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nuckolls.bob(at)aeroelect
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 Posted: Sun Jun 12, 2016 7:33 am Post subject: Review request for RV-9 Electrical System (G3X, GTN, etc) |
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At 02:36 AM 6/10/2016, you wrote:
| Quote: | --> AeroElectric-List message posted by: "gfb" <fly(at)bappos.com>
Initial thoughts on electrical system for my RV9, hoping to get some feedback from you folks. I do have some questions below but please feel free to chime in with anything else you feel is valuable.
- On the fence about some sort of avionics switch. Most of the big items installed don’t have on-off switches on them so I can’t keep them off during engine start. I understand the single point of failure problem so I have been avoiding it but it sure would be nice. |
What is the foundation for needing one in the first place?
| Quote: |
- Safe to put both trim motors on the same breaker? The GAD27 install manual recommends 2 5A breakers but does not indicate why. Seems like a waste of a breaker to me  |
"Safe" is an un-quantified term in aviation . . . airplanes
are dangerous as (at)#$$. We design and choose parts that drive
risks lower . . . but they are never zero. So the answer to
your question is, what are additional hazards for having both trims
disabled from the same fault? I suspect they are vanishingly
small. So if you're crowded for breaker space, then drive on.
Remember, 99.99$% of all breakers and fuses in vehicles of
any time sit there for the lifetime of the airplane never having
been called upon to do their job: protect a wire.
| Quote: | | - Any other spots I could save a breaker? I feel that I’m too granular with them but can’t figure out where to… trim… Maybe Nav+Strobes, Servos+GMC, AudioPanel+GTR20? |
Why not fuse blocks of generous size mounted out of the
way but in sufficient quantity to offer independent
protection for every feeder . . . with unassigned slots
as spares for future expansion?
| Quote: | | - What is the preferred method to set up the sd-8 to come on by itself? I have heard setting it's regulator voltage low is pretty common? What should that be set to? |
Why do this? When the low voltage light comes on
you say, "Aw shucks!". Finish the cup of coffee,
fold up the map, tell the passenger that the
little yellow light is no big deal. Then flip
a few switches to bring your second engine driven
power source into service. The hazards for main
alternator are not grounded in real risk to
comfortable termination of flight. It's the un
founded belief that it's a hazard that drive up
your pucker-factor.
- The e-bus relay should see a peak of 20A with typical below 15A. I’m considering the B&C with the 25A heat sink?
20A ebus? How did this get BIGGER than the SD-8 output?
Bob . . .
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gfb
Joined: 07 Jun 2016
Posts: 4
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| Posted: Sun Jun 12, 2016 12:46 pm Post subject: Re: Review request for RV-9 Electrical System (G3X, GTN, e |
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On avionics switch, would be interesting to have most avionics off while cranking the engine.
For the Trim breaker, Ray Allen indicates that a single 1A would be enough for both trim motors. This sounds odd considering Garmin recommended a 5A for each.
Yeah, I'm probably going to brake a few items out into fuse blocks. I'm thinking exterior lights get one, LRUs, Cabin Power, and maybe one other. Is it reasonable to have these fuse blocks have a pull breaker on the panel?
i.e. Ext Lights panel breaker for 20A goes to a fuse block behind the panel with fuses for each light.
Is this reasonable?
The e-bus has a peak that exceeds the SD-8. This only happens if everything is on at once, including the boost pump running, the radio transmitting, both servos moving and trimming, and a dead ibbs battery charging. This will likely never happen, definitely not for any amount of time longer than a second or so. The normal draw for the bus should be about 6A with it maybe going up to 10-15A on landing. I'm looking at the SD-8 as more of a battery extender than a full redundant alternator.
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nuckolls.bob(at)aeroelect
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| Posted: Mon Jun 13, 2016 4:17 am Post subject: Review request for RV-9 Electrical System (G3X, GTN, etc) |
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At 03:46 PM 6/12/2016, you wrote:
| Quote: | --> AeroElectric-List message posted by: "gfb" <fly(at)bappos.com>
On avionics switch, would be interesting to have most
avionics off while cranking the engine. |
The idea behind the avionics master switch
was birthed about 1965 when transistors were
first starting to show up in the radios.
Compared to today's hardware, batteries were
relatively soggy 'soggy' devices. We were
'killing' radios in brand new airplanes
before they left the factory.
The real physics behind those failures was
poorly understood. See http://tinyurl.com/hye6mpe
Today's batteries are more robust as are the
radios themselves. All things potentially
hazardous to the community of silicon devices
are well known and easily managed. Today,
there is nothing that any vehicular dc
power system can throw at a properly
applied transistor that puts the device
at-risk.
| Quote: | | For the Trim breaker, Ray Allen indicates that a single 1A would be enough for both trim motors. This sounds odd considering Garmin recommended a 5A for each. |
Breakers/fuses protect wires. You're
probably not going to wire with anything
smaller than 22AWG for which 5A is the
recommended protection. While 1A protection
for both is 'enough', 5A protection is not
'too much' based not on what's at the end
of the wire but based on the wire size alone.
The decisions for wire sizing/protection
are going to be simple . . . the more
difficult part is to craft an architecture
that produces the most failure tolerant
design based on the airplane's flight
handling qualities and how you intend to
use the airplane. In most instances, loss
of one or both trim systems does not
present an especially risky situation.
On the other hand, protecting each system
independently of each other is easy and
likelihood of ANY protective device being
tripped is exceedingly low.
| Quote: | Yeah, I'm probably going to brake a few items out into fuse blocks. I'm thinking exterior lights get one, LRUs, Cabin Power, and maybe one other. Is it reasonable to have these fuse blocks have a pull breaker on the panel?
i.e. Ext Lights panel breaker for 20A goes to a fuse block behind the panel with fuses for each light.
Is this reasonable? |
Why any breakers at all? They're expensive,
take up panel space, have no useful purpose
to the pilot in operating the aircraft. They
force you to route wires to relatively
unhandy places while increasing weight.
See http://tinyurl.com/hjkqsto
I proposed a shift from breakers back to
fuses in aircraft about 20 years ago. Since
that time, many failure tolerant airplanes
have been crafted with no breakers at a
substantial savings of weight, cost and
build time.
| Quote: | | The e-bus has a peak that exceeds the SD-8. This only happens if everything is on at once, including the boost pump running, the radio transmitting, both servos moving and trimming, and a dead ibbs battery charging. This will likely never happen, definitely not for any amount of time longer than a second or so. The normal draw for the bus should be about 6A with it maybe going up to 10-15A on landing. I'm looking at the SD-8 as more of a battery extender than a full redundant alternator. |
You've done a load analysis . . . great!
The loads you describe do not suggest the need
for a relay . . . a toggle switch will handle
them nicely. If a relay is necessary, something
like this is entirely appropriate to the task
http://tinyurl.com/hdmkvvt
Bob . . .
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gfb
Joined: 07 Jun 2016
Posts: 4
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| Posted: Tue Jun 14, 2016 11:14 am Post subject: Re: Review request for RV-9 Electrical System (G3X, GTN, e |
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OK, I think I see your point on not needing breakers and going with fuses but I'm not 100% there yet. For my personal sense of comfort I'd like to have items on the endurance bus on breakers. Does this seem like a reasonable compromise? It allows me to load-shed even further by pulling breakers if needed and uses much fewer breakers and panel space. Thoughts?
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gfb
Joined: 07 Jun 2016
Posts: 4
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| Posted: Tue Jun 14, 2016 11:24 am Post subject: Re: Review request for RV-9 Electrical System (G3X, GTN, e |
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Does the rest of the circuit seem reasonable?
Could you elaborate on the e-bus switch vs relay? Up to what amperage is it ok for it to be a regular switch? I was planning on using the S700 Carling switches sold by b&c and they claim 15A VAC but a quick search for them seems to bring up more failure posts than details on the switches. Is there a better option?
For the fuse blocks I'm looking at the ones sold by b&c and they come in different sizes. Does it make sense to have 2 or 3 to split up the load a bit? Where should they feed from? i.e. should I have one large wire for the firewall passthrough and then split that up somewhere between the fuse blocks?
Thanks!
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user9253
Joined: 28 Mar 2008
Posts: 1944
Location: Riley TWP Michigan
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| Posted: Tue Jun 14, 2016 2:10 pm Post subject: Re: Review request for RV-9 Electrical System (G3X, GTN, e |
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One advantage of relays is remote control. The relay can be located close to the battery. In the event of smoke in the cockpit or an imminent forced landing, power can be shut off at the source without having hot wires entering the cockpit. Switch the negative side of the relay like the battery contactor.
I can not remember the brand, but their switches used rivets to conduct electrons from inside of the housing to the outside. The problem was that the plastic case was part of the sandwich that the rivet held together. The plastic flowed over time which allowed the rivets to loosen. That led to resistance and heat and failure.
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Joe Gores |
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nuckolls.bob(at)aeroelect
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| Posted: Wed Jun 15, 2016 12:12 pm Post subject: Review request for RV-9 Electrical System (G3X, GTN, etc) |
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At 02:14 PM 6/14/2016, you wrote:
| Quote: | --> AeroElectric-List message posted by: "gfb" <fly(at)bappos.com>
OK, I think I see your point on not needing breakers and going with fuses but I'm not 100% there yet. For my personal sense of comfort I'd like to have items on the endurance bus on breakers. Does this seem like a reasonable compromise? |
Compromise for what? If a breaker ever opens it means
something is broke . . . and that something threatens
to set a wire on fire. You want to give it a second
chance?
What items in your airplane are high priority equipment
for comfortable termination of flight? From that list
of items, how many of them can fail in ways that do not
open a breaker? I can tell you that the vast majority
of equipment failures never open a breaker . . . if that
item is so necessary/useful that you're worried about
being able to reclose a breaker, then you'd better have
a plan-b . . . a back up for when the system decides
to take a vacation.
| Quote: | | It allows me to load-shed even further by pulling breakers if needed and uses much fewer breakers and panel space. Thoughts? |
Load shed? What's the e-bus for? The LAST thing
you should be doing in flight is running any kind
of mental gymnastics calculated to reduce risks
of dealing with some kind of failure. ALL such
things are done at THIS phase of your design
and fabrication. Should a necessary item go T.U.
then you go to plan-b. If the item is not necessary,
then there is no plan-B.
The idea behind the e-bus is to do a two-switch
load shed that either (1) does not overtax an
SD-8 or (2) produces a KNOWN endurance value
running battery only based on periodic capacity
checks of your battery.
Messing with any breaker or fuse in flight is a
demonstration of poor planning that should have
been managed during THIS phase of your project's
development. Put the fuse blocks out of sight and
out of mind. Deal with system difficulties as a
pilot with a plan . . . not an in-flight diagnostician
and maintenance technician.
Bob . . .
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nuckolls.bob(at)aeroelect
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| Posted: Wed Jun 15, 2016 2:38 pm Post subject: Review request for RV-9 Electrical System (G3X, GTN, etc) |
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| Quote: | | Could you elaborate on the e-bus switch vs relay? Up to what amperage is it ok for it to be a regular switch? |
Sure. Any generic toggle switch of any rating
is unlikely to be overstressed in your airplane.
| Quote: | | I was planning on using the S700 Carling switches sold by b&c and they claim 15A VAC but a quick search for them seems to bring up more failure posts than details on the switches. Is there a better option? |
The Carling switches sold by B&C are direct descendants of
those used by the hundreds of thousands on Cessna and Piper
aircraft for decades. What details are you lacking? Keep
in mind that nobody posts about their working switches . . .
only the ones that have presented some problem . . .
hence, a dozen posts of problems over a period of years
may have the appearance of describing a quality issue.
Switches, like every other device or bit of material
in your project, are subject to failure . . . nothing
lasts forever. Switches on personally owned airplanes
have nearly zero service stress compared to, say the
light switch in your bathroom.
When doing the failure modes analysis for your
project the questions to be asked are:
How can this part fail?
How will I know that it has failed?
Is the failure pre-flight detectable?
The answer to the third question drives your architecture and checklist decisions.
Does failure present some degree of elevated risk for uncomfortable termination of flight?
The answer to the fourth question drives your decisions for having a plan-b assuming that the part will fail.
The beauty and comfort to be secured by an artfully crafted FMEA is that you don't care if the part fails . . . it's not an issue of increased risk. It's only effect is a cost-of-ownership. You might upgrade a part because you're tired of replacing it . . . not because it #(at)%%(at)% near got you killed.
THAT is failure tolerant design . . . stone simple, cheap, easy to do.
| Quote: | | For the fuse blocks I'm looking at the ones sold by b&c and they come in different sizes. Does it make sense to have 2 or 3 to split up the load a bit? Where should they feed from? i.e. should I have one large wire for the firewall passthrough and then split that up somewhere between the fuse blocks? |
Generally, each block represents one bus. In figure
Z-13/8 you need three. Battery, Main and E-bus blocks.
Other architectures will call for more or fewer
blocks. Install blocks larger than needed today
to allow for easy, future expansion.
Bob . . .
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wgreenley
Joined: 09 Jan 2010
Posts: 100
Location: Dowagiac, MI
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| Posted: Thu Jun 16, 2016 5:38 am Post subject: Review request for RV-9 Electrical System (G3X, GTN, etc) |
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One place on my Cessna I have found a breaker VERY useful is the 12v power port (aka Cigarette lighter). Sometimes items with too big a draw get plugged in and blow the breaker, remove item, reset, and can still use other items in flight.
From: owner-aeroelectric-list-server(at)matronics.com [mailto:owner-aeroelectric-list-server(at)matronics.com] On Behalf Of Robert L. Nuckolls, III
Sent: Wednesday, June 15, 2016 4:11 PM
To: aeroelectric-list(at)matronics.com
Subject: Re: Re: Review request for RV-9 Electrical System (G3X, GTN, etc)
At 02:14 PM 6/14/2016, you wrote:
--> AeroElectric-List message posted by: "gfb" <fly(at)bappos.com (fly(at)bappos.com)>
OK, I think I see your point on not needing breakers and going with fuses but I'm not 100% there yet. For my personal sense of comfort I'd like to have items on the endurance bus on breakers. Does this seem like a reasonable compromise?
Compromise for what? If a breaker ever opens it means
something is broke . . . and that something threatens
to set a wire on fire. You want to give it a second
chance?
What items in your airplane are high priority equipment
for comfortable termination of flight? From that list
of items, how many of them can fail in ways that do not
open a breaker? I can tell you that the vast majority
of equipment failures never open a breaker . . . if that
item is so necessary/useful that you're worried about
being able to reclose a breaker, then you'd better have
a plan-b . . . a back up for when the system decides
to take a vacation.
It allows me to load-shed even further by pulling breakers if needed and uses much fewer breakers and panel space. Thoughts?
Load shed? What's the e-bus for? The LAST thing
you should be doing in flight is running any kind
of mental gymnastics calculated to reduce risks
of dealing with some kind of failure. ALL such
things are done at THIS phase of your design
and fabrication. Should a necessary item go T.U.
then you go to plan-b. If the item is not necessary,
then there is no plan-B.
The idea behind the e-bus is to do a two-switch
load shed that either (1) does not overtax an
SD-8 or (2) produces a KNOWN endurance value
running battery only based on periodic capacity
checks of your battery.
Messing with any breaker or fuse in flight is a
demonstration of poor planning that should have
been managed during THIS phase of your project's
development. Put the fuse blocks out of sight and
out of mind. Deal with system difficulties as a
pilot with a plan . . . not an in-flight diagnostician
and maintenance technician.
Bob . . .
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