alec(at)alecmyers.com
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 Posted: Mon Jun 08, 2020 3:59 pm Post subject: Physical construction of Z101 engine bus & battery bus |
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If the maintenance of the primary battery is “not always perfect” then isn’t it very likely that the maintenance of the backup battery is also “not perfect”?
If human factors are at play, then is it not likely that two batteries will get even more imperfect maintenance than one? After all, who needs to maintain their battery when there’s a backup battery installed?
On Jun 8, 2020, at 7:17 PM, Ken Ryan <keninalaska(at)gmail.com> wrote:
Thanks Charlie,
I am convinced that 2 alternators + 1 battery is superior to 1 alternator + 2 batteries. I am less convinced that if two alternators are not possible, a second battery adds virtually no value to the ship's overall reliability. Bob did make a pretty good argument, but it is founded on the assumption that the battery, when properly maintained, is utterly reliable. I have two problems with that assumption: 1) we know from Joe's experience that batteries can fail and 2) even with the best of intentions, maintenance (including battery maintenance) is not always perfect. To contend that having a fresh battery available at the flip of a switch adds no reliability to an electrically dependent aircraft still seems like a bit of a stretch to me.
Ken
On Mon, Jun 8, 2020 at 2:49 PM Charlie England <ceengland7(at)gmail.com> wrote:
Hi Ken,
I'll take a swing at it, but would suggest that it's not quite the right question.
I get your point, but the answer 'depends'. If using electronic fuel injection, the current demands mean that a single PC680 style battery (in 'new' condition) will only buy you about 30-40 minutes of flight time after alternator failure. A 2nd PC680 only adds another 30-40 minutes. On an engine that allows two alts, either of which can keep the engine running and the panel lit, a typical alt (ex: 55A Denso) is lighter than a 2nd battery, not much more expensive (actually much cheaper than an actual Odyssey branded PC680), will keep the engine running to fuel exhaustion, and has the additional benefit of allowing a return flight (in some range-dependent situations) without the need of maintenance while on the road. Not something I would ever consider after an alt failure with only a 2nd bat for backup. Add to the mix, the fact that while a 2nd alt is pretty much an install&forget item (just a startup check each flight similar to a mag check), any battery is a constantly degrading item that requires regular capacity testing to ensure that it has enough remaining capacity to give expected duration if called upon.
Now, with carb or mech fuel injection in a VFR environment, current demands might (likely will) be low enough that a single bat & single alt back up each other. In *my* opinion, that's where Bob's statement makes sense, and where your question, as framed, doesn't need asking.
So, I think you really have to define both the mission *and the hardware* to pick an architecture.
FWIW,
Charlie
On Mon, Jun 8, 2020 at 11:15 AM Ken Ryan <keninalaska(at)gmail.com> wrote:
Bob, you said:
"If you have TWO properly maintained batteries,
in all likelihood, you'll be carrying around
$twice$ the hardware with virtually no value
added to the ship's overall reliability."
Help me understand why the following statement is not correct:
If you have TWO properly maintained alternators,
plus a battery (sized for minimum endurance requirements)
in all likelihood, you'll be carrying around
$twice$ the hardware with virtually no value
added to the ship's overall reliability.
Ken
On Sun, Jun 7, 2020 at 6:56 PM Robert L. Nuckolls, III <nuckolls.bob(at)aeroelectric.com> wrote:
At 05:26 PM 6/7/2020, you wrote:
| Quote: | I am not speaking to any engine manufacturers recommendation (although the
Honda Viking manufacturer does recommend two batteries. I just thought
I was applying logic -- if dual alternator + battery (triple power source)
is desirable for electrically dependent engines, wouldn't that reasoning
imply that if the second alternator is not practical, a second battery
could be used as the third power source? I could ask you a similar question:
Are not two independent power sources (battery and alternator) sufficient
to meet the needs of the electrically dependent engine. Obviously you
saw value in adding the second alternator. Why no value in adding the second battery?
|
Excellent question.
It's a problem in energy budgets combined with
efforts to assure continued airworthiness.
The airplane cannot do without a battery if
you're going to have a starter. Depending on
your planned mission profiles, you will want
to size the battery (1) for cranking then
(2) minimum endurance in alternator-out modes.
This study gave impetus for the creation of
the endurance bus . . . a fast and predictable
way to economically tap known quantity of energy
stored in the battery's chemistry.
Z13/8 was a small but significant amplification
of that idea . . . <b>the second alternator's endurance
had no practical limits.</b> Hence, energy on the chemistry
just might be held completely in reserve for
descent and approach to landing.
Z13/20 (and the aux alternator option on
Z101) expanded the Z13/8 endurance opportunity
by a factor of 2.5 or better.
Okay, suppose the drive pad isn't available.
We are still charged with identifying and the
delivering to energy required to comfortably
terminate a worst-case mission.
This means that as a part of routine maintenance
the ship's chemistry needs to be monitored for
capability. We could certainly store that energy
on TWO devices but to what advantage? If we're laboring
under the notion that a battery can suddenly become
unavailable during one tank of gas, then we have
to assume that EITHER battery can roll over and
die . . . okay, how would that failure be
annunciated . . . how would remaining energy be
managed . . . ?
I think that's the scenario anticipated by the
folks that crafted that battery manager with a
full-wave rectifier that -anded- two, completely
isolated batteries together. Assume the alternator
has quit and some time later one battery
craps out. How does the pilot become aware of the
problem and what kind of energy juggling issues
are presented when the available energy drops
to half? This assuming he really knows that the
two batteries were performing equally and has
recently quantified their condition, he now
has to come up with a new "plan C?" and perhaps
declare an emergency.
This scenario first assumes TWO critical failures
during the consumption of one tank of fuel . . .
about 3-4 hour window. Part 23 certs don't
get concerned with dual failures at all.
Part 25 and heavier will wade into the reliability
quagmire with mountains of computer generated probability
studies that get 'worked' on until somebody
finally sprinkles the holy water and off they
go.
Ask Capt. Sullivan what he thinks about
reliability studies . . .
https://www.youtube.com/watch?v=HKJ1lIh2Cgk
So we're left to our own devices which in reality
are not so bad.
The short answer is: A diligently maintained
battery is the most reliable source of energy
on the airplane. Replacing it when ability
to store energy drops below some benchmark
(generally 75 to 80% of new) means that it
always cranks the engine and will provide a
quantified option for dealing with alternator
failure. Two batteries just doubles your
preventative maintenance labor. Further,
you need to decide if plan-b can reliably
depend on the sum total of energy in two
batteries . . . or will they be sized to
independently step up to the task? The
second option calls for 2X the battery
weight and volume; the first option complicates
calculations and switching operations for
carrying out a plan-b that shouldn't ever
happen. BOTH options still demand good
preventative maintenance.
Just as you KNOW fuel aboard when you launch,
you also need to know Watt-Hours aboard
no matter how many batteries you're carrying.
If you have TWO properly maintained batteries,
in all likelihood, you'll be carrying around
$twice$ the hardware with virtually no value
added to the ship's overall reliability.
Bob . . .
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