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al38kit
Joined: 13 Jul 2009
Posts: 21
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 Posted: Tue Oct 13, 2009 6:16 pm Post subject: 2 Alternator Split System |
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As I design my system, I have a question regarding which alternator to use to power the main flight/nav instruments...I'm planning an IFR all electric panel. I do plan to have back up instruments on the other buss.
I have the small B&C 20 amp gear driven alternator, and a 60 amp PP belt driven. This will be going on an IO520.
I plan to run the busses, with an interconnect in the event of failure of one of the alternators.
Which alternator should I plan to power the "more critical" buss?
Does it matter...?
Al
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MauleDriver(at)nc.rr.com
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| Posted: Wed Oct 14, 2009 4:08 am Post subject: 2 Alternator Split System |
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It's been awhile since I went through this thinking when I adopted Bob's
Z-14 design - split bus, dual alt, dual batt. But the conclusion I came
to 2 years ago (was it that long?) was that there are few if any failure
scenarios that would put you in a situation where your panel wouldn't
outlast your fuel. It was even challenging to come up with a scenario
where the aircraft couldn't be re-fueled and ferried to a repair station
(like home). The Z-14 can be a very robust system.
But I'll have to admit that I irrationally chose the Z-14 because I
liked its symmetry.
When I did my planning for 3 GRT HX's , G430W, SL30, GTX(Txpndr), TT AP,
PMX9000EX, and TT ADI for backup, I ended up with B&C 40 and 20 amp
Alts. It seemed to me that once you went to the Z-14, the size of the
Alts was entirely driven by the total load rather than any backup
(critical Alt) scenario. But I haven't thought that one thru in awhile
(rivet, rivet, sand).
Bill "dreaming of flying this dream ship sooner rather than later" Watson
RV-10
al38kit wrote:
| Quote: |
As I design my system, I have a question regarding which alternator to use to power the main flight/nav instruments...I'm planning an IFR all electric panel. I do plan to have back up instruments on the other buss.
I have the small B&C 20 amp gear driven alternator, and a 60 amp PP belt driven. This will be going on an IO520.
I plan to run the busses, with an interconnect in the event of failure of one of the alternators.
Which alternator should I plan to power the "more critical" buss?
Does it matter...?
Al
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p 67842#267842
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marcausman
Joined: 08 Feb 2007
Posts: 70
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| Posted: Wed Oct 14, 2009 5:09 am Post subject: Re: 2 Alternator Split System |
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I have this bus setup in my plane. If the 60A is on bus A and the 20A is on bus B, then you have to plan your loads accordingly so that loads on each bus do not exceed about 80% of the rated alternator capacity (just a rule of thumb). The busses normally run totally independently of each other, and only tie together if you have an alt fail on one of the busses, in which case you may need to load shed.
Some avionics, like most newer EFIS, have dual power inputs, and you can feed these off of both busses (the inputs are isolated from each other). In that case, I recommend that bus A be set to about .3 volt higher than bus B so that the power is drawn from the bigger alternator under normal conditions.
You can also install Comm1 on bus A and comm2 on Bus B, for example, just for redundancy too.
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_________________
Marc Ausman
http://www.verticalpower.com "Move up to a modern electrical system"
RV-7 IO-390 Flying |
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nuckolls.bob(at)aeroelect
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| Posted: Wed Oct 14, 2009 6:11 am Post subject: 2 Alternator Split System |
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At 09:16 PM 10/13/2009, you wrote:
| Quote: |
As I design my system, I have a question regarding which alternator
to use to power the main flight/nav instruments...I'm planning an
IFR all electric panel. I do plan to have back up instruments on
the other buss.
I have the small B&C 20 amp gear driven alternator, and a 60 amp PP
belt driven. This will be going on an IO520.
I plan to run the busses, with an interconnect in the event of
failure of one of the alternators.
Which alternator should I plan to power the "more critical" buss?
Does it matter...?
|
Hard to tell. An analogy to the framework of
your question might be, "I've got this
D8 caterpillar and a yard full of matching
implements . . . how should I use them?"
It depends . . . on whether your task is to
build a road up a rocky mountainside . . . or
to prepare your wife's flower beds for the
planting of pansies.
I've often reminded our bothers on the List
that electrical system failures of well qualified
hardware have not been high on the list of
really uncomfortable days in the cockpit. 99%
of all dark-n-stormy night stories that include
electrical system issues would not have
happened if the airplane had been fitted with
active notification of low voltage, a well
tended battery of known capacity, and a pilot
versed in Plan-B activities designed to deal
with alternator failure.
So the lucid answer to your question depends on
our understanding of your design goals. Why Z-14?
What was it about Z-18/8 or Z-12 that failed to
meet a perceived hazard to comfortable completion
of flight? You mention an IFR panel . . . which
doesn't help much. Every airplane I rent from
C-140 to A-36 has an "IFR Panel". Is it your
intent to spend a lot of time flying in unfriendly
environments like night time over rocks cloaked
in clouds? Z-20 is not well suited to a Lancair,
Z-14 is not suited to a Kitfox. The answer
to your question lies in a well considered
analysis of the airplane, mission, environment
and even perhaps your experience level and skills.
Z-14 was crafted as a means by which the most
demanding of flight environments and missions
could be accommodated. Z-14 is a STEP
UP from what you get in a Beechjet or a
King Air. Those airplanes have dual everything,
even dual autopilots. Interestingly enough
however, only ONE battery which demand continuous
closure of a cross-feed contactor and a much
more complicated procedure for dealing with
failures of major components.
So before we attempt to noodle through your Z-14
architecture, we need to know how Z-13/8, or
Z-12 failed to meet a requirement. Those systems
are lighter, lower cost of ownership, less
complicated and STILL offer system reliability
on a par with a Beechjet.
Bob . . .
---------------------------------------
( . . . a long habit of not thinking )
( a thing wrong, gives it a superficial )
( appearance of being right . . . )
( )
( -Thomas Paine 1776- )
---------------------------------------
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