of the surrounding fluid relative to the wing in use. It is well known that when such a wing is placed at a positive incidence to an airflow it will develop a pattern of flows in which vortex sheets, separating from close to the swept leading edges, will roll up to form two strong coiled vortex sheets approximately conical in form. Such sheets are indicated by references 7 and 8.Secondary vortex sheets, as indicated by references 9 and 10, may also form between the main sheets and the leading edges; however the size of the secondary sheets is very much smaller and consequently their effects are small also. The axes of vortex sheets 7 and 8 are indicated at 11 and 12, and it will be seen that they lie above wing 1 and inboard of leading edges 2 and 3: regions of low pressure are induced against the top surface of the wing underneath these axes. At all but very low incidences the lift distribution across the span of such a wing is as illustrated in Figure 2 and it is well known that as incidence increases, the peak lift values 13 occurring immediately beneath axes 11, 12 increase more rapidly than the lift value 14 which occurs over wing centreline 5.

Hpwever, relative motion between the wing and the surrounding fluid causes vortex sheets to be shed not only above the surface of the wing itself, but also downstream of the trailing edge of the wing.

The present invention arises from appreciating that the trailing edge vortex sheet can be modified with good effect. Figure 3 of the accompanying drawings illustrates such a vortex sheet formation schematically. As a result generally of the spanwise lift distribution illustrated in Figure 2, and particularly of the fact that maximum lift does not coincide with centreline 5, the sign of the vorticity of the vortex sheet 7 once shed downstream from trailing edge 4 varies across the span of the wing. Figure 3 illustrates the condition of the part 15 of the sheet just inboard of the line of maximum lift created by the axis 11 of the sheet and it will be seen that the sign of the vortex at this part of the sheet is opposite to the sign of the more forward and outward part 15a.At moderate and greater incidences this difference in sign has the effect of keeping axis 11 of vortex sheet 7 closer to leading edge 7, and thus further from centreline 5, than would be the case if the strength of part 15 of the sheet were less. This is disadvantageous: ideally maximum lift should occur at the centre.

The present invention proposes a simple modification to the wing which may delay and modify the formation of the trailing edge vortex, and thus allow the axes of the primary vortex sheets to lie closer to the centreline of the body. According to the invention an aerofoil body adapted in use to move relative to a surrounding mass of fluid, and to present a trailing edge and at least one swept leading edge relative to that motion, carries at least one strip-form device which projects rearwardly from the trailing edge so as to modify the vortex sheets generated by the interaction of the edges with the fluid and so to improve the lift-to-drag ratio of the body.The dominant action of the strips may be to modify the generation of trailing edge vortex sheets, and so bring about the result that the region of maximum lift generated by the leading edge vortices is closer to the centreline of the body than would otherwise be the case. This leads to less drag, and more generated lift, and hence a higher lift-to-drag ratio.

The body may be of triangular or "delta" shape in plan, and two strips may project from the trailing edge: the length dimension of each strip may lie parallel to the fore-and-aft centreline of the body and two strips may project from the trailing edge on opposite sides of the centreline. For a body of such plan shape each strip may project from the edge at a position substantially 0.6 of the distance between the centreline and the adjacent tip of the trailing edge. The projecting fore-and-aft length of such strips may be of the order of 35% of the trailing edge span, and the spanwise dimension of such strips may be of the order of 4% of the trailing edge span.

Such strips may lie substantially in the plane of the mean chord of the aerofoil body, or may alternatively project from the trailing edge of the body at an angle, either upward or preferably downward, to that chord.

Such strips may be flat but may also be twisted: for instance at the root of the strip, where it is attached to the surface of the body, the surfaces of the strip and the body may be parallel to each other, but the strip may be twisted along its length so that at its free end its surface lies in a perpendicular direction. The hand of the twist may be such that in proceeding from the root to the free end the inboard edge of the strip rises relative to the plane of the mean chord of the body.

In an alternative construction according to the invention the strip may be flat but the plane of its surface lies perpendicular to the plane of the mean chord of the body throughout, and in this alternative version the plane of the surface of the strip preferably lies parallel to the adjacent swept leading edge of the body.

The invention is defined by the claims, the content of which should be considered as forming part of the disclosure of this specification, and the invention will now be described, by way of example, with reference to the further accompanying drawings in which Figure 4 is a diagrammatic and perspective view of an aerofoil body and includes an enlarged fragment; Figure 5 is a plan view of the same body; Figure 6 is a plan view of another body; Figure 7 is a side elevation of yet another body, and Figure 8 is a perspective view of yet another body.

Figure 4 shows an aerofoil body in the form of a "delta" wing 1 with leading edges 2,3 and a trailing edge 4. Flatstrip-form devices 16,17 are attached to the body by rivets or other suitable means (indicated diagrammaticaily at 18) so as to project rearwardly from trailing edge 4 in a direction parallel to the fore-and-aft centreline 5 of the body. Reference 19 indicates the free, rearward tip of each strip and reference 20 the "root" of each strip, by which is meant the part of each strip coinciding with the trailing edge 4.

By describing the devices 16, 17 as "strip-form" or "strips", we mean devices of such form that the projecting length 22 is much greater than the width or spanwise dimension 21,which in turn is likely to be greater than the depth or thickness 40 (inset, Figure 4). While the length and width dimensions may as will be described be related to other parameters of the aerofoil body in a manner so as to optimise the invention, the depth dimension 40 may tend to be dictated by strength considerations: typically, the depth dimension will be one-tenth of the width dimension or less.A cross section through the projecting length of each device will thus typically be basically rectangular in shape, with the width dimension greater than the depth dimension, thus excluding from the invention for instance devices of near-circular cross section in which depth and width are of the same order of magnitude.

However the invention does not exclude devices in which the basically rectangular section is modified, for instance by rounding the corners, or causing the section to thin towards a point at opposite ends, or fitting a small keel 41 for extra strength (inset, Figure 4). As Figure 5 shows most clearly, in this version of the invention the strips are attached to the body in such a way that the flat surfaces of the strips lie parallel to the surfaces of the body.For a body of the dimensions shown, in which the lengths of leading edges 2,3 are equal and are each about 1.5 times that of the trailing edge 4, tests have suggested that especially good results are obtained when the spanwise dimension 21 of each strip is of the order of 4% of the total span of trailing edge 4, and when the length 22 of each strip from root to tip is of the order of 35% of the trailing edge span. The tests from which these conclusions have been derived have assumed an aircraft similar in form to the Saab Viggen, say, operating at subsonic speeds. However this version of the invention is, of course, not limited to bodies in which the dimensions and locations of the strips are restricted as just described. One versed in the art would rightly expect to vary both dimensions and location to suit aircraft of different shape or performance.

In the versions of the invention just described with relation to Figures 4 and 5 of the drawings, the centrelines 23 of strips 16, 17 have been parallel to the centreline 5 of body 1. In the alternative version of the invention shown in Figure 6 the strips 24,25 are again flat but are arranged edge-on to the body so that the planes of the strips lie at right angles to the plane of the chords of the body, and in addition the centreline of each strip lies approximately para llel not to the body centreline 5 but to the adjacent leading edge 2 or 3. Ideally such strips will be located somewhat closer to centreline 5 than would be the case if they were mounted on the wing itself tointeract with part 15a of sheet 7, because once behind the trailing edge the axis 11 of the sheet tends to change direction and become more nearly parallel to the centreline 5.

Figure 7 illustrates the possibility that the centrelines of strips such as 16, 17 may be angled upwardly (31) or downwardly (32) relative to the plane of the mean chord of body 1, instead of lying in the same plane. Tests suggest that by angling the centrelines upwardly or downwardly as indicated in this Figure, the lift-to-drag ratio of the body 1 may be decreased or increased respectively, in a manner perhaps similar to that achieved by known trailing edge controls. Obviously it would be desirable to be able to vary the angle to match different performances of the body in normal flight, and in particular to be able to raise a strip from a downwardly-deflected condition to a level one to improve ground clearance for an aircraft wing on landing.It is also within the scope of the invention that the strips could be movable, and used as some form of control for manoeuvring or trimming the aircraft in flight.

Figure 8 shows yet another version of the invention in which two strips (of which only one, referenced 26, is shown) project straight backwards from the trailing edge 4 so that the centrelines 27 of the strips lie throughout in the plane of the mean chord of body 1. However the strips undergo a right-angle twist between their root 20, where the planes of the surfaces of body 1 and strip 26 are parallel, and their tip 19. As Figure 8 shows, the "hand" of the twist is such that the inboard edge 28 of the strip rises between the root and the tip, and approaches closer to the adjacent leading edge 2. The possible beneficial effects of such a twist may be appreciated by referring again to Figure 3. It will be seen that the "hand" of the twist of strip 26 is the same as that of the trailing part 15 of vortex sheet 7, and a result of this similarity can be that a region of low pressure is induced against surface 30 of the strip, at least towards the tip end of the strip. There are both forward and upward components to the direction in which this surface is facing, so valuable extra thrust may be achieved by creating low pressure against it.

Claims (14)

1. An aerofoil body adapted in use for motion relative to a surrounding mass of fluid, and to present a trailing edge and at least one swept leading edge relative to that motion, and carrying at least one strip-form device projecting rearwardly from the trailing edge so as to modify the vortex sheets generated by the interaction of the edges with the fluid and so improve the lift-to-drag ratio of the body.

2. An aerofoil body according to Claim 1 of triangular or "delta" shape in plan.

3. An aerofoil body according to Claim 2 in which two strip-form devices project from the trailing edge, on opposite sides of the fore-and-aft centreline of the body and with the length of each device parallel to that centreline.

4. An aerofoil body according to Claim 3 in which each strip-form device projects from the trailing edge at a position about 0.6 af the distance between the centreline and the adjacent tip of the trailing edge.

5. An aerofoil body according to Claim 3 in which the fore-and-aft projecting length of the strip-form devices is about .35 of the trailing edge span.

6. An aerofoil body according to Claim 3 in which the spanwise dimension of each of the strip-form devices is about .04 of the trailing edge span.

7. An aerofoil body according to Claim 1 in which at least one such strip-form device lies substantially in the plane of the mean chord of the body.

8. An aerofoil body according to Claim 1 in which at least one such strip-form device projects from the trailing edge of the body at an angle to the mean chord of the body.

9. An aerofoil body according to Claim 1 in which at least one such strip-form device is twisted.

10. An aerofoil body according to Claim 9 in which the surfaces of strip-form device and aerofoil body lie parallel to each other where they are attached to each other, but in which the device is twisted along its length so that at its free end its surface lies in a perpendicular direction.

11. An aerofoil body according to Claim 10 in which the hand of the twist is such that in proceeding from the attached to the free end of the device the inboard edge of the device rises relative to the plane of the mean chord of the body.

12. An aerofoil body according to Claim 1 in which at least one such strip-form device is flat but the plane of its surface lies perpendicular to the plane of the mean chord of the body throughout.

13 An aerofoil body according to Claim 12 in which the plane of the surface of the device also lies parallel to the adjacent swept leading edge of the body.

14. An aerofoil body according to Claim 1, substantially as described with reference to the accompanying drawings.