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-   -   787 Lightning strike issues v FAA (https://www.pprune.org/rumours-news/627922-787-lightning-strike-issues-v-faa.html)

Grebe 10th Jan 2020 02:31

Originally Posted by A30_737_AEWC (Post 10658460)
Short answer maybe - but highly misleading.

Carbon fibre reinforced composites utilised in primary airframe structures are generally of the prepreg tape or woven cloth type. Cylindrical-structures may also be filament wound. Characteristic of all these forms is that the reinforcing fibres are continuous. Fibres are usually bundled in tows and woven to form the dry cloth reinforcement.

Polymer matrices reinforced with discontinuous fibres are generally employed for secondary/tertiary structures where structural integrity is not the primary design requirement.

You may be missing the point re lightning strike and composite. While prepreg tape is continuous fiber, when it comes to electrical conductivity - it doesn''t help UNLESS one figers out how to terminate each and every or at least most at both ends of the tape for electrical continuity to metal grounded structure. And tapes are typically laid in angles between each pass or layer eventually resulting in a near 90 degrees difference between x layers thus requiring termination to ground for each layer to be effective. This since a high energy strike can easily penetrate several ' layers ' of prepreg. Add to that the relatively low current capability of each ' layer' of prepreg tape makes the continuity issue of carbon long continuous carbon fibers pretty much a non issue re lightning strike. For a crude example - grind up or cut a bunch of cured prepreg fibers- fill a small plastic box with the cut fibers. Now put one probe in opposite walls of the plastic box and measure the continuity between the probes. or put a high voltage circuit output on the probes and do the same test instead of an ohmmeter.

clark y 10th Jan 2020 09:17

On the B787s my company operate, they never seem to lose static wicks with lightning strikes but there can be many (sometimes very many) exit points on the fastener heads. The Airbusses Iíve been around seem to lose static wicks.
If one looks in the main wheel well of a B787 youíll see a green wire about the size of a garden hose. Engineers have told me it is to help with the bonding.

clark y 10th Jan 2020 09:22

This reminds me of this thread-

Lightning strikes and FBW aircraft: Airbus

A30_737_AEWC 11th Jan 2020 10:43

Originally Posted by Grebe (Post 10658601)
You may be missing the point re lightning strike and composite. ................

Nope, not missing the point. Solely responding to the OP's single sentence post/response (# 26) regarding discontinuous fibres embedded in a non-conductive matrix. Not a typical method for fabrication of primary structures, most of which must also display adequate conductivity to address lighting strikes, p-static dissipation and the general electrical bonding requirements of airframe structures.

Now, to address yours...........

Woven and knitted dry fibre preforms for structural components (which is where the research/fabrication developments in composite primary structures have been moving in recent years) lend themselves towards having suitable conductive meshes/conductive 'fingers' at attachment points, etc. incorporated into them, ahead of their impregnation with structural resins (see 'resin transfer moulding', 'resin infusion', etc.). I'm not aware that these approaches have been validated in widespread use, but I can see the potential benefits.

Can you give me some real-world airframe examples where every lamina in a laminate (via the conductive fibres) is effectively electrically bonded to adjacent structure ? Open question. I haven't seen many solid monolithic laminates (fighter aircraft wing skin, 50 plies) or honeycomb panels (fibreglass/carbon/boron skins, aramid cores) with the kind of treatment you suggest. Probably because it's that airframe structures must be producible and are not 'science experiments'. The conductivity is typically effected through the outermost plies/surfaces or with conductive surface coatings/diverter strips/etc. grounded to the adjacent mounting/support structures.

Taking your point on grounding all lamina of a laminate (monolithic or honeycomb with facings), look at how non-metallic radomes survive lightning strikes. Not very well, usually. Conductivity and RF transparency (active/passive systems) are competing performance requirements, with the later usually trumping the former.

Grebe 11th Jan 2020 15:41

Can you give me some real-world airframe examples where every lamina in a laminate (via the conductive fibres) is effectively electrically bonded to adjacent structure ? Open question.
Guess I didn't make myself clear- my point was that UNLESS one could reasonably figure out HOW to ground both ends of a ' layer' of prepreg tape, the issue of continuity of a fiber layer was of no consequence re LIGHTNING STRIKE. I was not talking about structural issues.

I am not aware of any airframe that even tries to accomplish that re lightning protection. As to various methods eg imbedded mesh, foil, metal paths, conductive paint AFIK they all have their place. In some cases, re the protection of penetrations of composite structures by metal things like pumps, structure fittings, etc, copper plating on both sides and ' hole' of the penetration does work but is normally too expensive - complex to do for commercial production.

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