![]() |
Indulge me...
I'm inching closer to the point of having to commit to a power distribution architecture for my 10, and there are some fundamental considerations I have yet to resolve. I shall tap the Hive Mind and all will be good :D
My thoughts going in: some level of redundancy is essential for safety of flight in an electrically-dependent aircraft especially for electrically-dependent (no-vacuum) IFR flight. While the basic combination of alternator and battery offers some margin based on battery charge and health, this is too bare-boned to be prudent. Remaining energy in the battery at the time the light comes on may not be near enough and can never be precisely known as the battery ages, and in the event of a battery failure, the alternator may not behave properly to deliver steady, regulated voltage to the bus. A dual-alternator, dual-battery "Cadillac/NASA" system offers very comfortable margins - but at considerable cost and weight penalties, from my personal perspective. By process of elimination, the sweet spot is a dual-alternator or dual-battery architecture. The question is, which one is preferred, and why. I'd really like to hear some thoughts on the relative merits of doubling down on alternators vs. batteries, particularly as it pertains to RV-10 W&B and installation challenges. Thanks! (And if this has been hashed before, kindly redirect me to the thread.) |
Options
Are you going to be using electronic ignition/fuel injection? It changes the requirements if you are...and there have been quite a few debates about "what is best".
See this one: http://www.vansairforce.com/communit...151435&page=19 That being said, it all comes down to what level of risk you are comfortable with; you CANNOT design a perfect system so you need to be comfortable with what happens if things fail. I opted for a system based on the Z-14 detailed in The AeroElectric Connection. It it slightly different, having taken advice from several folks flying already with similar aircraft to my own. I am running EFII, so my system ended up being a dual alternator, dual battery, split bus, with an essential engine bus that is powered from both systems... |
During the panel upgrade, I chose to go with dual alternator (B&C pad mounted backup) and a single battery (EarthX) firewall mounted.
I also have a G5 with backup battery. I feel that in the -10 this worked best for me, as keeping as much weight forward is advantageous in terms of loading passengers and baggage. My CG is right at the forward limit with pilot and fuel, so I can load the seats and baggage area easily. I felt like the B&C stuff is very reliable, and this is a fairly hands off / maintenance free solution to electrons - no extra battery to maintain / check / replace. |
Yep - it has a been hashed several times. I offer:
- A well maintain battery is the most reliable single component in your airplane. Getting power out of it when you need it however can be problematic and should be your focus. I replace one battery every three years or on any occasion that I think it has been abused (e.g. master left on). This provides some assurance on battery health. - Taking the above a step further, a fault that takes down power from your panel is most likely not from a failed battery. As such, here is where a design that directly examines all the relays, switches, connections and such that can fail leaving you with a dark panel pays off. This simple example is your single avionics relay fails. - The impact from loosing the single alternator as the sole design focus, in my opinion, turns a blind eye to most “dark panel” lessons learned. - I have no faith in any alternator running without a battery. I’ll let other spin what they will on this but for my airplanes no way, no how will I design that into the electrical system. Bottom line: - Two alternator, single battery schemes are only marginally better than single alternator, single battery. The only risk mitigated is the loss of the primary alternator. This is not the leading reason for a failure giving you a dark panel. - If you jump to a two battery, single alternator foundation you can, with some thought, build a very robust power distribution that will have two hours or so of reserve capacity on the loss of the single alternator. - I use two PC-625 batteries, both on line for starting and normal operation, split out (opening the two master solenoids) as the first immediate action on any electrical fault to establish the most reliable configuration. One battery feeds Avionics #1, the other Avionics #2 but either buss can be selected to either battery. The objective is no single fault, without pilot action, will take down more than half of the panel (so one EFIS, one GPS and one COMM are always up). After the pilot examines the situation the other avionics buss can be restored by single pilot action. I did backfit a 20 amp standby alternator on the RV-10 in addition to two batteries as I wanted unlimited electrical reserve (as in full IFR flight to fuel exhaustion instead of two hours, or if need be a way to get home after stopping for gas). On reflection I decided to not install the second alternator on the new RV-8. One last consideration. I offer a thoughtful dual PC-625 (or similar) install eliminates the need for the plethora of EFIS backup batteries that now seem to be in fashion. My point - there is little weight penalty. Carl |
Yes
Thanks Carl for your help!
|
I went with dual alternators and single battery, with the EarthX ETX900 which is arguably oversized for my airplane but that was kind of the point. My Dynon screens have their own battery backup as well which is tested annually to show at least 45 minutes reserve or they get replaced.
Single alternator failure for me means continued flight on the backup, and replacement before further flight - it allows me to complete the in-progress mission as planned, but not launch into another. Dual alternator failure in a single flight is highly unlikely - and I treat it the same way I would treat an electrical fault behind the panel that suddenly started dumping smoke into the cockpit - hit the master/alt switch and make everything cold is step #1. Now - in my airplane that means the big fan up front stops making noise - so my step #2 consists of bringing up my dedicated backup power (wired direct from the battery prior to the master) to my electric fuel pumps, and with one standard mag on the engine I have power again. The ETX-900 will pull that single fuel pump for almost 2 hours, and it's the only load I have on the battery wired direct. The Dynon screen gives me a minimum of 45 minutes IMC capability to either get down or troubleshoot my electrical problem to bring at least one alternator and partial panel back online. In VFR conditions I can shut down the Dynon to save its internal battery power while I do a longer troubleshoot if necessary, and bring it back up later. The scenario of catastrophic battery failure or BMS disconnect or master contactor failure is handled by the alternator continuing to supply power to the panel without the battery online - I tested this shortly after Phase I, accidentally as it happened, but it worked just fine. I've got enough electrical load in my airplane that the alternator maintained a pretty stable voltage (it hunted up and down about a quarter volt each way) with the battery offline and kept it all going until I figured out that I had bumped my master switch accidentally. Later I tested it again (intentionally this time) under more controlled conditions and got the same result, so I'm happy with it. My only serious exposure to electrical failure is an IFR approach with dual alternator failure (or electrical fire/smoke), running on battery only for the fuel pump. I've got my Dynon screen but not my Garmin 430W or autopilot, so in that case I would have to complete a (non-legal but arguably safe, at least safer than the alternative) GPS approach somewhere by following my approach plate on the Dynon which will geo-reference my aircraft over the plate, and hand-fly it with altitude drop-downs. If the 430W is not the device which continues to emit smoke during troubleshooting, then I can bring it back online and fly a legal GPS/ILS approach anywhere at the expense of shortened runtime on my battery. You can't build a completely fault-tolerant electrical system in any airplane. There will always be some level of risk that has to be "acceptable" and that risk level is highly individualized. You should think about the likelihood of failure for various components, build in backups for the ones you consider critical, and accept the potential failure of the rest. |
Indulge me...
In my system I considered reliability/redundancy vs. weight/cost/safety. The common IBBS EFIS backup battery is a Li-Fe-Po4 chemistry, weighs 2 pounds and costs about $400. A PC680 is a lead/acid AGM chemistry, weighs 15 pounds and costs about $130. The IBBS must be mounted in the cabin, the PC680 can be firewall or aft fuselage. Both require periodic maintenance and replacement (IBBS requires annual load testing for continued airworthiness, and replacement following a failed load test). For my safety I decided not to have a Li-Fe-PO4 battery in the cabin. The only battery backup is the Garmin supplied one for the G5 backup EFIS.
For IFR I decided on the Z-14 dual battery/dual alternator architecture. The backup alternator weighs about the same as the obsolete vacuum pump/vacuum regulator/hoses/vacuum instruments that are replaced by the EFIS. My penalty is (15 lbs - 2 lbs)=13 lbs. On the plus side is a safer and more cost effective battery and battery installation. |
To re-state what might be obvious, dual bat is cheaper but heavier, and duration-limited by load an bat size & health (a potential big variable).
Dual alt will be more expensive (with a pad mount alt2) and lighter, with the advantage of unlimited duration. And contrary to Carl's belief ( ;-) ), it's not hard to avoid single-point failure with a 2-alt, single bat system. With most alternative engines, dual alt is both lighter *and* cheaper. Charlie |
Here we go!
Here we go again!:D
|
I don't think your electrical system would be very happy being powered from your alternator without a battery in the loop.
|
| All times are GMT -6. The time now is 04:55 AM. |