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On 10/11/2013 02:03 PM, GTH wrote:
> During the build of our project, we made a thorough study of the
> electrical system of the Rotax 914. We discovered that in some
> cases true redundancy is not achieved with the schematic provided
> by Rotax, because of the Rotax voltage regulator behavior. That's
> why we had to resort to dual battery installation.
I have that too. But in the original setup it is as close as you can
get to full redundancy.
> Concerning the fire hazard in a crash, it is admitted that skinny
> hot conductors (less than 5 amp) present relatively low risk.
I'm more concerned about sparks. Remember it is also a spark which
ignites the fuel in the cylinders. Any cable carrying current *will*
spark when ripped apart. And a glass aircraft won't bend but break, so
the risk is very high that cables will get sheered off (at the same
moment the fuel hoses break as well).
> Let's remember that at least one always hot conductor exists
> between the master switch and the main battery contactor, though
> the current is limited by the contactor coil.
That's true. In my ship it is the only "live" wire in the whole
aircraft. But you can eliminate the ability to spark if you use a 10
cent transistor to switch the contactor coil (and a diode to
extinguish the back flow current). So, there is now nothing that can
spark once the master switches are off.
Indeed, this is where I diverted from the original plans: I think that
keeping the stall warner and trim alive by routing them around the
master switch is plain stupid. Then you could keep as well the flaps,
the radio and a dozen other "essential" goodies alive.
My idea is to keep the electrical systems running until established at
"final" (whatever the target of the controlled crash will be) and then
fly the remainder without fuel and electricity. I should be close to
trimmed out for the intended landing speed and I can well do without a
stall warner if necessary. Anyway, I don't want to have any live wires
running through the ship at that moment (not to mention electric fuel
pumps that keep pumping out fuel once the crash landing has been
finished).
Frans
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