At first blush my thought is you should look to simplify. My guess is you have crossed that magical like between ?enhanced capabilities? and ?complexity induced risk?. I clearly understand the option to use two VPX boxes over one as you plan on a very electrically dependent engine, but the other option to achieve the same capability is to not use the VPX at all.
It's two PPSs, not two VP-Xs (the PPS is another product from Vertical Power which essentially replaces the battery/starter/alternator contactors with solid state equivalents).
I agree about simplifying if possible (I keep thinking "could a pilot that didn't build this figure out and safely fly this airplane without some advanced training?"), it's the path to that that I'm not seeing clearly
I also recommend you sharpen your pencil on load analysis. For example your lighting load numbers are way high if like most you plan on LED lighting, and you don?t included Pitot Heat.
Pitot heat is there (GAP26, sorry I used part numbers, just added descriptive names there), and I assumed "max" current to be the 12A that Garmin specified for initial heatup at -40˚C, but only 7A for typical as it draws less current once heated up.
The light numbers are the ones that I have some confidence in because I already have the lights - these are the Aveo Ziptips, and they do pull a surprising amount of current - a tad less than I put in (I put in the official numbers from their diagram), but still a lot.
Do you see anything in particular that seems wrong in the analysis?
My experience is a full up dual screen EFIS system with a TSO navigator will draw about 20 amps. Add to that intermittent loads like Pitot Heat, nav, strobes and Landing lights and you are well below the capacity of a standard 60 amp alternator. Your electrically dependent engine may push your numbers a little higher.
Interesting, you'd consider that strobes, landing and nav lights are intermittent? I thought anything that *could* be on for an extended period should be accounted for as continuous, even if it won't strictly be on all the time. The strobes do have a certain duty cycle which I didn't consider.
On the 20A estimate - that looks about right - consider that from my load analysis it's roughly:
11A for the EFII
About 12A continuous for the three-screen G3X plus idle GTN750
5A for the alt. field
About 6A for misc. avionics
That's ~34A typical current. A lot of the rest is lights, pitot heat, etc.
I also offer that you should consider the 20 amp B&C standby alternator instead of the 30 amp unit. It will fit much better and it is really all you need for power to extend battery capacity until fuel exhaustion. The standy alternator is just along for the ride if the primary alternator is still working (as in no current output), so your EarthX charging current limits are not an issue.
Thanks, that's a great hint about 20 vs 30, I haven't done careful consideration of mounting yet (other than seeing that I clearly will need to mod the rear battery mount in some way).
For W&B, unless you are adding AC or such mount both batteries in the normal aft position. I did a simple mod to the standard Van?s battery mount to accommodate two PC-625 batteries, two master solenoids and four 30 amp relays that feed the panel (two primary and two backup). If you use the EarthX batteries you may need to carry lead or other ballast in the back if flying with no back seat passengers or luggage - but your W&B will tell you all this.
Good point. I guess with two ETXs the tail weight will be only a few pounds below the standard (plus I'm mounting transponder and etc. back there as well).
Just because an alternator is capable of putting out 60 amps does not mean that it will. An alternator puts out voltage and current determined by the load. It is up to loads to take whatever current they want. Loads include batteries. Suppose that an aircraft is flying with all electrical loads shut off. Then the pilot turns on an indicator lamp rated at 0.1 amps. A 60 amp alternator will not force 60 amps through that little lamp and burn it out. The current is limited by the lamp resistance. The lamp will take whatever current that it wants: 0.1 amps. Batteries take what they want too.
A load will only take current from the source with the highest voltage. If two alternators are running simultaneously, they will not have equal voltage. The one with the highest voltage will supply all of the loads, the other alternator with lower voltage will supply none. There is an exception to this. If the main alternator is overloaded so much that its voltage sags, then when the voltage drops to that of the second alternator, then both alternators will supply current because their output voltages are then equal.
Listen to Carl.
Thank you for the explanation, much appreciated.
That makes sense, now that I'm thinking about generator theory.
I would like to compliment your method, i.e. the concept of mapping out architecture before proceeding to detailed wiring.
Thanks. I guess doing it that way made it simpler in my mind.
A minor detail; not everyone is conversant in Garminese, that strange language in which components are named in letter and number combinations. If you want review, it would be better to use descriptive labels. Some of us are way too lazy to look up the Garmin names.
Sorry, I realize that now - I've just updated the spreadsheet with descriptive names, hope that helps.
I lost faith in Bus Manager when I saw the fuel pump circuit has single points of failure in the wires and circuit breaker from the essential bus to the pump relay and in the relay itself. Are you showing two separate fuel pump relays? Buss Manager shows a single SPDT relay and if you connect it's common to both of your essential buses they will be tied together. Of course you could use a single DPDT relay but it would still be a single point of failure.
It's supposed to be a single relay, but I haven't settled on the specific way to wire it. My thought there was that, if the Bus Manager completely loses power, the pump on the other bus (on the VP-X) would be turned on. The single relay technically resists the failure of the Bus Manager as well, but possibly not a failure of the relay itself - with separate busses and separate ways to switch them, I'm hoping to eliminate that SPoF. Likewise with the ignition coils, if the Bus Manager essential bus loses power, it'll automatically switch to the other bus. The ECUs are supposed to be "smart" and work ok if the other dies, so no tricks needed there.
(but again, maybe I'm overthinking this
).
Auxiliary alternator capacity: For instance the B and C "20A" alternator puts out 32 amps at 3500 alternator rpm which is 2700 engine rpm on an O-360; I suppose O-540 has the same 1.3 alternator to crank rpm. In any case but especially with the EarthX battery I would use external voltage regulators with crowbar overvoltage protection.
Oh, of course - looking at some alternators now, it seems that the "60A" B&C (BC462) only outputs 41A at cruise, and the BC460 gives 60A only at 9000 alt rpm (no data on the ratio?). Also can't find data on the Plane Power output vs RPM.
Everybody has an opinion and one of mine is to keep it simple and go with something close to a Bob Nuckols circuit with no electronic circuit breakers. Bob's Z-14 is his dual battery dual alternator solution but you would want an essential bus so the injectors would be powered with both battery contactors de-energized.
Personally I'm planning a modification of Z-13/8 with a B and C 20A backup alternator. Z13 allows the backup alternator to charge the battery with the master contactor open. Full disclosure... Bob is opposed to this.
I'm installing dual SDS EFI who's ECU EFII uses until EFII's System 32 comes out. There are some differences in SDS's and EFII's implementation but for my SDS EM-5 system:
I'm probably going with the EFII System32 once it's out (I won't be done building before then anyway) - the data available on it is still partial, but I know it pulls a little more power: 11A total at high RPM, with 4.5 being the fuel pump. I don't have data on how much of the rest is the ECU vs the coils and injectors, so I took a wild guess 1A for the ECU and the rest for the coils.
Yes, I got some inspiration from Z-14, hence my essential busses, one connected to each power source (and half the EFII connected to each).
VP-X electronic circuit breakers can trip on transients that don't affect fuses and circuit breakers. This can shut down the transient loads we're interested in here; coilpacks and injectors. It's advisable to set the VP-X breakers higher than if a fuse or circuit breaker is used. How high, who knows, and that's a problem because it calls for high RPM testing. And a question; if the electronic breaker is set high, does that mean you need a bigger wire?... or you could set the VP-X really high and install an inline fuse or fusible link at the VP-X.
That's good to know.
I'm assuming no need for the thicker wire, since it's a transient - it'll have a bit more voltage drop at that time, but the transient would have to be really high for the voltage to drop enough that the equipment shuts down. As for thermal considerations, if it's a brief transient it shouldn't be enough to heat up the wire, I'd hope?
