Local ground vs return to source (common ground)

[rjcthree]

I'm laying out electrical, and realized that I'm back into heavy/automotive frame of wiring - that is, with the exception of under the dash, means common ground returns(to source). This would essentially double the length of wire the current would have to travel, resulting in having to step up in wire size to minimize losses(think in terms of a landing light). This is bad.

But how conductive is alodined surfaces in contact? I have to imagine bonding wing to fues will be required, but if I use the tip rib as the ground for position lights, taxi/langing lights, and strobes . . .

I've read that resistance along an airframe used as a common ground is 'negligable' if bonded correctly, but I'm not sure you can say that across the board. I'm also a little worried about corrosion induced by crrent flow.

Per the Vans' website, 5555 other builders have dealt with this one - but my search came up a bit dry, and I have yet to get into contact with a tech advisor . . .

Thoughts, please.

Rick 90432 - final tank fitting and conduit running (grr) prior to closure.

[jonbakerok]

There is a far greater chance that your single giant ground return wire will corrode or break than there is for all the thousands of connection points in your airframe to simulaneously corrode to the point that the electrons won't be able to find their way back to the battery. As far as alondizing, it is only a few atoms thick, is not an insulator, and the act of installing a fastener instantly removes it anyway.

[L.Adamson]

I used a larger ground wire (#8 if I remember) from the main firewall ground, to a junction box under the seats. From there, it splits to a grounding connection on the first rib of each wing. Lights at the wing tips are grounded to the wing. A heavier load such as my heated pitot, carries the ground wire back to the wing ground mentioned above. Many other grounds such as the auto-pilot servo behind the baggage bulkhead and in the wing, have ground wires running back to the seat junction box.

With aircraft wire being as many stranded as it is, I don't worry about corrosion or breaking. Since I've been doing electrical work in one form or another for many years, I'm quite satisfied with the way I've done it.

L.Adamson

[az_gila]

Originally Posted by rjcthree

....
I've read that resistance along an airframe used as a common ground is 'negligable' if bonded correctly, but I'm not sure you can say that across the board.......

In this case, "bonded correctly" is defined by the "bible".

AC 43.13 has some nice pictures and hardware detail on how to bond ternimals to the airframe to prevent corrosion effects

It's in Chapter 11, Table 11-15 and 11-15.

This long link might get you to the right chapter on theFAA web site...

http://rgl.faa.gov/Regulatory_and_Gu...r%2011.pdf

These methods have been well tested over time and do work...
There is no advantage to re-inventing the wheel here, which will drive up your $$$ and complexity...

Note that the exception to airframe ground is for the starter current (100+ amps?) - and this is handled with ground wires (or usually ground bonding braids) that connect the engine, motor mount, airframe and battery negative together as a low resistance return for the starter current.

gil A

[rv9av8tr]

An electrical / avionics engineer in our EAA Chapter recently told us that it is important to not use the airframe directly as a grounding point, in order to prevent ground current loops. Ground loops lead to RF noise generation which degrades communications and antenna performance. It's best to use a single point Star ground or Modified Star (multiple isolated grounds in various locations of the airplane, which all feed back to your single point ground via a wire).

He said especially your strobe power supply case, which can be physically grounded directly via the case to the airframe, should not be. Mount the power supply on a sheet of lexan and ground the case via a wire to your star ground.

I know that lots of folks use their airframes directly as multipoint grounds, but it's clear how that can lead to RF noise generation and degraded signals.

[Brantel]

Originally Posted by rv9av8tr

An electrical / avionics engineer in our EAA Chapter recently told us that it is important to not use the airframe directly as a grounding point, in order to prevent ground current loops. Ground loops lead to RF noise generation which degrades communications and antenna performance. It's best to use a single point Star ground or Modified Star (multiple isolated grounds in various locations of the airplane, which all feed back to your single point ground via a wire).

He said especially your strobe power supply case, which can be physically grounded directly via the case to the airframe, should not be. Mount the power supply on a sheet of lexan and ground the case via a wire to your star ground.

I know that lots of folks use their airframes directly as multipoint grounds, but it's clear how that can lead to RF noise generation and degraded signals.

I second this statement, As an Electrical/Automation engineer, I have delt with controls that have thousands of I/O and analog signals. Usually most noise related issues can be traced back to poor grounding practices which lead to ground loops.

I firmly believe in the star grounding method outlined above and any device that uses its metal case to provide shielding should be isolated to prevent a ground loop opportunity. If the device has a metal case and its ground terminal are internally connected, I would recommend case isolation and take the ground termninal back to the common star point.

[az_gila]

Originally Posted by rv9av8tr

An electrical / avionics engineer in our EAA Chapter recently told us that it is important to not use the airframe directly as a grounding point, in order to prevent ground current loops. Ground loops lead to RF noise generation which degrades communications and antenna performance. It's best to use a single point Star ground or Modified Star (multiple isolated grounds in various locations of the airplane, which all feed back to your single point ground via a wire).

He said especially your strobe power supply case, which can be physically grounded directly via the case to the airframe, should not be. Mount the power supply on a sheet of lexan and ground the case via a wire to your star ground.

I know that lots of folks use their airframes directly as multipoint grounds, but it's clear how that can lead to RF noise generation and degraded signals.

Moderation is needed here...

As the original poster said, he excluded the "stuff" behind the panel.

The Nuckell's approach of a single point ground behind the panel for avionics (the sensitive stuff) and airframe ground for the rest is a good approach... and is well proven.

Noise examples I've seen have been bad connections, sensitive ANR headsets (yes, found a case on a Piper yesterday) and multiple avionic grounds.

Keep the avionics separate, and you will be OK.... lots of history on this one... Anything else will add $$$, weight (small, in our case) and complexity.

gil A - and yes... I'm an Electronics Engineer too... dealt with sensors far more sensitive than our comm recievers working next to MegaWatt lasers...

PS .. all above comments above null and void for a glass plane....

[CESSNADON]

Airframes make a fine ground...It would take an eternity for the ground currents to cause troublesome corrosion on an airframe. I concur with the previous poster, Keep the avionics stack grounds connected to a single point and the other equipment, grounded at its mounting location.

[Ironflight]

There tens of thousands of metal airplanes flying around using the airframe as the ground, and it works just fine. I also agree that using a single point common ground for the avionics is a very good idea! Another point to make is that when you are wiring up audio stuff (headset jacks, comm radios, audio panels, entertainment sources), wire them exactly as shown in the documentation - probably the audio panel drawings will be the best - but I am willing to bet that in most cases, you'll be told to ground all shields at the audio panel end ONLY, and let the shields, jacks, and radios "float" at the other end. DO it that way, and the potential for hum and buzz is greatly reduced!

Just my experience with several aircraft...

Paul

[Rivethead]

Originally Posted by rv9av8tr

An electrical / avionics engineer in our EAA Chapter recently told us that it is important to not use the airframe directly as a grounding point, in order to prevent ground current loops. Ground loops lead to RF noise generation which degrades communications and antenna performance. It's best to use a single point Star ground or Modified Star (multiple isolated grounds in various locations of the airplane, which all feed back to your single point ground via a wire).

He said especially your strobe power supply case, which can be physically grounded directly via the case to the airframe, should not be. Mount the power supply on a sheet of lexan and ground the case via a wire to your star ground.

I know that lots of folks use their airframes directly as multipoint grounds, but it's clear how that can lead to RF noise generation and degraded signals.

It's exactly for this reason that I'm running the extra wire and taking everything back to the battery.