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  #21  
Old 01-22-2019, 07:23 PM
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mulde35d mulde35d is offline
 
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Default Garmin discussion

In regards to the GAD 27, I was speaking with a member of the G3X team who was knowledgable on the products. There is always the possibility that I mis-interpreted the conversation, but I am sitting back trying to think of how the GAD 27 could provide 12V power from a single battery if the battery itself is being drawn below 12V. The only method I can think is that the GAD27 would have an internal capacitor. This would stabilize power fluctuations and provide full 12V power for a very limited time should the battery drop below 12V. I guess that would have to be it if their is no backup power source, but I don't know the internals of the GAD 27 and I don't know anyone willing to open up a $500 component to see what's inside.

In response to Charlie's post, I appreciate the input on the start circuit. I originally had a fuse in line but after some conversations with a couple aircraft electrical engineers at work I pulled it since they convinced me it was pointless. I don't remember the conversation real well, but I am pretty sure it centered around the CB protects the wire from overheating due to pulling too much voltage. In the case of the 22 AWG starter wire if it were to accidentally ground out (the only time it would pull any voltage) then the bigger problem would be the active starter and not the 22 AWG wire heat. Hence no CB. Similar with the avionics relay, if the avionics master wire were to ground out it would simply close the relay, a pretty normal state. Hence no CB

I am curious, based on the comment about multiple failure points between the battery and avionics bus, how would one reduce those failure points while maintaining functionality and limiting complexity. Or is that simply the reality of the setup.
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  #22  
Old 01-22-2019, 08:59 PM
n982sx n982sx is offline
 
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Quote:
Originally Posted by mulde35d View Post
but I am sitting back trying to think of how the GAD 27 could provide 12V power from a single battery if the battery itself is being drawn below 12V.
Modern voltage regulators can handle this quite easily within limits. They will all have some dropout voltage below which they can't raise the voltage and there will also be limits on the amperage they can deliver.

In our case, the devices used will just draw more amperage at the lower voltage to deliver the target voltage and amperage. Not accounting for losses, here is a simplistic example. 2 amps at 12 volts is 24 watts. If you use a device that can deliver 2 amps of 12 volts from an input of 8 volts, it will draw 3 amps - or 24 watts - from the battery. Of course there is no free lunch as the conversion will not be perfectly efficient. The device will draw more than 3 amps and produce waste heat in this example.
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  #23  
Old 01-23-2019, 07:27 AM
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Carl Froehlich Carl Froehlich is offline
 
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Quote:
Originally Posted by rv7charlie View Post
SNIP

I'm another who has to disagree with Carl on the dual bat vs dual alt issue. I hear his argument, but dual bats doubles up on the most reliable accessory in the plane, and still results in finite (and variable with age) capacity. Alternators, on the other hand, have limited and very unpredictable lifespans. It's an even bigger issue with the current trend to electrically dependent engines, which often require quite a bit of electrical energy. Everyone should go with their (intelligently researched) comfort zone; mine is dual identical alternators (I have the luxury of mounting two).

Charlie
It seems I’ve failed to communicate. The battery is the most reliable element in your electrical system (assuming you have not abused it). It is the other stuff you add to it that adds all the potential single point failures.

The second battery is not solely for capacity - and is not simply two batteries running in parallel in lieu of a single, larger battery. The second battery and thoughtful design is to get independent power to the panel when some other component, junction or whatever fails.

Stuff breaks. Recognize this and design for “graceful degradation” and power restoration options, not for a dark panel.

I stand by my assessment; thoughtful dual batteries and single alternaor designs provide superior redundancy compared to single battery, dual alternator schemes. In most cases the weight penalty is small to none, especially if the risk mitigation is adding a bunch of avionic or engine backup batteries that spend most of their life just along for the ride. Adding a standby alternator is perfectly acceptable.

So - eyeball the single master relay, the single avionics relay, the single ground wire to the battery, etc. and figure out what happens if any of them fail.

All my comments assume the builder plans on flying IFR, and/or flying with an electrically dependent engine.

Carl
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  #24  
Old 01-23-2019, 12:48 PM
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mulde35d mulde35d is offline
 
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Default Failure modes

Quote:
Originally Posted by Carl Froehlich View Post
So - eyeball the single master relay, the single avionics relay, the single ground wire to the battery, etc. and figure out what happens if any of them fail.

Carl
Excellent, thank you for pointing out the failure points I keep hearing about. Now the fun part. Let’s divide these up into a couple categories and please correct me if I am wrong since these are great talking points.

Master relay failure. If it fails to off prior to engine start then no power is available. If it fails to off after the alternators are online then the alternators continue to power everything and no battery backup exists. If it fails in the on position then you can’t shutoff power. None of these failures seem to be a threat to life even in IFR. Compound failures of the master and alternators would be required for this to likely be a problem.

Single avionics relay failure. If it fails in the off position then you loose the avionics bus (partial panel situation). If it fails in the on position then everything still works until the Master and alternator are turned off. No threat to life in IFR as long as the main bus has the critical IFR instruments still available (G5 and magnetometer or compass as a minimum).

Single ground wire failure. The only way I could think this would fail is if it became disconnected from the battery or firewall. So check on preflight to minimize the risk. But if it does fail then revert to a battery powered G5 to maintain aircraft control if IFR. Possible threat to life, but very unlikely given a descent preflight and proper torque on the bolts.

My OPINION, and I emphasize opinion is that all of these degraded modes are acceptable since the risk is unlikely and some graceful degredation is built in to the setup.

My thought process was to maintain the essential IFR equipment on the main bus with equipment that could be shed quickly on the avionics bus. The backup alternator provides an unlimited supply of 30 amp power should the most likely failure occur (which I believe to be a primary alternator failure). If the primary and alternate failed in flight then I could turn off the avionics bus and communicate the problem to ATC over the remaining comm radio while continuing to control the aircraft on a partial panel. Should the main batter then run out of power then the G5 backup battery would provide basic pitch, roll, yaw, airspeed, and altitude information to control the aircraft for an additional 30 minutes. That seems like a lot of redundancy to me already.

Please don’t take this as criticism on your points since I really like thinking through and talking through failure modes. I think it makes everyone smarter on electrical which seems to be a complicated topic in experimental setups. Fortunately the basic electrical principles apply to everything. If anyone has additional failure point they see in the diagram let’s bring them up and we can discuss their possible effects.
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  #25  
Old 01-23-2019, 12:53 PM
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Quote:
Originally Posted by Carl Froehlich View Post
.....

All my comments assume the builder plans on flying IFR, and/or flying with an electrically dependent engine.

Carl
I think that is one of the biggest risk items.

If the last part of your sentence is not true by having a magneto then a lot of the rest of the system can be made simpler...
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  #26  
Old 01-23-2019, 01:00 PM
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Back to the Avionics buss.
Kinda like talking primer.
You will find that many including myself are not using an avionics buss or switch. I follow the thought that the modern stuff can handle it.
I do have a IBBS 6ah back up battery to keep the PFD from rebooting and as short period back up to the main battery.
Most of my panel will stay alive during a start unless prolonged then only the PFD stays alive.
Saves a few points of failure.
I guess I am a bit of a maverick since I also use fuses instead of breakers.
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  #27  
Old 01-23-2019, 01:00 PM
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Originally Posted by az_gila View Post
I think that is one of the biggest risk items.

If the last part of your sentence is not true by having a magneto then a lot of the rest of the system can be made simpler...
Gil, I am curious about your comment on the magneto. After researching the new P-Mags I was under the impression they run off aircraft power, but also have a built in alternator to power themselves should the aircraft power fail. Hence what the test switch is checking during run up. Wouldn’t that work just as well in a dual Pmag setup without the need for a magneto?
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  #28  
Old 01-23-2019, 01:06 PM
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Quote:
Originally Posted by mulde35d View Post
Gil, I am curious about your comment on the magneto. After researching the new P-Mags I was under the impression they run off aircraft power, but also have a built in alternator to power themselves should the aircraft power fail. Hence what the test switch is checking during run up. Wouldn’t that work just as well in a dual Pmag setup without the need for a magneto?
The Pmags run on aircraft's power up until about 800 rpm then the internal power takes over until rpms fall below 800 presumably at shutdown. Test switches remove aircraft power from the Pmag so you can see if it's self powering is working above 800rpm. In the run up block above 800rpm, his the switch and if the Pmag dies, then something is wrong internally. Otherwise you have no way to know if the internal power in working properly.
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  #29  
Old 01-23-2019, 01:21 PM
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Carl Froehlich Carl Froehlich is offline
 
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Jon,

Good thought process. Please take it a few steps further and also consider:

- High resistance contacts (in my simple example the battery ground). They work until they don’t - or melt on over current, and vibration is a contributor. This is the single failure mode of a somewhat recent twin (two alternators) going dark at night from a common buss bar connection failing. If you must do an avionics master at least consider using two - one for each side of the panel.

- While partial panel is a good last line of defense, I would not consider it acceptable as a planned prolonged mode for IFR. In this case I’d view the G5 as the instrument to keep the wings level as you restore power to the rest of the panel by throwing a switch to power from the other battery. In my plane one EFIS and one Nav/Comm retain power if one side is lost. The other side can be restored by a single pilot action.

- I would never consider running an alternator without a battery as acceptable. If you plan on this as a backup mode, please do a lot of testing.

- Look beyond component (switch, relay, etc.) as the only failure points. Wire junctions will bite you.

- Using procedures to mitigate design shortfalls seems to be the popular thing of late. I offer that any single fault should not prevent continued IFR flight with no pilot action. Panel restoration should be achievable with minor pilot action. Remember the pilot pucker factor needs to be considered.

At this point it seems you already made your decisions so I’ll step off my soapbox. I’m hopeful my comments will provide other readers some areas to examine as they consider their design objectives.

Carl
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  #30  
Old 01-23-2019, 01:35 PM
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Quote:
Originally Posted by mulde35d View Post
Gil, I am curious about your comment on the magneto. After researching the new P-Mags I was under the impression they run off aircraft power, but also have a built in alternator to power themselves should the aircraft power fail. Hence what the test switch is checking during run up. Wouldn’t that work just as well in a dual Pmag setup without the need for a magneto?
Yes, but I would vote for a single magneto based on less (no ) software involved.

Similar theory to not using a small Dynon unit as a back-up to a large unit if they are both running the same software.

IIRC the Pmags had a similar problem with a early design iteration.

I have seen different estimates of the gain of the second EI system and it is not that significant. A single system that gives variable timing over the fixed timing of a magneto is a big deal though
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Last edited by az_gila : 01-23-2019 at 01:38 PM.
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