Earlier this week, there was a little bit of discussion
about combining an oil line with a ground system in
a canard pusher aircraft. The line was to bring warm
oil forward for use as a cabin heat source. The writer
wondered if the same line could be used as an electrical
conductor to replace a 2AWG ground wire.
I could see how it might be done. The potential hazard
would arise from the single point of contact between
the liquid carrying tubing and the electrical connection
to that tubing. If that joint, and the one adjacent to
it were of impecable integrity, then no electrical arcing
and subsequent damage to the liquid carrying component
could occur due to poor conduction, overheating and
arcing.
I could see a copper strap looped around the tubing at
some appropriate location, soldered to the tubing and
formed into a tab where a wire could be bolted on
to carry electrons off to a destination separate from
the oil.
I suggested that the technique might save 2.5 pounds
in the total weight of the airplane and further that
the builder consider the trade off between the ease
and confidence of a tube and wire installation versus
taking on the task of making sure the dual use
installation was technically sound.
I expected to get a flood of mail about this . . .
I have received a few responses that run in this
general flavor:
>I've been mechanicing for a while and the general formula is to keep the
>electrical and fluid lines seperate. It introduces to many oppurtunities for
>sparking which would creat hot spots that would put holes in the tubing.
Understood. That philosophy operates under the assumption that
sparking and arcing WILL occur. If one designs a system wherein
arcing CANNOT occur, then the system is intrinsically safe.
For example, certain potential electrical energy levels are
ALLOWED inside a fuel tank because we understand the physics
that supports combustion and/or explosions. Saturated vapors
cannot combust due to lack of oxygen, ignition cannot happen
below certain energy densities within an explosive atmosphere,
etc. Automobiles have depended on these simple truths for
over 60 years and we've yet to see the ass-ends of cars
being blown off by their fuel gages.
Bureaucratic posturing and rewriting of the laws of physics
to support TWA 800 soothsayers not withstanding, there are
ways to bring potentially hazardous substances into close
proximity with potentially antagonistic phenomenon with comfort.
It's like defining the weight and ballance envelope for an
airplane, stay inside and your future is bright, venture outside
and risks multiply rapidly.
Rules of thumb, general formulas and other sage advice don't
have to consider anything except the stature of the authors,
their power to promote them, and our willingness to accept them.
The amateur built airplane arena is one of the few places
left were politicians and bureaucrats have yet to take a
strangle-hold on philosophy and technology. In this venue,
no idea is unworthy of consideration under the light and
magnifying glass of physics.
Personally, I'd have no problem fabricating such a system
and flying it with confidence. An amateur builder may want
to solicit the aid of one experienced in the mechanical
skills of putting the parts together. He might even consider
backing off the oil line and using the vacuum line (if
he's unfortunate enough to need one) to do the dual task.
I took on this issue to illustrate the precious value of
the freedom we have to do good science on our airplanes.
I'm sure I came off a bit wild-eyed a few days ago when
I responded to someone's query about an FAA inspector's
request to do a detailed weight and balance document for
an amateur built airplane. It's not that doing such a
document is a BAD idea, but it's not necessary from a
regulatory perspective. Nor does it have much value in
the future operation of the airplane . . . perhaps an
exercise with EDUCATIONAL value but certainly no more.
The requestor may have been genuinely interested in
advancing the builder's understanding of airplanes. No
matter what HIS/HER motivation, should some future up-n-
coming bureaucrat find reference to or even a copy of such
a document in an FAA file, there's an opportunity for
an educational exercise to take root and grow up as
a requirement.
Ben Franklin, at the signing of the Declaration of
Independence allowed as how, "We should all hang
together or most certainly we shall all hang
separately." The future of our craft and right to
practice it is delicately balanced on our ability
to "hang together." Fly comfortably my friends
but be watchful for the noses of camels circling
our collective tent . . .
Bob . . .
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