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  #1  
Old 06-11-2018, 06:05 AM
AviatorJ AviatorJ is offline
 
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Location: Oklahoma City
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Default Grounding Locations

I know the annoyance of chasing grounding issues and want to avoid that on this project. I am installing a lot of electronics and plan to have everything grounded on a large group of tabs (48) on the sub-panel. Here's the layout planned (All 2 ga)-

* Engine to Engine mount - Going to tie two of the sump bolts to the two tabs on the engine mount.
* Mount to Firewall - cable from one of the mount tabs to a buss bolt punched through the firewall.
* Firewall to Sub-panel - Cable from the back end of the bolt to the sub-panel which will have a 48 tab grounding block.
* Sub-panel to Battery - Cable ran to the back, connected to the negative terminal on the battery.

That's the gist of it. Everything that needs to be ground locally such as antennas will still be on the air frame but if it has a negative wire it will all come back to the sub-panel and grounded on one of the tabs. Questions-

- Do I need two grounding straps for the engine? I've read conflicting information.
- For low current items is there an issue with running a common wire in series? Example for my overhead lights where there are 4 different LED lights, plan to just daisy chain the ground... however the positives will run in parallel.
- Is there any benefit to having a small group of tabs either FWF or in the tail? There's only a handfull of items in each area so wouldn't be that difficult to either run a grounds back to the panel.
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  #2  
Old 06-11-2018, 07:26 AM
Mich48041 Mich48041 is offline
 
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Location: Riley TWP MI
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The engine mount should NOT be used to carry electrical current. There is nothing wrong with grounding the engine mount, but a ground strap or copper cable should connect the engine to firewall. If the ground strap is properly sized, then one strap is sufficient. The only reason to have two straps is if one of them was not properly installed or maintained (good crimps and tight fasteners).
It is OK to daisy chain the ground for LED lights provided that it is of sufficient size to carry the current of all 4 lights.
Devices that are sensitive to electrical noise, such as avionics and instrument sensors, should have independent ground wires connected to one common ground point. Do NOT ground audio jacks where they are mounted to the airframe. Use insulating washers for installing audio jacks.
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  #3  
Old 06-11-2018, 07:44 AM
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Carl Froehlich Carl Froehlich is online now
 
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Location: Dogwood Airpark (VA42)
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Ground the engine using the same gauge wire that you use for power to your starter. Ground the engine at the starter ground lug, not the engine case. The ground wire goes from the starter ground lug to the big brass bolt on your tab grounding block mounted on the firewall (tabs on the cockpit side of the firewall). No other engine or engine mount ground required, or desired.

Ground the battery(s) using the same gauge of wire going to your master relay(s) on the same big brass bolt on the cabin side of the tab grounding block.

For ease of wiring you could mount a second, smaller tab block on the bulkhead behind the panel. Run a #10 or so wire from it to the big brass bolt on the firewall tab block. Recommend any heavy loads go to the firewall tab block.

Carl
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  #4  
Old 06-11-2018, 08:12 AM
Canadian_JOY Canadian_JOY is offline
 
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Location: Ontario, Canada
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Quote:
Originally Posted by Mich48041 View Post
The engine mount should NOT be used to carry electrical current.
While I agree completely with this statement, I will point out that several highly successful aircraft have used the engine mount as the primary attachment location for the engine-to-airframe bonding strap. Recently I was looking at my hangar-neighbor's C-172M and found the engine mount had been neatly stripped of paint in one area and the bonding strap from the engine terminated on a steel clamp which covered the paint-stripped area. I had never paid much attention to 172's before (sorry, they may be popular but they just don't garner much of my attention) so hadn't noticed this before.

Carl's comments about the sizes of grounding wires are excellent, as his description of how to connect grounds in an aircraft with an aft-mounted battery. I would add that one can also use a flat braided bonding cable between the engine and airframe grounding point. Many people find this to be a more mechanically flexible alternative to typical stranded wire.
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  #5  
Old 06-11-2018, 09:16 AM
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Carl Froehlich Carl Froehlich is online now
 
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Quote:
Originally Posted by Canadian_JOY View Post
SNIP...
Carl's comments about the sizes of grounding wires are excellent, as his description of how to connect grounds in an aircraft with an aft-mounted battery. I would add that one can also use a flat braided bonding cable between the engine and airframe grounding point. Many people find this to be a more mechanically flexible alternative to typical stranded wire.
I use welding cable for all the #2, #4 and #6 runs. Very flexible so it mitigates fatigue failures from engine vibration, very durable (I put a torch to one to see what would happen - it just charged, did not burn) and less expensive than the aircraft wire alternative. I get it on Amazon.

#2 wire from the starter solenoid to the starter motor
#2 wire from the starter grounding bolt to the firewall ground bolt
#4 wire from each battery to that battery’s master solenoid, #4 from that solenoid to the common starter soleniod connection.
#4 wire from each battery ground to the firewall ground bolt
#6 wire for the alternator output

Carl
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  #6  
Old 06-11-2018, 10:34 AM
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jjconstant jjconstant is offline
 
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It's probably just me, but I found that when I had accounted for all the grounds I thought I would need, and installed a "forest of tabs" that accommodated that number, it was woefully inadequate. I would take the number of planned grounding tabs on your forest of tabs and allow for 50% more. I did like you are proposing and distributed a few, smaller forest of tabs, in a few strategic geographic locations behind the panel, instead of one huge forest, then daisy chained them all to a single common grounding point on the firewall. Also, having every tab occupied can be challenging from a dexterity point of view, especially if they are tucked away in awkward places. Having a few empty tabs makes access a bit easier. Having a small forest somewhere in in the tail or behind the baggage bulkhead seems like a good idea. Lights, strobes, ELT ground, etc.

Depending on the complexity of your panel, being able to group grounds can be helpful in trouble shooting and the inevitable upgrades that will come later in your airplane's life.

Bear in mind that I am by no means an expert, but have been through the original wiring process and 2 upgrades.
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Last edited by jjconstant : 06-11-2018 at 10:41 AM.
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  #7  
Old 06-11-2018, 11:40 AM
rv7charlie rv7charlie is offline
 
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There are a lot of things on certified a/c that get done better in the experimental world....

There's a pretty good guide to grounding practices in the Aeroelectric Connection book. The book also attempts to explain the whys of the technique; always helpful to know why something is done a certain way if you don't exactly follow a 'cookbook'.
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  #8  
Old 06-11-2018, 01:32 PM
AviatorJ AviatorJ is offline
 
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Thanks for all the great feedback, I have read through the Aeroelectric book, among several other common publications. This helped drive my decision to look at a distributed solution vs just putting everything on the airframe. Also validated the use of a tree of tabs on the subpanel, fire wall buss bolt, ect.

For positive side wire size I planned to do 2 ga from tail to starter contractor and through to starter. 6 ga for alternator B lead runs with ANLs in between. Specific to the grounding I need to read up more on the FWF components, I thought the starter and the alternators were grounding through contact with the engine case. That's why I thought you had to run a ground from the engine to somewhere on the airframe. However any positive wire size I'll match for the return ground.
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  #9  
Old 06-11-2018, 01:49 PM
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Mike S Mike S is offline
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Quote:
Originally Posted by AviatorJ View Post
I thought the starter and the alternators were grounding through contact with the engine case. That's why I thought you had to run a ground from the engine to somewhere on the airframe.
You are correct about the starter being grounded to through the case contact----an due to the rubber isolaters in the mount, you need a good solid ground source back to the airframe.

I do not think you need a ground cable running forward from the battery-----I did not have one, and had on problems. I suspect most RV 10s are the same.

Overall your scheme seems pretty good with regard to the FOT on the sub panel, very close to what worked for my build.
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  #10  
Old 06-11-2018, 02:06 PM
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Carl Froehlich Carl Froehlich is online now
 
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Quote:
Originally Posted by AviatorJ View Post
SNIP... I thought the starter and the alternators were grounding through contact with the engine case. That's why I thought you had to run a ground from the engine to somewhere on the airframe. However any positive wire size I'll match for the return ground.
As the starter is using the most current, it gets a ground as good as the #2 wire you have providing the +12vdc. Sure - it is grounded to the engine case and then you can ground the engine case to the firewall ground lug. I prefer, and recommend, using the nice ground lug on the starter for the engine ground.

Not a fan of ground straps under engine bolts. But shoot fire - I don’t use aircraft ground for the battery(s) or anything else. Many do and are happy. You decide what you want to do.

Carl
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