P-Stat and Grounding for external flight components 2

[MartinShane]

My company is involved in a project with an externally mounted (non-flight critical) composite structure.

This is outside of our experience, and though the company we are working with will bear the final responsibility for design and validation of the components, we are being looked to for recommendations on how to integrate shielding / grounding into the structure we will be producing.

I am familiar with lightning strike protective fabrics and surfacing films, but have no knowledge of their applicability to aiding static dissipation, and have been lead to believe that spray coatings as used for RFI / EMF shielding perform poorly in "P-stat" testing.

Does anyone have any suggestions on resources where I can better educate myself on the methods of dealing with static accumulation during flight, especially in composite structures, so that I can better understand and direct our material needs?

 

[rb1957]

not a lightning specialist, but ... i'd use a copper mesh for the outer ply and a grounding strap between the external structure and the airplane strucutre. you might be able to get away with using the fasteners if they have a good resistance.

Quando Omni Flunkus Moritati

 

[MartinShane]

Thanks for weighing in RB. I was leaning that way as well, I just don't have the experience in the subject to know whether it would be sufficient.

(off topic, The Possum Lodge does not usually conjure images of aerospace engineers for me =) )

 

[rb1957]

as George W (jr) might have said ... "what you can't fix with duct tape, ... well ... er ... but you can fix everything with duct tape, so ..."



Quando Omni Flunkus Moritati

 

[berkshire]

Martin Shane
You can put a bonding plate in contact with the Thorstrand or copper mesh, then screw into it to attach a static wick at the trailing edge.
Are you just dissappating static, or do you have to deal with the possibility of a serious lightning strike?
The entrance and exit points tend to do severe damage, regardless of what bonding system is used.
B.E.

 

[IRstuff]

We use a copper foil on the inside of a composite shroud on an aircraft. It supposedly works reasonably well, but a direct hit will still damage the shroud and require repairs.

TTFN
faq731-376

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[Compositepro]

Copper foil on the inside surface is strange. That will provide protection of structures behind the shroud and shield from EMI but will do little to protect the shroud from damage. Lightning passing through composite material results in a small explosion because most of the energy is dissipated in in a very small volume. Foil or mesh on the outside will just result in some surface scorching of the composite and vaporization of the foil in about a 2" diameter.

 

[IRstuff]

The customer is willing to accept that risk, and it eliminates the need to have an external connection for terminating the shroud, so the connection is inside the shroud.

TTFN
faq731-376

Need help writing a question or understanding a reply? forum1529

 

[MartinShane]

@Berkshire -
The primary concern as communicated by the customer is for static dissipation. They said that the test or standard that would be applied is fairly stringent.
No mention of direct lightning protection was made, and I don't know if this is due to the non flight-critical nature of the part, or if it was an over sight.
Either way, it sounds like consensus is that integrating a copper wire mesh (or foil) into the outside of the laminate and allowing for a ground connection from the shielding is probably sufficient, and can be a basis for providing either type of protection.

 

[Compositepro]

Lightning protection does add significant weight (copper is very dense) and is over kill for static dissipation. I'm not even sure that it would solve a static problem, since it would be under a layer of paint. An anti-static paint should be used.

 

[Sparweb]

There are some instructions for satisfying electrical conductivity, and static discharge, requirements in airframe and composite structure in AC 43.13-1B, Chapter 11, section 15 (IIRC). The size of the external mod may have some influence on the measures necessary to satisfy the authorities. If it exends beyond the overall dimensions of the aircraft then it could become the new preferred attachment point for lightning.

You can consider several ingredients: Electrically-conductive paint, copper mesh, CF with inter-woven aluminum threads, foil bonded directly into the laminate, metallic straps attached with fasteners through the laminate, and of course static-discharge wicks. My personal experience with these materials is that it takes some trial and error to make a good path with low resistivity. If your customer is very explicit with their requirements, then try asking them to elaborate on the exact conditions they need to avoid. It may help you set a realistic goal, rather than an expensive/perfect or cheap/inadequate one.

If you can get inside a "plastic" airplane you can get some ideas. Try a Diamond DA-40/42, Cirrus, Cesssna 300, or something similar. Digging into the Maintenance Manual and/or Strutural Repair Manuals of aircraft such as these will give you a shopping list of materials that may be helpful, and procedures for laying them up (written up as repairs in the SRM context). Of course the big 3 have their own ways of dealing with this; research what you have access to.

An area of the trailing edge of your part can be layed-up with features especially for the purpose of mounting a static-discharge wick, either by building up extra layers so you can sand down into the mesh/wire/foil safely, or securing the foil/wire on the inside where you can drill through and put a fastener into it from the outside.

Suddenly I find myself wondering: Has anybody come up with an electrically-conductive epoxy resin for wet-layups?



STF

 

[berkshire]

SparWeb (Aerospace)

Has anybody come up with an electrically-conductive epoxy resin for wet-layups?

Not that I know of, that will meet the strength and heat deflection curves, for most light aircraft.

Also is this the section you are referring to in 43-13
B.E.

11-193. LIGHTNING PROTECTION
BONDING. Electrical bonding is frequently
required for lightning protection of aircraft and
systems, especially to facilitate safe conduction
of lightning currents through the airframe.
Most of this bonding is achieved through normal
airframe riveted or bolted joints but some
externally mounted parts, such as control surfaces,
engine nacelles, and antennas, may require
additional bonding provisions. Generally,
the adequacy of lightning current bonds
depends on materials, cross-sections, physical
configurations, tightness, and surface finishes.
Care should be taken to minimize structural resistance,
so as to control structural voltage
rises to levels compatible with system protection
design. This may require that metal surfaces
be added to composite structures, or that
tinned copper overbraid, conduits, or cable
trays be provided for interconnecting wire harnesses
within composite airframes. Also care
must be taken to prevent hazardous lightning
currents from entering the airframe via flight
control cables, push rods, or other conducting
objects that extend to airframe extremities.
This may require that these conductors be
electrically bonded to the airframe, or that
electrical insulators be used to interrupt lightning
currents. For additional information on
lightning protection measures, refer to
DOT/FAA/CT-89-22. Report DOT/FAA/
CT 86/8, April 1987, Determination of Electrical
Properties of Bonding and Fastening Techniques
may provide additional information for
composite materials.

 

[kontiki99]

You may need to protect against P-Static. Depending on how and where the vehicle is operated, it can really disable the radios. The guy helped us, was not free.

http://www.jdchapdelaine.com/

We eventually bought a Dayton Granger Electrostatic Discharge Tester. Combined with other airframe bonding measures things have quieted down quite a bit.

In theory, there is no difference between theory and practice; experience suggests that in practice, there is.

 

[WKTaylor]

Some MIL aircraft with secondary composite structures [that I've dealt with] have (3) basic lightning protection schemes.

Dense flame sprayed aluminum coatings over a fiberglass outer ply. The aluminum coatings connect to attachment fasteners countersinks/fasteners that are also grounded.

A layer of monel, Inconel or CRES [thin] wire mesh between the structural plies and the outer fiberglass protective ply. Mesh diverts electrical energy to the grounded attachment fasteners.

Boron-epoxy fibers step-laminated onto a metal "picture frame. The metal frames tend to be lightning attractive/divertive; however, since the boron fibers are "grown" over a microscopic tungsten core fiber they provides a secondary electrical energy path [outer plies only].

These lightning-strike protected composite structures were usually over-coated with static dissapative coatings.

NOTES.
Lightning zoning is an ultimately critical step in this process. This step ensures that the typical lightning strike [in-out] locations and diversion pathways are well understood.
Composites structure integral fuel tanks present additional challanges for lightning and static disapation.

Regards, Wil Taylor

Trust - But Verify!

We believe to be true what we prefer to be true.

For those who believe, no proof is required; for those who cannot believe, no proof is possible.

 

[Sparweb]

Hi Berkshire:
So there is something out there? In development at least, even if it's not ready for aviation? Or are you thinking of something available commercially for other purposes?
There are so many ways to build composite laminates and structures, that maybe a lightly-loaded structure, such as a fairing, could be designed with such resin matrix, even it doesn't have significant strength/environmental resistance, given enough opportunities to experiment with it. If any "any" CF or glass weave could be made electrically conductive (even partially) by simply selecting a conductive resin, this would be a useful item in the toolbox.

STF

 

[berkshire]

There are several companies that make conductive resins for the pc board industry and others. Masterbond is one company that claims to have a conductive resin. Most of these that I have seen achieve this by loading the resin with metal powder. This tends to reduce the tensile and flexural properties.
It does not seem to affect the heat deflection curves that much.
A very lightly loaded structure like a fairing or boot could possibly be made using such a material.
B.E.

 

[Sparweb]

Thanks!

STF

 

[WKTaylor]

Coatings for Lighting Protection of Structural Reinforced Plastics. Part 1

http://www.dtic.mil/dtic/tr/fulltext/u2/883123.pdf

Coatings for Lightning Protection of Structural Reinforced Plastics. Part 2

http://www.dtic.mil/dtic/tr/fulltext/u2/894118.pdf

Regards, Wil Taylor

Trust - But Verify!

We believe to be true what we prefer to be true.

For those who believe, no proof is required; for those who cannot believe, no proof is possible.

 

[berkshire]

Wil,
The information you just posted is over 40 years old, is there nothing newer that is de classified yet?
Although it seems to be, that commercial aircraft companies are using these, or similar standards.
B.E.

 

[WKTaylor]

Oh, what You're looking for is the latest proprietary technology!! Good luck...

Start with basics and work toward solution for Your specific case.

Are You familiar with...?

DOT/FAA/CT-89-22 Aircraft Lightning Protection Handbook

http://www.dtic.mil/dtic/tr/fulltext/u2/a222716.pdf

NASA CR 3875 Fuel Containment, Lightning Protection and Damage Tolerance in Large Composite Primary Aircraft Structures

AGARD-CP-288 Effect of Service Environment on Composite Materials

http://www.dtic.mil/dtic/tr/fulltext/u2/a090311.pdf

Regards, Wil Taylor

Trust - But Verify!

We believe to be true what we prefer to be true.

For those who believe, no proof is required; for those who cannot believe, no proof is possible.