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flagstone(at)cox.net
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 Posted: Sun Jan 06, 2008 2:22 pm Post subject: Electrical System Reliability (and other ???) |
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Hi Bob:
I'm in the beginning phase of designing my electrical system. The following are the particulars of my setup:
Aircraft Mission - IFR (IMC is a likelihood), night and remote backcountry operations.
Engine - Lycoming IO-540.
Glass Panel - One Dynon D180 EFIS/EMS and one Dynon D120 EFIS both with internal battery backup. They are supposed to be able to cross-display via Dynon's D-SAB.
Avionics - Garmin GMA-340 Audio, SL30 Nav/Com, SL40 Com, GTX 327 Transponder, GPS 496, Dynon AOA/Heated pitot tube. All of the avionics were pre-wired by Approach/FastStack using their connecting hub. The GPS 496 has internal batteries as well as an external power source. I also have a ICOM IC-A23 handheld Com/VOR that I always carry with me.
Electronic Ignition - LASAR controller with dual Slick mags. One mag is equiped with a sensor for the LASAR system and the other is impulse coupled. Both are inop while the electronic system is operating. The system has the "Bush Kit" which is supposed to default to operating on the mags alone in the event of electrical or software failure. The controller is supposed to operate with voltages down to 5.5volts.
Vacuum System - None
Alternator/Regulator - B&C L-60 Alternator and B&C LR3C-14 Regulator.
Fuel Injection - Bendix style fuel injection with Airfllow Performance auxiliary fuel pump.
Electrical Controls - Three axis RAC trim system, Electric flaps, Electric cowl flaps.
Lighting - Dual Strobe lights, Nav Lights, Dual taxi lights, Dual landing lights, Dual "Type C4A" military style cockpit lights.
Autopilot - TruTrak Digiflight II two-axis autopilot.
Battery(s) - My intent is to use the Odyssey series battery(s).
Before I go any further with the design I need to make the following decisions:
1. Single or Dual Alternator. If dual alternator, size of second alternator and type of second regulator.
2. Single or Dual Battery. If dual battery, isolated systems or combined and capacity of each battery.
3. Glass Panel alone or Glass Panel with back-up "Steam Gauges", Airspeed, Altimeter, Turn Coordinator, Compass.
I have read through Sections 2, 3 and 17 of your book several times as well as the wiring diagrams and various articles on your site. These offer a great amount of information and advice which I'm sure would allow a knowledgeable person to make the proper decisions. However, my knowledge, experience and confidence are limited so I would like to confirm with you my interpretation of the information and get your confirmation/critique thereof. Here goes:
The first is that designing for complete battery failure i.e. a dead short that would take down the whole system in NOT necessary. This seems to be covered well on page 17-11 in a paragraph that contains the following, "Last, the antiquated concerns for battery "failure"....RG battery reliability....drives the probability of gross battery failure to zero." However, I would like to confirm that the Odyssey batteries would fall under the same class as "RG battery reliability" and that I would NOT need to design for battery failure given my intended mission and battery selection. Would you concur with that??
Regarding decision number #1: Your comments on several pages of sec. 17 clearly indicate your preference for dual alternators. My setup in heavily dependent on electrical systems for normal operations. Based on that and on your comments, I would opt for a second alternator using B&C SD-20 Alternator and B&C SB1B-14 Standby Controller. Given the need of a second alternator, the weight penalty between the 20amp and 8 amp models is 3 pounds. It would seem the 20amp would provide near normal operations for as long as it works should the primary alternator fail. Would you concur with that??
Regarding decision number #2: This has several sub-parts.
A - Your comments in sec. 17 seem to indicate that with a dual alternator setup as I opted for in #1, dual batteries with isolated systems are NOT necessary as a design point for in-flight electrical failure. I would like to avoid the dual battery contactors and the crossfeed contactor by using a single battery source. Would you concur with that?? If it makes a difference, the battery(s), regulators and LASAR controller will be located on the cabin side of the firewall.
B - One of the mission requirements is backcountry operations. Given that, one my design points was the ability to start the plane with a completely dead primary (cranking) battery, say as a result of leaving the master on. That is the reason I opted for the LASAR system with the bush kit. However, given my configuration I would need some sort of standby electrical power. To that end I am considering two options:
Option 1: Design in a permanent Non-Cranking (Aux) battery and bus as shown in fig. Z-35. to power the Fuel Pump, LASAR system, and one EFIS/EMS.
Option 2: Design in an external power plug for the fuel pump ONLY and carry a small battery with me during backcountry operations ONLY. With my configuration I do not need the EFIS/EMS or the LASAR system to function to just start the engine. The only thing I really need is a few seconds of fuel pump operation to pressurize the system in order to hand prop the engine. If I was backcountry I would have 12 volts of battery power someplace anyway.
My choice would be for Option 2. Would you concur with that???
C - Odyssey offers several batteries ranging in capacity form 14 to 28AH, in cranking power from 535 to 925CCA and in weight from 12.0 to 26.0 lbs. In the unlikely event of the same flight failure of both alternator/regulator systems opted for in #1, and according to the method used on page 17-6, any of the batteries would satisfy the requirements of my E-bus. However, I am not so sure about starting power for normal operations. My engine is fuel injected and high compression. My space limitations would preclude me from installing the 28AH/925CCA/26lb battery. That would leave two preferred batteries; The PC545 (14AH/545CCA/12.6lbs) and the PC680 (17AH/680CCA/15.4lbs). If necessary I could install two of either the PC545 or PC680 batteries wired together to act as one. My first preference would be a single PC680, second would be two of the PC545's and lastly two of the PC680's.
Here I would simply ask for your recommendation as to which way to go??.
Regarding decision #3: Well this started out to be a long and involved question but as I was typing out my reasoning and explanation, I made the decision to opt for the back up gauges. I intend to file IFR for trips and that means at some point I will probably be in IMC and there is just no way that I am going into IMC without them. Thanks for helping me get that one out of the way. Its been bothering me for months.
There are a couple of other questions I have:
On page 17-10 you say, "When you trade generators for alternators, loss of a battery contactor may well precipitate total loss of electrical power. Alternators, don't run well without a battery". Yet when I look at Figs 17-2,3,4,5,6, and 8, all of them place the Alt field switches on the main bus with the contactor in the path to the battery. If I understand your statement correctly, it seems that if you loose a single contactor in any except fig 17-5 & 6, Dual Batteries, you would lose all charging systems. Shouldn't there be a pathway to energize the Alt fields thru the E bus in case of a contactor failure. Is my reasoning correct?? It looks like Fig Z-13/20 does that for the Aux Alt only and Fig Z-12 doesn't for either Alt. Is that correct?? Wouldn't you want to be able to energize both Alt systems from the E-bus in case of contactor failure with a single battery??? Or alternatively, what about two contactors for a single battery??? They're cheap enough??? Would that work???
I really don't understand what P-leads are or the difference between P-mags and E-mags. What do I have with the LASAR system??? I looked but couldn't find any articles to explain mag operation or what they are. Could you point be to the proper article???
Thanks a lot Bob.
Mark Richards
[quote][b]
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bakerocb
Joined: 15 Jan 2006
Posts: 727
Location: FAIRFAX VA
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| Posted: Tue Jan 08, 2008 7:54 am Post subject: Electrical System Reliability (and other ???) |
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1/8/2008
Hello Mark Richards, You wrote: "I really don't understand what P-leads are
or the difference
between P-mags and E-mags. What do I have with the LASAR system???"
I admire your very ambitious design effort and your willingness to ask
questions. But the elephant cannot be eaten in one setting or with one
bite -- just keep nibbling away.
To answer one question and make suggestions:
1) P leads are the means of turning a conventional magneto OFF by preventing
it from sending high voltage electricity to the spark plugs.
The P lead electrical connection on the external surface of a magneto is
connected to the primary coil (hence the nomenclature "P") inside the
magneto. If the primary coil is connected to ground via the P lead, usually
by means of an "ignition switch", then the magneto is considered OFF and
will generate no high voltage spark.
2) If you will go to the emagair web site you will be able to learn the
difference between a P mag and an E mag. http://www.emagair.com/
3) Unison has a number of publications available on their Lasar ignition
system. Some they will send you for free, others will cost money. I suggest
that you contact them and request the free information and learn what is
available at cost.
http://www.unisonindustries.com/ourproducts/lasarelectronicignition.html
You need to do much more research on the Lasar system before you are ready
to make a decision on whether to use it or not.
Please continue to ask specific questions and we will do our best to help.
'OC' Says: "The best investment we can make is the effort to gather and
understand knowledge."
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