GB1593378A

GB1593378A - Aircraft surface structure

Info

Publication number: GB1593378A
Application number: GB52941/76A
Authority: GB
Original assignee: Lucas Industries Ltd
Current assignee: ZF International UK Ltd
Priority date: 1976-12-17
Filing date: 1976-12-17
Publication date: 1981-07-15
Also published as: NL187567C; NL7713936A; FR2374109A1; FR2374109B1; JPS6158359B2; JPS5378599A; SE437004B; SE7714234L; NL187567B; IN149367B; DE2756423C2; DE2756423A1; BR7708394A; CA1094526A; IT1088448B

Description

(54) AIRCRAFT SURFACE STRUCTURE (71) We, LUCAS INDUSTRIES LI MITED, a British Company of Great King Street, Birmingham B19 2XF, do hereby declare that the invention for which we pray that a patent may be granted to us and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to a method of protecting predetermined surface regions of an aircraft and to an aircraft surface region incorporating such protection.

It is known that certain surface regions of an aircraft, for example the leading edges of the wings, the engine air intakes, the leading edges of the tail structure, helicopter rotor blades, airscrews, engine compressor blades, and guide vanes, are subject in use to surface erosion and to impact damage, by rain, airborne sand, and ice and the like.

These surface regions may incorporate electrical heating devices to facilitate de-icing during flight, and of course the preservation of the mechanical, and where appropriate electrical integrity of these regions is of great importance. It is known to protect such surface regions against erosion and impact damage by providing the surface region with a synthetic resin covering, for example a covering of a polyurethane or neoprene material. Such coverings are suitable for relatively low speed applications, for example up to an approximate maximum of 450 miles per hour, this figure representing the velocity component normal to the surface in question. In order to provide surface erosion protection for higher speed aircraft, for example aircraft capable of entering supersonic speed ranges, it has in the past been proposed to provide protective layers, of stainless steel or nickel.The synthetic resin group of materials provide protection by virtue of their resilience and toughness, while the metallic coatings mentioned afford protection by virtue of their hardness. The synthetic resin materials suffer from the disadvantage that their usefulness is limited to relatively low speed applications, while the previously-proposed layers of nickel or stainless steel, while being suitable for a wide speed range, are both extremely difficult and extremely expensive to produce. Moreover, even though the protective metallic layers are extremely thin, it is extremely difficult and expensive to form them accurately to the complex surface shapes involved.

It is an object of the present invention to provide a method of protecting surface regions of an aircraft, and an aircraft surface region so protected, in a simple and convenient manner.

A method of protecting a surface region of an aircraft, in accordance with the invention, includes starting with a sheet or sheets of a super-plastic alloy engaging the sheet or sheets of super-plastic alloy, with the surface region to be protected, shaping the sheet or sheets of super-plastic alloy at a temperature such that the super plastic properties of the alloy are exhibited to follow the surface contours of the surface region to be protected, and securing the protective layer of super-plastic alloy so formed to said surface region.

Desirably the formed layer is secured to said surface region by means of an adhesive.

Conveniently the adhesive is applied to the surface region to be protected and/or the sheet or sheets of alloy prior to the shaping of the sheet or sheets.

Conveniently the sheet or sheets are preformed to the general shape of the surface region and are finally shaped in situ so as to conform to the surface contours peculiar to the actual surface region.

The invention further resides in an aircraft surface region having secured thereto a protective layer of a super-plastic alloy.

In one aspect of the invention, the aircraft surface region incorporates heater structure and the protective layer of super-plastic alloy overlics the heater structure.

in a further aspect of the invention the aircraft surface region incorporates strengthening fibres in its construction for example carbon filaments.

In a further aspect the invention resides in a protective shoe of a super-plastic alloy which is shaped to fit onto a predetermined aircraft surface region, said shoe being intended finally to be shaped in situ so as completely to follow the surface contours peculiar to the surface region for which it defines the protcctive layer. and said shoe incorporating a heater structure so that the protective shoe and the heater structure are applied, in use, to the predetermined surface region in the same operation.

Preferably the alloy from which the protective layer is formed is that known as: "SPZ alloy" manufactured by the Imperial Smelting Company Limited; or is a two-phase quaternary alloy having a fine grain microstructure stable at the tem erature of a super-plastic deformation consisting of zinc within the range 70-820/0 by weight and aluminium 30-18% by weight to which is added up to 0.25% by weight of magnesium and up to 2% by weight of one of the elements copper, nickel and silver.

In the accompanying drawings; Figure 1 is a cross-sectional representation of part of an aircraft wing in accordance with one example of the present invention; Figitre 2 is a view similar to Figure 1 of a modification; and Figure 3 is a diagrammatic representation of an aircraft.

Rcferring to the drawing, the wing ineludes an outer skin 11 which defines the airfoil shape of the wing. The desirability of protecting wing surfaces and other surface regions of the airframe against erosion and impact damage by rain, and airborne sand and ice, is well known. For example, the leading edge surfaces of wings, tail structures and engine air intakes guide vanes and compressor blades and the leading edges of helicopter rotor blades are susceptible to erosion and impact damage by rain, and airborne ice and sand, and thus it is desirable to take steps to preserve the mechanical integrity of these regions, and also, of course, their electrical integrity in the event that they are provided with surface heaters for dc-icing.Moreover where the component, for example a helicopter rotor blade, is formed from a material reinforced with fibres, i.e. carbon filiments, then clearly their integrity must also be protected. Thus in accordance with an example of the present invention such surfaces are provided with an outer protective layer, the layer 12 in the drawing, formed from a sheet or sheets of super-plastic alloy having the super-plastic properties found in many binary zinc/aluminium alloys but not having their susceptibility of moisture attack. This reduction is susceptibility to moisture attack is of course important where protection against rain erosion is one of the requirements.Tests made on aircraft parts fitted with surface protection layers of the superplastic alloy as described hereinafter show a good resistance to erosion under conditions of impact by rain, hail and sand.

A further benefit of using such superplastic alloys to produce the protective outer layer is that as a result of the super-plastic properties sheets of alloy can be worked easily to follow accurately the surface contours peculiar to the actual airfoil or other surface region of the airframe on which the sheet or sheets are secured.

The sheet or sheets of super-plastic alloy are relatively thin, of the order of 0.010 0.030 inches, the thickness chosen being consistent with need for the layer to be thick enough to prevent the impact shock of rain, ice and sand from damaging the adhesive bond between the layer and the surface being protected (where an adhesive is used) but thin enough to keep the weight of the layer within acceptable limits.

In order to apply such a protective layer 12 to a predetermined surface region 11 of the airframe several alternative procedures are possible, for example; the airframe component is detached and the predetermined surface region and/or sheet or sheets of the super-plastic alloy are coated with an expoxy resin adhesive 13. The sheet or sheets of alloy are then pressed onto the predetermined surface region so that they generally follow the shaping thereof. Thereafter, the components together with the sheet or sheets of alloy are raised to the temperature of superplastic deformation of the alloy at which temperature the superplastic properties of the alloy are exhibited, in an oven, and the sheet or sheets of alloy are then shaped to intimately follow the predetermiend surface region, using the surface region itself as the former. The shaping force is applied to the sheet or sheets of alloy by engaging with the outer surface of the sheet or sheets one or more air bags, and then inflating the air bags to apply pressure to the sheet or sheets so as to achieve a uniform final thickness of adhesive over the whole of the sheet or sheets.

Alternatively the airframe component carrying the sheet or sheets raised to the temperature of suitable plastic deformation can be placed in an impervious flexible bag which is then evacuated so that external air pressure presses the sheet or sheets of alloy to follow intimately the shaping of the predetermined surface region of the airframe component.

Where it is necessary to restrict the temperature to which the aircraft surface region is heated, the sheet or sheets of super-plastic alloy can be heated, separate from the surface region, to a suitable temperature in excess of the ideal forming temperature and can then rapidly be applied to and shaped on the cold surface region. As a further precaution a heat insulating layer can be introduced between the hot sheet or sheets and the surface region, the insulating layer being in place of and of a thickness equal to, the adhesive layer. The sheet or sheets are then shaped to the surface region, the sheet or sheets then being removed to permit the insulating layer to be replaced by a layer of adhesive, following which the sheet or sheets will be replaced so as to be adhesively bonded to the surface region.As is mentioned later, the aircraft region may require to be provided with a heater for use in aircraft de-icing. The heater layer may act as a heat insulating layer during shaping of the sheet or sheets of super-plastic alloy, but of course in this situation the insulating layer defined by the heater will be left in situ and the sheet or sheets will be adhesively bonded in position covering the heater layer. As a still further alternative the aircraft surface region can temporarily be provided with a cooling arrangement to prevent overheating of the surface region, during fitting of the sheet or sheets of super-plastic alloy.

As a further alternative where a series of theoretically identical surfaces are to be protected, for example the leading edges of the wings of a flight of identical aircraft then a plurality of protective shoes each formed from a sheet or sheets of super-plastic alloy can be produced using a former which is identical in shape to the theoretical leading edge shape of the aircraft of the flight. It will be recognised however that each aircraft in the flight will be slightly different from the others since it is individually constructed and so although each leading edge has the same theoretical shape it will in fact have surface contours, minor asperities and the like which do not materially affect its airfoil performance, peculiar to itself.Thus each protective shoe will fit anyone of the leading edges of the aircraft of the flight but none will follow accurately the peculiar surface contours. It is vital that the protective layer does follow the contours of the surface it is to protect in order that the layer of adhesive securing the layer to the surface is of even thickness and so exhibits constant adhesive properties over the whole area, it being recognised that for optimum adhesive bond strength, many adhesives must be present in a layer of predetermined thickness accurate to 0.1 mm. Additionally failure of the layer to follow local peculiarites in the surface could result in a void between the layer and the surface which would be an inherent weakness in the composite structure.Such inherent weakness would rapidly give rise to failure of the protective layer, under the extremely high impact forces of even small particles at sonic velocities. It is known that such a weakness would give rise to failure of the previously known nickel or stainless steel protective layers under the same operating conditions. Also, where heater layers are incorporated then such voids would retract from their performance and might give rise to failure due to overheating.

Since the protective layer is thin, it is dependent for its mechanical integrity on the adhesive bond and thus protective layer must conform accurately to the surface to be protected to ensure optimum adhesive bond strength. Thus although a stock of preformed identical shoes can be retained, in many cases each shoe must be subject to final shaping operation in situ on the actual surface to be protected. It will be understood that when using a preformed shoe the adhesive can be applied to the surface and/or the shoe. The final shaping operation of the shoe will be performed at the temperature of plastic deformation of the alloy and may involve either of the techniques described above. It will be understood that the intimate correlation in shape can be achieved with relative ease by virtue of the super-plasticity.

Where the shaping operation is performed with the adhesive in situ then it is of course a requirement of the adhesive that it has a sufficiently long curing time to permit the alloy and component to be raised to the correct temperature, and to permit the sheet or sheets of alloy to be formed to the correct shape before the adhesive cures to a point at which relative movement between the sheet and the aircraft component is prevented. In the event that a suitable adhesive is not available then it will be understood that the sheet or shoe of alloy could be finally shaped on the actual surface but without adhesive present, the sheet or shoe then being removed to permit application of the adhesive in temperature conditions suited to the adhesive and the sheet or shoe then being replaced so as to be secured by the adhesive.In such a techique a spacer representing the adhesive layer may be interposed between the alloy and the surface during final shaping of the alloy so as to ensure eventual conformity between the alloy and the surface when the spacer is replaced by the adhesive.

Some airframe surface regions incorporate heater structures 14 for de-icing the surfaces in use. It is of course important to preserve the electrical integrity as well as the mechanical integrity of such surface regions and thus surface regions can be protected by sheets or shoes of super-plastic alloy as described above. It is envisaged that in some instances, for example helicopter rotor blades, it may be desirable to incorporate a heater into the protective alloy shoe prior to the application of the shoe to the surface to be protected.In such cases the alloy shoe may be preformed to the general surface shape and may then have a heater structure 14 bonded on to its aircraft surface enging face prior to fitting the shoe to the surface. it will of course be necessary for the heater structure to be sufficiently flexible to conform to the surface contours of the airframe surface during the final shaping operation to ensure an even, void free thickness of adhesive. Various epoxy resin adhesives can be used and also phenolic resin adhesives and acrylic resin adhesives can be used if desired. Where complex surface shapes are involved the heater structure 14 may be fitted to the surface prior to the application of the sheets of alloy. In such cases the assembly of the sheets of alloy to the aircraft surface will be as described previously.

The adhesives used may obviously be painted or sprayed in liquid form or may be applied in the form of a paste or a prepared film.

A suitable super-plastic alloy is "SPZ alloy" manufactured by the Imperial Smelting Company Limited. The exact formulation of this alloy is not known to us.

However, it is believed that the alloy is definable as "two-phase quaternary alloy having a fine grain microstructure stable at the temperature of super-plastic deformation, consisting of zinc within the range 70-82% by weight and aluminium 30-180/u by weight to which is added up to 0.25% by weight of magnesium and up to 2.006Xb by weight of one of the elements copper, nickel and silver," and that many alloys within this definition may be suitable.

Reference is made herein to "the temperature of super-plastic deformation". It is to be understood that this temperature is in practice a range of temperature although a particular temperature within the range may prove preferable in practice. For example super-plastic deformation of a suitable alloy can take place between approximately 150"C and 260"C but it is desirable in some applications in order to match the qualities of the adhesive used, the nature of the airframe region, and/or the degree of forming required, to perform the final shaping necessitating super-plastic deformation at the minimum temperature at which permanent dcformation of the layer of alloy can be ensured.It will be understood that the protective layer may be applied to the aircraft component which has the surface to be protected either while the component is on the aircraft or where possible while the component is detached from the aircraft.

It will be recognised that the shaping of the protective layer to accurately conform to the contours peculiar to the actual surface to be protected is a desideratum which is virtually impossible to achieve in practice using the previously known non-superplastic metal protective members.

WHAT WE CLAIM IS: 1. A method of protecting a surface region of an aircraft including starting with a sheet or sheets of super-plastic alloy, engaging the sheet or sheets of super-plastic alloy with the surface region to be protected, shaping the sheet or sheets of super-plastic alloy at a temperature such that the super plastic properties of the alloy are exhibited to follow the surface contours of the surface region to be protected, and securing the protective layer of super-plastic alloy so formed to said surface region.

2. A method as claimed in claim 1 wherein the formed layer is secured to said surface region by means of an adhesive.

3. A method as claimed in claim 2 wherein the adhesive is applied to the surface region to be protected and/or the sheet or sheets of alloy prior to the shaping of the sheet or sheets.

4. A method as claimed in any one of the preceding claims wherein the sheet or sheets are preformed to the general shape of the surface region and are finally shaped in situ so as to conform to the surface contours peculiar to the actual surface region.

5. A method as claimed in any one of the preceding claims wherein the alloy from which the protective layer is formed is that known as: "SPZ alloy" manufactured by the Imperial Smelting Company Limited; or is a two-phase quaternary alloy having a fine grain microstructure stable at the temperature of super-plastic deformation consisting of zinc within the range 70-82% by weight and aluminium 30-18% by weight to which is added up to 0.25% by weight of magnesium and up to 2% by weight of one of the elements copper, nickel and silver.

6. An aircraft surface region having secured thereto a protective layer of superplastic alloy.

7. An aircraft surface region as claimed in claim 6 incorporating a heater structure and the protective layer of super-plastic alloy overlying the heater structure.

8. An aircraft surface region as claimed in claim 6 or claim 7 wherein the alloy from which the protective layer is formed is that

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. preserve the electrical integrity as well as the mechanical integrity of such surface regions and thus surface regions can be protected by sheets or shoes of super-plastic alloy as described above. It is envisaged that in some instances, for example helicopter rotor blades, it may be desirable to incorporate a heater into the protective alloy shoe prior to the application of the shoe to the surface to be protected.In such cases the alloy shoe may be preformed to the general surface shape and may then have a heater structure 14 bonded on to its aircraft surface enging face prior to fitting the shoe to the surface. it will of course be necessary for the heater structure to be sufficiently flexible to conform to the surface contours of the airframe surface during the final shaping operation to ensure an even, void free thickness of adhesive. Various epoxy resin adhesives can be used and also phenolic resin adhesives and acrylic resin adhesives can be used if desired. Where complex surface shapes are involved the heater structure 14 may be fitted to the surface prior to the application of the sheets of alloy. In such cases the assembly of the sheets of alloy to the aircraft surface will be as described previously. The adhesives used may obviously be painted or sprayed in liquid form or may be applied in the form of a paste or a prepared film. A suitable super-plastic alloy is "SPZ alloy" manufactured by the Imperial Smelting Company Limited. The exact formulation of this alloy is not known to us. However, it is believed that the alloy is definable as "two-phase quaternary alloy having a fine grain microstructure stable at the temperature of super-plastic deformation, consisting of zinc within the range 70-82% by weight and aluminium 30-180/u by weight to which is added up to 0.25% by weight of magnesium and up to 2.006Xb by weight of one of the elements copper, nickel and silver," and that many alloys within this definition may be suitable. Reference is made herein to "the temperature of super-plastic deformation". It is to be understood that this temperature is in practice a range of temperature although a particular temperature within the range may prove preferable in practice. For example super-plastic deformation of a suitable alloy can take place between approximately 150"C and 260"C but it is desirable in some applications in order to match the qualities of the adhesive used, the nature of the airframe region, and/or the degree of forming required, to perform the final shaping necessitating super-plastic deformation at the minimum temperature at which permanent dcformation of the layer of alloy can be ensured.It will be understood that the protective layer may be applied to the aircraft component which has the surface to be protected either while the component is on the aircraft or where possible while the component is detached from the aircraft. It will be recognised that the shaping of the protective layer to accurately conform to the contours peculiar to the actual surface to be protected is a desideratum which is virtually impossible to achieve in practice using the previously known non-superplastic metal protective members. WHAT WE CLAIM IS:

1. A method of protecting a surface region of an aircraft including starting with a sheet or sheets of super-plastic alloy, engaging the sheet or sheets of super-plastic alloy with the surface region to be protected, shaping the sheet or sheets of super-plastic alloy at a temperature such that the super plastic properties of the alloy are exhibited to follow the surface contours of the surface region to be protected, and securing the protective layer of super-plastic alloy so formed to said surface region.

known as; "SPZ alloy" manufactured by the Imperial Smelting Company Limited; or is a two phase quaternary alloy having a fine grain microstructure stable at the temperature of super-plastic deformation consisting of zine within the range 70-82% by weight and aluminium 30-18% by weight to which is added up to 0.25% by weight of magnesium and up to 2% by weight of one of the elements copper, nickel and silver.

9. An aircraft surface region as claimed in claims 6, 7 or 8 in which the component whose surface is to be protected incorporates strengthening fibres.

10. A protective shoe of a super-plastic alloy which is shaped to fit onto a predetermined aircraft surface region, said shoe being intended to be finally shaped in situ so as completely to follow the surface contours peculiar to the surface region for which it defines the protective layer, said shoe incorporating a heater structure so that the shoe and the heater structure are applied, in use, to the predetermined surface region in the same operation.

11. A protective shoe as claimed in claim 10 wherein the alloy from which the protective layer is formed is that known as: "SPZ alloy" manufactured by the Imperial Smelting Company Limited; or is a two phase quaternary alloy having a fine grain microstructure stable at the temperature of super-plastic deformation consisting of zinc within the range 70-82% by weight and aluminium 30-18% by weight to which is added up to 0.25% by weight of magnesium and up to 2% by weight of one of the elements copper, nickel and silver.

12. A method of protecting a surface region of an aircraft substantially as hereinbefore described with reference to the accompanying drawings.

13. An aircraft surface region protected in accordance with the method claimed in any one of claims 1 to 6 and claim 12.

14. An aircraft surface region substantially as hereinbefore described with reference to the accompanying drawings.

15. A protective shoe substantially as hereinbefore described with reference, to and as shown in, the accompanying drawings.