Airframe as Return Leg of Electrical Circuits

Discussion in 'Instruments / Avionics / Electrical System' started by GESchwarz, Sep 9, 2019.

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  1. Sep 14, 2019 #61

    pictsidhe

    pictsidhe

    pictsidhe

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    As with most technical subjects. If you aren't an expert, it's usually a good idea to copy what experts have done or recommend.
    RF electronics is something of a black art that takes a while to learn.
     
  2. Sep 14, 2019 #62

    PW_Plack

    PW_Plack

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    Even then, they won't know for sure. Metal joints can loosen over time, alternator brushes can wear and cause increased system noise, after testing has been deemed complete. And what if the inaccuracy of an instrument is so subtle as to not be obvious? "Noise" doesn't just mean something audible in the com. My Sport Copter has no separate ground for instruments, and this "noise" shows up as a rotor tach which shows about 40 RPM when the rotor is stopped but the engine is running. (It's accurate in the operating range when flying.) It also has a voltmeter which exaggerates system voltage drop when the lights are on compared to what's measured directly at the battery. A separate instrument ground would solve both these issues.

    If a best practice exists, you can experiment in a different direction, but you need to be confident your test program will find all the gremlins before they surprise you at an awkward time.
     
  3. Sep 14, 2019 #63

    Winginitt

    Winginitt

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    Time,corrosion,vibration,temperature,climate,wear......everything contributes to future inaccuracy and failure of all electrical sysytems, whether they be designed with a single ground or multiple ground system. You can't test whats going to happen in the future, only whats happening " currently". Some airplanes don't have metal fuselages, but rely on composites or wood. Some airplanes don't have radios.

    The only concern is interference, and all accomodations are being made in the hope that interference can be avoided. I'm simply saying a builder should build the safest system and test it to determine if he even has a problem before deciding to forgo that system. Secondary ground wires are easy to remove if testing shows some problem exists. As for future issues, you can't design a system and say that its never going to develop a problem.
    That includes single ground systems which are notorious for having radio problems in GA aircraft as they age.
     
  4. Sep 14, 2019 #64

    Dan Thomas

    Dan Thomas

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    1. Getting unused wiring out can be a hassle, and very often it's left in. It's usually clamped to multiple points on the airframe and is sometimes buried in bundles. I have experienced lots of hassle trying to troubleshoot some ailing system and being misled by unused wires. Tracing wires can be fun, and unused wires just add to the hassle. Getting old wires out of the leading edges of a wing, for instance, is no fun, and replacing a bad wire is worse. The manufacturers tend to clamp everything to everything, and much of the wiring is installed as the component is being assembled. Install the wire bundles, then rivet the rest of the skins on over them. I have found defective components in some really impossible places, a sign that the component was installed and wired before the airfame was closed up. That results in old, burnt-out components left in and the new one installed in an accessible location. The weight goes up. The homebuilder should try to think it out and design the system for better accessibility than that.

    Radio problems aren't restricted to bad grounds. Just about anything in the airplane can cause radio trouble. The dozens of power connections at the bus, breakers, switches, alternator and regulator, ammeter, and all the light fixtures, bulbs, motors, heaters and instruments can all cause tiny arcing that can drive a mechanic crazy. One loose screw somewhere can do it. Just installing the headset jacks in the metal panel can start all sorts of noise. They should be isolated. Adding extra grounds for everything can add more points for arcing or resistance.

    I have found single-point grounds under the instrument panel for the radios, and found them dirty, oxidized, corroded, loose, whatever. Maintenance, fellas. Maintenance. You cannot build a maintenance-free airplane.
     
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  5. Sep 15, 2019 #65

    Winginitt

    Winginitt

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    Winginitt :
    Secondary ground wires are easy to remove if testing shows some problem exists. As for future issues, you can't design a system and say that its never going to develop a problem.

    Dan Thomas :1. Getting unused wiring out can be a hassle, and very often it's left in. It's usually clamped to multiple points on the airframe and is sometimes buried in bundles. I have experienced lots of hassle trying to troubleshoot some ailing system and being misled by unused wires. Tracing wires can be fun, and unused wires just add to the hassle. Getting old wires out of the leading edges of a wing, for instance, is no fun, and replacing a bad wire is worse. The manufacturers tend to clamp everything to everything, and much of the wiring is installed as the component is being assembled. Install the wire bundles, then rivet the rest of the skins on over them. I have found defective components in some really impossible places, a sign that the component was installed and wired before the airfame was closed up. That results in old, burnt-out components left in and the new one installed in an accessible location. The weight goes up. The homebuilder should try to think it out and design the system for better accessibility than that. (I agree)Winginitt

    Winginitt: I can feel for you when you have to contort while being gouged by innumerable objects and cut and scraped by every sharp edge in sight. I would think that for the most part you are talking about those "professionally" designed and installed wiring systems in certified aircraft. A homebuilt doesn't have to be built in a manner that makes things hard to access. Its up to the builder. If a builder does a crappy or sloppy installation, he deserves whatever difficulties that arise later.

    With a homebuilt, probably most wiring is going to be forward of a pilots knees. The exceptions to that statement are for the most part items that failure isn't life threatening. An electric fuel pump in a wing tank would be an exception......and I would definitely want a dedicated ground for it.

    Most dedicated ground wires would be of a reasonably short length. They would be labeled and grouped together where they pass thru the firewall (from whichever side I chose).I would attach them to a ground terminal strip or a buss bar mounted behind the instrument panel or on the firewall. I would make the instrument panel easily removable or able to be tilted out for easy access. This way most all wires would be easily accessable and easily removable. Longer wires to an electric fuel pump (or?) would also be routed as needed, but with an emphasis on accessability. Then the buss bar would have a heavier lug with at least two wires. the wires would attach directly to the battery and to the fuselage. Maybe a third one to the motor if needed.
    Individual components would also have direct connections to the fuselage. Its not like there would be tons of long wires to deal with. A well thought out homebuilt should have access panels installed during the build so wires can actually be accessed without being a contortionist. The builder can do these things that the factory didn't do, because the builder isn't trying to pinch every production penny.

    Buss Bar 1 001.jpg


     
  6. Sep 15, 2019 #66

    AdrianS

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    One thing I haven't seen addressed on this thread is the difficulty of getting a good electrical connection to an aluminium part.
    There are ways to do it, but just bolting a ring terminal to sheet aluminium is asking for trouble in a few years time, IMO.
     
  7. Sep 15, 2019 #67

    dmar836

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  8. Sep 15, 2019 #68

    Dan Thomas

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    A few years can mean 70 years. I've worked on old airplanes like that where the original ground wire, screwed to the aluminum, is still working. The ones that eventually give trouble are the ones that use cad-plated steel screws; the cad eventually disappears as it sacrifices itself to preserve the steel, and then the screw rusts and the ground connection suffers. Strange, isn't it, that it's the steel that causes the problem, not the aluminum. In a few other cases it's the copper wire inside the crimp, again not the aluminum. That goes for a lot of structural fasteners, too. The rusty fastener should be replaced, as it can eventually cause corrosion in the aluminum if it gets bad enough.

    The 2024T3 light airplanes are made of is Alclad: it's coated on both side with a very thin layer of pure aluminum to protect the alloy beneath it. 2000 series aluminums are alloyed with copper, making them strong, but the copper accelerates any corrosion that starts. Pure aluminum oxidizes very quickly, and that oxide layer protects the aluminum after that.

    If a ground connection can last longer than I've been alive (66 years), it's good enough for me. If the circuit gives trouble it's usually easy enough to check that ground and clean it. Even at that, I've only run across a few cases like that in my career. It was usually at wingtips or other places where rainwater or snowmelt gets in. Water is the big problem.

    When it comes to lights, it's seldom the ground conncection. It's often the power feed, and very often the lamp socket. It corrodes and the contact button in the base develops a layer of oxide. The lamp's base buttons corrode, too.
     
    Last edited: Sep 15, 2019
  9. Sep 15, 2019 #69

    rv7charlie

    rv7charlie

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    Slight thread drift, but the subject of 'gas tight' is rightly cropping up here. In an earlier post, I said something to the effect that screws have 'the same non-gas-tight' issues as Faston style tabs. That statement was constructed poorly, and is a bit misleading. Both terminations, when properly executed using quality parts, are 'gas tight'. But there are subtle differences.

    The screw spreads its force over a much larger contact area (the ring) which can be a good thing in really high current situations (rare in today's equipment), but *can* have the downside of lower clamping force, due to that same large contact area. Now, with proper installation, the following *shouldn't* be an issue. But any counter-clockwise tension+vibration on the wire will be trying to loosen the screw. If that happens (or if the screw isn't properly tightened at installation), gas-tight is gone. There's also that pesky slot head screw that at least for me, has to be retrieved from down in the a/c structure a few times before it's finally started in the hole.

    My personal preference is a quality (as in, not a bubble pack from the local dollar store) tab style termination. No screw to lose. Quality tab style terminals will never lose their spring tension, so as long as they're fully installed on their tabs, the connection will be 'gas-tight'. 1/4" tabs are rated up to 24A continuous, so very few appliances require anything bigger. Did I mention no screw to lose?

    Not saying that screws are wrong; they've worked for decades and still do. But I just spent a couple of days troubleshooting a trim issue in my new-to-me RV-6. The builder used screw type terminal strips mounted to the seat ribs. It seemed like I spent half my time jerking with those little screws, and I wasn't even on my back in the dark like under an instrument panel.

    And for total thread drift: *Show your work!* (Meaning, document it well.) Even if you don't care about future purchasers of your a/c, you'll care about yourself in 4 or 5 years when you're under there troubleshooting or upgrading. I promise you won't remember what you did without good notes and drawings.

    Charlie
     
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  10. Sep 15, 2019 #70

    dmar836

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    It makes more sense to me to add accessory grounds only WHEN a problem is found rather than add the weight, cost, and complexity or individual grounding wires for each component and THEN test for problems as suggested above. That is proving the point and is a contradiction.
    This is where everyone doubles down and claims their rational is for the better good of all. Most homebuilts I've seen could benefit far more from more simplicity and less weight than from all the improvements than then necessitate themselves.
    As Pops says, "If it isn't there, it weights nothing, costs nothing, and can't fail."
     
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  11. Sep 15, 2019 #71

    akwrencher

    akwrencher

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    There is a very simple way to deal with this problem. Simply build a wood or composite airplane! The choice is made for you, have to run ground wires:D
     
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  12. Sep 15, 2019 #72

    pictsidhe

    pictsidhe

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    Vaseline helps prveent corrosion in electrical terminals. There are also 'proper' contact greases, but I always seem to be out. DO NOT under any circumstances use silicone grease. That usually causes problems. It sneaks in between the contacting faces. In fact, when I need a fix a contact that some genius silicone greased, my preference is to completely replace it if at all possible, it's so hard to remove. Dielectrics are insulators, and the silicone dielectric grease work very well for insulating...
     
  13. Sep 15, 2019 #73

    rv7charlie

    rv7charlie

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    I think 'dielectric grease' has a technically correct, but confusing name. Yes, it's insulating, but no, it will not insulate a properly installed connector. In fact, electrical codes typically *require* it to be applied to the big 'feeder' conductors in commercial and home wiring, which are almost always aluminum. (Yes, aluminum wire supplies power to your meter, and likely from the meter to any remote mounted breaker boxes.) Dielectric grease coats the bare end of the aluminum conductor before it's inserted in the termination and locked down with a big setscrew. Tightening the screw extrudes the dielectric and allows metal-to-metal contact. That leaves the 'edges' of the joint protected by the dielectric, which prevents galvanic corrosion between the dissimilar metals of the wire and terminal (hence, the 'dielectric' in the name). We get the same effect when used on ring terminals, and even with the gunk that auto parts stores sell us to use on battery posts.

    Trying to apply it on the outside of a joint is likely to be ineffective; it'll be almost impossible to force it into every crevice and keep water/oxygen away from the joint's edges. Not saying the joint will be bad, just that the dielectric won't be able to do its job properly.

    Charlie
    (no hotel stays lately, but lots of electrical/electronic 'wire twisting', both hobby and pro, from early teens to pushing 70)
     
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  14. Sep 15, 2019 #74

    Winginitt

    Winginitt

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    Dmar836 said: It makes more sense to me to add accessory grounds only WHEN a problem is found.

    Winginitt: So you would rather have something fail in flight than try to preclude it ? There are two things being considered here.

    1. Will the extra wiring cause problems with radio interference ? Thats not life threatening.
    2. Loss of ground connection causing failure of a necessary component. That will be life threatening

    Should I wait for my electric fuel pump to lose its ground connection, maybe my tachometer, fuel guage .....


    Dmar836: rather than add the weight, cost, and complexity or individual grounding wires for each component and THEN test for problems as suggested above. That is proving the point and is a contradiction.

    Winginitt: When you install any electrical system in an airplane you will have to test it. Whatever problems arise will have to be dealt with. In this conversation, the concern is that extra wiring "MAY"......."NOT WILL" create
    radio interference. It is not going to create any failure problem but "MAY" preclude you from a problem...especially later in the planes lifetime. If the small amount of additional weight of some wiring is that critical to the flight of an airplane, I suggest that the plane has more concerns than the weight of the wires. Cost is inconsequential. Complexity.....adding a ground wire circuit is the simplest part of any electrical system.


    Dmar386: This is where everyone doubles down and claims their rational is for the better good of all. Most homebuilts I've seen could benefit far more from more simplicity and less weight than from all the improvements than then necessitate themselves.
    As Pops says, "If it isn't there, it weights nothing, costs nothing, and can't fail."

    Winginitt: While simplicity is always a goal, its always trumped by safety and reliability. While I respect Pops mantra, it isn't always applied correctly. If its "not there", it can't fail......but it also can't help you when needed. Think in terms of a "life preserver". If your boat sinks and you don't have a life preserver.....it costs nothing,weighs nothing, and didn't fail...........but you are better off having it anyway.
     
  15. Sep 15, 2019 #75

    dmar836

    dmar836

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    There ya go. Let’s build!
    Dave
     
  16. Sep 15, 2019 #76

    pictsidhe

    pictsidhe

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    Contact grease is called dielectric grease in the USA? No wonder I'm confused! High voltage has less trouble jumping through grease. At low voltage, the silicone dielectric grease should be avoided, I've fixed a lot of bad contacts by removing it. I religiously smother new car battery contacts with vaseline. I never, ever have problems with them...
     
  17. Sep 15, 2019 #77

    rv7charlie

    rv7charlie

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    Just different names for the same product.
    A search for 'contact grease':
    https://www.google.com/search?q=Con...se&aqs=chrome..69i57&sourceid=chrome&ie=UTF-8

    Here's a link to the Permatex 22058 'Dielectric Tune-up Grease' TDS (product description and instructions for use), from the Permatex web site:
    https://441py33rout1ptjxn2lupv31-wpengine.netdna-ssl.com/wp-content/uploads/tech_docs/tds/22058.pdf

    All applications listed for this particular product are low (battery) voltage, except spark plug lead boots (very high voltage). Instructions are to clean the terminals, 'coat both terminals with grease', and re-assemble. Under 'material properties', its chemical type is 'silicone dielectric compound'.

    I'm not dissing the use of Vasoline on battery posts; I too have done it countless times. Just saying that my understanding of how dielectric grease is supposed to be used, and how it works, differs significantly from yours.
     
  18. Sep 15, 2019 #78

    pictsidhe

    pictsidhe

    pictsidhe

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    I have used two different types of electrical grease. One gets slathered onto contacts before assembly and does what American dielectric grease claims to do. Though I probably use vaseline more often as local stores carry it in large containers.... The other is for high voltage insulation, and has no place between conductors that you want to conduct. That stuff is dielectric grease in some foreign lands. For switches, I use real contact grease, others tend to kill the switch in short order.
     
  19. Sep 15, 2019 #79

    Dan Thomas

    Dan Thomas

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    Yup. I have installed a bunch of Artex 40 ELTs. Here's an excerpt from the installation manual:

    SUBTASK 25-62-30-450-001 B.Harness ELT D-Sub Plug Sealing1)Seal the D-Sub plug at the ELT end to prevent moisture from penetrating the connection, thus preventing water from beading up and causing bridging between connector pins resulting in possible activation of the ELT. Use the following procedure:

    NOTE: Perform the sealing process once all tests have been satisfactorily completed and all harness connections have been verified to be correct.

    a)Disconnect the remote switch harness D-Sub plug from the ELT.
    b)Separate the D-Sub housing halves.
    c)Inject Dow Corning® 4 Electrical Insulating Compound or an equivalent meeting MIL-S-8660C into the back side of the plug, such that the insulating compound surrounds the D-Sub pin area and covers the back of the plug.
    d)Reinstall the housing halves.
    e)Inject Dow Corning® 4 Electrical Insulating Compound or an equivalent meeting MIL-S-8660C around the male pins of the ELT receptacle.
    f)Connect the remote switch harness plug to the ELT.


    Dow Corning 4 is DC4, a silicone grease commonly found in aircraft shops. It keeps water and air away from the pin contacts without hurting conductivity.

    The connector used here is a 9-pin connector just like we used to use to connect monitors to computers.
     
  20. Sep 16, 2019 #80

    rv7charlie

    rv7charlie

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    OK, we may have a 'failure to communicate'. That sounds like a 'sealant' or 'coating'. Something like this?
    https://www.3m.com/3M/en_US/company...ing-FD-15-oz-Can/?N=5002385+3294648449&rt=rud

    That stuff linked would be used for weatherproofing stuff that's exposed to weather, direct burial, etc, but the types I'm familiar with will harden or cure into a rubbery shell that has to be peeled or cut away to get to the joint, if it later needs to be opened. Agree that you wouldn't typically coat the terminals with that before assembly. Dielectric grease might dry out a bit with age, but shouldn't ever harden or 'cure'. It also wouldn't typically be painted over the joint; you usually just see it where it squeezes out of the joint as it's tightened, where it keeps moisture/oxygen from attacking the edge of the mating surfaces.

    Perhaps the problem joints you've found had the wrong substance used at assembly.

    One use of (of a particular variation specific for high voltage insulating sealant) was in TV repair shops, back in the dark ages when the picture was displayed on something called a 'tube'.

    Charlie
     

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