>> I think the only place that wire size is an issue is the high amp
>> portion of the system & then only because the large wire gets
>> stiff & hard to route. This applies to systems with batteries in
>> the fuselage, etc. On the other hand, all new airplanes are going
>> to 28v systems, as far as I can tell.
Most of all "new" airplanes have been 28v for decades, the
type certificiated ones that is. The trend was started
when Cessna observed that buying one kind of thing in one
voltage version only was less expensive than buying the
same thing in two versions. Even the lowly C-150 went to
28v. It had almost nothing to do with weight since all of
the 28v hardware with the exception of wire was the same weight
as the 14v stuff. It was 95% driven by purchasing economics.
Many of my readers building big Glasairs and Lancairs would
LIKE to go 14V but their engine came with EXPENSIVE 28V
alternators and starters installed. EVERY voltage sensitive
part they have to purchase is uniquely "aircraft" which
will never be priced according to consumer driven economics
nor will they experience the product improvements we enjoy
in an unregulated, free market atmosphere . . .
>> . . . . This will surely affect the
>> future availability of avionics for replacements & etc. Remember,
>> the avionics manufacturers can pack more "stuff" on 28v boards than
>> they can 14v boards & can create smaller packages for transmitters.
The differences in electro-goodies between voltages is trivial
to none. The major drivers of volume and weight have to do more
with packaging and human interface aspects. A transmitter, for
example, can be quite tiny except for the need to get heat out
of it. While a 28v transmitter may be a couple of percent
more efficient than its 14v cousin, it's by only a very few
percent . . . given that the output stages have similar
efficiencies and output power, their size doesn't materially
change with voltage. BTW, most small signal stuff in avionics
needs to run a voltages much below 14v . . . this is good and
bad . . . it allows for power conditioning to take all the noises
and perturbations off DC power before it's applied to sensitive
electronics . . . it also drives up parts count and volume
of the system without much effect on its overall efficiency.
>> The only items from the auto industry are the alternator & the
>> battery & perhaps some lights.
. . . . and relays, electronic controlled fuel injection
systems, ignition systems, fuel pumps, blowers and fans,
contactors, and most important LOW COST SEALED GAS RECOMBINANT
LEAD ACID BATTERIES.
. . . . .The voltage regulator is now mostly
>> in the alternator. 28v alternators & batteries cost about twice as
>> much. Even the emergency starting issue is perhaps not such a big
>> deal as batteries can be hooked in series, even while in autos.
>> This would require two autos, however. This is all about a dead
>> heat right now, it seems to me, but don't forget that it is very
>> easy to step down voltage for lights & etc.,
. . . not really. It's the same problem for lights as it is for
radios . . . power conditioning of some type between the
bus and the working parts of the product. More parts count
and less efficiency. Your nav lights are the most energy
consuming system on the airplane . . . while a starter takes
a lot of POWER for 5 seconds (200A X 11V X 5S = 11K watt-seconds)
the nav lights are 6A X 14V X 7200S = 604K watt-seconds for
a two hour flight). Having a 30% efficient starter isn't nearly
as bad as having an 80% efficient lighting system when you
start tallying up the ENERGY budget required to utilize each
system.
Bob . . .
////
(o o)
< Independence Kansas: the >
< Jurassic Park of aviation. >
< Your source for brand new >
< 40 year old airplanes. >
http://www.aeroelectric.com
|