nuckollsr(at)cox.net
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 Posted: Sun Oct 15, 2006 1:22 pm Post subject: short course on bus loading (was Q on Z-13 and E-Bus) |
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At 09:31 AM 10/15/2006 -0500, you wrote:
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One thing I find useful when you get bogged down in M-bus vs E-bus vs
B-bus divisions is to go flying and start turning off everything
electrical. Eventually you realize just how little you really need to
have, unless you're in IMC with ice. For instance, the turn coordinator
is sometimes presumed to be important, but in VFR, it's just not needed at
all, so it doesn't have to be sitting there on the e-bus drawing all that
current.
Also, remember in the event of an off-field emergency landing, you want to
be able to shut off your fuel pump and some other things you've currently
got wired to your battery.
Dave Morris
At 11:27 PM 10/10/2006, you wrote:
>I'm approaching the time where I need to spend some time on this board
>but forgive the somewhat basic nature of the questions.
>
>I'm starting to work on installation of the Z-13 architecture in an RV-7,
>IFR with dual GRT EFIS, and Garmin stack (SL-30/480/327), P800B audio
>panel, and TT DigiFlight II AP.
>
>I've done the load analysis and have come up with the following ( do have
>a full spreadsheet with each phase of flight listed against Normal Ops,
>Main Alt Fail, and Battery only):
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Start out supposing that you're going to put ALL the electro-
whizzies planned for your project into a nice ol' restored
but original architecture C-170. The Main bus is all you got.
Figure out if you have sized the alternator to carry the loads
under various flight conditions and still have 15A or so
headroom for battery recharging.
If you're going to have a plain-vanilla, Z-11 or an SD-8
enhanced Z-13, you figure out what gizmos are useful
for the en route mode of flight, i.e. ENDURANCE MODE.
Add up CONTINUOUS loads on these devices and then
see if you have sized the battery for ALT OUT ENDURANCE
with NO ASSIST for the design goals you've established.
Keep in mind that:
(1) battery capacity is NOT a linear function of load
vs rated capacity. The apparent capacity falls as loads
increase primarily due to internal resistance of the
battery.
(2) the BEST way to assess battery capacity to a
design goal is to CapCheck it under the en route
load determined above.
(3) if you're a Z-11 system, the what's in the battery
is all you have. Leave 25% or more of your real capacity
for approach to landing. Knowing the required real
capacity under Plan-B operations is how you size your
battery selection.
(4) if you're a Z-13 system, then the e-bus can be
loaded to the full capacity of the SD-8 while holding
100% of the battery's stored energy in reserve for
approach to landing.
E-bus means ENDURANCE-bus . . . its design goal
is to prevent an emergency by encouraging well
crafted Plan-B and preventative maintenance
programs for the properly sized battery.
Battery-Bus loads fall in two classes:
(1) things necessary to keep an electrically
dependent engine running. When you have bad
smells in the cockpit, it's useful to be able
to turn EVERYTHING off and not have the engine
quit. Don't forget to consider Battery Bus
loads in the endurance modeling described above.
(2) things useful on the ground during loading,
unloading and storage support. Clocks, utility
lighting, battery maintenance, etc.
Every item not needed for alternator-out endurance
ops for 99% of your planned flight regimes goes
on the Main-Bus. Yes, if you find yourself in bad
weather and the main alternator craps, you do have
an emergency. That's Plan-C . . . get on the ground
and or out of IMC ASAP. Plan-B numbers are irrelevant.
Keep in mind that you can always bring the main bus
back up and run anything in the airplane . . . but
now you have an emergency and Plan-C is in effect.
Rethink your distribution of loads based on this
philosophy and see what it looks like.
Bob . . .
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