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finn.lassen(at)verizon.ne
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 Posted: Sun Oct 22, 2023 6:35 am Post subject: Rivets as ground path |
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I posted this on VAF but no answers yet. Perhaps Bob has experience on this?
My RV-4, even though first flight was in 2021, was a project where fuselage (firewall) was probably mainly riveted in the 90's. Could be 30 years ago. I know there are a lot of rivets from the SS firewall to skins and longeron angles, but how well do those do over time as ground paths?
My common ground point for the airframe is a SS screw through the firewall with star washers -- forest of tabs on cockpit side and maybe 4" of 8 AWG to battery negative pole on engine side. Flexible ground strap from battery negative to engine block near alternator. Also ground path from engine via engine mount to airframe. No problems with ground voltage drop when starting.
Using airframe as ground for some items like flap motor, lights, strobes and antennas.
Gradually over the 100 or so hours since first flight in 2021 I'm seeing various odd electrical issues and of course ground is the first thing that comes to mind.
Finn
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nuckolls.bob(at)aeroelect
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| Posted: Sun Oct 22, 2023 11:39 am Post subject: Rivets as ground path |
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At 09:34 AM 10/22/2023, you wrote:
| Quote: | I posted this on VAF but no answers yet. Perhaps Bob has experience on this?
My RV-4, even though first flight was in 2021, was a project where fuselage (firewall) was probably mainly riveted in the 90's. Could be 30 years ago.
I know there are a lot of rivets from the SS firewall to skins and longeron angles, but how well do those do over time as ground paths? |
A properly sized and driven rivet SWELLS in the bore
hole forming a very gas-tight union of the mated materials.
In the case of the firewall, as you've observed, there
are LOTS of them. Any electrical resistance between the
firewall and other structure is comprised of many
resistors in parallel. The combination of these
joints will add up to an exceedingly tiny number.
| Quote: | My common ground point for the airframe is a SS screw through the firewall
with star washers -- forest of tabs on cockpit side and maybe 4" of 8
AWG to battery negative pole on engine side.Flexible ground strap from battery negative to engine block near alternator.
Also ground path from engine via engine mount to airframe. No problems
with ground voltage drop when starting. |
Probably okay. I would rather see battery(-)
to firewall ground stud; crankcase to firewall
ground stud.
| Quote: | | Using airframe as ground for some items like flap motor, lights, strobes and antennas. |
Which is as is should be. Except for batteries
mounted on the other end of the airplane from
the engine, the airframe is never expected to
carry major OPERATING currents. And in the
case for cranking currents, ground return
losses in the airframe are of no practical
consequence.
Grounding of peripheral accessories you cited
will be even less affected. The most IMPORTANT
feature for a carefully crafted ground systems is
avoidance of ground loops in VULNERABLE avionics
as described in Z15 of the 'Connection.
| Quote: | Gradually over the 100 or so hours since first flight in 2021 I'm seeing various
odd electrical issues and of course ground is the first thing that comes to mind. |
Can you elaborate?
Bob . . .
////
(o o)
===========o00o=(_)=o00o=========
< Go ahead, make my day . . . >
< show me where I'm wrong. >
=================================
In the interest of creative evolution
of the-best-we-know-how-to-do based
on physics and good practice.
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finn.lassen(at)verizon.ne
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| Posted: Sun Oct 22, 2023 1:49 pm Post subject: Rivets as ground path |
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On 10/22/2023 3:38 PM, Robert L. Nuckolls, III wrote:
| Quote: | At 09:34 AM 10/22/2023, you wrote:
| Quote: | I posted this on VAF but no answers yet. Perhaps Bob has experience on this?
My RV-4, even though first flight was in 2021, was a project where fuselage (firewall) was probably mainly riveted in the 90's. Could be 30 years ago.
I know there are a lot of rivets from the SS firewall to skins and longeron angles, but how well do those do over time as ground paths? |
A properly sized and driven rivet SWELLS in the bore
hole forming a very gas-tight union of the mated materials.
In the case of the firewall, as you've observed, there
are LOTS of them. Any electrical resistance between the
firewall and other structure is comprised of many
resistors in parallel. The combination of these
joints will add up to an exceedingly tiny number.
|
OK. So if you have never seen that as a problem over age (corrosion/vibration), I'll rule that out.
| Quote: | | Quote: | My common ground point for the airframe is a SS screw through the firewall
with star washers -- forest of tabs on cockpit side and maybe 4" of 8
AWG to battery negative pole on engine side. |
| Quote: | Flexible ground strap from battery negative to engine block near alternator.
Also ground path from engine via engine mount to airframe. No problems
with ground voltage drop when starting. |
Probably okay. I would rather see battery(-)
to firewall ground stud; crankcase to firewall
ground stud.
|
Why? Want the shortest path from battery to starter and back. Via a ground stud adds a connection.
| Quote: | | Quote: | | Using airframe as ground for some items like flap motor, lights, strobes and antennas. |
Which is as is should be. Except for batteries
mounted on the other end of the airplane from
the engine, the airframe is never expected to
carry major OPERATING currents. And in the
case for cranking currents, ground return
losses in the airframe are of no practical
consequence.
Grounding of peripheral accessories you cited
will be even less affected. The most IMPORTANT
feature for a carefully crafted ground systems is
avoidance of ground loops in VULNERABLE avionics
as described in Z15 of the 'Connection.
|
Headset/mic jacks isolated from airframe. Now, I do need to review my engine monitor grounding.
| Quote: | | Quote: | Gradually over the 100 or so hours since first flight in 2021 I'm seeing various
odd electrical issues and of course ground is the first thing that comes to mind. |
Can you elaborate?
|
I've been logging data from pretty much every single flight via engine monitor (the Dynon D100 EFIS also has air data logging but more elaborate to download data than just removing a USB thumb drive). Engine monitor logs 37 channels of data each second. Of particular interest has been (engine) bus voltage (because of occasional voltage alarms during flights). Most recently (again) voltage drifted higher and higher from average of 14.5V prompting me to finally put old alternator together and restoring internal connection between three small rectifier diodes from windings to voltage regulator (feeds field winding) and replacing my schottky diodes with a relay between Alternator B+ and battery. Relay controlled by Alt breaker. That resulted in alternator output dropping to about 13.9V. Seems to indicate that original path from battery +, master relay, alt breaker to alternator voltage regulator had a 0.6V drop over it, a lot for 3.4A and much less at higher RPMs. Could of course also be difference between voltage regulators. More work to be done there.
Other issue is the wideband oxygen sensor board https://ecotrons.com/accurate_lambda_meter/wideband_controller_alm-board/ analog output logged by engine monitor more and more frequent spikes to full lean. Replaced the LSU 4.9 sensor but didn't improve. I have an Arduino Mega 2560 that I use for trim tabs display and controlling spotlight servos. It also has a data logging shield (to SD Card) that I enabled. When I remembered that the ALM-board also has digital output I starting logging both analog (LP filtered) output and digital output from it. Digital data also has glitches -- needs more programming work to figure out if it's the board going bad or an issue with engine monitor logging.
Then there's occasional engine controller and monitor issues.
Just a bunch of "little" things that made me wonder if I have intermittent ground issues, occasional voltage spike issues (short enough not to trigger OVP module) and not get logged.
Finn
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