AU1767292A

AU1767292A - Boundary layer control

Info

Publication number: AU1767292A
Application number: AU17672/92A
Authority: AU
Inventors: Mark Anthony Braniff
Original assignee: Short Brothers PLC
Current assignee: Short Brothers PLC
Priority date: 1991-05-30
Filing date: 1992-05-27
Publication date: 1993-01-08
Also published as: WO1992021560A1; GB2256253A; CA2106850A1; GB9211225D0; EP0585305A1; GB9111620D0; GB2256253B; IL102048A0

Description

BOUNDARY LAYER CONTROL

The present invention relates to boundary layer control and is particularly although not exclusively concerned with the control of the laminar flow of air in a boundary layer adjacent a face of a component of an aircraft exposed to high speed air flow during flight.

It is well known to provide control over a boundary layer adjacent an aerodynamic surface exposed to high speed airflow by injecting air into the boundary layer or by withdrawing air from it. In boundary layer by injection of air, high speed air is blown into the boundary layer to prevent or control separation, that is to say, detachment of the flow from the aerodynamic surface with which it has been in contact. In boundary layer control by the withdrawal of air from the boundary layer, separation is prevented or controlled by removing the layer which would otherwise separate. In addition to preventing or controlling separation, withdrawal of air is also used to prevent or control boundary layer transition, that is to say, the change from laminar to turbulent flow in the boundary layer and is of particular interest in maintaining laminar flow over surfaces of components required to have low drag characteristics.

It has hitherto been proposed for boundary layer control to drill a multitude of small holes in the skin of the component and to provide a ducting system connecting the skin to a vacuum or low pressure source. A plenum chamber is provided beneath the skin to provide a uniform distribution of low pressure to the holes provided in the skin. There is however the disadvantage that the component structure is weakened by the provision of a large multiplicity of holes and by the plenum chamber and a more complex structure is required to compensate for this weakening of the structure.

It is an object of the present invention to provide a component which can be subjected to boundary layer control by withdrawing air from the boundary layer through the face of the component, which does not suffer or does not suffer to the same extent from the disadvantage of the prior proposal and which can be more readily fabricated to stringent design requirements.

According to a first aspect of the present invention, there is provided a low drag component responsive to movement relative to a surrounding gaseous medium to produce at a front face of the component laminar flow in a boundary layer adjacent the front face, the component including a cellular face region which includes the front face and which has an open- celled structure which permits gaseous fluid flow through the front face and between the front face and a rear face of the face region, and gaseous fluid conveying means at the rear of the face region to permit or cause gaseous fluid flow from or to the boundary layer to control laminar flow in the boundary layer.

By "open-celled structure" is meant a cellular structure having a multiplicity of intercommunicating cells obtained by aggregation of particulate material or by the displacement of material by a dispersion or like technique from a body of material in liquid phase followed by a solidifying step.

In an embodiment of the invention hereinafter to be described, the face region is made from a porous permeable thermoplastics material. Preferably, the face region is produced by powder sintering the thermoplastics material.

In an alternative embodiment of the invention hereinafter to be described, the face region is made from a porous permeable composite material formed from a thermoplastics matrix material reinforced with one or more non- thermoplastic reinforcing materials. Preferably the face region is produced by powder sintering the composite material.

In an embodiment of the invention hereinafter to be described, the face region is in the form of a facing component part, the component further comprises a base component part having a front face adjoining the rear face of the facing component part, and the gaseous fluid conveying means comprises channel means at the interface of the facing component part and the base component part.

In embodiments of the invention hereinafter to be described, the channel means provides for gaseous fluid flow through the rear face of the facing component part at predetermined locations over a selected area of the facing component part. The locations may be discrete locations over the selected area.

Furthermore, the facing component part may then be so manufactured that the density of the open-celled structure varies over the selected area, producing a maximum resistance to gaseous fluid flow at each of the discrete locations and a minimum resistance to gaseous fluid flow between the locations to provide for a uniform or substantially uniform flow rate through the face of the facing component part over the selected area. The variation in the open-celled structure is preferably obtained during powder sintering of the thermoplastics or composite material.

In an embodiment of the invention hereinafter to be described the thermoplastics or composite material is presented as a sheet for powder sintering with the thickness of the sheet being increased in zones to be aligned with the discrete locations over the selected area whereby powder sintering of the sheet produces a more densely packed open-celled structure in these zones, offering higher flow resistance than between the zones. The thickness in the zones may be increased by the superposition of a succession of additional layers of material of successively reducing size.

In one specific embodiment of the invention hereinafter to be described the channel means take the form of a plurality of channels which are formed in the rear face of the facing component part and which serve as passageways for the gaseous fluid. Each channel has impermeable walls except at the discrete locations at which the wall is permeable to permit flow of gaseous fluid into or from the channel.

In another specific embodiment of the invention hereinafter to be described, the channel means take the form of a plurality of channels which are formed in the front face of the base component part and which serve as passageways for the gaseous fluid.

The channels provided in the rear face of the facing component part or in the front face of the base component part may be arranged in spaced parallel relation across the width or along the length of the component and may be so dimensioned as additionally to serve as plenum chambers .

Preferably, the component further comprises one or more manifold elements which extend across a rear face of the base component part in a direction transverse to the parallel channels and openings are provided in the base of each channel for flow communication between the channel and the manifold. The manifold or each manifold may take the form of a top hat stiffening element which is bonded to the rear face of the base component part to impart structural rigidity to the component as well as serving as a manifold.

In an embodiment of the invention as hereinafter to be described, the base component part is formed with a recess, the channels are formed in the base of the recess and the facing component part fits within the recess between shoulders of the base component part formed by the recess.

In a still further embodiment of the invention hereinafter to be described, the face region is in the form of a facing component part, the component further comprises a base component part having a front face adjoining the rear face of the facing component part, the gaseous fluid conveying means includes a plenum chamber in the front face of the base component part and the chamber so extends over the rear face of the facing component part as to provide a uniform fluid flow inducing pressure over a selected area of the facing component part. The facing component part may then have a cellular structure which is of uniform density over the selected area so as to provide for uniform gaseous fluid flow through the front face of the facing component part. In yet another embodiment of the invention hereinafter to be described, the base component part has a cellular structure formed by wall portions which extend across the base component part from the front face to the rear face and which provide bounding surfaces for an array of juxtaposed cells which terminate in open ends at the front face. The gaseous fluid conveying means takes the form of openings in the wall portions to provide flow communication between adjacent cells and manifold means are provided in flow communication with the cells of the base component part.

In each of the embodiments of the invention hereinafter to described the front face of the facing component part may advantageously be ribbed to improve laminar flow in the boundary layer.

According to a second aspect of the invention, there is provided an aircraft including a low drag component according to the first aspect of the invention.

According to a third aspect of the present invention, there is provided a boundary layer control arrangement comprising a low drag component according to the first aspect of the invention and suction means communicating with the conveying means to cause gaseous flow from the boundary layer through the front face, through the face region of the component from the front face to the rear face and through the conveying means.

According to a fourth aspect of the present invention, there is provided an aircraft including a boundary layer control system according to the third aspect of the invention. According to a fifth aspect of the present invention, there is provided a method of controlling laminar flow in a boundary layer adjacent the front face of a component according to the first aspect of the invention comprising the step of applying suction to the fluid conveying means to cause gaseous fluid to be withdrawn from the boundary layer through the front face of the component and through the face region of the component between the front face and a rear face of the component and the conveying means.

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which:-

Fig 1A is a schematic isometric view of a part of an aircraft wing showing the location of a component according to the invention

Fig IB is a view corresponding to that shown in Fig 1A of part of an aircraft wing showing a component according to the invention in a form different from that shown in Fig

1

Fig 2A is a schematic isometric view from below of a facing component part of a component according to an embodiment of the invention, in which air flow channels are formed in the rear face of the component part

Fig 2B is a schematic section, taken on the line II-II in Fig 2A, of a part of the facing component part shown in Fig 2A with a base component part for supporting it

Fig 3A is a schematic representation of a processing step in the formation of a facing component part of a component according to the invention, in which material in sheet form and of variable thickness is powder sintered to produce a constant thickness component part of variable density

Fig 3B is an enlarged schematic view of a portion of the sheet to be sintered and as shown in Fig 3A

Fig 3C is a schematic cross-section of the built up sheet shown in Fig 3B, prior to powder sintering

Fig 3D is a schematic cross-section of the sheet shown in Fig 3A after powder sintering

Fig 3E is a graph of the vacuum experienced at the face of a component according to the invention where the sheet forming the facing component part is of uniform density

Fig 3F is a graph of the flow rate of air through a facing component part according to the invention where the component is made from a powder sintered sheet in the manner illustrated in Figs 3A to 3D

Fig 4A is a schematic isometric view from below of a facing component part of a component according to a further embodiment of the invention, in which air flow channels are formed in the rear face of the facing component part in a configuration different from that shown in Fig 2A

Fig 4B is a schematic section of the facing component part shown in Fig 4A, taken on the line IV-IV in Fig 4A, together with a base component part by which the facing component part is supported

Fig 5A is a schematic isometric part view of a component according to a still further embodiment of the invention, in which air flow channels are formed in the front face a base component part of a component according to the invention

Fig 5B is a schematic section of the component shown in Fig 5A taken on the line V-V in Fig 5A

Fig 5C is a schematic perspective view of an aircraft wing, illustrating a location for the component shown in Figs 5A and 5B

Fig 6 is a schematic isometric view of a part of a base component part of a component according to another embodiment of the invention, formed with channels extending transverse .to the channels in the base component part shown in Figs 5A and 5B

Fig 7A is a schematic isometric view partly cut away and viewed from above of a component according to a yet another embodiment of the invention

Fig 7B is a sectional scrap view of the component shown in Fig 7A, taken on the line VII-VII in Fig 7A

Fig 7C is a schematic perspective view of an aircraft wing, illustrating a location for the component shown in Figs 7A and 7B

Fig 8A is a schematic isometric view partly cut away of a component according to a further embodiment of the invention

Fig 8B is a schematic section of the component shown in Fig 8A taken on the line VIII-VIII in Fig 8A Fig 9A is a schematic isometric view of a component according to yet another embodiment of the invention, and

Fig 9B is a schematic part sectional end view of the component illustrated in Fig 9A

Referring first to Fig 1A, an aircraft wing section 11 is shown, which includes at its upper leading edge a component 12 made in accordance with the invention. The component 12 is connected to a suction source (not shown) via conveying means hereinafter to be described for applying suction to the component 12 to withdraw air from the boundary layer adjacent the upper surface of the wing at the leading edge to control flow in the boundary layer, with the aim either of maintaining laminar flow throughout the chordwise extent of the upper surface of the wing section 11 or of shifting the boundary layer transition point on the upper surface downstream as far as possible. As will be seen the component 12 takes the form of a sheet of uniform thickness which extends spanwise and fits into a recessed part of the wing structure.

In an alternative embodiment of the invention illustrated in Fig IB, the wing section 11 embodies a component 12¹ which extends completely over the upper leading edge of the wing section 11. The component 12¹ is feathered at its leading and trailing edges and would normally be used for boundary layer control only and not serve as an effective structural part of the wing section.

Referring now to Fig 2A and 2B, a low drag component 12 according to the invention comprises a facing component αart 13 and a base cα ϋonent oart 14. The facing component part 13 is produced as hereinafter to be described with reference to Figs 3A to 3F by powder sintering of a thermoplastics material in particulate form optionally reinforced with one or more non- thermoplastics reinforcing materials to produce a porous permeable thermoplastics sheet of uniform thickness. The facing component part 13 has a front face 15 and a rear face 16 and is produced as shown with channels 17 formed in the rear face 16.

As best seen in Fig 2B, each channel 17 is of semi¬ circular cross section with bounding walls 18 which are impermeable except at discrete openings 19 where localised areas of the upper wall are permeable. The formation of the channels 17 is preferably made during the production of the part 13 in the powder sintering process, although if desired they may be formed subsequently.

As shown in Fig 2B, the facing component part 13 is bonded at its rear face 16 by an adhesive 20 to the base component part 14 , which is of a non-porous impermeable material and which serves to provide structural strength for the component 12 and to close off the channels 17. The channels 17 terminate at closed ends 21 and are connected at their open ends to a suction source not shown.

In use, suction is applied to the channels 17 to produce a low pressure at the discrete openings 19 where the channel walls are permeable with the result that air is drawn through the front face 15 of the facing component part 13 and through the component part 13 between the front face 15 and rear face 16 into the channels 17 for discharge at a discharge point at the suction source. The component 12 as described with reference to Figs 2A and 2B is mounted as shown in Fig 1A in the upper leading edge of the wing section 11 and the withdrawal of air through the front face 15 serves to maintain laminar flow in the boundary layer during flight of the aircraft as hereinbefore described.

It will however be apparent that the withdrawal of air from the boundary layer through the front face 15 in the component described with reference to Figs 2A and 2B varies over the withdrawal area, with a maximum flow of air in regions of the face 12 aligned with the channel openings 19. A component 12 of this simple form may nevertheless be found to be adequate for some purposes.

In order however to provide for suction to be applied at discrete locations over a selected area of the face 15 while at the same time providing for a uniform withdrawal of air through the face 15, the density of the facing component part 13 can be varied so as to offer increased resistance to flow at each discrete opening 19, reducing to a minimum resistance to flow in the region between the opening 19. The production of a facing component part 13 having these characteristics will now be described with reference to Figures 3A to 3F.

Referring now to Fig 3A, a layer 22 of a thermoplastics material in particulate form is shown supported by a conveyor 23 which is arranged to be advanced through the nip of a pair of heated rollers 24 and 25. The rollers 24 and 25 are preset to provide a gap 26 which will cause the layer 22 when advanced on the conveyor 23 through the rollers 24 and 25 to be compacted to the required particulate packing density to produce a powder sintered sheer at the outlet side of the rollers 24 and 25. As shown to an enlarged scale in Fig 3B and in the cross section in Fig 3C, the layer 22 of particulate material has superposed on it at spaced locations 30 circular localised further layers 27, 28 and 29 of thermoplastics material of circular form with progressively reducing diameters, thereby to provide at these spaced locations 30 a build up of particulate material with a maximum thickness at the centre of each location 30. During processing, the layer 22 is advanced through the rollers 24 and 25 which cause the material to be sintered with the regions at the locations 30 being compressed to a higher packing density than the regions between them ro produce as illustrated in Fig 3D a constant thickness porous permeable thermoplastics sheet 31 having a variable particulate packing density with maximum density at the locations 30.

In Fig 3E is plotted a curve which illustrates the surface vacuum variation resulting from connection of the localised openings 19 to a vacuum source, while Fig 3F illustrates the uniform flow rate through the face 34 of the sheet 31 by virtue of the increased packing density at the locations 30.

The facing component part 13 described with reference to Figs 2A and 2B is preferably produced in a single processing step to provide in addition to the channels 17 and openings 19, increased density at locations 30 aligned with the openings 19 in the channels 17 so as to achieve a uniform or substantially uniform flow rate over the face 15 of the facing component part 13.

In the embodiment of the invention now to be described with reference to Figs 4A and 4B the facing component part 13 takes the same form as that described with reference to Figs 2A and 2B and is produced in the same manner as that described with reference to Figs 3A to 3F, except insofar that the channels 17 are replaced by an arterial channel 34 extending for the full length of the part 13 with branch channels 35 leading to radial channels 36 which terminate in closed enlarged ends 37. The channels 34, 35 and 36 have impermeable walls and the enlarged ends 37 are left in part permeable to provide openings 19 corresponding to the openings 19 in the channels 17 of the facing component part 13 described with reference to Figs 2A and 2B.

The packing density of the facing component part 13 is varied over the area of the facing component part 13 in the same manner as that described with reference to Figs 3A to 3F with the maximum packing density and therefore maximum resistance to flow being provided over the openings 19 and the minimum packing density and lower resistance to flow being provided in the regions between the openings 19.

As best seen in Fig 4B, the facing component part 13 is bonded to the base component part 14 to close off the channels 34 to 36 and the enlarged ends 37. The arterial channel 34 is open at its ends for connection to a suction source (not shown) via appropriate manifolds.

In a further embodiment of the invention illustrated in Figs 5A to 5C, the component 12 is formed from a facing component part 13 and a base component part 14. The facing component part 13 is of uniform thickness and is produced by powder sintering particulate thermoplastics material to a packing density which is uniform throughout. The base component part 14 which is of non- porous impermeable material is formed with a recess 38, the base of which is formed with a plurality of channels

39 which extend across the base of the recess in spaced

parallel relation and which communicate through openings

40 with a manifold 41 for connection via an outlet 42 to a suction source (not shown) . The facing component part 13 fits, as shown, into the recess 38 so as to be flush with the surfaces 43 and 44 of the shoulders 45 and 46 which flank the recess 38. As best seen in Fig 5B, the channels 39 are of substantial depth so as to serve as plenum chambers to provide for a uniform distribution of low pressure throughout each channel.

The component 12 illustrated in Figs 5A and 5B may with advantage be used for boundary layer control in the upper surface of a wing section as illustrated in Fig 5C. In use, suction is applied by the suction source to the manifold 41 and via the openings 40 to the channels 39 causing air to be withdrawn from the boundary layer adjacent the upper surface of the wing section 11, through the face 15 and the body of the facing component part 13 to effect boundary layer control as hereinbefore discussed.

In yet another embodiment of the invention illustrated in Fig 6, the component 12 comprises a facing component part 13 of the same form and produced in the same manner as the facing component part 13 in Figs 5A and 5B and a base component part 141 of non-porous impermeable material formed with a recess 38 corresponding to that shown in Fig 5A, the base of which is however formed with an array of channels 39¹ which extend in spaced parallel relation in a direction at right angles to that of the channels 39 in the embodiment described with reference to Figs 5A and 5B. A manifold 41 is in like manner bonded to the rear face of the base component part 14 and communicates with the channels 39¹ via openings 40 formed in the base of the channels.

The component 12 which has been formed using the facing component part 13 as described with reference to Fig 5A and the base component part 14¹ as described with reference to Fig 6 may alternatively be used for boundary layer control at the upper surface of the wing section 11 as in the position illustrated in Fig 5C.

In still yet another embodiment of the invention illustrated in Figs 7A to 7C, the component 12, shown inverted in Fig 7A, comprises a facing component part 13 provided with channels 17¹ formed in the rear face 16 in the same manner as the channels 17 of the component part

13 of Fig 2A. The base component part 14 is of uniform thickness and made from a non-porous impermeable material and is bonded to the rear face 16 of the facing component part 13. To the rear face 42 of the base component part

14 are bonded two manifolds 43 and 44 which are in communication with the channels 17' via openings 45 formed in the base component part 14.

The channels 17¹ are closed at each end and formed in the rear face of facing component part 13 in such a way as to leave them with permeable walls throughout. As a result, when suction is applied to ducts 46 and 47 leading to the manifolds 43 and 44, air is withdrawn through the front face 15 of the component part 13 into the channels 17¹ through the openings 45 into the manifolds 43 and 44 and then to a discharge point at the suction source.

The component 12 as illustrated in Figs 7A and 7B is suitable for embodying within the surface of a wing section 11 in the manner illustrated in Fig 1A or at other positions in an aerodynamic surface of the aircraft, such as for example at the lower leading edge surface of the wing section 11 as illustrated in Fig 7C.

It will of course be appreciated that the channel 17¹ in the embodiment of the invention described with reference to Figs 7A and 7B may be formed with impermeable walls except at discrete locations where they are permeable and form openings 19 and the packing density of the material forming the facing component part 13 may then be arranged to vary over the area of the face 15 to provide for a uniform flow of air through the face 15 as in the embodiment of the invention described with reference to Figs 2A and 2B and 3A to 3F.

In yet a further embodiment of the invention illustrated in Fig 8A and 8B, the component 12 comprises a facing component part 13 of uniform thickness secured to a base component part 14 of non-porous impermeable material and has a recess 48 within bounding walls 49. The recess 48 extends substantially throughout the surface area of the component 12 and it is formed with an opening 49 for connection to a suction source. The facing component part 13 is secured to the walls 49 of the base component part 14 by headed screws 50 which pass through countersunk holes in the facing component part 13 and are screwed into threaded holes provided in the walls 49. The recess 48 serves as a plenum chamber to provide an even distribution of low pressure throughout the effective area of the facing component part 13. In this construction, the facing component part 13 is unsupported over the recess 48 and is reinforced preferably with a non-thermoplastic reinforcing material combined with the thermoplastic material or by providing an external perforated open weave carbon fibre fabric sheet bonded to it.

In all of the embodiments of the invention hereinbefore described with reference to Figs 1A and IB to Figs 8A and 8B, the base component part 14 is formed as a solid structure reinforced in some embodiments by manifolds. In order to provide a lightweight structure of sufficient strength for use as a component in an aircraft surface structure it may be appropriate to make the base component part 14 of a cellular structure and with this in mind there is shown in Figs 9A and 9B a component 12 having a base portion 14 of this form.

Referring now to Figs 9A and 9B, it will be seen that the facing component part 13 is in the form of a sheet of uniform thickness. It is produced by the sintering process described with reference to Figs 3A to 3F but simplified to have a uniform packing density throughout. The base component part 14 on the other hand comprises a cellular core 141 having wall portions 142 which provide bounding surfaces for an array of cells 143 which in the embodiment illustrated are of hexagonal cross section and form what is termed a "honeycomb core". Each of the cells 143 of the cellular core 141 are in communication with each other through openings 144 formed in the walls of the cells and the component is completed by enclosing the cellular core within an enclosing cover 145 which is provided with an outlet duct 51 for connection to a suction source.

The component 12 of the embodiment illustrated in Figs 9A and 9B may be mounted in an appropriate position in an aircraft surface structure to provide boundary layer control over the surface in the same manner as that proposed for the embodiments of the invention hereinbefore described, with suction being applied to the suction duct 51 to withdraw air from the boundary layer through the front face 15 of the facing component part 13 through the cells 143 and the openings 144 to the duct 51 and through the duct 51 to a discharge point at the suction source.

In all of the embodiments of the invention hereinbefore described with reference to the drawings, the face 15 of the facing component part 13 may be ribbed to improve laminar flow across it. Such ribbed configuration may be formed at the time the facing component part 13 is produced in the powder sintering process or the face 15 machined after production of the part 13.

The facing component point 13 is preferably made by powder sintering a thermoplastic material in particulate form. Examples of suitable thermoplastics material for the facing component part 13 include polyether ketone, polyether ether ketone, polyaromatic ketone, polyphenylene sulphide, polyamide-imide and thermoplastic polyimide, polyether-imide, polyurethane and polyethylene.

The base component part 14 requires to be non-porous and impermeable and may be made of any of the following material:-

(i) A carbon/thermoplastic composite where for example the thermoplastic is polyether ether ketone,

(ii) A carbon/epoxy resin. (iii) An aluminium alloy.

The cellular core 141 is preferably made from a non- porous impermeable sheet of any of the following materials:-

(i) A thermoplastic such as polyether ether ketone.

(ii) A polyester fabric/phenolic resin.

(iii) A fibreglass/phenolic resin.

(iv) A NOMEX/phenolic resin (NOMEX being a registered trade mark for an aramid fibre paper impregnated with various resins to produce a structural material. By "aramid" is meant an aromatic polyamide polymer.

(v) An aluminium alloy.

The cellular core 141 of the base component part 14 of the embodiment described with reference to Figs 8A and 8B may, if desired, be in the form of an open-celled plastics foam which may be manufactured from any suitable thermoplastics material. Examples of suitable thermoplastics materials include polyether ketone, polyether ether ketone, polyaromatic ketone, polyphenylene sulphide, polyamide-imide, thermoplastic polyimide, polyether-imide, polyurethane and polyethylene. It may for example be formed from the same thermoplastics material as that from which the facing component part 13 is formed.

The facing component part 13 in all the embodiments of the invention hereinbefore to be described is made from a porous and permeable thermoplastics material. It may however be combined with a non-thermoplastics material to form a composite material which is then subjected to sintering.

The reinforcing material combined with the thermoplastic material may be in any of the following forms:-

1. Spheroids, micro balloons, powder, particles of ceramic such as silica or particles of carbon.

2. Fibres of carbon, glass, boron, steel, ceramic or the like.

The reinforcing material may be added in various quantities to impart (i) strength, (ii) stiffness (iii) to reduce the net coefficient of thermal expansion or (iv) to vary the flow resistance properties of the material.

Boundary layer control using a component according to the invention possesses the following material/structural characteristics: -

(a) The porous permeable thermoplastic facing component part 13 is readily produced by a sintering process.

(b) The porosity of the material forming the facing component part 13 may be varied over a given area by control over the sintering process, when required.

(c) The material of the facing component part 13 may be made impermeable by over compacting (ie particles completely fused together) , when required. (d) The material forming the facing component part 13 may be shaped during the sintering process.

(e) The thermoplastics material may be combined with high performance fibres (carbon, kevlar, glass, boron etc) to produce a lightweight structural component. The fibres may be continuous, chopped or woven.

(f) A range of fillers may also be incorporated at the sintering stage to improve stiffness and vary flow characteristics

Figs 1A and IB illustrate possible locations of the component on a typical aerofoil section. The size and location of component 12 may readily be determined by experimentation and from the function of the underlying structure. Many conventional applications (ie wing surface, control surfaces, nacelles and the like) would however require redesigning.

A number of alternative embodiments of the invention have been described with reference to Fig 2A and 2B to Figs 9A and 9B. They may require integrating the air conveying system into the aircraft structure which may require an insert to transmit wing loadings. The porous permeable thermoplastic facing component part 13 may be configured with more conventional composite and metallic materials in order to increase load bearing capabilities. The design of the stiffening elements (blades and tophats) may be so configured that they act in a secondary role as air duct systems, hence reducing the complexity of the sintering process (ie a relatively flat porous component part 13 with uniform flow capacity may suffice) . Figs 5A and 5B illustrate a variation in that a plenum chamber is incorporated, allowing a thermoplastic outer facing component part 13 with uniform density to be utilized, significantly simplifying the sintering process. The basic differences between this arrangement and that of the prior proposal is the use of a porous permeable thermoplastics material in place of drilled holes. The plenum chamber may be machined or fabricated from alloy or alternatively moulded in conventional reinforced thermoset or thermoplastic materials. In addition, the manifolds in the form of tophat stiffening elements may be bonded to the plenum bottom surface to impart structural rigidity as well as serving as a manifold, directing air to the vacuum source.

Figure 6 illustrates a further structure, again incorporating plenum type chambers. The direction of these chambers and the underlying manifolds have been interchanged. This may be necessary in order to align with the surrounding structures load paths.

In Figs 7A and 7B, the plenum chamber effect has been incorporated into the porous thermoplastic facing component part 13 by the use of channels 17¹. These channels or chambers may be machined after sintering or formed during sintering by dedicated tooling. In order to create closed channels or pathways, the sintered skin is bonded to a carbon fibre composite or light alloy base component part 14 predrilled to permit air transfer to co-cured or bonded tophat manifolds 43 and 44. These manifolds serve as stiffeners as well as being an integral part of the vacuum source network.

In Figs 8A and 8B the component is simplified by utilizing a reinforced porous thermoplastic facing component part 13 mechanically fastened to an underlying base component part 14 shaped to produce a plenum chamber when assembled. The most significant aspect of the design is the structural nature of the thermoplastic facing component part 13, replacing the role of the tophat manifold stiffeners. The reinforcement may be incorporated at the sintering stage (ie continuous fibre, discontinuous long fibre, chopped fibre or filler powders) . Alternatively, the part 13 may be stiffened by secondary bonding with for example, perforated mesh, open weave carbon fibre or drilled laminates, to form a sandwich arrangement.

In Figs 9A and 9B the porous thermoplastic facing component part 13 is combined with a honeycomb core 141 and backing skin 145 of light alloy or advanced composite to form a sandwich structure capable of flowing air whilst resisting structural loads. The backing skin 145 must be sealed to the porous facing component part 13 at the periphery in order to maintain vacuum integrity. The individual honeycomb cells 143 are interconnected by openings 144 to allow lateral airflow.

Boundary layer control by components according to the present invention lead to the following advantages:-

(a) Environmental - The increase in aerodynamic efficiency should reduce fuel consumption significantly and hence reduce toxic emissions.

(b) Chemical Resistance - The proposed porous permeable thermoplastic facing component part 13 is resistant to chemical attack from the majority of liquids associated with the aviation industry (ie fuel, de- icer, cleaning solvents and the like) . (c) Uniform Flow - Control of the sintering process will produce a facing component part capable of flowing air through the total surface area rather than at individual drilled holes as with systems hitherto proposed.

(d) Resistant to Blockage - The major problem associated with the prior proposals utilizing drilled holes concerns a performance drop off with time due to blockage of the holes by dust and insects. The porous thermoplastic facing component part cannot be blocked easily since the pores are an order of magnitude smaller than the drilled holes and present a torturous path to incoming particles.

(e) Cost and Replaceability - The proposed structures will be relatively cheap to manufacture due to the single piece nature of the structure and consequently should permit easy in-service replacement depending on the function of the underlying structure.

(f) Aerodynamic Drag - It will be possible to mould riblets into the air wet surface during the sintering process. Such riblets in themselves promote laminar flow, reduce drag and consequently reduce fuel consumption.

(g) Weight - The porous permeable thermoplastic facing component part 13 may be combined with high performance fibres (carbon, kevlar, glass, boron etc) . Integrated designs can reduce the requirement for ducting and additional stiffening of the underlying structure and it is anticipated that little additional weight would be incurred by implementing such designs.

(h) Impact Resistance - This is an inherent property possessed by thermoplastics in general which may be exploited in this particular application of the invention.

(i) Colour - A wide range of colours are available since the proposed boundary layer control device cannot be painted due to the nature of its operation. The thermoplastics material may be coloured during the sintering process producing a material which requires no touch up work after drilling, cutting or the like.

The following applications of the invention are envisaged although any air wet surface requiring boundary layer control may be treated by this method:-

(1) Aircraft wings

(2) Nose cowls

(3) Nacelle lipskins

(4) Control surfaces

(5) High performance cars and motorcycles

(6) Fuselages

Claims

1. A low drag component responsive to movement relative to a surrounding gaseous medium to produce at a front face of the component laminar flow in a boundary layer adjacent the front face, the component including a cellular face region which includes the front face and which has an open- celled structure which permits gaseous fluid flow through the front face and between the front face and a rear face of the face region, and gaseous fluid conveying means at the rear of the face region to permit or cause gaseous fluid flow from or to the boundary layer to control laminar flow in the boundary layer.

2. A component according to claim 1 wherein the face region is made from a porous permeable thermoplastics material.

3. A component according to claim 2 wherein the face region is produced by powder sintering the thermoplastics material.

4. A component according to claim 1 wherein the face region is made from a porous permeable composite material formed from a thermoplastics matrix material reinforced with one or more non-thermoplastic reinforcing materials.

5. A component according to claim 4 wherein the face region is produced by powder sintering the composite material.

6. A component according to any of claims 1 to 5 wherein the face region is in the form of a facing component part, wherein the component further comprises a base component part having a front face adjoining the rear face of the facing component part, and wherein the gaseous fluid conveying means comprises channel means at the interface of the facing component and the base component part.

7. A component according to claim 6, wherein the channel means provides for gaseous fluid flow through the rear face of the facing component part at predetermined locations over a selected area of the facing component part.

8. A component according to claim 7, wherein the locations are discrete locations over the selected area.

9. A component according to claim 8, wherein the f cing component part is so manufactured that the open- celled structure varies over the selected area, with a maximum resistance to gaseous fluid flow at each of the discrete locations and a minimum resistance to gaseous fluid flow between the locations to provide for a uniform or substantially uniform flow through the face of the facing component part over the selected area.

10. A component according to claim 9, as appendent to claim 3 or 5 wherein the variation in the open-celled structure is obtained during powder sintering of the thermoplastics or composite material.

11. A component according to claim 10 wherein the thermoplastics or composite material is presented as a sheet for powder sintering with the thickness of the sheet being increased in zones to be aligned with the discrete locations over the selected area whereby powder sintering of the sheet produces a more densely packed open-celled structure in these zones offering higher flow resistance than between the zones.

12. A component according to claim 11 wherein the thickness in the zones is increased by the superposition of a succession of additional layers of material of successively reducing size.

13. A component according to any of claims 6 to 12 wherein the channel means take the form of a plurality of channels which are formed in the rear face of the facing component part and which serve as passageways for the gaseous fluid.

14. A component according to claim 13 wherein each channel has impermeable walls except at the discrete locations at which the wall is permeable to permit flow of gaseous fluid into or from the channel.

15. A component according to claims 6 to 12 wherein the channel means take the form of a plurality of channels which are formed in the front face of the base component part and which serve as passageways for the gaseous fluid.

16. A component according to claim 13, 14 or 15 wherein the channels are arranged in spaced parallel relation across the width or along the length of the component.

17. A component according to any of claims 13 to 16 wherein the channels are so dimensioned as additionally to serve as plenum chambers.

18. A component according to claim 16 as appendent to claim 15 further comprising one or more manifold elements which extend across a rear face of the base component part in a direction transverse to the channels and wherein openings are provided in the base of each channel for flow communication between the channel and the manifold.

19.^' A component according to claim 18 wherein the manifold or each manifold takes the form of a top hat stiffening element which is bonded to the rear face of the base component part to impart structural rigidity to the component as well as serving as a manifold.

20. A component according to any of claims 15 to 19 wherein the base component part is formed with a recess, wherein the channels are formed in the base of the recess and wherein the facing component part fits within the recess between shoulders of the base component part formed by the recess.

21. A component according to any of claims l to 5 wherein the face region is in the form of a facing component part, wherein the component further comprises a base component part having a front face adjoining the rear face of the facing component part, wherein the gaseous fluid conveying means includes a plenum chamber in the front face of the base component part and wherein the chamber so extends over the rear face of the facing component part as to provide a uniform fluid flow inducing pressure over a selected area of the facing component part.

22. Component according to claim 21, wherein the facing component part has a cellular structure which is uniform over the selected area so as to provide for uniform gaseous fluid flow through the front face of the facing component part.

23. A component according to any of claims 1 to 5 wherein the face region is in the form of a facing component part, wherein the component further comprises a base component part having a front face adjoining the rear face of the facing component part, wherein the base component part has a rear face and wall portions which extend across the base component part from the front face to the rear face and which provide bounding surfaces for an array of juxtaposed cells which terminate in open ends at the front face and wherein the gaseous fluid conveying means takes the form of openings in the wall portions between adjacent cells and manifold means in flow communication with the base component part.

24. A component according to any of the preceding claims wherein the front face of the facing component part is ribbed.

25. A boundary layer control arrangement comprising a component according to any of the preceding claims and suction means communicating with the conveying means to cause gaseous flow from the boundary layer through the front face, through the face region of the component from the front face to the rear face and through the conveying means.

26. A method of controlling laminar flow in a boundary layer adjacent the front face of a component according to any of the preceding claims comprising the step of applying suction to the fluid conveying means to cause gaseous fluid to be withdrawn from the boundary layer through the front face of the component and through the face region of the component between the front face and a rear face of the component and the conveying means.

27. An aircraft including a structural component according to any of claims 1 to 24.

28. An aircraft including a boundary layer control system according to claim 25.