>Sir Rabbit,
>
>I had occasion to talk with a professional aviation electrician today about
>the proposed wiring scheme for my GS. Took the opportunity to ask your
>question. This is what he said (illustrated by a visit into the hangar to
>show samples of welding cable he removed from a Super Cub, as well as the
>aviation cable). The main safety difference is that the insulation of
>welding cable, which is black rubbery stuff WILL support combustion and
>gives off very noxious, toxic fumes (think burnig rubber tires and worse)
>while the aviation cable insulation will not not burn.
The insulations are indeed different . . . tefzel and cousins
don't support combustion but they give off equally noxious if
not downright toxic fumes when the wires they contain are overheated.
We're talking red-heat type temperatures here too . . . not impossible
to generate in a battery cable with an RG battery but very difficult.
>
>Another difference is the size of the individual wire strands; much coarser
>in welding cable, very fine in aviation cable.
I'm thinking the "welding" cable he was comparing was not welding
cable but some other material. All the welding cables I've worked
with are designed to be VERY flexible and able to withstand a lot
of abuse (trucks run over them, they get handled a lot). The wire
stranding is VERY fine in order to achieve both flexibility and resistance
to flexure stresses. If the cable he was showing you had fewer,
heavier strands, it WASN'T welding cable . . . got it off a golf
cart maybe?
>This makes a difference
>because the prefered route of electrical conduction is on the surface of
>the conductor and the finer the component strands in a cable, the more the
>surface area to conduct electricity and the lower the resistance.
Total BS. So called "skin effect" comes into play in the megahertz
frequency range and is insignificant below tens of megahertz. It
doesn't happen at DC.
>He couldn't offer any specific comparison in terms of Ohms per foot however.
>Lastly, the connectors are available for the aviation cable but do not fit
>on the #2 welding cable as well.
Perfectly suitable connectors are available for ANY size wire.
>Many resort to soldering which produces a
>low resistance joint at first but which leads to fatigue failure presenting
>as increasing resistance across the solder line until it fails altogether.
Again, not consistent with the physics of the matter.
See <http://www.aeroelectric.com/ArtBigTerm/big_term.html>
I've been soldering things together for over 35 years including
some wiring that carried over 1000 amps . . . I can't even
imagine the failure mode being described here.
>It seems to me that with a well planned electrical system (assuming battery
>on the firewall) you ought only to need 3-4 feet of #2, so the cost
>difference is only on the order of $10. Go with the good stuff.
True . . . ACS gets about $3.75/ft for #2 and we get $1.75 when we
have it in stock (which reminds me, I'm about out and need to chase
down anohter spool). However, for airplanes with the battery in
back, runs are longer. Canard pushers with battery on opposite end
of airplane from engine need two long runs. Here the weight is
a bigger factor. #2 welding cable is heavier and bulkier but it's
very soft and much easier to work with. In fact, if you want to
run #2 wiring throughout the cranking circuit, I would RECOMMEND
welding cable be used for connections to the battery. These connections
tend to be short jumpers between battery and ground or battery and
contactor. #2 aircraft wire is so stiff that it's hard to work with
on short bend radii and stresses it's terminals more. Use #2 aircraft
wire everywhere else but nice, flexible battery connections are very
nice to work with. The afformentioned hazards are about as remote
as your wing falling off so for my money, it's a toss up. If you
can stand the weight difference and wish to use REAL welding cable
(go buy new stuff from a welder's supply), I don't perceive any
great sins happening here.
>Switching circuits here, I need a bit of guidance. Auto engines with auto
>type ignition systems rely on external power so the design for such a plane
>must include a separate, uninterruptible power supply that will function
>right through an alternator failure. I would like to talk to people who
>are flying with automotive style ignition systems to hear how they've dealt
>with this. I am not on the RV net, nor any other systems.
Every airplane has an un-interruptable source of power . . . called a
battery. This pre-supposes that the battery is maintained well. We
tend to treat batteries pretty badly in airplanes . . . run 'em until
they refuse to crank an engine any more. By that time, their usefulness
as a backup power source has been gone for months. Run
periodic capacity checks on a battery, and replace it at 50% of
capacity (or some higher number you choose). If you don't run
capacity checks, then put a new battery in every two years. For
airplanes with total dependence on electrical power, we suggest
two half-sized batteries with alternator-out loads distributed between
the two batteries depending on system needs. In this case, we suggest
replacing one battery every year. This way, you always have one battery
less than 1 year old, no battery is more than 2 years old.
If you can go "total electric panel" there are options for replacing
the vacuum pump with a 20 amp aux alternator. This permits total
redundancy of power generation and storage for very little weight
penalty compared to classic alternator/battery/vacuum pump
installations.
Bob . . .
AeroElectric Connection
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(o o)
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