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Electrical System Design and Build

No, it is on to on the buy before I start list.
I have reviewed a few excerpts from others who have it.

Tim

Once you get the book and read the subject matter, you'll find that the author has little use for the various electronic bus managers, for a number of (pretty good) reasons.

He also champions the idea that the electrical system should be 'failure tolerant' (my words), in that any single avionics-related failure shouldn't be a reason for early termination of a flight, or even be a cause for alarm. Aircraft equipment and mission define the features (backups) of the avionics and electrical system. A byproduct of this philosophy is that you don't need access to the circuit protection devices in flight, which means that (gasp!) automotive style (ATC) blade fuses work just fine, and are lighter, and radically cheaper than a/c style breakers (a complete set of power supply buses can cost less than a couple of a/c CB's). FWIW, Van's seems to have gotten the memo, with the -12.

My take, and hopefully, worth at least what you paid to read it...

Charlie
 
leok,

I'm planning on using an Arduino for climate control and lighting as well. Do you intend to share your code for the Arduino climate control system? I'd be interested. I was thinking about implementing multi zone temperature control, but am not sure where the temp sensors would best be located (this is way in my future anyway).

I also wanted to do the seat heaters with the Arduino. Basically all comfort-related functions, I planned to do with Arduino, and a Bluetooth dongle to allow a custom phone app to control heat and lighting.
 
You should download and read the VP-X installation manual. Anyone who says it is a single point of failure does not understand it or how it is wired. The manual clearly lays out ways to wire backup circuits for critical devices, and also how to use backup batteries for additional redundancy. Additionally, the VP-X Pro is designed with two independent systems as part of the core architecture. I could go on...

There are always people who want to go old-school and that is fine. But in general, for those with an open mind, I believe new technology has great benefits over old stuff. :)
 
I agree with Carl and Walt and Charlie. Use circuit breakers or better yet, fuses. It is unlikely they will become obsolete. If they break, the aircraft owner can fix the problem herself without grounding the aircraft. Van's Aircraft put fuses in the RV-12 for a reason. Over 500 are flying.
No doubt the VP-X performs as advertised. But for me, simpler is better and less expensive. I know how to replace a fuse, but do not know how to repair electronics.
 
Once you get the book and read the subject matter, you'll find that the author has little use for the various electronic bus managers, for a number of (pretty good) reasons.....

I have had that impression, based on other comments. My concern about the VPx is more about product longevity.

You should download ....
There are always people who want to go old-school and that is fine. But in general, for those with an open mind, I believe new technology has great benefits over old stuff. :)

I like a lot of the VPx fail over, dual bus architecture. I have downloaded and read the manuals, including a lot on the new PPS. However, it has a lot of complexity, for the fault tolerance.

I now waffle mentally if this is over analyzing it. A simple bus bar, with two alternators connected to it and a battery. Set the voltage for the primary alternator a half volt higher then the backup alternator. At startup, turn on the backup alternator first to test it, then the primary alternator. I effectively have a three tier system. Lose the primary alternator, I have to manual drop some load (such as pitot heat), but the system is super simple. Would use stuff that I can almost guarantee to find a replacement for in thirty years.

Like I said, I am still at the waffle stage. I really like ECBs, I like the technology... But sometimes, pure simple just wins.

Tim
 
I really do appreciate the thoughtful critique of my design. I spent many years as an engineer and engineering manager. I always ran my thoughts by others and encouraged others to do the same. There is no way one person can approach a design from as many angles as a group can. So in that spirit let me give a shot at your comments

Nice work with lots of planning. There may be a few gotchas though.

1. BATTERY BUS (SWITCHED). If you are forced to shut of the master in flight you have no power to the bus and lose fuel pump and electronic ignition #1.

Yes, absolutely true. My thinking was that the loss of the master (and/or both alternators) it was time to get on the ground. The fuel pump is backup for the engine driven pump during take off and climb. Not required to get safely on the ground when power is limited to battery. The same for EI #1. EI #2 will get me on the ground. Would I really want to run 2 electronic ignitions when power is limited to what is in the battery? I think this is sound reasoning, but am open to others thoughts.


2. If switching the E-BUS switch from VPX to battery there may be a momentary open that would reset some of the avionics. If done in IFR or an emergency this would not be good. At least test for problems.

The only time I would switch the E-buss on is if both alternators went down or the master relay craped out. Power would already be dead. (I would still have the G5 as it has it's own battery backup.) At that point it is intended to be a fast easy way to unload non-critical items with a single switch.
Garmin allows two power feeds into each instrument separated by internal diodes. Power feed to the instruments is power in #1 through the master/VPX, and power in #2 through the E-buss. You can see that in the pin outs for the critical instrument diagrams. There is no cross talk between the power feeds.
Powering the E-buss all of the time is possible. However, since I have Avionics #1 and the E-buss on the same switch (DP/ST) leaving the E-buss on all of the time is not possible as currently wired. I will think on that one for a while to see if there is a better alternative.


3. Might take the E-BUS backup power feed off the fuse and use the fuse box input stud with a fuse link. Would not use one fuse for so much important stuff. Have seen fuses melt and fuse connections get hot and distorted.

This comment/question has me really thinking. I have the blade fuse in the rear behind the rear bulkhead. If I used a resettable breaker on the IP I could at least bring back some power if the breaker tripped. It would really be a bad day if I lost both alternators and then popped the E-buss. All I would have left at that point is the G5 and no EI ...... that needs some rethinking!!

I had been considering that I needed to move at least one of the ignitions to a direct battery feed based on reading the Light Speed Ignition installation manual. The ignitions are one thing I do not have wired yet since I haven't purchased the engine. So easy to change.

Thank you for your comment!


4. 7 SWITCHES. Three are down on and four are up on. The aircraft standard is all down for off and up for on. I see what you may be doing and it is you who is flying your design, but non standard can cause confusion issues when emergency decisions are made. Would recommend consistency.

Here is the thought process. (Page 19 of the power point is the switch group in question) In the normal run/cruse condition all switches are up. The bottom row is all on, the top row is all off. They are rocker switches that show red when down, so in normal run no red shows. If any switch is pushed to the down position, it is an abnormal situation and red shows on the top of the switch. i.e. boost fuel pump on is down (showing red) reminding me to turn it off as appropriate.
Again, my logic tells me this is simple and appropriate. As always I am open to other thoughts.

5. I would remove the G5 from any external bus inputs to keep true independence. A single point failure could take out the entire set of Garmin flight instruments. I know that the Garmin experts say it can/will not happen, but do you want to bet your life on it flying IFR. True independent redundancy would be preferred.

The G5 connection to the system is through the CAN buss. Physical damage to the CAN buss wiring/daisy chain can take the buss down. However communication is based on packets of info being transmitted with appropriate headers. The loss of the CAN buss or corrupt information on the buss have no effect on other buss devices. This is the same buss technology that has been used in automobiles for about 50 years.
MY larger concern is that by taking the G5 off the buss you loose all of the backup functions available from the G5 which are GPS, AHRS, and auto pilot functionality.

A single large diagram of all the power circuits may find other small potential problem areas. A lot is riding on the VPX. Hope it is reliable and glitch free.

I don't disagree here on the VPX. I would really be bummed if ever had to rip it out and reconfigure to a conventional power system.
I do like what I have, and think I have a reasonably well thought out implementation plan. I like the flexibility and diagnostic capability and think it a good trade off. I also like solid state components.
I appreciate the comment on one large diagram. The (virtual) paper I have been working with, as well as my evolving thought processes led to several smaller diagrams. What is important is that I can find and identify every wire from the diagrams. In the spread sheet, I also listed what is on each buss so I can see the overview there.

One last comment: I am close to a finished plan for my specific aircraft, but am always open to thoughtful feedback. Usual disclaimers, I do not suggest this is appropriate in whole or part for anyone else's project. I will be testing and looking for issues before and well into flying. I will try to follow up in this thread over time.

George
 
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Here is a question I was asked this week, and I do not have a good answer. :D
I would like to use VPx, but in the last ten years they have had multiple model changes.
My next plane will be my last plane, and I hope to fly for thirty more years.
So, if you plan to fly for a long time, do you treat the VPx like your avionics and plan to replace it? Or do you abandon such advanced technology and go with simple single pole circuit breakers which have been the same for forty years and will likely still be the same in another forty?

Tim

My response to your comments is that early adopters are often stranded with obsolete products. Fast model changes are a fact of innovation with fast moving development. The question in my mind is are we far enough into this technology that changes will come less frequently and will VPX stick with the current product for a long time. Or at least consider backward compatibility with the next offering.

I don't see ECBs becoming obsolete. Only time will tell on the current VPX offerings.

I also see no reason to attempt to change anybody's mind. As stated elsewhere, if you see no value in the added features of the VPX don't get one. If you do find the features compelling, and plan to use them, the money difference is not that great after you piece together a conventional system.

I do (with respect) challenge the refrain "single point failure" as used in the context of the VPX. If I understand the architecture of the VPX Pro properly, there are two fully separate internal electronic busses. Other than one power wire attaching to the box, there is no "single" failure that would take the VPX completely out.
Good planning will split redundant critical functions between these. I also don't feel that I have spent any significant money, only thought, logic and a few wires to add layers of redundancy while incorporating the VPX Pro. I'm not trying to be a pain here, just accurate. Please correct me if you think I am wrong.
 
leok,

I'm planning on using an Arduino for climate control and lighting as well. Do you intend to share your code for the Arduino climate control system? I'd be interested. I was thinking about implementing multi zone temperature control, but am not sure where the temp sensors would best be located (this is way in my future anyway).

I also wanted to do the seat heaters with the Arduino. Basically all comfort-related functions, I planned to do with Arduino, and a Bluetooth dongle to allow a custom phone app to control heat and lighting.


Sure, no problem, here is the link to the program file:

https://drive.google.com/file/d/0BxZ4BsCO5FFgeVlHSTlYanN4MXM/view?usp=sharing

Just a few comments; The YouTube series by Paul McWhorter is excellent instruction on how to program the little beast. https://www.youtube.com/channel/UCfYfK0tzHZTpNFrc_NDKfTA

He is a school teacher in Texas, and does a great job in explaining the programming. I found tuning the end points of the movement to be an iterative process. You don't want the servos to be in a stalled condition at the end of movement. The numbers in my program will not likely work in anyone else's. I used 1K pots for my control. If you use other values the numbers would also have to change. Without a shield, a single Arduino can control up to 6 servos. I have seen shields that will handle up to 12. I also added a connection shield to mine as it has better wire attach points and a breadboard where I could add the power circuit to drive the servos in a nice neat package.

The addition of climate control should be an easy thing with a little playing around. All you would have to do is tie a thermistor into the servo position loop. Oh, and also find a position in the aircraft to place the thermistor that gave reasonable performance. I do plan on playing with that some time in the future ... maybe.

So good luck. I would like to hear how it goes.
 
Now you got me thinking about a VPX. This could get expensive. Especially if you tell me the VPX sends data to the G3X over the CANbus.

Well sorta, there is a RS232 link (not CAN) to the G3X. Control of the electrical (on/off) can be done on screen or through a physical switch.
 
Thanks very much!

I used an Arduino a few years ago to semi-automate a small (10-gal) brewery. I started off with it doing temperature control for the mash. there was a PID controller library that someone wrote that I used. It worked pretty well in testing. I ended up mostly running the brewery manually because I wanted to make beer more than I wanted to spend time automating the system, but I still have a couple of Arduino boards and plenty of accessories sitting around. I also got a bluetooth serial link running with a basic Android app to monitor the temperatures and set the setpoint, so I have the basics of what I want to do with the airplane complete. Once I have my project ready to go, I intend to share the code.

I don't want to put any safety related or high-current systems on the Arduino itself (even with a relay breakout board), so if I go with a VPX, I will just use the Arduino to send signals to the VPX for things like seat heaters and maybe cowl interior temp control (if I go with cowl flaps). Lighting and climate control will be via a relay board with an option to override critical lights like panel and pilot map illumination. Nav lights and strobes will be strictly through the main power system, probably.


Sure, no problem, here is the link to the program file:

https://drive.google.com/file/d/0BxZ4BsCO5FFgeVlHSTlYanN4MXM/view?usp=sharing

Just a few comments; The YouTube series by Paul McWhorter is excellent instruction on how to program the little beast. https://www.youtube.com/channel/UCfYfK0tzHZTpNFrc_NDKfTA

He is a school teacher in Texas, and does a great job in explaining the programming. I found tuning the end points of the movement to be an iterative process. You don't want the servos to be in a stalled condition at the end of movement. The numbers in my program will not likely work in anyone else's. I used 1K pots for my control. If you use other values the numbers would also have to change. Without a shield, a single Arduino can control up to 6 servos. I have seen shields that will handle up to 12. I also added a connection shield to mine as it has better wire attach points and a breadboard where I could add the power circuit to drive the servos in a nice neat package.

The addition of climate control should be an easy thing with a little playing around. All you would have to do is tie a thermistor into the servo position loop. Oh, and also find a position in the aircraft to place the thermistor that gave reasonable performance. I do plan on playing with that some time in the future ... maybe.

So good luck. I would like to hear how it goes.
 
. SNIP
I do (with respect) challenge the refrain "single point failure" as used in the context of the VPX. If I understand the architecture of the VPX Pro properly, there are two fully separate internal electronic busses. Other than one power wire attaching to the box, there is no "single" failure that would take the VPX completely out.
Good planning will split redundant critical functions between these. I also don't feel that I have spent any significant money, only thought, logic and a few wires to add layers of redundancy while incorporating the VPX Pro. I'm not trying to be a pain here, just accurate. Please correct me if you think I am wrong.

Leok,
Correct - loss of the power lead or the ground lead and you lost the VPX and everything it runs. Just like if your single Master solenoid or Avionics Master switch failed on a legacy electrical distribution design.

As you point out you can do a work around by adding dedicated busses that bypass the VPX (or perhaps use two VPX boxes) and I encourage anyone using a VPX like box to do so. I'll stick with what I consider a simpler, cheaper and more robust approach to achieve "no impact or at most graceful degradation" when any single component fails.

I'm now stepping off my soapbox,
Carl
 
interconnects

Leo,

Sent you an email concerning some of the GMA 245 interconnects. Please let me know if you didn't receive it.

Regards,
J. Baker
RV8 Finishing
 
Recieved the email

I did receive it. It is on my work email and with the weekend, well, I didn't see it until today. It will take a few days to research and comment back.
 
A detour

Leo,

I want to jump in here and thank you publicly for the use of your overhead console mold, which you generously made available to me and even covered significant shipping expense to get this to a complete stranger with the only stipulation that I pay it forward to the next user.

I have made my overhead (it was as you warned me a lot of work to finish the tool surface since I didn't vacuum bag it.) It weighs about 3lb 8oz. I was very pleased. The mold is at its new home in Jack Phillips' hangar at W91, where he is himself almost ready to pass it along to the next fellow in line. Thanks from both of us for sharing your labor! Also appreciate you making your electrical design deliberations available to the VAF clan in this thread. Good community:)
 
Bill,

Glad to hear things went well with the mold. It is even better to see that it has another user. It beats having it kicking around the shop gathering dust until I threw it out to make room.

By the way if anybody wants the mold to the center stack close out or center console (look in my build log to see what the parts looks like) , I'll make the same deal ... just pass it on is someone else wants to use it. :cool:

Makes me wish I spent more time getting a perfect surface.
 
Leo,

Sent you an email concerning some of the GMA 245 interconnects. Please let me know if you didn't receive it.

Regards,
J. Baker
RV8 Finishing

Based on your comments I checked the GMA245 drawings and found you are correct.
I have (had, fixed now in my drawings and I will repost a corrected version) pins 5 and 20 at the GTN650 P1003 connector switched (Lo MIC should splice into Lo COM) and the same with the GMA245 connection to pins 36 and 17 on the GTR20 P2001 connector (again Lo MIC should splice into Lo COM)

I think the MAPMX reference on the GTN650 to G5 RS232 connection was just a copy error on my part as I copied/pasted blocks to construct the drawings. I missed deleting the MAPMX label. I find no reference in the G5 installation manual that indicates how the RS232 should be configured as. I will check with GARMIN to see what they recommend and let you know after I talk to them.
 
Updated file

I updated the wire diagrams based on new information. I added a fan output and configuration module to the GTN-650. The new link is:

https://drive.google.com/file/d/0BxZ4BsCO5FFgcFN5TVZjcWdNZVE/view?usp=sharing

Garmin can be a little frustrating in relation to connectors for their avionics. Some come with the component, some are purchased separately as install kits, some are not available at all to the experimental installations. If they are reading this, it would be a good thing to simplify for us builders. Specifically the GTN-650 has no connector kit available, at least through Aircraft Spruce. They sell it only with a pre-fabricated harness. And, if you don't want a pre-fab harness ... well .... Stein to the rescue!

Stein set me up with the install kit from a GTN-650. It includes 4 connectors, a rack mount, a config module and the back plate/fan. Other than a handling fee, I will get a credit from Stein when I purchase the GTN-650.

The GTN-650 is now fully installed.

Thank you to Stein.
 
You are welcome

You are very welcome Alex.

Just be aware I have yet to test all of the connections. One correction I have yet to make is the power leads to the GTN650. There are 6 pins directed to power in. I mistakenly thought they were primary and back up power (in sets of two for the Nav/Comm/GPS like other Garmin instruments) and they are that way in the diagram.
In reality they are all primary power in, so need to be spliced together. Three sets of two. They are that way because of the size/power limitation on the high density pins. I will end up feeding back up power through a diode for redundancy.
 
Primary Power Diagram

Why am I a year behind in these conversations ????

Thanks for this detailed presentation of your electrical connections and the discussion. I had one question about the primary power diagram. At least in the latest I've found, you are powering your Battery Buss (Switched) through your master switch. This leaves a variable load, including the Fuel Pump, going through the switch contacts. Why didn't you connect to the switched side of the Battery Relay? I would expect the relay contacts to be considerably more robust and reliable than the switch contacts.

Thanks much
Ken
RV-9A
N729KT (Reserved)
 
Hey Ken,

Here is my thought process to answer your question on why I show the "switched battery buss" connected directly to the battery and fed through a DPDT "Master Switch" instead of through the master relay.

The key is that I have dual electronic ignition rather than magnetos. For that reason I needed a direct from the battery feed for one of the ignitions. No relays that could fail in flight. I also need the ignition to be off when the master is off, so no independent separate power switch to the ignition. Since the master will typically only be switched on/off on the ground, I will always know if there is a switch problem before I fly. It is highly unlikely for a switch to fail while in the on/off position without being cycled. When the master is off, I always know the ignition is dead/off.

The other loads tied to the "switched battery buss" are all loads I might want live with only the master on (i.e. prior to engine start) and also do not want going through the VPX (another possible failure point). I suppose you could make the argument that I could add another battery buss fed by the master relay moving all other loads there so the power did not flow through the switch. That would relieve some load from the switch contacts.

Anyway, as always I am willing to be schooled by others.
 
Hi Leok,

Forgive me if you've already dealt with this, but one thing caught my eye in that post: 'I also need the ignition to be off when the master is off'. Does the system include a 'smoke in the cockpit' procedure? Traditional action for smoke is typically 'master off' first (engine continues to run), then if smoke clears and comfort zone allows, try to find the culprit. Though troubleshooting is best left for ground ops.

If a typical pilot suspects fire in the cockpit and instinctively flips the master off, does your engine stop?

Charlie
 
Blind spots

Charlie,

I have done a lot of technical writing, product and production system design in my career and have found there will always be someone with a different perspective or greater knowledge in some area that will point out something you didn't think of. That is one of the reasons I put this thread out there ... well

You are correct. In the design as it stands (and is already mostly in place, but can be adjusted) there is no way to turn off the master and keep the big noisy fan going. And I agree master off, is one way of quickly killing all electrical quickly. I don't fly big high end stuff so I'm not sure if that would apply to more complex aircraft.
So now I need to ask myself is what I have acceptable? Much better to work through possible responses in my shop than in the air. This is one I dismissed (and perhaps should not have) since every circuit is protected by a fuse or electronic CB. What would be the response with smoke of unknown origin in the cockpit? Rather than trying to reason it out here, I appreciate the comment and will look hard at what I have to see if I should make some changes.
 
I just went through this and ended up with three busses and three switches. A master bus, an avionics bus and an ignition bus. In case of a fire or problem, the master goes off, leaving me avionics and ignition. If not solved, the avionics bus goes off, leaving only the ignition with power. In my case, each bus can be separately switched from Left, Right or both batteries: I am using relays/contactors.

I would not want to be without a way to kill everything except the ignition, being ignition dependant. &$*^ happens and you need a way to isolate things if you want to keep the fan spinning with dual EI.

Larry
 
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I'm also going through the same planning process. I think I've settled on a dual bus, dual alternator, single battery system. Primary and endurance bus, with the e-bus only hosting all bits to keep the fan spinning (and pitot heat) and an alternate feed path to the battery. This does mean that an emergency requires you to close 'E-BUS ALT FEED' before turning off the master.

I'm considering the effects of leaving the relay-controlled alt feed closed in normal operations, allowing you to shut off the master with no effect to the engine. So far I can't think of any downsides to this but I'd be happy to hear what others think. ETA: http://forums.matronics.com/viewtopic.php?p=195860 Looks like this was already discussed and determined to be fine :D
 
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A few updates to this thread

Two updates to my earlier posts.

Keeping the engine running with electronic ignition when the master has been turned off is resolved. I went with the CDI-2 system with the back up battery. With total loss of the buss power the CDI-2 switches to it?s own back up battery. I can restore ship power by turning on the E-buss to extend the CDI-2 battery, or leave it off to provide isolation.

The second item is a complete validation of the wiring diagram I posted for the G3X system. All functions as intended. I did confirm with Garmin one add that might be desired. ADS-B weather and traffic from the GTX-45R to the main display(s) is by serial port with a dedicated wire. I have that wire in place for the PFD, but not the MFD. The PFD will not share the data with the MFD. In other words, connect a serial port from the GTX-45R to each display you want ADS-B In displayed.

Last update is still coming. I had a very steep learning curve configuring each device to properly communicate. I will update the wiring diagram with config information for each device as soon as I get a chance. Hopefully that will help anyone doing their own interconnects or upgrades.
 
Now flying

Another update.

Now flying with 11 hours on the aircraft. Everything is working, talking and playing nice. The only issues I have had that are wiring related have been with engine sensors. I got lazy when I got that far and did not do pin out drawings since the hook up was per the Garmin GEA24 pin out drawings ?. what could be easier?

I hooked up the oil pressure and fuel pressure sending units to 12V instead of 5V destroying $200 worth of sending units. I also hooked up all of the CHT and EGT thermocouples reversed, because hey, red wires are always positive, right? No cost there, but some head scratching and research to find the reference for type K thermocouples polarity. My recommendation.. do the drawing!

The take away from this post is that the interconnect file I posted is correct for my set up, and hopefully, might provide a starting point for someone else. I still need to go back and add the configuration notes to the file and repost. Some day when I have more time.
 
Another update. <snip>
Some day when I have more time.

Congratulations on flight!! I found that time has not gotten more available after first flight.

Each homebuilt is a prototype, everything is not perfect! Keep us informed of your progress in the verification/validation phase.

You know what they say . . "there is a time in every project when you fire the engineers and go to production" or some variant of that. :D
 
Congratulations on flight!! I found that time has not gotten more available after first flight.

Each homebuilt is a prototype, everything is not perfect! Keep us informed of your progress in the verification/validation phase.

You know what they say . . "there is a time in every project when you fire the engineers and go to production" or some variant of that. :D

So what do you do Bill when the producer is also the engineer? :D
 
I am <SNIP> this older thread since I still get requests for my files. They are still available to anyone who asks. Click the links in the thread and Google emails me to give access.


Two years of flying, with nearly 300 hours, lots of long cross countries. All I would have done differently on the avionics is to run a serial port, rs232 connection tfrom the Xponder to the PFD to get the ADSB weather feed on that screen as well as the MFD.
 
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Thank you

Leok, that is a lot of work and posting it all for us to digest is a thankless and time consuming task.

For now I'm learning about the VPX system and will be putting one in my RV-7 as I take it all apart and put it back together with a new panel, engine and prop.

Thank you. I found your build log and like your set up. I'll likely be going a very similary direction with my RV-10 build that will start in the near future. :cool:
 
Redundency, or load sharing?

I am WAY behind you in this, but the thinking is beginning. As to the two alternators; one 60a and the other 30a: what is the total expected electrical load in your aircraft? I'm asking because I'm considering a back-up alternator too. Do you plan for the primary 60a alternator to be able to carry the entire load? Is the back-up really a back-up, or is it carrying a part of the system all the time?
I'm thinking about the early light twins that had two engines, but neither of them was really capable of powering the ship alone.
 
Typical load and back up alternator

If I add every load maxed out in the ship I would need 85 amps. That would include transmitting on every radio at the same time, seat heaters on, and heated pitot on, as well as every light and running the flap motor, which will never happen.

My typical load in flight is less than 15 amps. That is with all of the exterior and interior lights on as well as the avionics. If I add the 10 amps in for the heated pitot that is still less than 30 amps.

I have actually used the back up alternator one time when the field plug in the primary came loose. No issues. I flew home like nothing happened where I could trouble shoot in comfort. The only difference I see is a slightly lower buss voltage than normal as the regulator is set slightly lower.

Leo
 
Ok, thanks.

There are interesting design philosophies in operation here. In a related, but unrelated vein, all current Boeing jets have a single rudder surface with multiple actuators, except the 777, which has a split rudder with separate actuators. So, which contingency makes you lose more sleep: one rudder actuator fails and locks the entire rudder so the other actuator can't move it, or one actuator fails and now you only have half a rudder? You can thought experiment this all night long, though I'm sure Boeing has data.

As to modeling an electrical system, is it worth carrying a back-up alternator when the primary can carry the entire load? Or, if your max anticipated load is, say 80a, then why not carry two 40a alternators and split your devices so either half can get the job done?

I'm not sure there is any one ultimately right answer. Thanks for all your schematics. May your back-up be forever superfluous.
 
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