GB2295594A

GB2295594A - A lighting diverter for aircraft

Info

Publication number: GB2295594A
Application number: GB9524487A
Authority: GB
Inventors: Richard Brian Morris
Original assignee: Chelton Electrostatics Ltd
Current assignee: Chelton Ltd
Priority date: 1994-12-01
Filing date: 1995-11-30
Publication date: 1996-06-05
Anticipated expiration: 2015-11-30
Also published as: GB2295594B; GB9524487D0

Abstract

A lightning diverter 1 is provided for protecting an aircraft body part (eg randome) 3 from lightning, which comprises a material having a positive temperature coefficient of resistivity in excess of that of a metallic material. The material, e.g. carbon black loaded ethylene vinyl acetate, exhibits the PTC effect as the diverter is heated by lightning current and as the line resistance of the diverter increases with temperature, an electric field is established at the surface of the diverter sufficient to ionise air. As the resistance of the diverter increases above that of the ionised air, the lightning current is ejected from the diverter into the current path provided by the ionised air above the diverter. Various cross-sectional shapes of diverter are disclosed such as triangular (figure 2), inverted-T (figure 4) and rounded (figure 5). The diverter is stated to be cheap to manufacture and can withstand multiple strokes. <IMAGE>

Description

A LIGHTNING DIVERTER This invention relates to lightning diverters for diverting lightning away from a body to be protected therefrom, and in particular relates to lightning diverters for diverting lightning away from certain parts of an aircraft body, such as the radome, panels and blades constructed purely of dielectric materials.

Lightning diverters are widely used for protecting sensitive parts of an aircraft body from lightning strokes and divert lightning away from these parts of the aircraft safely to ground. An aircraft's navigational and communications instrumentation, including radar antennae, which includes many metallic parts is often housed at extreme positions of the aircraft such as in the nose or tail. These parts and other extremities of the aircraft such as the wing tips are, by virtue of their position, the most likely to be struck by lightning. The part of the aircraft which houses the radar and communication equipment, such as the nose radome, is, of necessity, made from a material which is transparent to electromagnetic radiation and its insulation can easily breakdown (flashover) in the presence of the intense electric fields which immediately precede a lightning strike.

Thus, the instrumentation housed in the radome is extremely vulnerable to lightning strikes and without protection lightning can easily pierce the radome and cause extensive damage to both radome and instrumentation.

To prevent lightning penetrating the radome, it is known to use a number of regularly spaced metallic strips extending from the nose tip rearwardly over the radome to ground positions on the aircraft. These lightning diverter strips protect the instrumentation by ensuring that the lightning leader attaches to one or more of the strips in preference to a metallic object within the radome.

A known lightning diverter strip comprises a thin metal foil, for example as described in US Patent No.

4583702, to Baldwin. However, the strip vaporises on carrying the return current from a lightning stroke and therefore can only be used once. Furthermore, the metal foil tends to interfere with the transmission of RF radiation.

Another known lightning diverter comprises a metal bar which can withstand more than one lightning strike but at the expense of increased drag and weight. Furthermore, the bars can deform during heavy strikes which causes the radome to buckle. They also interfere with RF transmission.

Another known lightning diverter comprises a thin strip of oxidised aluminium particles finely deposited on a flexible dielectric substrate. This arrangement is an improvement over the metal foil strips in that it interferes less with RF radiation, but it can still burn out after a single lightning strike.

Another known lightning diverter strip is a so-called segmented diverter strip which consists of a series of thin conductive disc-like segments fastened to a strip of resistive material. In the presence of an intense electric field, generated by the approaching lightning leader, the air in the gaps between the segments becomes ionised, providing a conductive path for the return current. One problem with this arrangement is that occasionally the air between two adjacent segments fails to ionise, for example due to the presence of water between the segments, so that current is carried by the resistive backing strip which consequently burns out. A further problem with this arrangement is that it is relatively complicated to manufacture and relatively expensive.

There is therefore a need for an improved lightning diverter which is capable of withstanding multiple strokes, is reliable and cheap to manufacture.

According to one aspect of the present invention, there is provided an aircraft body part including a lightning diverter for protecting the part from lightning, the diverter comprising a material having a positive temperature coefficient of resistivity.

The positive temperature coefficient of resistivity of the material is greater than that of a metallic material as the material is heated by lightning current. The term 'metallic material' means metals or materials substantially consisting of metal whose positive temperature coefficient of resistivity, if any, remains relatively low in the temperature range of interest.

Preferably, the resistivity of the diverter material in the absence of current is selected such that a lightning stroke attaches to the diverter in preference to the part of the aircraft to which the diverter is attached.

Preferably, the resistivity of the diverter material increases to a value, when lightning current is flowing therethrough, such that the electric field in the direction of current flow is sufficient to ionise air above the material before the current becomes sufficient to vaporise the material.

In a preferred embodiment, the resistivity of the diverter material has a fractional change with increasing temperature of at least 0.02 per OC at at least one value of temperature as the material is heated by lightning current.

Preferably, the resistivity of the diverter changes by a factor of at least 10 as the material is heated by lightning current, and more preferably by a factor of at least 100.

Preferably, the diverter has an initial resistivity in the absence of current selected such that a lightning strike attaches to the diverter in preference to said part at at least temperatures between -100C and +500.

In one embodiment, the diverter is a strip of the material and has a line resistance between -100C and +50C of between 0.02 and 2 megohms per metre in the absence of current.

The diverter may comprise a composite material and in one embodiment, the diverter may comprise an insulating material having conductive particles dispersed therein. The insulating material may comprise an elastomer and the elastomer may be ethylene vinyl acetate. The conductive particles may comprise carbon.

In an alternative embodiment, the diverter may comprise a semiconductor material having conductive particles dispersed therein.

Advantageously, one embodiment of the diverter comprises a strip which may have a substantially flat inner face for securing to the outer surface of the body part and an outer surface including an elongate ridge formation. The ridge formation serves to discourage water residing on at least part of the outer surface of strip. The outer surface of the strip may be shaped to encourage liquid to flow in a direction transverse to the length of this strip. In one embodiment, the elongate ridge formation comprises a camber between the edges of the strip. Preferably the radius of curvature at the ridge is relatively small so as to discourage water films residing thereon. Advantageously, such a ridge will also serve to concentrate the electric field in this region. In one particular embodiment, the strip has a generally triangular cross-section over at least part of its length.

The diverter strip preferably has a width of between 0.5 mm and 5 mm.

According to another aspect of the present invention, there is provided an aircraft body part including a lightning diverter for protecting the body part from lightning, the diverter comprising a composite material, for example, an insulator or semiconductor material having conductive particles dispersed throughout the volume thereof. Preferably, the conductive particles are dispersed so as to provide local variations of resistance in the material. In a preferred embodiment the resistance variations are such as to produce local electric field regions at the surface of the material sufficient to ionise air when the material is conducting lightning current, while the average resistance of the material is generally less than that required to ionise air for the same value of lightning current. Preferably the resistance variations occur on a microscopic scale.

Further embodiments of this aspect of the invention may include any one or more features described above in relation to the first aspect of the invention.

Alternatively, the diverter may comprise a conductor material dispersed with insulator throughout its volume.

According to another aspect of the present invention, there is provided a diverter strip for protecting an aircraft body part from lightning, the strip having an inner face for securing to the outer surface of the body part and an outer surface including an elongate ridge formation.

The elongate ridge formation may be formed at one edge of the strip or between the edges. The ridge provides a raised portion at the outer surface of the diverter so as to discourage water from residing on at least part of the strip, which otherwise tends to impair the effectiveness of the diverter. Part of the outer surface may be sloped towards an edge of the strip. The sloped portion may be curved and may be either concave or convex. Alternatively, the sloped portion may be substantially flat. Advantageously, the ridge formation serves not only to discourage water films from residing on that part of the strip, but also serves to enhance the electric field intensity at the outer surface of the diverter to help ensure that lightning attaches to the strip in preference to any other site.

In one particular embodiment, the diverter strip has a generally triangular cross-section over at least a part of its length.

Further embodiments of the diverter strip may comprise any one or more of the features of the above described diverter included with said aircraft body part.

According to a further aspect of the present invention, there is provided a diverter strip comprising a material having a positive temperature coefficient of resistivity, having a line resistance in the absence of current of between 0.02 and 2 megohms per metre between -100C and +50C, and a resistivity having a fractional change with increasing temperature of at least 0.020C1 at at least one value of temperature as the material is heated by lightning current.

Further embodiments of the diverter strip may include any one or more of the features of the above described diverter included with the aircraft body part.

According to another aspect of the present invention, there is provided a method of protecting an aircraft body part from lightning comprising the steps of applying to said body part a diverter comprising a material having a positive temperature coefficient of resistivity, to divert lightning away from said body part.

Further embodiments of the method comprise the step of applying to said body part a diverter which further includes any one or more of the features described above in relation to the diverter included with the aircraft body part.

The essence of the present invention lies in the provision of a lightning diverter consisting of a material having a positive coefficient of resistance with temperature (PTC) and/or of a material comprising an insulator or semiconductor having conductive particles dispersed therein. It is important that the material exhibits certain properties under the intended operating conditions and as long as the material exhibits these properties, no further limitations on selection of the material are imposed.

The properties which the material should exhibit within the intended range of operating temperatures are as follows.

(a) For a given diverter geometry, the resistance of the material in the absence of current flowing through it should be sufficiently low so that the lightning leader attaches to the diverter in preference to the body or body part to be protected.

(b) In the case of a PTC material, the coefficient of resistance of the material with increasing temperature must produce a positive change in resistance in the presence of current sufficient to establish an electric field at the surface of the material equal to or greater than that required to ionise air before the temperature of the material increases to a value which vaporises the material. In the case of a material comprising an insulator or semiconductor with conductive particles dispersed therein (which may also be a PTC material) the material must produce local electric field regions in the presence of current sufficient to ionise air at the surface of the material, at a mean value of current flowing through the material that can be withstood thereby without substantial damage.

The initial resistance of the diverter is selected to be lower than the resistance of the surface to which it is attached in the temperature range in which it is desired to protect the body from lightning. Most lightning strikes occur at temperatures between -100C and +5 C, although they may occur outside this range. The normal operating temperature range for aircraft is typically from -600C to +500C although again, it can be outside this range depending on the ambient temperature, and the speed and shape of the aircraft. The lower limit of the resistivity of the surface of the body to be protected may be set by, for example, surface films of water or ice or antistatic paint and has a value typically in the range of 0.02 to 2 megohms per metre.

The principle of operation of the diverter comprising a PTC material is as follows. As the diverter begins to carry current during the leader attachment phase, the temperature of the diverter increases due to ohmic heating. As the diverter has a positive coefficient of resistance with temperature (PTC), the increase in temperature causes the resistance to increase at a rate such that the material attains a value of resistance sufficient to cause the development of an electric field at or above the value required to ionise air, before the temperature of the material, due to ohmic heating, reaches a value sufficient to cause irreversible damage to the diverter or to vaporise the material.

Once the electric field at the surface of the material is sufficient to ionise air, the current flowing through the diverter is ejected from the diverter and flows along the conduction path provided by the ionised air, safely to ground.

Thus, the invention provides a diverter which can withstand multiple lightning strokes, since the return current is not carried by the diverter itself.

Furthermore, because the ionisation of air above the diverter does not depend on the presence of conductive discs (as with the segmented diverter), the surface of the diverter can be smooth, without undulations in which water or other surface contaminants may be entrapped, thereby more reliably ensuring that an ionising field is established at the surface of the material.

In a preferred embodiment of the invention, the diverter is formed as an elongate strip having a triangular cross-section. Advantageously, the triangular cross-section provides a ridge at the outwardly extending apex thereof, which intensifies the electric field in this region to ensure that the lightning attaches to the diverter as opposed to any other site, optimises the shedding of water films that may be present on the diverter, in service, and facilitates application of the diverter to the surface of the body part to be protected, while retaining a relatively streamlined profile.

Examples of embodiments of the present invention will now be described with reference to the drawings in which: Figure 1 shows a plurality of diverter strips in accordance with a preferred embodiment of the invention applied to the radome of an aircraft, Figure 2 shows an expanded view of one of the diverter strips shown in Figure 1, Figure 3 shows a cross-sectional view of the diverter strip shown in Figure 2, Figure 4 shows a cross-sectional view of an alternative embodiment of a diverter strip, Figure 5 shows a cross-sectional view of another embodiment of a diverter strip; Figures 6a to 6d illustrate the operation of an embodiment of the diverter during the leader attachment phase and high current phase, and Figure 7 is an example of a graph of the resistance-temperature characteristics of a PTC material suitable for an embodiment of the invention.

One embodiment of the diverter comprises a conductive polymer. The conductive polymer may comprise, for example, a carbon black loaded elastomer such as ethylene vinyl acetate (EVA). In one embodiment, the material is ethylene vinyl acetate loaded with 10 to 25 percent by weight of carbon with low specific area, for example in the range of 100 to 300 m2 per gram. Materials with a higher specific area tend not to exhibit the PTC effect.

Preferably the material has a specific (block) resistance in the range from 5 to 100 ohm cm, for example 20 ohm cm. The material has a high positive coefficient of resistance with temperature in excess of 2 percent per OC, for example in the region of 8 percent per OC, above the normal operating temperature.

The initial line resistance for the diverter is moderately low, for example in the range of 0.02 to 2 megohms per metre, eg 0.1 megohm per metre and is selected to mask the conductive effects of antistatic paint and water films on the radome surface, without seriously interfering with RF transmission.

Advantageously, the cross sectional area of the diverter strip can be as small as 4 mm2. Thus, relatively little material is required to manufacture the diverter, so that it can be lightweight and relatively inexpensive and furthermore, it presents a relatively small cross-section to electromagnetic radiation transmitted across the radome. With this cross-sectional area the diverter strip is capable of channelling high energy lightning pulses of about 200 kiloamps with only superficial damage to the diverter.

Referring to Figure 1, a plurality of diverter strips 1 according to a preferred embodiment of the present invention, are shown positioned around the nose radome 3 of an aircraft, extending from the nose tip 5 to the back of the radome 7, where they make contact with the conductive shell or frame of the aircraft fuselage 9.

Figures 2 and 3 show a preferred embodiment of a diverter strip 1 in more detail. The diverter strip 1 has a triangular cross section 11 which provides a ridge 13 running along the length of the strip 1.

Advantageously, the ridge 13, which in this embodiment has a small radius of curvature, serves to discourage the build up of water films and other deposits, such as icing, on that part of the strip 1. Furthermore, the small radius of curvature of the ridge 13, serves to intensify and so concentrate the electric fields at that part of the strip. As can be seen from Figure 3, the upper portion 15 of the diverter strip 1 lies above the surface of the water film and/or ice layer 17, which covers the surface of the substrate 19 to which the diverter 1 is attached. This ensures that lightning attaches directly to diverter through the medium of air only. Furthermore, the exposed surfaces 21 and 23 either side of the ridge 13 facilitate positioning the strip 1 on the radome surface 4 or other surface to which it is to be fixed.

Figures 4 and 5 each show an alternative cross-sectional geometry for a diverter strip 1. The geometry shown in Figure 4 has the appearance of an inverted 'T', with the upper part including a triangular cross-section 11, which forms a ridge 13 whose apex, again, has a small radius of curvature to discourage water films residing on that part of the strip 1. Alternatively, the top of the ridge could be flat. The height 'H' of the strip 1 is sized such that at least part of the strip 1 lies above the level of the water film 17 covering the substrate 19 to which the strip 1 is bonded or otherwise fixed.

Advantageously, the lower, flat part 25 of the strip 1 adjacent the substrate 19 is of sufficient width to provide a good area of contact with the substrate 19, for adhesion strength, whereas the upper portion 15 of the strip 1 provides a ridge 13 which presents a relatively small cross-sectional area to the slipstream to reduce drag when the aircraft is in flight.

The cross-sectional geometry 11 of the strip 1 shown in Figure 5 is essentially triangular, wherein the sides 23, 21 of the triangle adjacent the apex 13 are convex, and the apex 13 has a smaller radius of curvature than the sides 23, 21, thereby providing a ridge between the edges 27, 29 of the strip 1. The height 'H' of the strip 1 is sized such that at least part of the strip 1 lies above the level of the water film 17 covering the substrate 19 to which the strip 1 is bonded or otherwise fixed.

Diverter Operation The detailed operation of the lightning diverter will now be described with reference to Figure 6, which illustrates the stages of lightning attachment and diversion, and with reference to Figure 7, which shows the resistance-temperature characteristics of a particular embodiment of a suitable material.

The first stage in the lightning leader attachment phase is shown in Figure 6a, which shows the electric field condition at various points on and in the vicinity of an aircraft nose radome 3 immediately preceding a lightning strike. This initial stage can be described using electrostatics only, ie., no current is flowing in the diverter 1.

In this initial stage, the nose radome is placed in a uniform external electric field E. The presence of the radome modifies the external electric field E in the vicinity of the radome. The electric field condition is such that the electric field EA at the end 2 of the diverter strip 1 is greater than the electric field EB at the tip 5 of the nose radome 3 which is greater than the background electric field E (ie. EA > EB > E) The next stage of the leader attachment phase is shown in Figure 6b, in which the field intensity EA at the end 2 of the diverter 1 reaches the corona threshold value (3 megavolts per metre at sealevel) so that a corona discharge is established at the end of the diverter 1. Current i begins to flow through the diverter 1 and increases prior to leader attachment.

In the third stage, shown in Figure 6c, the lightning leader attaches to the diverter 1. At the same time, the electric field EB at the tip 5 of the nose radome 3 begins to decrease. The resistance of the diverter strip 1 rises rapidly due to the PTC effect, and the voltage drop AV along the length of the diverter 1 increases by virtue of both the increasing current i flowing through the diverter 1 and its increasing resistance with temperature (through ohmic heating). This doubly increasing voltage drop AV very rapidly establishes an electric field at the surface of the diverter 1 which is sufficient to ionise air. Thus, an ionised air channel above the surface of the diverter is created.

Figure 6d shows the high current phase which immediately follows the creation of an ionised air channel above the diverter strip at the end of the leader attachment phase. Once an ionised air channel has been established above the diverter strip, the lightning current is ejected from the diverter 1 into the ionised air channel which then provides a low resistance conduction path to ground (provided by the aircraft structure 9) to carry the high current return stroke.

Referring now to Figure 7, the particular material used for the diverter strip was ethylene vinyl acetate impregnated with carbon powder, and was formed from a batch designated 'EXPO26', obtained from Whitaker Technical Plastics Limited, U.K. The particular strip under test had a length of 345mm.

The resistance-temperature characteristics of the material shown in Figure 7 are divided into three distinct regions which illustrate the switching action of the diverter.

In region A, to the left of the dashed line 10, the diverter strip has a relatively low line resistance which gradually increases from about 0.06 megohms per metre at -600C to about 0.15 megohms per metre at about SOC.

In region B, which lies between the two dashed lines 10 and 12, the line resistance of the material increases rapidly with increasing temperature from a transition temperature at between 0 and 100C to a temperature of between 50 and 600C where the line resistance is about 7 megohms per metre. Accordingly, in region B, the resistance increases by a factor of approximately 50. Region B is the active region in which the material effectively switches from a low resistance state to a high resistance state. In this region the coefficient of resistance with temperature defined as 1 dR - 0.081 C R where R is the line resistance of the diverter strip, and dR is the change of resistance with temperature.

dT In region C, to the right of the dashed line 12 in Figure 7, the resistance of the material reaches a maximum value between about 60 and 800C and then begins gradually to decrease with increasing temperature.

The material whose resistance-temperature characteristics are shown in Figure 7 is suitable for civil aircraft applications in which the operating temperatures range typically from -60 to +200C.

However, the temperature at which most lightning strikes occur is between -10 and +50C. It is important that the diverter has a resistance to ground, at the temperature at which a lightning strike occurs, sufficiently low so that the lightning leader will attach to the diverter strip in preference to any other neighbouring site, eg the surface to which it is fixed. At the same time, the resistance should be high enough not to interfere significantly with RF transmission. Both these criteria are met in region A of Figure 7, where the material has a relatively low resistance over the range of temperatures at which lightning is most likely to occur.

In the event of a lightning strike somewhere in the temperature range of region A, current passing through the diverter strip, during the leader attachment phase, causes ohmic heating of the material so that the temperature of the material increases into region B. At the transition temperature shown as J in Figure 7, the resistance of the material increases rapidly. As the current of the leader increases, the temperature and hence resistance of the material increases until at some point within the active region B, the electric field along the surface of the strip will reach the breakdown field of air, (typically 3MVml) and a low resistance conduction path of ionised air will form above the strip.The current flowing in the diverter strip will then be ejected into the ionised air and the return current which can be of the order of many kilo amps will be safely conducted to ground by the ionised air channel. The temperature and therefore the resistance of the strip will subsequently decrease and return, for example, to region A. The diverter will then remain in the low resistance state ready to divert another lightning strike. The current at which ejection occurs may be of the order of 50 amps and the diverter may switch from the low resistance state to the high resistance state in a time of about 1 to 10 microseconds.

An important aspect of conductive particle impregnated insulators in achieving the desired effect lies in the manner in which air outside the material is ionised. The conductive particles dispersed in the material have interparticle distances which vary on a microscopic scale. Accordingly, on a microscopic scale, the resistance of the material varies and has properties of a percolation network which has fractal characteristics. Thus, when a current is passing through the material, the electric field induced in the material in the direction of current flow will also vary on a microscopic scale. Hence, the electric field required to ionise air will first occur in those local regions of relatively high resistance.Because, on a random statistical basis, there are many high resistance regions throughout the material, the overall effect is that the electric field necessary to ionise air will be reached at a value of current much less than the current which would otherwise be required if the resistance throughout the material was constant on a microscopic scale. Accordingly, the material creates an ionised conduction path above the surface thereof at moderately low power levels, and well before the material reaches a temperature which would cause damage to the material or cause the material to vaporise.

If the material reaches a temperature above the active region (e.g. region B in Figure 7), where the resistance of the material tends to a constant value or starts to decrease, and the current has not yet been ejected from the material, it is unlikely that the current will be ejected in this region. Thus, it is important that the material is designed so that the lightning event occurs in either regions A or B but not in region C. If the event occurs in region B, then preferably the temperature at which it occurs is such that the resistance of the material increases by an order of magnitude before reaching the temperature of maximum resistance of the material: the upper threshold temperature of the material, for example, as indicated by K in Figure 7.Furthermore, it is preferable that the resistance of the material changes by two orders of magnitude in the active region (region B).

Of course, as the resistance of the material increases when the diverter is active, the diverter will have even less effect on the transmission of RF radiation.

Although the preferred form of the diverter is a linear strip, the diverter may be formed in any other suitable shape. Furthermore, the diverter may have any suitable cross-sectional geometry, for example square, rectangular, circular, semicircular, polygonal etc, although triangular is preferred for the reasons given above.

The diverter can be applied to any part of an aircraft where it is desirable to do so, for example, the tail or wing tips, and its shape may be tailored appropriately.

The diverter may also be used in other applications, for example for other aircraft, such as missiles, rockets, and weather balloons, and may also be used to protect ground based installations.

Modifications to the various embodiments of the diverter as herein described will be apparent to those skilled in the art.

Claims

CLAIMS:

1. An aircraft body part including a lightning diverter for protecting the part from lightning, the diverter comprising a material having a positive temperature coefficient of resistivity in excess of that of a metallic material.

2. An aircraft part as claimed in claim 1, wherein the resistivity of said diverter material in the absence of current is selected such that a lightning stroke attaches to the diverter in preference to the part of the aircraft to which the diverter is attached.

3. An aircraft part as claimed in claim 1 or 2, wherein the resistivity of the diverter material increases to a value, when lightning current is flowing therethrough, such that the electric field in the direction of current flow is sufficient to ionise air above the material before the current becomes sufficient to vaporise the material.

4. An aircraft part as claimed in claim 3, wherein the resistivity of said diverter material has a fractional change with increasing temperature of at least 0.02 oC-1 at at least one value of temperature as the material is heated by lightning current.

5. An aircraft part as claimed in claim 3 or claim 4, wherein the resistivity of the diverter changes by a factor of at least 10 as the material is heated by lightning current.

6. An aircraft part as claimed in claim 5, wherein said factor is at least 100.

7. An aircraft part claimed in any preceding claim, wherein said diverter has an initial resistivity in the absence of current selected such that a lightning strike attaches to the diverter in preference to said part at at least temperatures between -100C and +50C.

8. An aircraft part as claimed in the preceding claim, wherein the diverter is a strip of the material and has a line resistance between -100C and +50C of between 0.02 and 2 megohms per metre in the absence of current.

9. An aircraft part as claimed in any preceding claim, wherein said diverter comprises an insulating material having conductive particles dispersed therein.

10. An aircraft part as claimed in claim 9, wherein said insulating material comprises an elastomer.

11. An aircraft part as claimed in claim 10, wherein said elastomer comprises ethylene vinyl acetate.

12. An aircraft part as claimed in any one of claims 9 to 11, wherein said conductive particles comprise carbon.

13. An aircraft part as claimed in any one of claims 1 to 8, wherein said diverter comprises a semiconductor material having conductive particles dispersed therein.

14. An aircraft part as claimed in any preceding claim, wherein said diverter is shaped as a strip, the strip having an inner face for securing to the outer surface of the body part and an outer surface including an elongate ridge formation.

15. An aircraft part as claimed in claim 14, wherein the strip has a generally triangular cross-section over at least part of its length.

16. An aircraft part as claimed in either claim 14 or 15, wherein at least part of said outer surface is curved in cross-section.

17. An aircraft part as claimed in claim 14 or 15, wherein the width of said strip is between 0.5mm and 5mm.

18. An aircraft part as claimed in any preceding claim, wherein said body part comprises any of a radome, a wing, a fuselage tail, a tail fin and a tail plane.

19. A diverter strip for protecting an aircraft body part from lightning, the strip having an inner face for securing to the outer surface of the body part and an outer surface including an elongate ridge formation.

20. A diverter strip as claimed in claim 19, wherein said strip has a generally triangular cross-section over at least a part of its length.

21. A diverter strip as claimed in either claim 19 or 20, wherein at least part of said outer surface is curved in cross-section.

22. A diverter strip as claimed in any one of claims 19 to 21 and further including any one or more of the features of the diverter defined in claims 1 to 13 or 17.

23. A diverter strip comprising a material having a positive temperature coefficient of resistivity, having a line resistance in the absence of current of between 0.02 and 2 megohms per metre between -100C and +50C, and a resistivity having a fractional change with increasing temperature of at least 0.020C1 at at least one value of temperature as the material is heated by lightning current.

24. A diverter strip as claimed in claim 23, and further including any one or more of the features of the diverter defined in Claims 1 to 7 or 9 to 17.

25. A lightning diverter disposed adjacent a body to be protected from lightning, the diverter comprising a material whose composition is substantially homogeneous on a macroscopic scale and which is capable of establishing an electric field sufficient to ionise air adjacent the diverter when the diverter is conducting lightning current.

26. A lightning diverter as claimed in claim 25, wherein the resistance of the material increases to above that of ionised air as the diverter conducts lightning current.

27. A lightning diverter as claimed in claim 25 or 26, wherein the diverter comprises a composite material.

28. A lightning diverter as claimed in any one of claims 25 to 27, wherein the diverter comprises a material having a positive temperature coefficient to resistivity in excess of that of a metallic material.

29. A lightning diverter as claimed in any one of claims 25 to 28, wherein the diverter includes any one or more of the features of the diverter defined in claims 2 to 24.

30. A lightning diverter disposed adjacent a body to be protected from lightning, the diverter comprising an insulator or semiconductor material having conductive particles dispersed throughout the volume thereof, said material being capable of establishing an electric field sufficient to ionise air adjacent the diverter when the diverter is conducting lightning current.

31. A method of protecting an aircraft body part from lightning comprising the step of applying to said body part a diverter comprising a material having a positive temperature coefficient of resistivity in excess of that of a metallic material to divert lightning away from said body part.

32. A method as claimed in claim 31, wherein the diverter comprises any of the features of the diverter defined in claims 2 to 30.

33. A method as claimed in claim 31 or 32, wherein said aircraft body part comprises any of a radome, a wing, a fuselage tail section, a tail fin and a tail plane.

34. An aircraft body part substantially as hereinbefore described with reference to and illustrated by any of the drawings.

35. A diverter strip substantially as hereinbefore described with reference to and illustrated by any of the drawings.

36. A lightning diverter substantially as hereinbefore described with reference to and illustrated by any of the drawings.

37. A method of protecting an aircraft body part substantially as hereinbefore described with reference to and illustrated by any of the drawings.