---From another list-server . . . some discussions about architecture choices
with PM alternators like Rotax.
>I guess I got confused, I thought the 912 was like some auto conversions (and
>your C206 plane you mentioned) which need a battery to power the electronic
>ignition in case of total electrical failure. I wonder why the 912 powered
>Europa (or was it the Katina) had dual electrical systems to satisfy the FAA
>when it was certified.
>-----------------
I think it had something to do with the fuel system options. Low wing
airplanes need electric pumps to keep engine running. So, to be certifiable
in all installations, a second electric supply was necessary. For a high
wing airplane (I presume the KitFox will stay in the air with gravity flow)
the second electrical requirment goes away.
>Failure concerns:
>When I was first studying your Fig 1 (SA 2/93) I had many questions most of
>which have been answered after I obtained your book. Any way there were two
>reasons for considering two batteries 1) to provide power to the engine in
>case of total electrical system failure, which you point out is not necessary
>with the 912. And 2) To provide the ability to find a landing place after
>electrical failure using a backup battery. The second battery is desirable
>because I expected to be close to the limit of the stock alternator. With
>total alternator or alternator and main battery failure I would like to have
>20 or 30 minutes of essential bus life. With main battery failure I would
>like to have more time, which should be possible if the alternator is
>operating.
What size battery do you plan? It should be quite possible to get HOURS
of battery only operations from the single battery. Even if you don't
install an essential bus, you should be able to get the airplane's minimal
draw down to less than 3 amps. A 17 a.h. battery at 50% capacity will carry
3 amps for over 2.5 hours. Replace the battery at regular intevals BEFORE
it quits cranking the engine and you can probably count on much more than
2.5 hours.
>I would prefer to have the following critical items during day flight:
>nav/com, turn coordinator, voltmeter and fuel pump for landing.
>Of course I would not have starter, clock and hobbs since they would be on
>the main battery only.
>For a night failure I would only add the instrument flood, and hope for a
>lighted place to land. I am considering a switch to add the landing light to
>the essential bus (and removing it from the main buss) for this failure mode
>so I can have a safer night landing.
Not necessary. If you're so far out that you have depleted the main
battery by the time the airport is in sight, having the landing light
on the essential bus is a mute point. If you've been a good steward of
limited resources (e-bus loads very light) then when the airport is
is sight, you turn the battery master back on to show whatever external
lights you like and/or operate the landing light. The essential bus
concept is primarily a MINIMUM loads feature for EXTENDED flight. Once
the airport is in sight, you can expend remaining battery however you
choose. It's your willingness to conserve enroute energy that determines
availability of lights for landing, not bus structures or even dual
batteries.
>With regard to the essential/main buss isolation, I am wondering if the
>pilot action can be reduced to one action instead of multiple actions. For
>example the way I understand it:
>A) Alternator fail to charge indication, action: 1) remove alternator from
>system, and wait for example E below.
An alternator failure is annunciated in seconds after the fact. I set
low volts warning lights for 13.0 volts . . . you won't have to wait.
>B) Alternator over voltage, action after OV device trips, action: wait for
>example E below.
Again, LV warning is only seconds behind the event . . .
>C) Main battery or main bus short circuit failure, action:
How will this happen? Think through the mechanisms that have to occur
for "main battery" or "main bus" shorts? These two points are often
brought forward in builder's FMEAs; they try to allow for it but
it might be unnecessary. If I were to hand you a box full of tools and
say, "here, go short that bus or short that battery, how would you go
about it? . . . and assuming you did figure out a way for the shorts
to happen, what's the likelihood that the same events will happen
in flight?
> 1) turn off master switch,
> 2) turn off alternator,
> 3) turn on essential bus feed.
>
> D) Main battery open circuit failure, action 1) turn on essential bus feed.
This is one condition that can happen due to broken wire and/or failed
battery master contactor. This would be the only compelling reason to
go to Figure Z-1 style architecture . . . but let's suppose you are Z-3
and you do loos the battery contactor. What's the major concern? Are you
going to have a hand held radio (if not what happens when the panel
mounted
radio goes poof?). What are you going to do if the landing light is
burned out (they do fail with some frequency . . . usuall when you
want them).
The point of this thought process is that lots of things builders do to
take care of this condition or that emergency are overshadowed by other
possiblities that are covered only buy dual systems (i.e. hand held
radio)
and proficiency (for night operations without a landing light).
>E) Main battery low voltage indication, action: 1) turn off master switch,
> 2) turn on essential bus feed.
This is the almost immediate indication at time of alternator failure.
If you're only planning occasional "late arrivals" with no exended night
flight, I'd turn the whole system off immediately. Navigate by hand
held or
pilotage until airport in sight and then turn battery master back on to
use whatever you need. Lots of builders seem to want things to be
running
enroute and then run out of snort needed for approach and landing
which is much more critical. For VFR enroute, I'd turn everything OFF
util airport is in sight.
>Question: I know this was discussed but the final answer does not come to
>mind. Does an alternator need to have a battery as a load?
The PM alternators on the ROTAX engines can produce power without
a battery to sustain them but they often need a minimal amount of
battery snort to get the regulator up and running.
>A circuit could
>be designed to provide power only from the alternator if a battery is not
>required (in the unlikely case of battery failure in flight).
This is something Paul M has mentioned from time to time, putting a large
capacitor in the system to keep the alternator output smoothed and the
regulator happy if the battery is off-line. This can work with a some
field excited alternators and any of the Permanent Magenet alternators
like the ROTAX. In your case, if you're in flight and the battery
contactor
opens, you won't get a low voltage warning light. Depending on the design
and quality of your regulator, you may see some increased instability
of voltage (flashing panel lights, etc) but for the most part everything
keeps running. Note that Figure Z-3 includes the capacitor.
Now, the current configuration of either Z-1 or Z-3 does not allow for
alternator operations. If you'd like to configure for that kind of
operation,
you'll need to do some testing to make sure that the regulator you choose,
size of the filter capacitor, and combination of loads will work. Had
one reader do this a couple years back and found that his
alternator/regulator
combination was too jumpy without a battery on line and lightly loaded.
Running nav lights with the battery off line offered sufficient stability
for his emergency procedures to define a technique for continued flight
alternator only.
In this case, plan for separate altenrator and battery switches. Make the
battery switch two pole as shown now so that the alternator is ON any time
the battery is ON. Then a second switch for alternator only. I'll help
you map out a test plan for writing your alternator-only operations
checklist.
In your case, I'll suggest you NOT worry about an essential bus
architecture.
or standby battery. If you'd like to have the e-bus feature, it can
certainly
be added . . . use Figure Z-3 (it shows wiring and ov protection unique
to PM alternators) and modify it to add the essential bus structures.
>BTW, Isn't the diode assembly in Fig Z-2 reversed?
>
Yes it is . . . good for you! So far, only a half dozen folk
out of 2,500 readers spotted it. I think we got that fixed with
Rev 7 books.
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