Van's Air Force
Don't miss anything! Register now for full access to the definitive RV support community.
Priming and Grounding
- Thread starter dfmoeller
- Start date
Ground wire connection
Similar dumb question. The interior surfaces of my wing are primed. I'm installing a landing light, and the plans call for a ground wire terminal to be bolted to a rib, so I removed some primer to get a good connection.
However, I saw a photo of a similar connection with the terminal connected to the primed rib. How does that work? Am I worrying about nothing?
Similar dumb question. The interior surfaces of my wing are primed. I'm installing a landing light, and the plans call for a ground wire terminal to be bolted to a rib, so I removed some primer to get a good connection.
However, I saw a photo of a similar connection with the terminal connected to the primed rib. How does that work? Am I worrying about nothing?
GusBiz
Well Known Member
Experiment
Doug, adding to what Walt said, if you really want to know, build a 10 minute mock up and measure what the effect is. You will soon see for yourself and it will be the correct answer. Because what Walt wrote is best practice. He never said there will not be any connection, however it maybe a terrible connection, at least in the end you will know.
PS. If your grounding a strobe, you can go with a self tested experiment. If your grounding your electrical bus, just go with best practice. I am sure you can see why.
Doug, adding to what Walt said, if you really want to know, build a 10 minute mock up and measure what the effect is. You will soon see for yourself and it will be the correct answer. Because what Walt wrote is best practice. He never said there will not be any connection, however it maybe a terrible connection, at least in the end you will know.
PS. If your grounding a strobe, you can go with a self tested experiment. If your grounding your electrical bus, just go with best practice. I am sure you can see why.
Gus,
My concern was actually a bit more subtle than it might have appeared. Your suggestion to build it and measure the resistance is a good one - the obvious solution.
As a 30 year plus electrical engineer who has worked in the aircraft design, flight test, and mod industries over a good portion of that time (although that was several years ago), I take AC43.13 to be gospel. My concern wasn't the primary grounding to the structure (as shown in the 43.13 diagram that Walt posted), rather the creation of non-bonded ground paths and ground loops caused by having more than two layers of metal with primed feying surfaces riveted together (consider a wing spar with double spar caps. In that situation, priming of the feying surfaces isolates the web in the middle. The rivets through the assembly only make contact on the outsides of the assembly via the undersides of the heads, leaving the bonding to the web to be kind of doubtful as it relies only on incidental contact with the rivet shaft. This is kind of similar to what happens when 2 fully primed parts are riveted together. Will that be enough to guarantee bonding? I suspect that it will, but we will see.
I will do a short experiment and post the results, if anyone is interested.
Doug
My concern was actually a bit more subtle than it might have appeared. Your suggestion to build it and measure the resistance is a good one - the obvious solution.
As a 30 year plus electrical engineer who has worked in the aircraft design, flight test, and mod industries over a good portion of that time (although that was several years ago), I take AC43.13 to be gospel. My concern wasn't the primary grounding to the structure (as shown in the 43.13 diagram that Walt posted), rather the creation of non-bonded ground paths and ground loops caused by having more than two layers of metal with primed feying surfaces riveted together (consider a wing spar with double spar caps. In that situation, priming of the feying surfaces isolates the web in the middle. The rivets through the assembly only make contact on the outsides of the assembly via the undersides of the heads, leaving the bonding to the web to be kind of doubtful as it relies only on incidental contact with the rivet shaft. This is kind of similar to what happens when 2 fully primed parts are riveted together. Will that be enough to guarantee bonding? I suspect that it will, but we will see.
I will do a short experiment and post the results, if anyone is interested.
Doug
On the RV-12 that I'm building, as well as priming the contact surfaces, I have been dipping the pulled rivets in primer just before installing them. Some other builders have done this too for added corrosion resistance, but not many as far as I know. After seeing your original post, this suddenly seemed like a bad idea. However after testing the resistance between rivets at various points in the wing, the electrical connectivity appears to be OK. Time will tell whether it's good enough. I'd be interested to see what your experiment shows.
Walt
Well Known Member
There is generally an adequite electrical bond created by the large number of fasteners (rivets) in an aircraft structure. Solid driven rivets have high forces in the hole ID which create a good bond. If you are installing fasteners wet, unless the paint is dry (should not be for "wet" installation) the paint will be pushed out of the contact area, it then seals the area but there is still metal to metal contact. If you are using the airframe ground for things like lights and strobes it will not be a problem. There will be "some" resistance in the airframe ground circuit which will cause a small voltage drop in the airframe, but your landing light really doesn't care if it sees 14.5 volts or 14.4 volts (in this case the airframe would be dropping 100mv and the airframe resistance would be 10 miliohms for a 10 amp landing light). AC43-13 has acceptable voltage drops for wires and basically you can use the same figures for airframe resistance.
Last edited:
Wayne Gillispie
Well Known Member
Use a stainless bonding brush and an alodine pen for good long term low resistance.
