john_rv10(at)yahoo.com
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 Posted: Tue Mar 11, 2008 7:00 am Post subject: Critique please Bob and other learned members |
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Bob,
First, thank you for your outstanding contribution to the OBAM community, especially for the AeroElectric Connection, this email list, and the sample wire book which we have used to craft our design. We are building an all electric panel, IFR RV-10, and in the development of our thoughts and knowledge about and around our electrical design, your input, and that of the other learned people on this email list, have been invaluable. You have all helped us move from a position of zero knowledge to a position where we now have some idea of what questions to ask.
We struggled around Z-14, Z-13/8 and finally Z-12 to settle on our current design, and we would welcome comments from you and others on any aspect of our design, but especially on the specific question areas noted below.
Our mission is cross country flight with IFR capability, mostly with two POB and the ability to take large amounts of luggage as we tour Australia.
We concluded that the alternator was the unit with the highest probability of failure in service and decided early on that we would go with 2 alternators.
We have calculated that we have added about 55 lb to the basic VANs design empty weight, primarily in the panel, console, overhead console, second alternator and second battery. We have elected to maintain design gross weight which limits baggage in the baggage compartment to 45lb
95% of the time my wife and I will fly in the front seats. We are both light by VANs standards, and that combination plus the added weight indicated a need to move some weight forward. We have decided to go with 2 PC680 batteries instead of the standard battery, and the W & B works out theoretically perfect with the second battery relocated forward in the centre console. The console battery, power buss and power switches are all co-located in the centre console with a spark isolation barrier between the electrical switches and the fuel system.
This is a long introduction, but necessary I think for you to understand why we have gone the way we have.
We began by trying to fit Z14 to our perceived needs, then Z13/8. The electrical load analysis shows that in IFR conditions with the pitot heater on, we will draw about 17 amps continuous on the power circuit and also on the combined endurance and avionics circuits. The max/intermittent draw on each circuit is about 37 amps. We weren’t happy with either of these being loaded onto the 20 amp system as part of normal operations, especially if that system was to be our endurance circuit. Z14 and Z13/8 also both required that we run an extra fat wire back to one of the batteries.
Our decision finally was that we should concentrate on designing the system for robust normal operations with the minimum parts count for reliability, weight and cost reasons, and make the back up systems as simple as possible. We decided to go with a modified Z12 design because it keeps the noisy wires firewall forward and away from the panel, it uses one less contactor to use both batteries to crank the engine, the day to day loads are left with the larger alternator, and there is one less fat wire needed when the batteries are down the back. Normal operations will be to run with both battery contactors energised, the main alt on, the endurance and avionics busses fed from the main buss, and the aux alt resting on standby.
Failure modes would be treated as follows:-
Main alt failure - turn off main alt, turn on aux alt and adjust loads to suit
Much less likely failures:-
Main battery contactor fails closed in flight – no action needed till landed
Main battery contactor fails open in flight – The alternator load is balanced by the aux battery which needs to be always on in flight. The main battery relay can be opened to bring the main battery back on line
Aux battery contactor fails closed in flight – no action needed till landed
Aux battery contactor fails open in flight – No action necessary, but aux relay can be opened if desired
Both alts fail – Open main or aux battery relay to run min endurance load approx 3.5A. Close main and aux batt contactors to conserve power for landing phase
Fuse link between power buss and endurance buss blows – Open main or aux batt relay to feed endurance and avionics busses
Alternators:- We plan to go with the Plane Power 60A alternator for the main alt, primarily because of its inherent simplicity from an installation and operations perspective. We plan to leave the aux alt turned off unless the main alt fails, which means it should rarely get used. The SD 8 seems perfectly designed for this task and there is about a 3 lb penalty with the SD 20, but the SD 20 will give better peace of mind should the main alt fail in IMC. So, we plan to go with the SD-20 subject to your critique, and this leads us to our first question:- Is it OK to leave the SD 20 installed but not running? Will this cause any problems with the SD 20? Are the generic ford regulator and OVM adequate and appropriate to this task with the SD 20?
Power supply to the alternator fields:- Do you have any concerns with the alt fields supplied as depicted – ie main alt field via 22AWG fuseable link through 18AWG wire to the 5A CB, then 20AWG to switch and the main alt field. Does it matter which side of the switch the CB goes in this case? Would a 5A SCB be better?
Ground system:- We plan to use your forest of tabs either side of the firewall for everything on the panel and firewall forward, with the engine crankcase grounded to the through firewall stud. We have nominated local grounds for items such as landing lights, pitot heater, etc. We think we should ground both batteries locally. Is this correct, or should we handle battery grounds some other way with this design?
Fuse tray:- Are there any risks in placing the endurance buss, avionics buss, regulator and OVM in close proximity to each other on a hinged fuse tray under the panel? Is there a preferred way to connect the two busses together when in such close proximity?
We currently plan not to fit the diode between the main buss and the endurance buss because the SD 20 gives us the time needed to systematically reduce loads as needed should the main alt fail. Do we create any problems for ourselves not having an isolation relay or diode between the main buss and the endurance buss?
Bob, in appendix Z you comment that the ACS-OFF-L-R-BOTH-START keyswitch is not suitable for electronic ignition. I have also read somewhere that if you use this switch with an electronic ignition you disconnect the ground at the back of the switch. We are confused on this matter, so would you please explain whether you can use this switch or not with a future electronic ignition, and what you have to do to make it work properly? If it can’t be used, would you please explain what problem it causes? Do the same issues apply with the potential future installation of a Pmag?
Bob and others, we thank you for the time you take to look at our design, which is a works in progress. We will value any comments you care to make.
John and Jenny Cleary and Peter Bowman,
Australia
Canopy and doors 90% done and 90% to go
Trying to finalise the wiring plan so we can drill the holes and fit the cable attachments prior to painting inside
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