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Dennis,
In
simple terms, folks use 28 volt systems because you get twice as much power to
operate electrical goodies at the same system current (Power = Volts x Amps).
Since the current is lower for a given power requirement, you can use smaller
wires and the other components tend to be smaller and lighter in general. As a
practical matter, 28 volt systems are not quite that efficient, but do offer
significant weight savings for high electrical loads.
The
"down" side is that 28 volt components tend to be harder to find, more
expensive, and much more limited in choice/variety of components. Between cars,
boats, and the model world there is an almost endless variety of 12 volt
components to tickle your every fancy. Except for the high end avionics
typically used in business jets and commercial aircraft, avionics and other
electrical components for GA and experimental aircraft are also available in 12
volt versions. When electrical components each drew the power of a small toaster
(and radiated an equivalent amount of heat), GA and experimental airplane
builders went to 28 volts just to cover the electrical power demand--and run the
cooling fans... Today, only the hydraulic pump, landing/taxi lights and pitot
heat draw any significant power--and some of the new LEDs and gas-discharge
lights use less than 30% of their incandescent counterparts. Avionics have truly
gone on a power diet, and some draw less than 10% of the power that equivalent
equipment of 20 years ago used. You can still overload any system, so
I'd strongly recommend you make a list of the electrical components you intend
to put in your new plane, and total their power. Considering that you're
unlikely to ever use all of them at the same time, estimate your maximum and
"steady state" power requirements under various flight conditions (night, IMC,
etc) against your respective battery and alternator capacities. Bob
Knuckols' "Aeroelectic Connection" is a very good reference for this and other
electrical considerations.
Jim
White and I installed 28 volt systems in our two Lancair IV-P's. It was a
decision we made in 1998, and we've "paid" for it several times over. We built
"all electric" airplanes with EFIS panels, significant redundancy and back up.
Given the actual loads, we're well over powered for everything except
electric anti-ice, and probably well short of the power needed for
that--assuming it's ever perfected. If I had it to do over, I'd go
with a 12 volt system-it's one of the very few things I'd do differently if
starting to build an IV-P today. I considered changing, but the mile or so of
wiring in my plane was sized for 28 volt and corresponding current; changing to
12 volts would require replacing much of it...not worth it. The moral of
this: once you start building/installing electrical components, you probably
need to stick with whatever you decide.
Hope
this helps.
Bob
Pastusek
... I am
an electrical neophyte .....anybody want to explain the Pro's or Con's of
these too me ....its decision time ...
Dennis Shy
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