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 9, 2019 #1

    GESchwarz

    GESchwarz

    GESchwarz

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    My airplane is primarily rivited aluminum construction. It is my understanding that the metal airframe can be used as the return leg of DC electrical circuits. My question is, is it worth is to use the airframe instead of dedicated return wires, given all the bonding consideratons of an electrified airframe?
     
  2. Sep 9, 2019 #2

    litespeed

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    Personally,

    I would make all the wires return as earths to the source. Yes it involves more wire and thus weight. But if using aircraft wire, the penalty is small.

    Just like with boats, electrolysis is a problem and can lead to electrical failures and corrosion of the airframe. Just another area you can reduce potential failure points. It also makes diagnosing faults a lot easier, no chasing bad earths.
     
  3. Sep 9, 2019 #3

    TFF

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    Depends. Fly in any certified airplane and most have grounds through the airframe except sensitive stuff or stuff turned on an off by ground. Depends on each item.
     
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  4. Sep 9, 2019 #4

    BJC

    BJC

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    TFF: I have no disagreement about what you wrote wrt certificated airplanes, but I am in violent agreement with litespeed wrt E-AB; there are advantages to having returns to a common ground that, IMO, are worth the very slight added weight.


    BJC
     
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  5. Sep 9, 2019 #5

    TFF

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    With delicate electrical stuff , it usually helps cut noise in radios. I learned this summer that dedicated ADSB units transmit about 3 times the watts of a regular transponder. Think of that on fiberglass planes. Things like alternator and starter tend to be flow through. Alternators tend to have a lead to ground at a central point for noise but charge voltage is through the engine ground. The ground line is not near the gauge of the hot wire. Electric sensors on the engine tend to be through the airframe ground. It’s a roll with the punches of best practice. Boats have dedicated ground, metal or glass. Lights? If I was to do a nice glass panel, I would be buying one of the custom wire harness with the circles and arrows showing me how. A five wire airplane like mine, if it works, I’m happy.
     
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  6. Sep 9, 2019 #6

    Angusnofangus

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    I'm not an avionics guy, but I can tell you that larger airplanes (aluminum) use the structure of the aircraft for many grounds.
     
  7. Sep 9, 2019 #7

    Derswede

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    I run a mobile "ham" radio. Having installed radios in lots of vehicles, I have gotten to the point that I will have a return ground run. I also fuse both legs of the power leads and have ferrites on both legs as well. RF noise is suppressed by using such. Voltage drop is also minimized and the DC power is much cleaner. Weight wise, a single 6ga wire for ground is not that heavy.

    Derswede
     
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  8. Sep 9, 2019 #8

    Aerowerx

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    One mistake I have seen many times, made by newbees in the electronics world, is to have a common ground return for signals. They think that since it is just 1 MHz, or even 10 MHz that it doesn't matter. Yes it does, if you consider that those signal pulses require components at 5, or even 7 times the fundamental to maintain their square shape. This means that the return line must run along in the same cable or at least in a pair of wires.

    Consider also a strobe at the wing tips. It will be pulsing on and off. It is far better to run a pair of wires, preferably twisted, out to the wing tip. A single wire with frame return forms a big loop antenna, radiating (and picking up) all kinds of trash.

    From an electrical engineering viewpoint (with 50 years experience), just bite the bullet and run a twisted pair of wires.
     
  9. Sep 9, 2019 #9

    12notes

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    The best way to not have to chase ground loop problems later is to have all the grounds in the same place. Headset noise, flickering lights, or intermittent electrical problems will drive you crazy trying to find them, it's best to eliminate one of the major causes with design. With as little systems and weight as there is in a typical homebuilt electrical system, ground return wire is cheap insurance at a minimal cost in weight.

    Larger aircraft have enough wiring that it's better to get rid of the extra weight by removing all those ground wires, and either deal with the problem with filters or some other method probably impractical on small homebuilts. I'm not familiar enough to know what that is.
     
  10. Sep 9, 2019 #10

    Dan Thomas

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    All certified metal airplanes (aluminum or tube-and-rag) use the airframe for the ground path. And they work just fine for about 20 years, when small amounts of oxidation start raising the resistance a tiny bit, encouraging the electrons to find easier paths. They will do that through stuff that can cause some hassles. For instance, you have a strobe on the tail, with its power supply in the aft tailcone, and the power supply's ground is to the airframe. It all works well until some of the riveted seams get a little corroded, so the electrons start traveling along the VOR and COM antenna shielding. Causes trouble with radios and noise in the headset. Alternator noise is often due to deteriorating ground paths between the alternator case and battery ground: corroded and oily ground cable terminals, corrosion and paint between the engine mount and airframe, corrosion and maybe some fretting between firewall sheets. Piper used to use the engine mount itself for the alternator's ground path instead of a cable to the firewall, and the steel mount tubing got magnetized from the constant DC running though it; which screwed up the compass readings.

    You have to think about weight. Avionics ground wiring to a common point won't add much, but if your battery is in the tailcone and you add a #4 ground cable between the battery and some common ground point at the firewall, you will add pounds of weight for no good reason.

    Remember, there are plenty of 1950s and '60s airplanes out there that use airframe ground returns, and aside from the avionics, they work fine as long as their ground connections are kept clean. 60-year-old airplanes. How many of you will be flying your next creation 60 years from now? One has to keep things in balance: Perfection is nice but it costs a lot more money, it takes a lot longer to build, and it ends up weighing more. Is it worth it? Does the added cost and weight justify the tiny bit of added safety? You can keep adding small bits of safety until the airplane is too heavy, or you are bankrupt, or you're to old to fly.
     
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  11. Sep 9, 2019 #11

    BJC

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    That is another problem.


    BJC
     
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  12. Sep 10, 2019 #12

    Rockiedog2

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    My lithuim batt is between the rudder pedals and grounded to the galvanized firewall. Everything electric is within about 16" either front or rear of the firewall so it's the common ground. No lights.
    That's pretty simple, cheap, light and easy to get to.
     
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  13. Sep 10, 2019 #13

    BBerson

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    I watched a lithium battery go off like a blowtorch a few months ago. Burned for several minutes. Large RC model crash, the battery was ejected and burned about two feet away from the plane from hitting the earth, I guess.
    They usually torch off normally only while charging , I think.
     
  14. Sep 10, 2019 #14

    Aerowerx

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    Metalic Lithium will react quite violently in the presence of moisture. Even the moisture in the air. What happens is that once the casing is compromised--POOF!
     
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  15. Sep 10, 2019 #15

    litespeed

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    Just because they did it that way before is not a good enough reason when we know it causes issues.

    Yes, old aircraft did it that way, yes they survived. But we know it causes interference in electronics, we know it causes corrosion in airframes over time. We know electrical connections are likely to fail over time with a common earth.

    In a typical homebuilt we are not needing large amounts of extra wire if well designed for earthing the circuits. A few pounds at most with quality wire. The cost is tiny overall. We generally seek the best solution we can afford- this is really cheap insurance- why not use it?

    How much time and money goes into ensuring all your electrics and electronics work reliably and interference/noise free? What is the real cost of failure inflight or even just to resolve it on the ground?

    What cost to having a airframe with corrosion issues later in its life?

    The solution is easy, cheap and with only a small weight penalty.

    Its up to the builder but why scrimp and cause problems you know will eventually occur later?

    Use best practice and its hard to go wrong.

    Fuse all circuits, earth returns and quality wire.

    Fire is also a nasty that loves electrics- any reduced risk is golden. The better the wiring the less likely burning can be a option.

    But the builder chooses, what do you choose?
     
  16. Sep 10, 2019 #16

    Rockiedog2

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    LOL

    I KNEW that was coming! In that case BBerson I think the gas and oil fire I'm trying to get out of will be a lot worse than the battery out there somewhere burning.

    Yeah I'm aware of the history of problems. The research indicated that over charge could increase the fire hazard, or something like that. The ones I use are EarthX; got one between the rudder pedals on both my operational planes. They "claim" to have a battery management thing on theirs that prevents those problems. Who knows? But that and the history satisfied me and the 12# loss was big too. My 5 or so years experience with 2 of them has been all positive. So far. Way better than any of the other batts I've ever used. Stout, light, and never discharge from sitting.

    I notice some of our more knowledgeable types; particularly some engineers, seem to get all wrapped up in what may be minutely possible. And some of us lesser educated but maybe more experienced types are ok with going with what may be more probable. The acceptable degree of probability depends on experience and personal tolerances.

    Can't go wrong with the engineer's approach.
     
    Last edited: Sep 10, 2019
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  17. Sep 10, 2019 #17

    Rockiedog2

    Rockiedog2

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    Aw naw.
    And we got lotsa humidity here in Mississippi too!
    Thanks. I'll let you know if I get burnt up feet.
     
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  18. Sep 10, 2019 #18

    BJC

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    Nothing to add .... just thought that your comment was worth repeating.


    BJC
     
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  19. Sep 10, 2019 #19

    BJC

    BJC

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    Back when all homebuilts were scratch built, a good friend built 6 or 7 homebuilts, most in less time than others took just studying the plans. He said that engineers seldom finish an airplane, because they spend all of their time “improving” the design rather than building it. (Some of you guys remember Dennis, the Englishman.)

    I must admit that I resemble that remark.


    BJC
     
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  20. Sep 10, 2019 #20

    BBerson

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    The model airplane batteries are the most dangerous. I don't know about EarthX, could be harmless. I don't know if they recommend a vented box or not.
    I doubt air moisture sets them off, crash damage and overcharging is my concern.
    My comment was just an observation, for consideration by those that might use a lithium model airplane battery in the cockpit for any reason.
     

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