Bonding a Fiberglass Airplane...

Discussion in 'Instruments / Avionics / Electrical System' started by wsimpso1, Oct 16, 2018.

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  1. Oct 16, 2018 #1

    wsimpso1

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    Some texts, including Bingelis, indicate that airplanes should be bonded by running ground straps to everything in an attempt at keep the airframe at the same potential. This all sounds fussy and complicated... I am a mechanical engineering geek, and can go on and on about structures, vibration, fabrication. Electrons, well, I can not see them, and do not really get them... So I need help and hope that the rest of us can use the discussion.

    Then I read Nuckolls' AeroElectric Connection... He tells us to run power out and then back to a common ground connector that bridges both sides of the firewall and has the battery negative cable and engine strap all attached. He also tells us that these ground systems running to everything are invitations to ground loops and noise galore...

    So, what am I to believe? Anybody have any experience on doing the systems either way, or better yet, both ways? How did it work? I would like to hear the pros and cons of both schemes...

    Billski
     
  2. Oct 16, 2018 #2

    BJC

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    That is the cheap way to wire an airplane made of conductive material. (Not fiberglass.)
    That is the way to wire an airplane made of non-conducting material. (Fiberglass.)

    It also is the way to minimize curculating ground currents, which are the primary cause of problems in avionics and audio circuits.

    Believe Bob.


    BJC
     
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  3. Oct 16, 2018 #3

    lr27

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    In the RC world, when we're worried about noise we:
    -twist wire pairs
    -use small ceramic caps near vulnerable points
    -use chokes, ferrites, or whatever you want to call them.
    Not sure how much of this applies, but I'm sure some does. I've used the first two methods myself. It's probably better to do some calculations with the caps, but I just tried them and they worked.

    I'm wondering how much trouble static charges are with non-conductive airframes, whether static dissipative materials are used to deal with them, and how vulnerable avionics are. Certainly there's a lot of concern with electrostatic discharge with other sorts of electronics. Don't know how much this ties in with your question, though my guess is that it does. Wood and, obviously, metal airplanes would be less vulnerable. Carbon is somewhat conductive as well, though if you zap it with a really high current, it will more or less explode into a big bundle of fibers, minus the epoxy. But I suppose that's getting into lightning strikes and hitting power lines, so maybe I'm getting too far afield. I've seen a little mushroom cloud of burnt epoxy before...
     
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  4. Oct 16, 2018 #4

    lr27

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    I haven't thought about this before in regard to aircraft, but if you've got a thin layer of fiberglass with conductive stuff on either side, like carbon or metal (or maybe even just aluminum powder in dope?) that's a high voltage capacitor. As in big fat sparks! Probably good if there's at least a little bit of conductivity between the layers. In the electronics world, we would wear slightly conductive heel straps with a 10 megaOhm resistor between us and the strap. I had to worry about some of these issues when designing electronics packaging. Where you REALLY have to get paranoid about electrostatic discharge is for equipment to be used in a hyperbaric chamber. Anything with a spark can be deadly. Possibly some of the same issues are present if you've got an oxygen system that leaks in the wrong place.
     
  5. Oct 16, 2018 #5

    Hot Wings

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    It's been a while.....But IIRC when he says 'bonding' he is talking about something different than grounding. Bonding is the connection of 2 parts to eliminate electrical potential even though there is normally no current in the system. Grounding is done to complete the electrical path for a current.

    I don't have enough mastery of the subject to properly explain so:

    https://okigihan.blogspot.com/p/bonding-and-grounding.html
    https://en.wikipedia.org/wiki/Electrical_bonding
    https://www.google.com/url?sa=t&rct...25_899-1.pdf&usg=AOvVaw1qx_7QD06ebGheLRsPZwkY
     
    Last edited: Oct 16, 2018
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  6. Oct 16, 2018 #6

    BJC

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    Bill:

    On a related topic:

    Some experimentation with grounding fuel filler necks embedded in fiberglass was done, and the conclusion was that the most effective way to prevent static discharge while fueling is to wipe the fiberglass surface around the filler neck with a damp rag just before starting to fuel.

    Having the neck bonded to a conductor that extended into the fuel tank made no difference.

    Note, though, that to pump fuel, you still will need to input a “yes” into the self-service pump when asked if you have grounded the airplane.


    BJC
     
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  7. Oct 16, 2018 #7

    Hot Wings

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    Please don't take this as a challenge to the statement, but I'd be real interested in a link or some other source for the data. This is a subject that directly effects me. I'm working on a Quickie and have read a lot on this subject over the years. Some good, some not worth the ink and some that I haven't the expertise/experience to judge.

    My engine mount/tank filler ring and some other bits of isolated metal are bonded together to a wire running back to the tail wheel. It will be attached to a static wick that will drag on the ground and eventually be connected to a remote grounding post for use during filling.

    I've got some good fire stories from my early youth..............:gig::shock:
     
  8. Oct 16, 2018 #8

    Aerowerx

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    Well, Billski, I am an electronics engineering geek.

    There are two things here.

    For the electrical current that runs all the systems, you need a path for the current to flow back to the source. This is best done with a wire. Be sure to size the wire for the current load. Do not connect it to the frame. That is asking for connection problems, particularly if the structure is metal and the wiring is copper. And be sure to fuse both sides of the circuit at the source (that is, the battery).

    The second thing is preventing static build-up in certain parts of the aircraft. This is done by making sure the various parts are connected electrically, again with a wire. Although it can be a relatively small gauge compared to the systems wiring. Here you could use copper wire if you seal the joints from moisture, or stainless braid is common.
     
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  9. Oct 16, 2018 #9

    BBerson

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    If you fly in the clouds the fiberglass might need conductive mesh and bonding between parts to allow lightning to pass through.
    Some controversy if that is mandatory or not.
    I don't know if your concern is radio static, lightning destruction, or fuel tank explosion.
     
    Last edited: Oct 17, 2018
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  10. Oct 16, 2018 #10

    BJC

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    I don’t have a link. IIRC, the experimentation was presented in an old Stoddard-Hamilton newsletter. The static accumulates on the surface of the fiberglass structure. A wet rag eliminates it. Bonding / grounding don’t help because the fiberglass is not conductive.

    There are lots of well-meaning statements wrt static and fiberglass airframes that are not based in physics, so don’t believe any of them, including this one, until you have independently validated them.

    Stoddard-Hamilton used a NASA grant to develop a Glasair III that would withstand a lightning strike. They succeeded by embedding a conductive (might have been copper, I don’t recall) mesh in the outer lamination, bonding everything together. They also used static wicks. It worked, but added significantly to the kit cost. The test airplane still is flying, but they never sold a single kit that included the lightning protection option.

    Please test the surface of the structure around the filler neck, in very dry conditions, and let us know if you find enough static there to generate a spark to a grounded filler hose.

    Thanks,


    BJC
     
  11. Oct 16, 2018 #11

    plncraze

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  12. Oct 16, 2018 #12

    BJC

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  13. Oct 16, 2018 #13

    Himat

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    To decrease the build-up of static charge across the surface doping the material in the surface coating, paint, may work. Doping in this case is adding a very small amount of a substance that modify the “plastic” paint to make it slightly conductive. Commonly done with engineering elastomers, that is plastics, to reduce build-up of static charge on the finished product.

    I would think some paint manufactures already have modified their paint this way to some customers/markets.
     
  14. Oct 16, 2018 #14

    Himat

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    It is possible to dope many engineering polymers, “plastics”, to make them slightly conductive. This was considered/done on a plastic chassis for some electronics I was involved in designing. The modification, doping, of the material was to prevent static charge build up during manufacturing and handling to damage the electronics when mounted. If this is a viable and sufficient method to control static charge on an airframe I do not know.
     
  15. Oct 16, 2018 #15

    Hot Wings

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    I'll put that on my testing program sheet. Dry conditions are pretty easy to come by around here. I too remember the Stoddard-Hamilton project and can't remember if they used copper or aluminum mesh. I would like to have been there for the testing. :ban:
     
  16. Oct 16, 2018 #16

    poormansairforce

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    Couldn't one use an aluminum metallic "primer" coat under the color coat to act as a conductor to discharge any high voltage regions? Maybe leave some bonding surfaces to connect some grounding straps?
     
  17. Oct 17, 2018 #17

    Lucrum

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    Am I reading this correctly, that you're suggesting not using a metal airframe as a common ground? As in two wires for every circuit, one power & one ground?
     
  18. Oct 17, 2018 #18

    wsimpso1

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    Yeah, foil on each side of insulators can make a capacitor, but why would we do that?

    Paint acting as a capacitor? I doubt it. To make a capacitor, you need poles, with one attached to a plate, then an insulator, then another plate attached to the other pole. To store charge on it, you need to apply opposite polarity to the poles. Metallic paint is a lot of tiny plates but no big plates. While you have non-conductor between the flakes, no conductors are attached to the flakes to put charge on them. I suspect that while you can always try to store electrons on the surface, your total capacity would be pretty small. Not much of a capacitor...

    Jim Weir of RST states in his book on internal antenna that they work fine when silver dope and metallic paints are put on over them, with the explanation that the individual particles of metal are apparently electrically isolated from the outside and from each other by the resin in the paint. Metal parts that have largest dimensions smaller than 1/4 wavelength do not interfere with antenna, so frequencies would have to get mighty high to interact with paint flakes... Jim also tells us that graphite fiber skins will prevent an internal antenna from transmitting because each fiber is a wire that runs across the whole part, and you have many long wires. These are known to suck off electro-magnetic signal...

    Graphite fiber as a capacitor? How do you get ahold of the fibers with charge? They are inside a layer of resin...

    In the ammunition industry, we wore conductive shoes and kept the shops at 100% humidity. Manufacture of mix for primers and tracers, and then assembling those products... I have also visited Hercules and a blasting inititiator plant in the Hudson Valley - they also all wear conductive shoes and run 100% humidity...

    Billski
     
  19. Oct 17, 2018 #19

    Mad MAC

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    I know of one metal type certified aircraft where they bonded the all the fuel lines in the cockpit area together as a solution to radio issues. so beware even where you have a row of metallic components.
     
  20. Oct 17, 2018 #20

    TFF

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    Bonding avionics and alternator generally are wired to a common area if not post. Aircraft avionics are finicky things. Many times there will be multiple grounds, just in case. As for a glass plane, you pretty much have to cluster grounds to terminal strip bars. As for lighting protect or radio interference, airliners and such bond in a mesh so it acts like a faraday cage. I have seen lighting strikes where composite panels turned to fabric; the resin completely burned away. Biggest piece about the size of a side of an RV wing; for one reason the mesh was not bonded electricity to the plane. Somehow it took the whole exit. The exit is where the big damage is. All flight controls, doors and such have bonding wires so all have same potential. Most GA planes are not built like that.
     

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