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humptybump

Well Known Member
Most things ground to a grounding bus (which is easy for things in the panel). But what about the autopilot servos and a remote magnetometer or AHRS ?

It is "OK" or "a bad idea" to ground the servos direct to the airframe near the servo location ?

When a magnetometer is installed in the tail, It is "OK" or "a bad idea" to ground the unit to a near location on the airframe ?

Is this a vendor specific question ?
 
I grounede all of my "remote" electrical items (lights, servoes, etc.) at a convenient nearby airframe part. So far, no problems.
 
I ran all my grounds to a common terminal block because the GRT, auto pilot, flaps, and trim harnesses already had the long ground wires. I was already running the other wires. The radios required it in the install manual, so I did it.

But can't answer if grounding locally is a bad thing or not. It was just convent for me to not take that chance.
 
There are some technical reasons not to ground locally, like you might cause some corrosion where the parts are riveted or bolted together. Who knows if this is true or not.

The reason I put a forest of ground tabs on my firewall was to eliminate ground loops, and thus noise in my headset.

However, I did ground my position and landing lights on a plate nut riveted to the last (outboard) rib and I grounded the strobe pack right next to the strobe pack and brought the strobe wire shielding back to that same ground. That eliminated the noise in my headset from the strobe discharge.
 
N941WR said:
However, I did ground my position and landing lights on a plate nut riveted to the last (outboard) rib and I grounded the strobe pack right next to the strobe pack and brought the strobe wire shielding back to that same ground. That eliminated the noise in my headset from the strobe discharge.
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I did the same but added a grounding strap (short jumper wire) from the wing to the fuselage to ensure a good direct electrical path from the wings into fuselage. I know not needed but at day job that is standard practice. Makes sense when have large assemblies that are painted before final assembled since paint will not allow direct metal to metal and not assuming the fasteners will provide the grounding path.
 
Using a ground wire for the fuel sensors is a good way to keep from having problems with those units. Most read errors are likely due to airframe grounding of the units.
 
I'm a big fan of dedicated ground wiring and ground buses. It's a proven way of doing aircraft wiring that ensures good electrical grounds. I've wired up 4 different RVs that way.

However, my old Cherokee (new owner, still based at my airport) has been flying for nearly half a century with airframe grounds all over the plane including the battery under the back seat simply grounded to the airframe, and to this day has never had any grounding-related electrical issues.
 
Well, with the exception of the lights, which already have an airframe ground in the fuselage, everything else goes back to the ground bus.

Ive decided the servos will have "local ground". If it doesn't work out I'll run the extra ground wire. Lets just call this "my little experiment".

What I found interesting, when investigating this topic was that Dynon has said in forum posts that local ground to the airframe for the servos is OK. MGL said in an email that they want the servo ground to go back to panel. (I'm not using either vendor's servos. This was just a point of reference for my question.)
 
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My advice is to ground all avionics, intercoms, radio and "active" devices on a common ground buss like the forest of tabs up near the firewall. I also recommend that this be the point where the battery ground connects and the engine ground strap. Not saying that if you don't do this that you will have noise issues, it just helps make sure you don't

"Passive" devices are almost always OK to ground locally. (LED's without constant current drivers, regular incandescent lights, standard pitot tube heaters, etc)

MGL and Dynon both use stepper motors in their servos. Stepper motors can be huge noise sources since they typically operate with square waves on their motor leads and square waves are big noise generators. There are ways to limit the amount of noise that a square wave can produce by controlling the slew rate of the wave. Not sure if Dynon does this or not.

The biggie is to ensure that your audio system is grounded only where the manufacturer says to ground it especially the shields and the headset jacks!

Again not saying that if you don't do this that you will have noise issues, it just helps make sure you don't. The general idea should be to minimize ground loops where possible.
 
Brantel said:
MGL and Dynon both use stepper motors in their servos. Stepper motors can be huge noise sources since they typically operate with square waves on their motor leads and square waves are big noise generators.
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Thanks Brian.

I am using TruTrak but am planning a series of ground tests before I button everything. I'll add a test card to see if noise is immediately apparent.

I figure, at this point, it would be easy to add the long ground wire. At the same time, I am curious so I will run the test.
 
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I went with single point ground as well. I just didn't want to be chasing phantom ground loop problems around, especially with something that is attached to the control system. I am a only accomplished enough electrically to have finished my plane with lots of help, but I've already had to deal with radio transmissions sending an induced voltage into my EGT wires, so I'm not interested in the same thing happening to my auto pilot servo wires.

I had a friend with an early Navaid unit and he had to disconnect the auto pilot before any radio transmission or it would turn left:eek:

It might not be a problem, but if it is, it's probably easier to eliminate the possiblitiy now, than try to do various fixes when all you want to do is fly:D

I tried to use "best practices" whenever I learned what they were.
 
Corrosion

N941WR said:
There are some technical reasons not to ground locally, like you might cause some corrosion where the parts are riveted or bolted together. Who knows if this is true or not.

......
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It is not a "might" issue, it is quite true.

However the fix is to use the correct hardware per AC43.13

Pages 8 onwards here defines the correct hardware to minimize corrosion.

http://www2.tech.purdue.edu/at/courses/at308/Technical_Links/Ac43-13-1B/CH11_15.pdf

In our usual cases a cadmium plated AN washer will do the job as an interface between dissimilar materials.
 
My TruTrak servos, strobe power packs, heated pitot, and nav lights are grounded locally. EVERYthing else goes to the ground tabs on the firewall. I have no electrical issues and no radio noise. :)
 
Rupester said:
... EVERYthing else goes to the ground tabs on the firewall. I have no electrical issues and no radio noise. :)
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Strobes are grounded at the powerpack because the manufacturer said to...
Pitot is grounded to the rib...
Landing lights are grounded at the rib...
EVERYTHING else is grounded at the firewall... :eek:
NO noise :D
May ground the baggage LED light locally too when it gets installed...
 
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