Aircraft Lightning Protection 11

[jonhoward]

I am currently working on a secondary wing structure for a military aircraft. The structure will be primarily made of carbon fiber/epoxy. The mounting features to the aircraft are 7075 aluminum that will be bonded to the carbon and the fairings will be glass/epoxy which will be quarter turn connected or something very similar to the carbon main structure. I am wondering if anyone knows any good resources for handling lightning protection of such a structure or can offer any other potentially helpful information. Thanks

 

[Compositepro]

Carbon and glass composite will explode where struck by lighting, usually on its way to some metal part under the skin. Metal wires can be cowoven into carbon and glass fabrics. My personal opinion is that the best aproach is an expanded copper or aluminum foil bonded to the outside surface. Astroseal and Delker make the foils. Several prepreggers have adhesive products with inbedded foils. The foil will conduct the current away from the stike location. But the foil will still get vaporized for a diameter of about 3 inches.

 

[Kenneth]

thread327-21528
thread16-20511

 

[rb1957]

a different concern ... you're planning to bond 7075 to carbon ... this'll cause galvanic corrosion (unless you protect the 7075 is some meaningfull way). for this reason most people use Ti for fittings that will contact carbon.

good luck

 

[jonhoward]

How do you go about deciding how many plies of this hybrid material or mesh must be applied for appropriate protection (is there a method or set of standard calculations)? I guess I was sort of hoping that there was a way to protect the structure without having to replace it following a lightning strike, as I would expect if the mesh does disintegrate upon being struck.

As for the 7075 to carbon bond I believe that we are going to be using a phos anodize prep. The military also suggested PER MIL-DTL-81706A, CLASS 1A (ALODINE 1201) however I believe this detail is more for components that are not going to bond to the structure. All of the AL/Carbon contact areas we will have isolation plies of glass.

Thanks again to all that have offered there insight....

 

[IRstuff]

Reality check:

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/lightning2.html#c4

Even if your wing were solid metal, there probably would be localized damage after a strike. Your best approach is to determine how other people are dealing with this, e.g., they may be designing the wings to be modularly repairable.



TTFN

 

[BrianR]

The danger of CFC panels exploding when hit by lightning seems a much lower now than what it was and this is supported by test results on commercial jets. A lightning strike blows a small hole in CFC much like it does in metal. Probably manufacturing techniques prevent the ingress of water which was the main cause of explosion.

Modern aircraft designs, Boeing & Airbus, generally do not now use embedded mesh in panels and rely on the conductivity of CFC and lightning wicks bonded to prime structure through metal straps. Mesh is used to provide ground planes for radio antennas and EMC shielding where it is needed but not otherwise.

Protection of fuel tanks is another matter and there are specific requirements there.

The corrosion issue referred to by rb1957 is very important as 7xxx alloy is at the opposite end of the galvanic scale to CFC and Ti. This guarantees corrosion if any water gets into the joint - ever. The Al is sacrificial to the CFC so if the smaller part (eg a fastener) is the sacrificial one then it will slowly disintergrate. This is why Ti fasteners are used and thin fibreglass layers to seperate the materials at the join to break the electrical path for galvanic corrosion. Of course you now need to have a bonding strap on the now insulated panel to account for lightning.

I have listed a section taken from MIL-STD-464A below that shows where you can obtain guidance on lightning protection for military aircraft.

Society of Automotive Engineers, Inc.
ARP1870 Aerospace Systems Electrical Bonding and Grounding for Electromagnetic Compatibility and Safety
ARP4242 Electromagnetic Compatibility Control Requirements, Systems
ARP5412 Aircraft Lightning Environment and Related Test Waveforms
ARP5413 Certification of Aircraft Electrical/Electronic Systems for the Indirect Effects of Lightning
ARP5414 Aircraft Lightning Zoning
ARP5415 User’s Manual for Certification of Aircraft Electrical/Electronic Systems for the Indirect Effects of Lightning
ARP5416 (Draft) Aircraft Lightning Test Methods
ARP5577 Aircraft Lightning Direct Effects Certification

 

[dropmaster]

My $.02:
When I worked at Cessna as a structural liaison engineer, we had a lightning strike class. They brought in some composite Citation 10 panels that had been struck by a lightning generator. They were experimenting with differing the paint thickness to see what the effects were. The results were amazing. Thin paint caused only minor pits in the paint because the lightning didn't have to fight through the paint to get to the metal mesh (I think it was called Astrostrike - I know Scaled Composites uses it) embedded in the composite surface. A panel that had thick paint had a large hole blown through it. The point of this exercise is to not only control the paint thickness during manufacture, but to put into the maintenance manuals to control the paint thickness if a hole in the panel is patched or if paint is touched up in the field.

Chase

http://www.dropmaster.com

 

[Higgler]

I worked at Raytheon when they made the Starship all composite. Google showed me this website and how Raytheon solved their lightning problems. The aircraft was too expensive to produce and led to its' demise. Raytheon executives still fly in one I believe.

http://www.aviatorservices.com/starship_history_3.htm

This was accomplished by using a combination of fine wires in the first layer of composite skin and a ground-plane system to shield the electronics, allowing the lightning current to flow through and out, leaving only minor surface and cosmetic damage at the strike point.

If you plan on placing antennas in your structure, the lightning protection can affect them greatly.

kch

 

[JETA4GA]

I am looking if anybody would know about data/references where I could find the rate of lightning strike events on Cessna Single Engine Recip a/c (172, 182, 206)over a few years in North America (preferably) and, elsewhere in the World; Thanks for the help.

 

[David823]

My 2 cents.. Lightning qual for me was to design a path for the strike to dissipate into the part and route to a safe exit location. I do all the faa req tests including direct effects.

Several approches, screen (something like stainless or coated copper), or a spar along the outside edge of the part.

My experience is the strike will take the straightest path of least resistance to lowest potential.

My requirements are safe flight to land and repair.
Hope this helps.

 

[BrianR]

One rule that we observed with lightning was that it is lazy! It takes the shortest path to get somewhere, not necessarily the one of lowest resistance. It will spark across a gap when there is a low resistance, but longer path around it and blow through a radome to the antenna behind it rather than attach to a diverter if the antenna is much closer.

 

[Higgler]

Don't forget, current moves through the lowest impedance, not the lowest resistance. Impedance includes the inductance and capacitance in series and in parallel with the resistive path.
Hence a simple wire is a poor lightning protection atop a house due to it's high inductance. 6" wide wire braid is 1000x better.

kch

 

[Kenneth]

From Lightning a weighty issue for the 787 at

http://seattlepi.nwsource.com/business/277220_air12.html:

We always planned to deal with this issue, but we did not anticipate the complexity," acknowledged Boeing's Scott Strode, head of 787 development and production.

 

[BrianR]

Higgler, I agree that a wide braid strap is a much better ground lead than a wire no matter how thick but the inductance still increases with length and when the L/W ratio is much greater than 4 then it all starts to even out. Still, more square inches is better.

 

[Higgler]

Haven't heard the L/W ratio >4 before. I bet those inductance formulas for wires would be useful to give hints. I don't recall what the national electrical code says about wire braid size, but I'd bet its in the 4-12" width range.

My exgirlfriend had a lightning experience last summer whilst at her computer. Her keyboard caught fire in a lightning strike.
Her boss spent $50K in lightning protection and a recent lightning strike took out her computer last week (washington dc area). It did it without exploding her keyboard this time. I told her to buy a "wireless keyboard, wireless mouse" or put me in your will.

sorry, I diverged from air to ground lightning strikes.

kch

 

[BrianR]

I don't have the references with me at present but can get to them if needed. Calculate the impedance (inductance) of a flat strap and the impedance of a round wire. This is the basis for the choice of bonding straps in aircraft and you find that the strap is much lower at first but two curves start to converge after L/W > 4. I think you would treat braid as an equivalent strap based on the total of the surface areas of the strands.

 

[MechanicalChef]

There is an article on this subject in the current issue of Advanced Composites magazine.

http://www.compositesworld.com/hpc/issues/2006/July/1366/1

 

[BrianR]

MechanicalChef, that is a very interesting article.

 

[kontiki99]

I'm not an expert, but from what I've read, square inches doesn't matter as much as surface area of a conductor. Lightning is generally modeled as a high frequency A/C. It's generally conducted on the outer surface because of skin effect.