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Ohms law makes the answer pretty simple, Tim; As a simple example, a 12 watt
load at 12 volts requires 1 amp and the commensurate wire size - a 12 watt
load at 24 volts draws 0.5 amp and requires one-half the wire size. The
weight per unit length is approximately proportional to the wire size (there
are some small differences in this as the insulation weight doesn't exactly
half). A double voltage system can theoretically cut your total aircraft
wire weight in half, though realistically due to wiring components such as
lugs, connectors and etc, 40% reduction is probably more like it. Some of
the big poles in the tent can really save weight such as the starter
cables - unfortunately, you're still going to have to use the standard size
RF cables so no weight savings are available there.
IMHO, there just isn't enough total wire weight in a Lancair (at least the
two seaters) to put up with the aggravation of going 24 volt. Too much
airplane stuff is, right or wrong, designed for 12 volts and is a lot easier
to find when replacement parts are needed from the factory or the local FBO.
Double (or higher) voltage systems really only make sense when applied to
fairly large aircraft - there you're talking about miles and miles and maybe
tons of wire - where a 40% weight saving is worth the effort.
Some of you might be interested in the fact that the B-1 (the program where
I misspent most of my 30 years with Rockwell) is primarily an AC powered
aircraft (it had only two small 15 amp-hour batteries on board) and it uses
a double voltage system (230/400 volts AC, 3 phase) instead of the more
common 115/208 volt system. The higher voltage system on an airplane of this
size saved a huge amount of weight (and if we'd have had to use twice the
wire size, we probably couldn't have gotten it all in the airplane!).
Dan Schaefer
LML website: http://www.olsusa.com/Users/Mkaye/maillist.html
Builders' Bookstore: http://www.buildersbooks.com/lancair
Please send your photos and drawings to marvkaye@olsusa.com.
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