nuckolls.bob(at)aeroelect
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 Posted: Tue Mar 03, 2015 6:19 am Post subject: GRT autopilot servo wiring |
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At 21:41 2015-03-02, you wrote:
After hours searching the net, stumbled on your ad for 22awg triple twisted pair shielded tefzel wire.
Is the price $3.50 per 10ft increment, or $3.50/ft in 10ft increments, or is it in 10ft pieces? I need about 50 ft.
Perfect! Except GRT sez 20awg for power and ground in their servo schematic. Wire runs are about 20 & 25 ft.
Can I run extra 18awg outside the shield to augment power and ground?
That was 3.50/10=foot increment or 0.35/ft. I still have
some of that wire and we can talk about it if it proves
useful and necessary to your task.
GRT and others say not to run next to antenna coax because of magnetic coupling to serial I/O wires..
There is no foundation in physics for this assertion. It's
a "ol' mechanics tale" that may indeed have roots in a
EXPERIENCE wherein interference problems manifested due
to (1) poorly installed antenna system and/or (2) failure
of the victimized appliance to possess certain levels
of immunity to such interference. That's what DO-160
is all about. If all systems are designed and installed
with a rudimentary understanding of antagonist/victim
relationships, proximity of system wires adjacent to
coax feeders is not the stuff from which bad dreams
are made.
GRT schematic says max 2 ft max for 20 awg ground, use 12 or 14 for 20 ft max run. Just the ground???
Your skepticism is not without foundation . . .
Can you give me a link to download the installation
manuals for the GRT products?
Seawind sez signals through the hollow left longeron and noisy power items through the right longeron.
I spent 45 years herding electrons in everything from
C-150 to BE-800 and I can tell you that I've never
encountered a situation where, in spite of observing
a few basic rules, a 'noisy' wire was tormenting
'signal wires' due to a failure-to-separate by
the folks who had the duty of getting wires from
one place to the other on the airframe. The challenges
just to find volume and pathways were not trivial.
DO-160 and dozens of similar design standards
relieved us of the burden for identifying and accommodating
noisy/signal wires that shared the same airframe.
I have been avoiding electrical wire in the tunnel beneath the floor which carries 5 1/2" fuel lines, 6 control cables, 6 push/pull cables, 3" heater duct, and one Halon fire suppression tube so far...
Is the space getting full? Are there mechanical
issues for achieving good support of wire bundles
that share the space?
Fuel boost and bilge pump are in there too,but can be routed out sideways. I guess I can grit my aging teeth and run the servo wires through the tunnel, crossing things like the strobe and alternator wires at right angles.
Due to replacing composite stabilizer with larger graphite unit, VOR dipoles are at the wingtips. Their coax shares the bundle with AOA tubes, pitot tube, camera coax, landing light, nav light, Aileron Trim, Flap position, Facet pump, and STROBE wire. Will the shielded strobe wire screw up the VOR or Aileron trim?
No reason for them to . . .
The data from the EIS unit mounted in the fuselage the engine transmits all the engine data forward through one serial wire (no serial in)--If shielded autopilot servo twisted pairs are so twitchy next to com antennae coax, shouldn't this lone wire also be shielded?
EXCELLENT observation and question!!! Modern ground
based vehicles run twisted pair, high speed data
all over the vehicle with virtually zero notice paid
to shielding or separation. I cannot speak to the
talents of the EIS designers but I can assert that
any misgivings they or anyone else might voice
are the products of poor understanding of the
nature of 'noise' and how easy it is to design
for worst case.
Let me see the installation data for the servos
and let's resolve your shielded wire procurement
question first.
Bob . . . [quote][b]
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