Ground to frame

[Peter_SE]

I´m redoing the electrical system on my KF2 and the previous owner had a wire from batt negative behind the seat to chassie of the plane.

Then the ground bus behind instrument panel was also grounded to the frame.

What is the preffered way?
1. Route a wire from battery negative directly to ground bus
2. Ground the frame

The starter needs separate wire for ground if the frame is not grounded

[jonstark]

In common parlance it’s referred to as “airframe ground”. Save the weight of running heavy wiring when it’s unnecessary.
Ground to the frame.

Jon

[avidflyer]

With my Avid Flyer rebuild, I mounted the battery as far back in the tail for CG reasons. I had to run a fairly large cable for the positive side of the battery, on the negative side, I ran a much smaller ground wire, and also grounded to the frame near the battery. Works fine JImChuk

[aviator79]

The frame usually works fine for a ground. However, consider what troubleshooting will look like when your widget isn't working right. Where is that widget grounded? Can you find it? can you reach it to troubleshoot and fix it? A common ground at the firewall, with a dedicated wire from everything back to ground adds a pretty negligible amount of weight to a small airplane, and ensures you aren't relying on any dodgy fasteners that might be contaminated with paint, oil, polybrush, oxidation, etc.

Also, current is a conserved quantity. All the current that leaves the battery comes back into it. All the current that goes into your starter comes back out. So your ground wires should be the same size as your power wires. They are handling the same amount of current.

[PaulSS]

I know the Eurofox uses the airframe for ground return (with a connection to the airframe close to the battery) but I will be using the central ground method. I've done far too much reading on the subject and have changed my mind so many times but I reckon the elimination of ground loops, easier fault diagnosis and no interference between various user units outweighs (excuse the pun) the extra lbs and work of running wires to a common ground.

[David47]

So that grounding system you have Paul is exactly what I'm doing. Got that idea from the excellent book by Bob Knuckols, which you can download for free from his website. I guess the only issue is it can become a single point of failure, but then if the grounding is done properly, that shouldn't be an issue. Also, removes any ground loops that may affect other stuff, particularly comms.

[Peter_SE]

I ended up grounding the battery to the frame and then ground all components to the same point in the frame behind the firewall.

Thanks all!

[jonstark]

I ended up grounding the battery to the frame and then ground all components to the same point in the frame behind the firewall.

Thanks all!

I recommend that you separate any audio or signal grounds onto their own stud or buss.

Jon

[Peter_SE]

Naah!!....i changed my mind and will ground the way that Paul suggest. The only pint where B- will ground the fram is the firewall passthrough bolt.

Or

I´m also thinking of isolating the passthrough bolt from the frame...

Hmm..!

[PaulSS]

I don't think you want to isolate the pass-through bolt as that will ground the firewall (and frame) to the central ground and, then, back to the battery.

I'm going to use one of those 'forest of tabs' ground buses. This will be bolted to the firewall. The big stuff, like the engine grounds and battery ground will go to the big pass-through bolt and everything else will just attach to the tabs.

Forest of tabs (https://www.bandc.aero/groundblock2448-tabfirewallkit.aspx)

[PaulSS]

I've done a bit more reading on the subject of 'star' ground systems and I am even more convinced than I was before about using this type of setup. I thought the 'forest of tabs' bus was connected to the firewall but I was wrong and it should be isolated and then wired to the pass through bolt with a single wire. The pass through bolt is bolted to the firewall to ensure it makes contact there. I was also thinking of grounding the battery to the airframe using a tab near the battery but this, apparently, would be another ground return and potentially a ground loop source.....so the only wire going to the battery negative will be the one from the pass through bolt.

I'm still waiting and dreaming and obviously just have 'book learning' on this subject but it certainly seems the way to go with modern avionics.

This is my final basic system design (well, it's not mine at all, just re-drawn :) )