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<< Lancair Builders' Mail List >>
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For Ted Noel and others:
My LIV equipment list is virtually the same as yours with dual
alternators, dual batteries, dual busses, SFS equipment and Garmin 430.
I based my electrical design on Bob Nuckoll's work with modifications.
1) I disagree with Bob about the need for an avionics switch, and
included it if for no other reason than to eliminate any bone of
contention with the avionics black box suppliers if I have equipment
problems.
2) The avionics bus is fed by either the A bus or the B bus through an
A-off-B switch.
3) In case of contact failures or in the unlikely chance that both
alternators check out, I have bypass feeds via selector switches
("emergency feed") from batteries to the busses. Either battery can
supply either buss. They are wired for only 20 amps, and require that
the aircraft be powered down to minimal power consumption or the fuse
back by the battery box will let go. The contactors pull about 1 amp
each, so to prolong battery life in the event of dual alternator
failures, I can use the emergency bypass lines.
4) There is one failure that Nuckolls missed. If you leave the master
on and flatten the batteries (I would never do such a thing, would you?)
in the dual battery dual bus system, the ground power contactor connects
with only one battery, and there being no power in the other battery,
the second battery contactor can not be engaged because the battery is
flat. The fix is a diode to feed power to the second battery (though a
power resistor to prevent the inrush current from frying the diode) that
puts some charge into the second battery sufficient to permit the second
battery contactor to be engaged.
I read Mike DeHate's comment about internal alternator failure. It
points out the need for some way to isolate the alternator via a fuse
failure or contactor. I think a large power fuse (which should be
included anyway) would be adequate, but maybe not if the internal short
is intermittent (vibratory). It bears thinking about, and suggests that
alternator contactors are worthwhile, and could be energized by a two
pole switch that also energizes the alternator field. Alternately, if
one alternator has a short, the alternator and associated bus can be
isolated, avionics fed from the other bus, and the flight continued with
the remaining bus while the alternator and associated battery commit
hari kari. However, I would think the alternator fuse would prevent
excessive fireworks if the current levels get too high. Comments
appreciated.
For backup instrumentation, I have assumed that the Sierra Flight
Systems equipment checks out for some reason, and so I have included the
following:
1) Airspeed indicator
2) Altimeter
3) Electric artificial horizon (a bit expensive but cheap compared to a
vacuum system)
4) Turn Coordinator (required for the S-Tec autopilot)
5) Whiskey compass
I was persuaded that the VSI is not necessary for backup. The SFS
system has a VSI displayed, and moreover with the SFS way point selector
and display, one can plan descents merely by placing the velocity vector
on the way point (at a specified height above the ground) and just aim
for the way point. Also, for the Garmin 430 I have a Garmin VOR/LOC/ILS
display which also functions as a backup GPS as well as VOR/LOC/ILS.
Hope this helps. If you have additional questions about the architecture
of the power distribution system, let me know.
Fred Moreno
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LML website: http://www.olsusa.com/Users/Mkaye/maillist.html
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Please send your photos and drawings to marvkaye@olsusa.com.
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