nuckolls.bob(at)aeroelect
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 Posted: Fri Aug 27, 2010 5:38 am Post subject: Z-12 questions |
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At 11:52 PM 8/26/2010, you wrote:
| Quote: | Bob,
I know you’re a very busy man, so I hope I’m not imposing too much by asking you a couple of questions. I’m hoping you can help shed some light on these issues, for which I’ve been unable to find answers: |
No problem, it's what we do.
| Quote: |
1. My RV-8 will utilize a Z-12-based architecture, including a 60A main alternator, and a 20A backup alternator from B&C mounted on the vacuum pad. I am using the standard LR-3 and SB-1 alternator controllers. My first question is: Does the “Aux Alternator” switch normally stay ON during flight, so that the Aux alternator actually provides power to the Main Bus all the time? Or should the switch stay OFF, only to be switched on by the pilot in the event of a Main alternator failure? |
The SB-1 alternator is specifically designed for STANDBY service
and is normally installed with a setpoint down around 13.5 volts.
Therefore, the second alternator installed for standby service is
not intended to either share loads with the main alternator nor
is it intended to charge the ship's battery.
It's your option, you can leave it ON or OFF at all times for normal
flight . . . if left ON, the SB-1 regulator will interpret the
normal bus voltage of 14+ volts as "too high" and will completely
relax thus driving the SB alternator field voltage to zero.
In the event of main alternator shut down for any reason, the
bus voltage would normally fall to battery support levels
that start at about 12.5 volts and goes down from there.
If the SB alternator is ON, this depression in bus votlage
will cause the SB-1 regulator to wake up and bring the
standby alternator into service. It will annunciate the fact
by illuminating the SB ALT loaded light. If total loads on
the SB alternator exceed 20A, the SB-1 will flash the warning
light . . . the pilot is advised to reduce ship's electrical
loads until flashing stops.
If left OFF, you'll get a LO VOLTS warning light that
prompts you to turn the sb alternator ON and
then do the load-shedding exercise.
| Quote: |
2. My interpretation of the Z-12 architecture indicates that in the event of a Main alternator failure, it would be the pilot’s job to manually load-shed the electrical system so that the total load was below 20A. This is because the Aux alternator would feed the Main Bus (and thence the Endurance Bus), so in the event of an Aux alternator-only operation, most of the Main Bus items would have to be turned off (or not used). My Endurance Bus will pull about 10A, so I’d like to feed the Endurance Bus directly from the Aux alternator (in addition to the normal feed path from the Main Bus). That way, I could perform a quick, pre-selected load-shed operation merely by turning off the Main Alternator switch and turning on the Aux Alternator switch. (And, of course, I’d also have the E-Bus Alternate feed from the Batt Bus, as a last-ditch backup.) So my second question is: Is this a reasonable modification to the Z-12 architecture? My goal is a simple, robust system, but I’d like the convenience and pilot friendliness of a dirt-simple “load shed” procedure, rather than a manual load-shed -- and having to ration power to the Main Bus items. |
Then you're not talking about Z-12 (originally crafted to
be a drop-in for type certificated aircraft that (1) doesn't
have an e-bus and (2) would be exceedingly difficult under
FAA rules to shuffle bus structures around. The system you're
describing looks more like this
http://tinyurl.com/24j8gh4
This is a more robust version of Z-13/8 where the 20A
alternator is installed primarily to support an electrically
dependent engine. No e-bus is shown because if you have
a robust second alternator, there's no likelihood of needing
to operate battery-only . . . that's two major failures on
a single tank of fuel. Hence the simple direct connection
for the sb alternator to the battery such that battery
contactor failures are covered.
"Last ditch" ???? You're spending too much time worrying
about things that only happen in Hollywood's notions of
how airplanes work. Modern alternators properly integrated
into your airplane are very low failure rate items.
It is unlikely that you will EVER find experience a
pressing need to drop to SB alternator or a battery-only
ops mode over the lifetime of your airplane. In 25 years
of working with the OBAM aviation community, I've had
but a handful of readers report that the s/b alternator
or e-bus ops were found useful and functioned as advertised.
If the airplane you're flying now is a TC aircraft, then
it has no e-bus, no sb alternator, might even be a rented
airplane where you have little knowledge of and zero
control over ship's maintenance. This has always been
my personal situation having never owned an airplane.
So when I walk up to the airplane, I do it with the
mind set that I don't care if the electrical system
is going to work 100.0% of the time or not. I've got
stuff in my flight bag that will allow me to operate
any of the airplanes I fly in "J-3 mode" with a goal
of aviating to airport of intended destination whether
or not anything on the panel works.
http://aeroelectric.com/Pictures/Misc/Failure_Tolerance.pdf
No matter what electrical system is installed in your
OBAM aircraft, the stuff in your flight bag should be
similarly selected. If you've got smelly stuff in the
cockpit that's suspiciously like wire or electrical
accessory smoke, you might just want to shut everything
off, dig out the REAL stand-by goodies and keep on truck'n.
Actually, I've not turned a panel mounted nav radio
on since I purchased my first hand-held GPS in '95.
The $100 Magellans do 99% of everything I need the
nav radio to do . . . how far, which direction,
and ETA.
Your questions give the distinct impression that
you don't have much trust in the hardware for which
you're crafting an architecture. Lack of trust comes
from some combination of three conditions:
(1) You KNOW you're installing certified junk on
an airplane constrained by regulation to be forever
less than the best we know how to do.
(2) You don't personally possess a demonstrable,
experience-based data set the confirms the reliability
enjoyed by the OBAM aircraft community.
(3) Your understanding of the system is diluted
with a lot of extraneous noise exemplified by the
majority of dark-n-stormy-night stories in the
flying rags and the nail-biter scenarios dreamed
up by Hollywood writers to make the actors look
like the gods of failure mitigation.
If you're crafting an airplane with an electrically
dependent engine where all the engine support
comes from the battery bus, then Z-08 is recommended.
If your engine is not electrically dependent
then Z-12 as depicted is recommended.
When and if that low voltage warning light ever
comes on, know that this is NOT an emergency
You can finish your cup of coffee and decide
what, if anything, you plan to do about it.
The point is, there IS A PLAN that probably
requires repositioning less than a half dozen
switches until the light stops flashing.
Hence, I'll suggest that your quest for "dirt
simple" responses to a "last ditch" scenario
are probably not a good investment of emotional
capital or time.
You need to tell us more about what equipment
is installed on what kind of engine for the
advice to me more specific.
Bob . . . [quote][b]
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