Today's Message Index:
----------------------
1. 07:35 AM - Rivets as ground path (Finn Lassen)
2. 12:39 PM - Re: Rivets as ground path (Robert L. Nuckolls, III)
3. 02:49 PM - Re: Rivets as ground path (Finn Lassen)
4. 04:34 PM - Pilot accessible B lead CB (John Bright)
Message 1
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| Subject: | Rivets as ground path |
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
Message 2
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| Subject: | Re: Rivets as ground path |
At 09:34 AM 10/22/2023, you wrote:
>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.
>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.
>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.
>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.
Message 3
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| Subject: | Re: Rivets as ground path |
On 10/22/2023 3:38 PM, Robert L. Nuckolls, III wrote:
> At 09:34 AM 10/22/2023, you wrote:
>
>> 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.
>> 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.
Why? Want the shortest path from battery to starter and back. Via a
ground stud adds a connection.
>> 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.
>> 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
>
> Bob . . .
>
Message 4
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| Subject: | Pilot accessible B lead CB |
Until now I have posted to Aeroelectric List on the website but I got locke
d out and have not been able to fix that so this comes by email and I suppo
se that will work OK.
Advanced Flight Systems, a subsidiary of Dynon, builds instrument panels fo
r experimental aircraft and recommends a pilot-accessible resettable circui
t breaker for the alternator B lead, which is contrary to Aeroelectric Conn
ection's recommendation of a robust fuse, current limiter, or fuselink clos
e to the master contactor.
I emailed AFS tech support to ask why.
=========
Their response:
(We had a customer) cook his instrument panel (due to) a runaway alternator
.
We believe that the alternator field and B+ lead wires chafed and shorted t
ogether inside the regulator (Dual EarthX batteries with a B&C alternator),
causing the alternator to become self powering.
(The) panel was equipped with both an alt field switch and a pilot accessib
le alternator CB (but) due to human factors only the field was shut off, an
d this was insufficient to stop the alternator.
(Exacerbating) the problem was that the aircraft used an EarthX (LiFePO4) b
attery, which is designed to internally disconnect itself in an overvoltage
event so that it doesn't explode. (This) removed the largest load on what
had by then become an unregulated alternator, causing the voltage to spike
to 100V. This in turn caused massive unrepairable damage to the Advanced Co
ntrol Module as well as repairable damage to the EFIS displays and Avidyne
IFD navigators.
Important to note that the EFIS displays did survive and provide primary fl
ight instruments on the backup battery until landing, but they needed repai
r afterward before they could boot again.
An AGM battery, in contrast, has no provision to disconnect itself and does
not have the same risk of fire that an (LiFePO4) battery does. In the same
scenario with an AGM battery, it would weather the runaway alternator by h
eating up some. Because the battery would remain in the loop, the overvolta
ge the avionics would have seen would not have been nearly as dramatic or d
amaging.
I would say in the event of an AGM battery being used..., Nuckolls' favorin
g of a firewall mounted ANL... is understandable. Given what we have seen,
use of an (LiFePO4)... battery means a pilot accessible breaker should be s
trongly considered. Ultimately, this is a matter of pilot choice, and there
is no right or wrong answer.
=================
Seems doubtful the regulator was other than B&C as "... this was a spare NO
expense (spared) aircraft. The panel... had dual AF-5000 EFIS displays, IF
D-440, IFD-550. Dual battery (EarthX) with a B&C alternator.
I have never heard of a B&C regulator failing to stop an OV event. Obviousl
y the B+ feeder does not pass through the regulator so that part of the AFS
response above is incorrect.
Curious to me the voltage only went to 100 and is described as a "spike".
There's a lesson there that due to Human factors the B lead CB was not pull
ed.
Without the opportunity to inspect the incident aircraft and the EFIS logs
the exact cause becomes speculative but I would be interested if Bob or oth
ers can comment. Maybe someone is even familiar with the incident aircraft.
John Bright
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