>Sorry if I didn't explain my thinking very well. I had just
>recovered from a hardware failure on the computer. 12Gb of
>lost data and 2 days of nail biting recovery and deadlines to meet!
Ugh! Make me shiver just to think about it. We just bought
a CD Rom writer to back up the dual hard-drive setup on our
business machines so that we have 3 copies of most critical
work product and records. Hope you get recovered okay.
>My instinct is to allow for isolation of everything that stores
>energy, particularly in an aeroplane.
Can't disagree . . . certified ships in US have a master
battery contactor that unhooks EVERYTHING except a few,
carefully architectured battery feeds and we try to keep those
BELOW 5 amps . . .
>I 'phoned RD and they suggested
>a battery isolation contactor with a rating such as to isolate
>the starter if the starter solenoid should jam, which, they told
>me, can lead to rapid overheating of the starter cable. It all
>started with a discussion of cable gauge for the starter cable
>given the long run now suggested.
History has shown us that the major hazards are from battery
meltdown. With modern, lightweigh RG batteries comming on
line, a continuous, heavy current drain to battery depletion
is likely to overheat one or more cells to the point of
penetration of a plastic enclosure. Of course it takes
a VERY healty starter to stand the abuse of cranking until
the battery dies. In a Europa size aircraft, 4AWG cranking
wires will pretty much quiet concerns for overheated wires.
>For most of it's life the contactor is energised. I therefore
>set up the sample I had bought on the bench to check the
>coil current at maximum voltage. After a short while the
>contactor was too hot to touch. Whilst this may not be a
>problem on the firewall, I wasn't happy to mount it under
>the luggage bay where the battery is now fitted.
Hmmmm . . .most continuous duty contactors do indeed run
warm. The contactors we stock and recommend draw 0.92
an amp at 14.4 volts . . . about 13 watts of heat to dump.
The coil resistance calculates out to 15.6 ohms.
Just stuck a thermocouple on one and powered it up. We'll
see how warm it gets.
1 hr later the case temperature is up around 65C and the
current is down to .65 amps. Knowing copper's coefficient
of resistance to be on the order of .004 (0.4% per degree
C) we can calculate that the coil temperature is running
about 130C. Modern magnet wire insulations are good for
175C or thereabouts. With the current down to 0.65
amps, the power disipated by our contactor has fallen
to just over 9 watts.
65C (150F) is indeed too hot to touch but doesn't begin
to put much thermal pressure on composite materials.
I'll bet it can get that hot inside a wing sitting
out on a sunny ramp.
>My view is that the isolation should be as near to the item
>to be isolated as possible.
Sure . . . battery feeds disconnects should indeed be
as close to a battery as practical.
>The starter cables follow the same
>route as the fuel lines. A remote possibility of trouble but
>still a possibility!
Same route? As long as they're not bundled together, there's
no hazard to the fuel line. In tests I've seen the REAL
hazard to fuel lines happens NOT from local heating of
a wire going into self destruct but later after the wire
separates. Then there's a possiblity that the fuel line
becomes a conductor for the heavy current across the
wire's open path.
A real hazard in our airplanes? No . . . starters don't
draw much current and wire sizes to handle these currents
continuously are not hard to come by. As long as we
don't bundle the wires together tightly with fuel
lines, there's no hazard to contemplate.
>Given that I wished to isolate the battery locally, it
>occurred to me to use a parallel contactor of lower rating
>once the engine had started and I checked out a 70A type
>of very high reliability that I had used before. In this way
>the main contactor is only used to start the engine and the 70A
>type used basically to carry the battery charge current in
>normal use .... the latter runs barely warm.
>A simple circuit determines when each should be in use. This is
>tied in with automatic bus isolation of the avionics and Navaid
>wing leveller during starting. The manufacturers of the latter
>particulary request it . . .
Ask them if they can comply with DO-160 power requirements.
If they say "yes", then ask them what devices in the airplane
are know to generate spikes, surges or other manner of
gremlin that exceeds DO-160 stress levels. If they say
"no", suggest that you have it on good authority that complying
with those requirements is no big deal. Further, given
that experimental airplanes are now 20% of the single-
engine fleet in the US (and more than half the modern
fleet), elevating one's "experimental" product to modern
aircraft capabilities is a VERY good thing to be doing. . .
> . . . . although there are differing views as to
>whether it is needed on the former with modern avionics and
engine systems.
. . . the differences are only in the era's in which we
practice the art. Some folk are still firmly rooted in 1960.
>Maybe I have missed something, or I am being over cautious
>but that is the thinking!!
Maybe a little. Obviously, what you describe is functional.
Have you provided for redundant power paths to essential
goodies?
>And yes, Bob Nuckolls book is essential and excellent!
Thank you.
|