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nuckolls.bob(at)aeroelect
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 Posted: Wed Nov 15, 2023 6:17 pm Post subject: Power, signal, and Coax - How should they be bundled? |
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At 04:29 PM 11/15/2023, you wrote:
| Quote: | --> AeroElectric-List message posted by: "wsimpso1" <wsimpso1(at)comcast.net>
Here is my little problem. I am trying to wire my fiberglass airplane with internal antennas properly. I have little insight on electromagnetic compatibility. Please help.
I know that I am supposed to keep wiring for DC power to stuff like motors and electronics separated from data, but am struggling with knowing what is what and how much they matter. |
Not true. "Data" carried on RS or CAN style
serial signals are DESIGNED to live happily
with other energy paths in a complex system.
That's what good data management is all about . . .
if a product is qualified to go into an airplane,
it is assumed to be TESTED per some agreed
upon protocol for RESISTANCE to EXPECTED
antagonists.
I worked in general aviation electrical/avionics
systems for 40 plus years. Not once did
I encounter a situation where the manufacturer
warned against routing any of their products
wiring separate from any other wires in the
airplane. To do so would be a written admission
of their failure to design a product INCOMPATIBLE
with their target market.
Same thing goes for cars, over the road trucks,
all manner of heavy machines. High speed data
systems are EVERYWHERE. Do installation
instructions for any accessories proposed for
use in your project include such warnings? Yeah,
flux gates are a special case cause they're
task is to resolve VERY TINY angular position
of earth's magnetic field. But signal paths
out to the flux gate should require no special
treatment for electro-magnetic compatibility
with the rest of ship's systems.
Throughout my experience in experimental aviation,
I've encountered several instances where a
product was adversely affected by radio
frequency energy from transmitters. Poorly
designed products.
On the certified side of the house, I had
a case on Hawker-Beech 800 aircraft where
a poorly conceived HF antenna caused a
gross failure of every starter generator
controller in the airplane. Turns out that
the 30+ year old controllers were grand-fathered
onto current production never having been
qualified to ANY reasonable compatibility
requirements! Spent a week in Little Rock
getting some brand new aircraft sold off
before a Dec 31 deadline to meed sales
targets! Man . . . that was a can of
worms!
Bottom line is that much of what's circulated
as sage advice for configuring wire bundles
in your project is not founded in good
engineering or physics. Ignore it. Concentrate
on good craftsmanship and don't worry about
things that go squeak and bump in the night.
Bob . . .
////
(o o)
===========o00o=(_)=o00o=========
< Go ahead, make my day . . . >
< show me where I'm wrong. >
=================================
In the interest of creative evolution
of the-best-we-know-how-to-do based
on physics and good practice.
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Voyager
Joined: 30 Jun 2020
Posts: 77
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| Posted: Thu Nov 16, 2023 4:45 am Post subject: Power, signal, and Coax - How should they be bundled? |
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In general, I agree with Bob that this issue is overblown by some, but I am not quite as cavalier as is Bob in this regard. I don’t have any experience with aircraft wiring, but I do have 30+ years of experience with industrial controls and data acquisition and we had a lot of issues with EMI/EMC and noise over the years.
In the airplane I am building, I am running power wires down one side of the fuselage and signal wires down the other where possible. I also try to avoid bundling power wires with signal wires and also where they cross try to cross them at 90 degrees, again where possible and not a huge inconvenience. I am not saying to jump through hoops, but no reason to run a GPS cable beside the cable to the A/P servo in most cases as it can easily be avoided.
At least one manufacturer, Garmin, does provide guidelines and warnings in regard to wiring. Here is a short except, shouldn’t invoke copyright concerns since this is short and for education, from the Garmin G3X installation manual. This is from chapter 2 Installation Preparation:
“2.3.1 Wiring Harness InstallationUse cable meeting the applicable aviation regulation for the interconnect wiring. Any cable meeting specifications is acceptable for the installation. When routing cables, observe the following precautions:
• All cable routing should be kept as short and as direct as possible.
• Check that there is ample space for the cabling and mating connectors.
• Avoid sharp bends in cabling.
• Avoid routing near aircraft control cables.
• Avoid routing cables near heat sources, RF sources, EMI interference sources, power sources (e.g., 400 Hz generators, trim motors, etc.) or near power for fluorescent lighting.
• Route the GPS antenna cable as far as possible away from all COM transceivers and antenna cables.
• Analog Input wires routed too close to spark plugs, plug wires, or magnetos may result in erratic readings.”
There are also special precautions for the magnetomers, as Bob mentioned, that are covered elsewhere in the manual, but the above are the general suggestions for wiring of all Garmin LRUs.
Most equipment is fairly tolerant, but I will also say that the 60s and 70s vintage Cessna and Pipers that I have owned and flown were train wrecks in regards to EMC/EMI. I don’t think I’ve flown one that didn’t have issues such as hearing the beacon faintly in the headsets or general static and poor audio quality. These may have been caused by repairs and field modifications, but I don’t know as I’ve never had the privilege to fly a brand new GA airplane. On my build I am trying to do things reasonably in accordance with Garmin’s recommendations above and also in accordance with what I learned several decades ago in a course called “Grounding and Shielding” taught by Dr. Tom Van Doren from the University of Missouri-Rolla. I brought him in to teach his course at the Fortune 500 company I worked for as we always had issues in the field with large installations that had hundreds of sensors and high voltage and high current industrial heaters and motors.
Tom showed us that very simple practices that add virtually no cost to the installation (using twisted wires for power and ground for example) add huge benefit.
So, no need to obsess as Bob says, but I do think it pays to do a few good practices such as what Garmin recommends. Particularly with devices like GPS antennas and cables that are dealing with very, very small signals and even with standard audio signals if you want a clean signal with minimal background static.
Matt
Sent from my iPad
| Quote: | On Nov 15, 2023, at 9:19 PM, Robert L. Nuckolls, III <nuckolls.bob(at)aeroelectric.com> wrote:
At 04:29 PM 11/15/2023, you wrote:
| Quote: | --> AeroElectric-List message posted by: "wsimpso1" <wsimpso1(at)comcast.net>
Here is my little problem. I am trying to wire my fiberglass airplane with internal antennas properly. I have little insight on electromagnetic compatibility. Please help.
I know that I am supposed to keep wiring for DC power to stuff like motors and electronics separated from data, but am struggling with knowing what is what and how much they matter. |
Not true. "Data" carried on RS or CAN style
serial signals are DESIGNED to live happily
with other energy paths in a complex system.
That's what good data management is all about . . .
if a product is qualified to go into an airplane,
it is assumed to be TESTED per some agreed
upon protocol for RESISTANCE to EXPECTED
antagonists.
I worked in general aviation electrical/avionics
systems for 40 plus years. Not once did
I encounter a situation where the manufacturer
warned against routing any of their products
wiring separate from any other wires in the
airplane. To do so would be a written admission
of their failure to design a product INCOMPATIBLE
with their target market.
Same thing goes for cars, over the road trucks,
all manner of heavy machines. High speed data
systems are EVERYWHERE. Do installation
instructions for any accessories proposed for
use in your project include such warnings? Yeah,
flux gates are a special case cause they're
task is to resolve VERY TINY angular position
of earth's magnetic field. But signal paths
out to the flux gate should require no special
treatment for electro-magnetic compatibility
with the rest of ship's systems.
Throughout my experience in experimental aviation,
I've encountered several instances where a
product was adversely affected by radio
frequency energy from transmitters. Poorly
designed products.
On the certified side of the house, I had
a case on Hawker-Beech 800 aircraft where
a poorly conceived HF antenna caused a
gross failure of every starter generator
controller in the airplane. Turns out that
the 30+ year old controllers were grand-fathered
onto current production never having been
qualified to ANY reasonable compatibility
requirements! Spent a week in Little Rock
getting some brand new aircraft sold off
before a Dec 31 deadline to meed sales
targets! Man . . . that was a can of
worms!
Bottom line is that much of what's circulated
as sage advice for configuring wire bundles
in your project is not founded in good
engineering or physics. Ignore it. Concentrate
on good craftsmanship and don't worry about
things that go squeak and bump in the night.
Bob . . .
////
(o o)
===========o00o=(_)=o00o=========
< Go ahead, make my day . . . >
< show me where I'm wrong. >
=================================
In the interest of creative evolution
of the-best-we-know-how-to-do based
on physics and good practice.
|
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wsimpso1
Joined: 04 Nov 2018
Posts: 31
Location: Saline MI
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| Posted: Thu Nov 16, 2023 7:41 am Post subject: Re: Power, signal, and Coax - How should they be bundled? |
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Before we get too far into implying that I listen to poor sources, my one and only source of advice on all of this has been "The AeroElectric Connection" by Bob Nuckolls, 12th edition. Specifically Chapter 16, page 16-16
"(3) Avionics and audio systems should not share wire bundles with DC power distribution"
Am I to ignore that? Antenna cables and GPS data seem to be avionics, while the lights and servos and ELT seem to be DC power stuff...
There was also talk within chapter 16 of interference between audio and other systems, magnetic field interference, high currents in COAX, and lots of other examples including the warning that shielding will not keep down magnetic field interference.
Now I am asking for help from this forum on details of exactly how to wire some potentially noisy DC power, some DC potentiometer signal, and some RF so they do not mess with each other. I have padded clamps, mounting studs, terminals, spools of wire and cable, etc. Advice received will be acted upon and soon.
Since the aft fuselage is a difficult place to rearrange wiring in later, I want to arrange it now so it is unlikely to ever need rearrangement later.
Details in the aft fuselage:
My trim motors are DC power, and like all of the others are only certified for splash/condensation resistance. No data bus. They each have a DC electric motor with a stall current draw of 1 amp fed with two 24AWG leads, a high and low reference voltage and a sense wire. I suspect the DC power feeds, while modest current, could be noisy. I do not know or even begin to understand if EM interference could begin to be an issue with the potentiometer wiring adjacent to any of the rest of this stuff.
My aft position/flashing light are LED's, not certified. All switching is done in the cabin - the LED set is cycled between low and high power. The manufacturer (FlyLEDS) tells us the wiring is noisy, and so recommends a two wire shielded cable, with the shield grounded in the cabin.
My ELT is aircraft certified hardware. There is a single conductor for GPS position data to the ELT inside a shielded cable with power and ground wires, and the shield is grounded in the cabin. I have no doubt that this hardware and cable is fine by itself.
Three RG400 cables from two COM and one GS antenna. Antenna are copper tape and ferrite balun per common practice in fiberglass airplanes.
So my question for aft fuselage stuff: Would I be wise to isolate any of these from any of the others?
Best case - everything can go in one bundle;
Easily accepted case - two bundles and please say which conductors/cables in each bundle;
Difficult case - three bundles with DC power in one, servo potentiometer leads in one, and coax in third.
Does anything else require shielding?
Going forward to the baggage bay, we include lots of DC power including batteries, contactors, starter and ground cables, Dynon's ADS-B, COM radio, data platforms, transponder, data bus, etc. Again, three bundles going around the people to the instrument panel is more difficult to execute, two is way doable.
What wires/cables should not be bundled with what other wires? Will I be at low likelihood of noisy radios and messed up signals with two bundles? Do I really have to figure out a third path between panel and baggage bay?
Billski
| nuckolls.bob(at)aeroelect wrote: | At 04:29 PM 11/15/2023, you wrote:
Bottom line is that much of what's circulated
as sage advice for configuring wire bundles
in your project is not founded in good
engineering or physics. Ignore it. Concentrate
on good craftsmanship and don't worry about
things that go squeak and bump in the night.
Bob . . .
|
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user9253
Joined: 28 Mar 2008
Posts: 1908
Location: Riley TWP Michigan
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| Posted: Thu Nov 16, 2023 11:54 am Post subject: Re: Power, signal, and Coax - How should they be bundled? |
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A trim servo position potentiometer is extremely unlikely to cause any
interference because the output signal changes very slowly as the motor
runs. When the motor stops running, the position signal is steady DC. Other
antagonists that transmit pulsing signals are more likely to cause
interference. Since both the positive and negative signal wires of the trim
position sensor are run together in one cable, any transmitted or received
interference in one wire is canceled by the other wire.
In other words, don't worry about the trim position indicator circuit.
Circuits that are more likely to cause interference are the magneto P leads
and strobe light power wires. Also make sure that audio circuits are
grounded at one end only. Microphone jacks should NOT be grounded locally.
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wsimpso1
Joined: 04 Nov 2018
Posts: 31
Location: Saline MI
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| Posted: Fri Nov 17, 2023 6:01 am Post subject: Re: Power, signal, and Coax - How should they be bundled? |
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Thanks for the knowledgeable responses.
It looks like nothing I have in the back end is terribly sensitive, and the thinking around the topic is mostly fuss over nothing. So, in the aft end of the fuselage, it is looking like this:
Starboard side will get all three antenna cables, 3 sets of 3 sense wires from the trim motors, and both the three wire shielded cable and RJ11 for the ELT;
Port side will get 3 pairs of trim motor power wires, and the shielded two wire LED power cable.
Further forward is more up in the air, but it is looking like we will add fat wires and any noisy stuff on port side while starboard will get data, DYNON system cable, etc. Centerline is already tied up with flight controls, fuel lines, and air handling.
It is also looking like I will be making twisted pairs for the trim motor power and twisted triplets for trim sensing, This more for neatness and keeping things straight than for noise control. Onward to system numbering etc.
Thanks again.
Billski
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