Bonding a Fiberglass Airplane...

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

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  1. Oct 17, 2018 #21

    lr27

    lr27

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    You can get static dissipative wax and spray. Not sure how well they hold up to weather, though. The ESD wax holds up on the floor ok. I suppose for airplanes one has to think about corrosion when considering doping the materials. Maybe a couple of square feet of doped paint around the filler? If you wanted to be really paranoid, I guess you could touch the nozzle and the doped paint at the same time, before filling. The static dissipative material has to touch something that's grounded to dissipate the charge. Kind of like scuffing your feet across the carpet and then touching the doorknob with a piece of wood before grabbing it with your hand.

    When evaluating something that's supposed to be static dissipative, you can't just touch it with VOM (voltohm meter) probes. You have to have a reasonable amount of area, and maybe even a little pressure. As I recall, we had a special instrument with flat metal probes that we'd press onto a surface to check it. I suppose if you put a large enough piece of metal on the surface of the material, you could check it with a VOM.

    I once made a safety cover with Plexiglas that was supposed to be static dissipative, but it turned out that only the outer surfaces were. I had to run a bunch of bolts with washers through to electrically connect the inside and outside. With the dissipative material, point contact didn't seem to be enough.

    I think red plastic gas tanks are supposed to be dissipative, but I don't know if that goes right through or if it's just the surfaces. If the latter, I don't know what measures there are to connect the inside and the outside. Maybe that's getting too paranoid.

    From a web search, it appears that there's a vast literature about static charges, sparks, and fuel. But I didn't run across any practical distillation, other than not getting back into your vehicle while refueling, and putting your gas can on the ground before filling.
     
  2. Oct 17, 2018 #22

    lr27

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    At one time, aluminum house wiring was introduced. The connections weren't always solid, particularly after a bit of corrosion, so aluminum wiring apparently caused some fires before they figured out how to work with it correctly. One of the ways to deal with it was to use a conductive grease, which you can get at Home Despot, if I recall correctly. Not sure if this becomes an inspection and maintenance issue for airplanes, though. If memory serves, I was using the stuff to ensure a ground connection in an outside light fixture stayed solid. There are also things like wire nuts and connectors which are rated for both aluminum and copper, but that doesn't really apply here.

    While it seems to me that the silver (aluminum) paint I've tested isn't particularly conductive, I wouldn't stick a silver painted wooden fork in an outlet to prove it. And who knows what kind of paint additive would change that? That's something I didn't explore when I was fooling around with ideas for van de Graaff generators. It's probably not relevant for aircraft, but it turns out India ink is somewhat conductive, enabling you to do some amusing things with high voltage and ping pong balls. Maybe if mixed with latex paint...

    Let's look at the carbon fiber in the resin. Say the carbon is in a molded structure with .003" of epoxy between it and the outside. I've seen a figure of 500 volts per mil for the breakdown voltage of epoxy. In flight, triboelectric effects can generate voltages in the tens of thousands of volts on aircraft surfaces[SUP]1[/SUP]. I'm no expert, but I suspect that if you have 20,000 volts on the surface, it won't be long before the carbon has a few thousand volts of potential as well. If you have another layer of carbon inside, separated by a core, and no electrical connection, you have a big, high voltage capacitor. You don't need to hook up a battery or generator to either surface. Consider that people report being zapped after leaving a ceramic cup of coffee, with a metallic coating on the outside, near a van de Graaff generator for a while. Our capacitor is much larger than a small Leyden jar of that sort. We may have fasteners or metal filler necks or something that will serve as electrodes as well.

    A capacitor with good insulation can hold charge for a long time. Even the foam peanuts I stuck to the ceiling by charging them with a van de Graaff stayed there for 10 or 20 minutes. So perhaps you'd run into the problem if you gassed up before ordering your $100 hamburger. My VDG was far from awesome. I never tracked down all the current leaks.



    1. For an example, see page 3 of: http://www.ema3d.com/downloads/AEP_3.pdf
    A CRITICAL REVIEW OF PRECIPITATION STATIC RESEARCH SINCE THE 1930'S
    AND
    COMPARISON TO AIRCRAFT CHARGING BY DUST
    R.A. Perala
    Electro Magnetic Applications, Inc., Denver, Colorado, USA



    The aftermath of the incident you saw seems very similar to what I saw on a smaller scale. (The hum of thousands of volts and MANY amps of AC current passing through the carbon is awesome in the original sense, even from a few hundred yards away. Ditto the mushroom cloud.) As far as most GA planes not being built like that, most are made of aluminum, so it's only going to be a huge issue with other materials, I think. Even if the gaps have paint, you're probably only talking a few hundred volts building up.
     
  3. Oct 17, 2018 #23

    Hot Wings

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    Now I have a use for all those EKG patches I've saved :gig:

    Somewhere in the before years when I started researching this I ran across an article that said there was a federal law requiring 50 meg ohms/inch or less for plastic gas containers. I've never have been able to verify that 'fact'? I do remember taking my Fluke to Walmart...:whistle:
     
  4. Oct 17, 2018 #24

    Topaz

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    Bill, Tony Bingelis' article, "Electrical Bonding of Aircraft", from Sport Aviation pre-2000, or Sportplane Construction Techniques, page 325, is the "way it's done" as far as I've ever seen or heard. Might seem a little "fussy", but it's the only reference of which I'm aware for our little airplanes.
     
  5. Oct 17, 2018 #25

    Chilton

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    I am not sure how relevant it would be for a composite materials airframe, but in the past the wooden airframes ran a bonding cable (actually a piece of copper strip) between all the metal components, so hiinges, bellcranks etc in each wing were connected, and then they all joined up at a central point. That may be enough with a non conductive structure.

    On the other hand I remember about 1999 a glider out of the uk flying between 2 clouds being struck by lightning on one wingtip, exiting via the other tip, and the structure disintegrated instantly leaving both pilots falling strapped to their seats. It was a trial flying lesson so the student got his first glider flight and first parachute descent at the same time.
     
  6. Oct 17, 2018 #26

    Chilton

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  7. Oct 18, 2018 #27

    wsimpso1

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    Now that is the first time I ever heard of fusing both legs - I can speculate, but would rather you elaborate on why it is needed.

    I guess that is the big question for me. Do we need bonding? Bob Nuckolls is critical of schemes with a whole network of ground straps connecting all of the airframe together as a source for ground loops... What happens to a fiberglass bird if we use a central ground for the accessories and skip bonding hinge halves to each other and skip bonding control surfaces to the foils and so on? Somebody please explain what we are really up against? Here.

    Billski
     
  8. Oct 18, 2018 #28

    wsimpso1

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    Well, I sure do not want any of those but I am trying to be realistic.

    Radio and electronics function is what I had been aiming at;

    Thunderstorm avoidance is a priority without even needing to worry over lightning, so let's consider that one covered;

    Fuel tank explosion? Tank grounding with the conductor arranged to penetrate the fuel surface has been talked about, and BJC talked about how Stoddard-Hamilton's (Glasair) research. I do not even know which way to go there. I guess I will have to seek out the article...

    Billski
     
  9. Oct 18, 2018 #29

    wsimpso1

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    Work done by various folks on internal antennas has shown that virtually all paints are transparent to RF radiation. The thought put forward to explain that is the metallic flakes are very small and electrically isolated from each other by the resin of the paint. That also makes the paint a pretty good insulator. Now if the paint flakes were all electrically connected, then the paint would be conductive, and internal antennas would be shielded by it, much like graphite fiber composites do with internal antennas.

    Billski
     
  10. Oct 18, 2018 #30

    lr27

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    My guess is there's some NASA publication or mil spec discussing just what to do about bonding and non-conductive composites. Finding it might be a challenge.

    While poking around a little, looking into this, I found a description of charges building up on non-conductive surfaces and then discharging through a corona. The latter leading to radio noise. Probably looks really cool too, if you're flying in the middle of nowhere on a moonless night and you can turn off every last light in or on the airplane.
     
  11. Oct 18, 2018 #31

    wsimpso1

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    I was trying to get us talking about fiberglass airplanes. And yes, Nuckolls warns us that using a metal airframe ground is not a slam dunk, and talks up his ground bus... Experience with them or not with them?

    Billski
     
  12. Oct 18, 2018 #32

    BBerson

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    My fiberglass Grob doesn't have any bond straps on the ailerons or anywhere. Maybe the aluminum fuel tank is bonded by a ground wire. I don't see how bonding could be of use on fiberglass. Faster metal planes are apparently bonded with wire braid straps usually on the ailerons, I think. And they have static wicks screwed on the trailing edge of the wing tips or ailerons.
    I first touch the fuel nozzle to the metal fuel cap and then remove the cap for fueling.
    I saw a Cherokee with a burned wing around the fuel cap area. No doubt from a 5 gallon plastic can filling. My boss had a fire from defueling out the drain port. The fuel flowing causes static arcing.
     
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  13. Oct 18, 2018 #33

    wsimpso1

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    I will spend some time on this topic...

    Saint Elmo's Fire is real.

    Billski
     
  14. Oct 18, 2018 #34

    lr27

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    If you cover your aircraft, say every couple of inches, with sharp pins sticking straight out, it probably won't build up much voltage. ;-)
     
  15. Oct 18, 2018 #35

    Lucrum

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    Absolutely, I've seen it many times on the windshields of older aircraft I used to fly.
     
  16. Oct 18, 2018 #36

    Himat

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    The plastic is made slightly conductive by doping it with a small amount of material that modify the dielectric property of the plastic. The plastic do not become a conductor, but get a resistance low enough that high static charges do not build.
     
  17. Oct 18, 2018 #37

    Himat

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  18. Oct 18, 2018 #38

    Himat

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    All AC housing electrics are done that way in Norway, but that is another story.

    If different metal parts are not connected together they will float, that is all have their own potential. Something that may not matter until something is done to the surface of the plane to reduce the build-up of static charge.
     
  19. Oct 24, 2018 #39

    markaeric

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    My only up close and personal experience as an avionics tech among a few other positions I've held where I had to be concerned with bonding and grounding is with metal biz jets (with the occasional composite fairing - parts which used a copper mesh), so there will obviously be differences that can't apply to your plane, but I'll try to stay relevant.

    Since you don't have the benefit of being able to use the airframe as ground, I'd suggest running all your equipment grounds separately (as in not spliced together) to a low-impedance bus bar as close to your battery as possible. In the case of a metal airplane, that's what the airframe basically is. I suspect you probably won't have avionics using an ARINC 429 bus, but maybe a few pieces communicating over some other digital bus (and perhaps a few analog signals). If these call for shielded wiring, first off pay attention to shielding termination requirements (terminated at one or both ends), and if they call for single-ended termination, try to do it at the end closest to your ground bus. This applies to shielded analog signal wires too.

    I can't really see the sense in bonding every metal component together on your plane. They didn't need to do it on wooden aircraft as far as I'm aware. But of course, do it where it makes sense, such as fuel filler to ground.


    The following is a little pet idea I like that is kind of relevant which I hope to use when I eventually build my own plane with limited electronics: Where appropriate, locate the circuit breakers above or below the switch for the equipment that it operates. For example, the breaker for the navigation lights is below the nav light switch. For one, I think it would make identifying and managing the misbehaving circuit quite a bit easier than having them on the opposite ends of the instrument panel. Secondly, the wiring ends up being a bit simpler and cleaner by bringing the power bus closer to the switches and then having a short hop from the breaker to the switch. Clean and efficient wire routing should make you happy.
     
  20. Dec 11, 2018 #40

    rv7charlie

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    Hi Billsky,

    What I'm about to say is difficult to do in a public forum without seeming disrespectful to others, but some of the stuff in this thread is a bit off the mark. Not unusual on the interwebs; I've unintentionally given out bogus info myself, on occasion.

    Anyway, you're right to question the idea of fusing the ground returns. It must be done on circuits that have a potential referenced to neutral, or 'ground', like 208VAC & higher service in the USA. But there's no reason (and some good reasons not) to fuse the ground side of a single ended power supply. Especially in an airplane. Opening the positive lead in a negative-ground system does everything you need, and fusing the negative side just adds another set of failure points.

    I don't think you got resolution on the 'where to ground' question. On metal a/c, grounding 'locally' works fine for stuff like lights, flaps, etc. The ground loop issue typically shows up with avionics; that's where the single point grounding becomes important.

    With a fiberglass plane, there will obviously be no local grounding. So you get your choice: run a heavy wire from the main battery ground, out each extremity, and use it to make 'local ground' points as if in a metal a/c. Or, do 'home runs' from each electrical appliance to the central ground point. If you choose the former, you'd still need to pay attention to avionics ground routing; carrying all avionics grounds to the neighborhood of the battery negative.

    To wrap my head around the whole ground loop/electronic noise issue, I like to think of it this way: If a 'noisy' device (like a strobe) has to share a ground path back to the battery with a small signal device (like an intercom), the electrical noise that rides back toward the battery kinda 'piggybacks' on the tiny intercom return, and the intercom's power supply sees/hears it as noise, so you hear it in your headset. If the two devices take different paths back to the battery negative, it's much more difficult for the big noisy device to affect the little quiet one.

    Hope some of that is helpful,

    Charlie
     
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