George,
Thanks for the time you took to offer this feedback. It's really helpful to me!
Nice diagram and load charts.
I have been watching your system design progress and most of the problems I had seen have been addressed. I have a few ideas, not that they must be followed.
1. SD-8 alternator. You might call B&C. The regulator needs good cooling to achieve maximum available output. Under the cowling may be too hot for the best results.
Thanks, I'll ask about that.
2. The main alternator current limiter is too small. Most alternators easily put out more than advertised current. Recommend 70 amp.
I'll defer comment on this one -- Joe has a response below and I have no experience with it, so I need more study to figure it out.
3.The Auxiliary Alternator warning light will always be on with the switch turned off. Unless you plan to leave on, the solution would be a double throw switch that would also turn off the light circuit when not being used. A warning light on all the time would be a nuisance.
I'll have to go back to Bob's notes on this light. I may be missing the details off the top of my head, but I think the design goal is for that relay to always be closed, but I could be wrong. I understood this light to function in two capacities -- it would come on if the crowbar removed power from the relay, and it would also be on if the switch was off with power applied, serving as a reminder to turn the switch on for normal operations. It also serves the dual purpose of an idiot light if I leave the master on after flight (which I've NEVER done
).
4. The 20 gauge essential buss jumper is too small for a continuous 15 amp load of more than 2 seconds. I would recommend a 14 or 16 gauge jumper with no protection. Same for the 22 gauge main buss jumper. Protection is unnecessary, especially if a very short run.
The topic of fusible links between my buses has caused me tremendous angst in the past week. I frankly don't know what to do. The distance between E-Bus 1 and E-Bus 2 is about 18", same for Main Bus 1 and MB2. E-Bus 2.
Main Bus 2 will draw 3A continuous and 4.5A max, E-Bus 2 will draw 2.9A continuous, 8.2A typical max, and 16.7 theoretical max (everything on at once, which is unlikely). I used the typical max to size the wires, so that gives me a figure of 24 AWG for M-Bus 2 and for 20 AWG E-Bus 2 for 18" wire runs. Since I don't plan to use wire smaller than 22AWG, I up-sized the MB2 feed to 22 AWG as the fuselink, and stepped up the rest of the wire 4 sizes to 18 AWG. On the EB2 feed I used 20 AWG fuselink and 16 AWG feed wire. If this is all wrong I'm certainly willing to scrap it and go with something else. If I removed the fuselink, wouldn't I just have the wires sized at 22 and 18 respectively? I'm really confused about this part of the design.
5. The 30 amp inline fuse for the E-buss alternate feed relay is just adequate for full load. Would put a 40 amp with the wire size used or would recommend a 40 amp current limiter. (same thing different package)
This is another area I'm having trouble understanding. Bob recommends 30A fuse, the manufacturer specs a 10A fuse. I can't figure out what the electron flow would be across that fuse. Would it get 60A from the main alternator when the battery contactor is closed? It seems to me that unless there was somewhere for those electrons to go (i.e. a short in the SD-8 or relay), they wouldn't flow towards the SD-8. But again, I'm an electro-novice, so I don't know what I don't know.
6. The main alternator and auxiliary field breakers should be easily accessible and identifiable by feel.
Roger that.
7. Recommend all Honeywell TL quality and/or type switches. The Carlin can give problems, especially on inductive loads. Fast-on are nice, but a quality switch is more important.
I'll take a look at those. I really haven't given a ton of thought to switches yet, other than load ratings and function.
8. Have no provision to power up the avionics prior to engine start. Would be nice for IFR. Set up radios, check ATIS, insert flight plan, call for clearance, etc.. A small/medium Lithium could be used.
Couldn't I just turn on the E-Bus Alt Feed switch, which should power the Endurance Bus directly from the battery?
9. I would not use a Lithium for the main battery, especially IFR all electric plane. Time will tell. (I'm sure to start a fire storm for that suggestion)
Thanks -- a few people offered me the same advice and I'm still thinking about it. I was firmly opposed to lithium batteries until Vans came out with it as standard equipment in the RV-12IS; that made me reconsider it as a viable option. I still have some research and thinking to do before I commit to a battery.
10. Would recommend a Honeywell double throw switch for the avionics switch. (extra contact area). Why the diode, unnecessary voltage drop and can control current with the avionics switch. Breaker switches can be problematic. Does the avionics switch circuit between the busses need protection? You could put a 14 gauge fuse link on each end, but I would not bother.
Can you explain how a double throw switch provides extra contact area? Is it a DPDT that has both switches wired together, or something else I'm not considering?
E-Bus 1 & 2 draw a typical max of 21.9A and a theoretical max of 29.6A. Given those figures, I thought I needed a switch rated for 30A but had trouble finding one. The CB switch is rated for 35A, so that's how I landed on it. The distance between MB1 and EB1 is a matter of inches; if it includes a trip to the switch it will be on the order of two feet total. I'm not sure if that requires circuit protection or not. I initially had a relay there to solve the amperage rating issue, but I tried to get fancy with it in an early design which just complicated the issue, so I opted for a CB switch in the name of simplicity. However, I'm not sure I achieved that goal. I've been flying several different Beech products lately, and they have extensive use of CB switches, so that has certainly biased me toward a CB switch, but I don't have any experience maintaining one. Perhaps putting the relay back in (in a less fancy way) is the better solution? That would reduce the power wires to a few inches, and remove the need for circuit protection. I think...
I don't see a Honeywell switch rated for more than 20A, but again I'm not sure I've done my calculation correctly. Do you have a part number for the switch you recommend?
11. Would run the strobes through a breaker rather than a fuse. Have seen multiple strobe fuses melted. This is documented, but not fully explained.
The fuse to CB comparison is another item that I don't have a firm grasp on. I know not all fuses are the same, as in some are slow blow, and others are not. I'm installing Pulsar LED strobe lights, and the install instructions recommend three conductor 20 AWG shielded wire to power supply, and "Budget 5A per strobe light input." Since I have three strobe lights, I've budgeted 15A, but according to table 11-3 in AC 41.13, 16 AWG wire should be used for 15A CB or 10A Fuse. I have a note to pose this question to AeroLEDs, but it's still something I need to learn.
12. You have fuses for the GTN650. There is no way to reboot the radio. This is done with the breakers. Suggest easily accessible, small breaker panel with the necessary breakers.
Why would I need to reboot the GTN650? Is there a function that requires it, or is it a trouble shooting step of some sort?
You have the diagrams, now for the install. Be sure and use high quality connectors. Be careful of chafing areas and protect well. With the large number of connections a Daniels crimper, used red/blue crimper, tefzel strippers would be worth the investment, but not necessary with verification of proper results. Get the proper solder sleeves. Work on perfecting your techniques. Follow techniques in the manuals. The install is only as good as the harness and connection quality.
Looks like you are on the right path. Hope I am helping and not being too critical.
George Meketa
RV8, 16 years
Thanks George -- I really appreciate your feedback!