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1. 01:25 PM - Re: Re: Z101 switch state question: engine and main bus shutdown (Robert L. Nuckolls, III)
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| Subject: | Re: Z101 switch state question: engine and main |
bus shutdown
>Bob,
>For me, the critical factor here is not independent control of the
>battery; it's that the aux alternator is effectively playing the
>role of the internal generation system of a magneto; supplying
>electrical energy to the engine, even when the entirety of the
>airframe is made electrically 'cold'. As drawn in Z101:
What accessories will be fed from your
proposed aux bus loads that you would
not want to power in combination with
keeping the engine running?
The fundamental premise for the aux bus
is to power accessories most useful
for continuation of flight with minimal
energy demands. Primarily of concern
where battery-only ops are part of
the plan-b, i.e. no aux alternator.
When an aux alternator is present,
then battery-only ops are so statistically
remote that battery selection can be
minimized for weight and volume . . .
assuming that plan-b load analysis
(and perhaps even flight testing)
demonstrates that aux alternator will
support necessary loads.
>the aux alt cannot be working unless the aux bus is 'hot'. So,
>if we make the airframe 'cold', we're left with the battery running
>the high current-required engine control/injection system.
Electrically induced smoke in cockpit is
an exceedingly tiny risk. So aside from
finding yourself on short-final-to-the-rocks,
when would you expect to want the electrical
system completely 'cold'? This condition
would call for the engine to be shut down
as well.
So what's your FEMA/Plan-B analysis show?
Will your anticipated aux alternator not carry
the engine plus minimal flight accessory loads?
>My choice on my project was to have both the output
>and the control source tied to the *engine* bus, so
>if muscle memory causes me to shut down the airframe
>in an emergency, I can react in the same way that I
>trained in old mag/carb aircraft. This leaves the aux alt
>supplying current to the engine bus.
Muscle memory? When did this replace a
well conceived and practiced check
list? When faced with a need to reduce
energy consumption in a Z101 architecture,
I would think that line item 1 would be to
switch ENGINE BUS ALTERNATE source feeder ON.
Item 2 would be to switch AUX BUS ALT FEEDER
and AUX ALTERNATOR to ON. 3. Set master switch
to OFF to kill any potential main alternator field
drain and eliminate battery contactor drain.
These steps carried out on the very compact
DC POWER management panel will assure smooth
transition to the Plan-B condition with no
interruption of power to either engine or
aux bus loads.
>Running an automotive style high pressure fuel injection
>system/computer/ignition places significantly higher demands on the
>electrical system, making battery-only operation a poor choice if
>there's any way to avoid it. Real world in-flight tests using a
>good-condition PC680 has demonstrated a max endurance of about 40
>minutes of battery-only operation, even if the rest of the airframe
>loads are minimal. Reading the PC680 capacity specs aligns with
>real-world experience. Not acceptable, when it's so easy to work around it.
If you plan an aux alternator then without
a doubt, due diligence is required to
confirm that it meets DESIGN GOALS for
continued flight sans main alternator.
Unlike the legacy considerations for sizing
the main alternator, one doesn't have to
consider excess capacity for battery
recharging. You can PLAN for 100%
utilization of alternator capacity for
flight ops. Ideally, the aux alternator
would happily carry those loads. I.e.
battery energy is not a factor for
planning.
I'm not seeing any value for plan-b ops
sans AUX BUS. Energy to Comm, primary nav, panel
lights and transponder should not
represent a terrible concern for
aux alternator energy management.
Been out of pocket for a few days. Sons
came to trim some trees around the house
and yours truly spent a few hours dragging
bush and trucking loads to the dump.
Feel like I went too many rounds in
bar-room brawl!
Bob . . .
////
(o o)
===========o00o=(_)=o00o========
< Go ahead, make my day . . . >
< show me where I'm wrong. >
================================
In the interest of creative evolution
of the-best-we-know-how-to-do based
on physics and good practice.
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