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  #11  
Old 03-24-2018, 07:28 PM
Nihon_Ni's Avatar
Nihon_Ni Nihon_Ni is offline
 
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Quote:
Originally Posted by DanH View Post
Work through basic failure analysis. Wires and devices fail short or open. Make a wire list, mentally short or open each of them, and think about the result.

For example, let's short wire #6. The 60A current limiter opens. With no power on wire 22, the e-bus alt feed relay switches to NC, which feeds electrons to the short via 10. The e-bus alt feed relay might buzz, but if it has much ON time (or simply welds its contacts), the 30A e-bus arm breaker will pop. The automatic system automatically unpowered everything.

Is there a recovery? SOP would have the pilot first switching the alternate feed, then the master. Given an open 30A breaker, doing so would net nothing. Add a 30A breaker reset, and it just pops again. Ok, so now throw the avionics master switch, then reset the breaker. Maybe the avionics relay opens, or maybe it doesn't. With a dead short on 6, relay coil voltage is probably too low to operate, and even if it works, it's just another buzzer circuit, like the one Gerald described for the alt feed relay with its switch at 2-3.

Gotta do the homework.
Dan, as always, your posts are very insightful and helpful. I had thought through appliance failure modes, but I didn't think through all the dead short modes. The one you point out is pretty bad, and I totally missed that one.
I'm going to redesign the E-Bus feed, but even the changes I was thinking about fixing the problem Gerald pointed out wouldn't have addressed this short. I'll review my design wire by wire. Thanks!
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Rob K
真喜志友幸
RV-8 under construction as of Sep 30, 2012
Tail kit completed on Feb 8, 2012 (180 hrs)
Wing kit mostly completed, fuselage under construction as of March 6, 2014 (750 hrs)
Paid up for 2018
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  #12  
Old 03-24-2018, 07:55 PM
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Nihon_Ni Nihon_Ni is offline
 
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Quote:
Originally Posted by Mich48041 View Post
Get rid of that 30 amp breaker feeding the
E-Bus relay. It requires an always hot wire
running into the cockpit. In case of a
forced landing, that hot wire will ignite
fuel. Mount the E-Bus relay within 6 inches
of the main battery contactor. Double
insulate always hot wires where they pass
through the firewall.
Bob's Z-13 and his Avionics Switch designs both have the E-Bus feed on a fuse, so that's why I added circuit protection to mine. I decided to change it from Bob's fuse to a CB switch to keep the appliances on the E-Bus from draining the battery when the engine was off due to the way I've wired the relay as NC. That's a good point about keeping an always hot wire out of the cockpit.

Quote:
How can you energize the E-Bus relay if the
main bus has no power? Connect the coil to
the always hot terminal.
The E-Bus relay is NC. That was to enable auto switching function in the event of an alternator causality.

Quote:
Do NOT connect the E-Bus to the normally
closed terminal of the relay. The only way
to shut off the E-Bus is to energize the
relay which will run the battery down when
not flying.
Which is why I put the E-Bus Arm CB Switch upstream of the relay.

Quote:
Notice that the avionics relay is connected
in parallel with the battery contactor. The
relay contacts will carry starter current (and main bus current)
causing something bad to happen. Replace
that relay with a diode.
I guess I don't understand the benefit of adding a diode there. I understand a diode to be like a check valve in function. How will that protect starter current from reaching the E-Bus?

I just checked Bob's design for an Avionics switch, and he does in fact have a diode between the Main Bus and the Avi switch. I replaced the switch with a relay because my E-Bus max draw is 29 amps, so that means I need a switch rated for 35 amps, correct? I thought a relay would be easier than that size of a switch. (The distance between those two buses is only inches.)

Quote:
It is a bad idea to have two fuses in series.
Chances are both will blow, not just one.
Or the wrong one could blow. Get rid of that
15 amp fuse between the two E-Buses.
Those buses are about 5 feet apart. As Charlie suggested above, I'll look into fusable links there instead of the fuse.

Quote:
Why not use Bob Nuckolls' Z-13/8? Many have
tried to improve upon his architecture but
few have succeeded.

The problem with modifying a proven design is
that you might miss a problem that will only
show up when something else fails.
I did use his design. However, I needed to modify it because it didn't completely suit my needs. I wanted dual P-mags, which his design didn't have. I don't need the LV module that he designed because that capability is in my G3X. I also wanted an avionics switch, so I followed his optional design for that, but used a relay instead of a switch. Unfortunately, his avionics switch design is more generic and doesn't use a relay for the E-Bus, so I combined the two designs as I thought best. I strayed from his design in other areas that I will undo (Alt field circuit). So I couldn't simply follow his design without modification and end up with an airplane that I wanted.

Thanks for taking the time to offer your insights. I knew nothing about electrical when I started this project and I've learned a lot in the process, but clearly I have a lot more to learn.
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Rob K
真喜志友幸
RV-8 under construction as of Sep 30, 2012
Tail kit completed on Feb 8, 2012 (180 hrs)
Wing kit mostly completed, fuselage under construction as of March 6, 2014 (750 hrs)
Paid up for 2018

Last edited by Nihon_Ni : 04-10-2018 at 04:55 AM.
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  #13  
Old 03-24-2018, 09:35 PM
Mich48041 Mich48041 is offline
 
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Location: Riley TWP MI
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The reason for a diode is to prevent current from flowing from the battery
through the E-Bus to the main power bus and then to the starter motor when
cranking the engine. Yes, starter current will flow via the normal path, but part
of the starter current will also flow through the E-Bus circuit. Either a diode is
required, or else an automatic way to open the avionics relay during starting.
It is hard to say how much current will flow through those relays during
starting. But a starter can draw hundreds of amps. Chances are that the relay
contacts will be overloaded.
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  #14  
Old 03-25-2018, 09:24 AM
Mich48041 Mich48041 is offline
 
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Schematic below uses a double pole momentary push button start switch
to disable the avionics relay during engine start. The diode in your drawing
is not wired correctly. Actually the diode is redundant because there is
already one connected to the start contactor. But if you want a second
diode, connect it to ground as shown below.
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Last edited by Mich48041 : 03-26-2018 at 09:51 AM.
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  #15  
Old 03-25-2018, 08:30 PM
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Ed_Wischmeyer Ed_Wischmeyer is offline
 
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* I've got an RV-9A that I fly IFR and an RV-8 that I don't. The -8 has a very responsive Dutch roll mode in turbulence, so I don't think it would be a good IFR plane, regardless of what equipment is in it. Double check that you really want to fly your -8 IFR;
* When time comes to update your databases, you'll want to be able to power the GTN650 and the G3X touchscreens without powering up the whole system and draining the batteries. Trust me, really worth doing;
* Rear seat heater? You'll end up with a really heavy plane if you add in all kinds of stuff like that;
* Consider the GTN750 instead of the GTN650 if you are going to go IFR, as the keyboard is nicer;
* Seriously consider having a separate switch to disable the servos and the electric trim, just in case there's a runaway condition.

Enjoy!

Ed
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RV-8 (steam gauges), RV-9A at KSAV (Savannah, GA; dual screen G3X with autopilot, GTN 650)
Previously RV-4, RV-8A, AirCam, Cessna 175
ATP CFII PhD, so I have no excuses when I screw up
2018 dues paid
Retired - "They used to pay me to be good, now I'm good for nothing."
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  #16  
Old 03-26-2018, 03:05 AM
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Nihon_Ni Nihon_Ni is offline
 
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Location: Fredericksburg, VA
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Quote:
Originally Posted by Mich48041 View Post
The reason for a diode is to prevent current from flowing from the battery
through the E-Bus to the main power bus and then to the starter motor when
cranking the engine. Yes, starter current will flow via the normal path, but part
of the starter current will also flow through the E-Bus circuit. Either a diode is
required, or else an automatic way to open the avionics relay during starting.
It is hard to say how much current will flow through those relays during
starting. But a starter can draw hundreds of amps. Chances are that the relay
contacts will be overloaded.
Thanks Joe. In the schematic I posted, as soon as power is applied to the main bus by closing the battery contactor, the avionics relay would close breaking the path between the main bus and the endurance bus for engine start. After start, I would remove the ground by throwing the Avionics Master switch. That's why I didn't think I needed a diode. That normally closed feature of the avionics relay was part of my design goal, which I patterned after a type certified airplane, but I didn't get the details right so I'm going to change that function.

The schematic you posted of the starter diode isn't showing up. Could you PM me the link?

Thanks again!
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Rob K
真喜志友幸
RV-8 under construction as of Sep 30, 2012
Tail kit completed on Feb 8, 2012 (180 hrs)
Wing kit mostly completed, fuselage under construction as of March 6, 2014 (750 hrs)
Paid up for 2018

Last edited by Nihon_Ni : 03-26-2018 at 03:37 AM.
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  #17  
Old 03-26-2018, 03:34 AM
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Nihon_Ni Nihon_Ni is offline
 
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Quote:
Originally Posted by Ed_Wischmeyer View Post
* I've got an RV-9A that I fly IFR and an RV-8 that I don't. The -8 has a very responsive Dutch roll mode in turbulence, so I don't think it would be a good IFR plane, regardless of what equipment is in it. Double check that you really want to fly your -8 IFR;
* When time comes to update your databases, you'll want to be able to power the GTN650 and the G3X touchscreens without powering up the whole system and draining the batteries. Trust me, really worth doing;
* Rear seat heater? You'll end up with a really heavy plane if you add in all kinds of stuff like that;
* Consider the GTN750 instead of the GTN650 if you are going to go IFR, as the keyboard is nicer;
* Seriously consider having a separate switch to disable the servos and the electric trim, just in case there's a runaway condition.

Enjoy!

Ed
Thanks for the input Ed.

Yes, I am going to fly the -8 IFR. I appreciate your perspective, but I'm way too far down this path to switch horses. All planes are a series of compromises, and the -8 has the right set of compromises for the missions I want, which includes the ability to fly IFR.

That's a good point about being able to power up the system on the ground. I know that during the build I'll want to have the ability to do ground power ops for extended periods of time, and I haven't exactly figured out how to do that yet. For database updates perhaps I could just use the back up battery and let it recharge during the next flight. Does the database update take long enough that I need to be concerned about the 3Ah backup battery going flat during an update cycle? If so, perhaps I could retain the ability for longer ground power runs that I'll figure out for the build phase. I'll have to work on a solution to that issue.

The rear seat heater is 14 ounces. I doubt that will make the airplane so heavy that I won't enjoy it, but it certainly will make the plane warm enough that my GIB will enjoy it. I'm not doing a full interior, but I definitely need some creature comforts in the back seat, such as USB ports and seat heat. Happy wife, happy life!

The keyboard is definitely nicer on the GTN750. I've used both a -650 and -750 in flight on a friend's dual equipped airplane. For me the price and panel space are more important features than the ease of functionality.

The reason for all of my CBs is because I want a way to disable individual appliances. In the G3X, the trim wires through the servo, so if I get a runaway trim situation, I'll disable the trim motor by pulling the auto pilot servo CB.
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Rob K
真喜志友幸
RV-8 under construction as of Sep 30, 2012
Tail kit completed on Feb 8, 2012 (180 hrs)
Wing kit mostly completed, fuselage under construction as of March 6, 2014 (750 hrs)
Paid up for 2018
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  #18  
Old 03-26-2018, 03:36 AM
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Ed_Wischmeyer Ed_Wischmeyer is offline
 
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* Double check what the G3X has for a Hobbs meter equivalent. If you don't have an oil pressure switch, you'll get register lots of time spent fiddling with the avionics;
* Add in wires for a trickle charger or a Genius battery charger. That $99 charger saved a dead battery that was killed by leaving the avionics on too long during a database upgrade. And that should provide enough power for the database updates without drawing on your batteries. I don't recall the times for the updates, but IIRC the longer ones can be the better part of an hour -- 4 GB can take a while;
* Sure, you can do runaway trim with a CB, but it's easier to find a CB switch. Your mileage etc...

Ed
__________________
RV-8 (steam gauges), RV-9A at KSAV (Savannah, GA; dual screen G3X with autopilot, GTN 650)
Previously RV-4, RV-8A, AirCam, Cessna 175
ATP CFII PhD, so I have no excuses when I screw up
2018 dues paid
Retired - "They used to pay me to be good, now I'm good for nothing."

Last edited by Ed_Wischmeyer : 03-26-2018 at 05:49 AM.
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  #19  
Old 03-26-2018, 06:50 AM
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Nihon_Ni Nihon_Ni is offline
 
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Default Version 10

Thanks again to everyone who offered feedback. The discussion brought out weak points in both my design and my knowledge and I'm working to fix both of them. I've created a new version of my schematic to solve the issues brought up so far. Although I haven't had a chance to do all the homework that Dan suggested, I thought I'd update the post with the corrections I've made to date. I seriously doubt this is the final version, but it's definitely better than the last.

I've given up on the automatic switching feature of the E-bus relay I was attempting to design. I've also given up on the NC feature of the avionics master and replaced it with a CB switch. I think that's a simpler solution, and adds a layer of fault protection for the example Dan highlighted, but it doesn't have a closed circuit failure mode. In this design the E-Bus Alt Feed relay could serve as a back up Avionics Master in the event of a Avi Mstr switch failure.

I've changed the Bus 1 to Bus 2 circuit protection to fusible links, but I'm not certain I've done that correct. Looking at my load analysis, E-Bus 2 uses 8.2 typical max and 16.7 max. That wire is 3 ft long, so sizing that wire for 10 Amps continuous indicates 20 AWG. This size becomes the fuselink, and I step up the wire 4 AWG to 16 AWG, correct?

I revived the Battery Bus and moved the right P-Mag to it. That adds about 8 feet of extra wire to run from the Battery Bus to the CB panel so I still have to weigh that decision. That length of power supply wire probably needs a fuse at the hot end even though that doubles protection on that circuit. The instructions dictate 18 AWG power supply wire, so I added a 10A fuse on the Battery Bus. I need to chat with the folks at E-Mag about that set up. Their material specifies a CB, so I need to check with them about the upstream fuse, or if a 3A fuse will be sufficient in lieu of a CB. I also need to discuss the idea of placing both P-mags on the same bus for the reason that Charlie pointed out. I could easily move the right P-mag to E-Bus 2 as it's already located under the CB Console, but I'm not sure that's the best solution.

I'm considering moving all the CBs from the F-865 Console to somewhere on the panel. That would shorten the distance between buses 1 and 2 as well as the distance between the CBs and the Battery Bus (probably on the front side of the fire wall). I'll need to work out panel space issues and see what other ripples that change will have.

Thanks again for everyone's input.

Edit: Updated 3/27 with the most current version.



For those who are wondering, I used PowerPoint to produce this drawing as well as the individual component drawings. I experimented with a couple of the CAD programs others have used, but I had trouble getting those programs to do what I wanted. I'm familiar with pptx and use it nearly daily at work, so I just switched to it for my electrical drawings.
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Rob K
真喜志友幸
RV-8 under construction as of Sep 30, 2012
Tail kit completed on Feb 8, 2012 (180 hrs)
Wing kit mostly completed, fuselage under construction as of March 6, 2014 (750 hrs)
Paid up for 2018

Last edited by Nihon_Ni : 03-27-2018 at 07:57 AM.
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  #20  
Old 03-26-2018, 07:14 AM
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Nihon_Ni Nihon_Ni is offline
 
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Quote:
Originally Posted by Ed_Wischmeyer View Post
* Double check what the G3X has for a Hobbs meter equivalent. If you don't have an oil pressure switch, you'll get register lots of time spent fiddling with the avionics;
* Add in wires for a trickle charger or a Genius battery charger. That $99 charger saved a dead battery that was killed by leaving the avionics on too long during a database upgrade. And that should provide enough power for the database updates without drawing on your batteries. I don't recall the times for the updates, but IIRC the longer ones can be the better part of an hour -- 4 GB can take a while;
* Sure, you can do runaway trim with a CB, but it's easier to find a CB switch. Your mileage etc...

Ed
Ed, thanks for the additional input.

Good point about the G3X Hobbs function. I just checked and it does have a total time record feature that can be set to oil pressure, engine RPM >100, or GPS determined flight. I'll have to think about if I want that to be the only source of aircraft time, or if I want an analog meter too (I already bought a Hobbs meter). At some point, the G3X will be upgraded for something else whereas the analog Hobbs meter would likely be in service for the life of the airplane. I know Hobbs meters fail too, and total time could be recorded in the logbook when it's switched from one meter to another, so I guess it's just a matter of preference. I'm generally in the "Less is More" camp, so my inclination is to sell the Hobbs meter and just use the G3X time record. I'll have to stew on that one for a while.

I know the Earth-X battery has a specific charger requirement, but I don't know what it is. I'll have to figure that out and see if I can use it as you described. The backup battery probably won't last for an hour, and it doesn't power the GTN-650, so after more thought it's not really an option.

Good point about finding a CB vs a switch. More to think about!
__________________
Rob K
真喜志友幸
RV-8 under construction as of Sep 30, 2012
Tail kit completed on Feb 8, 2012 (180 hrs)
Wing kit mostly completed, fuselage under construction as of March 6, 2014 (750 hrs)
Paid up for 2018

Last edited by Nihon_Ni : 03-26-2018 at 07:23 AM.
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