/First off, let me say that I always find your posts
/to the RV list to be informative, well thought out
/and they very clearly explain the topic at hand. I
/have to tell you that I have a special 'Bob Nuckolls'
/folder where I keep all of your posts.
Thank you . . I'll try to remain worthy of your
attention and trust.
>>When in doubt as to temperature rise (wire passes though
>> a hot section of the airplane or is buried in a bundle
>> of wires) pick the next larger AWG number for the circuit.
/Shouldn't this be a 'smaller' AWG number (larger diameter)
/instead of a larger AWG number?
You are correct of course. I got wrapped around the semantics
axle there . . I'm trimming it up for submission to Sport
Aviation and caught the error too. It has always irritated
me that both American and British "gages" are reversed . . .
bigger numbers are smaller products . . . . Of course the
guy(s) who set that up are all dead now but I's sure like
to give 'em a piece of my mind!
/In an aluminum airplane, what is the rule of thumb for
/calculating the resistance of the airframe when one
/connects the 'ground' of an electrical device to the
/airframe. I know aluminum is not as good a conductor
/as copper, but the 'effective' diameter of a fuselage
/or a wing is quite large compared to the diameter of
/most of the wiring on an airplane.
A very good question. My experience suggests that using
the airframe as common ground introduces small and
USUALLY insignificant voltage drops for most systems.
Audio systems are generally the only thing sensitive enough
to pick up aluminum airframe ground loops . . ya gotta
INSULATE microphone and headset jacks from airframe where
they mount lest you introduce alternator noise into the
system.
The biggest problem with using airframe as ground is not
the gross ability of all that aluminum to carry current.
I have problems finding places to attach a wire, especially
in monocoque construction. How do you grab sheet metal
in a way that will reliably carry 200 amps. Here's a case
where a doubler for spreading current out on the skin of
a structure is as valid as a doubler for spreading mechanical
loads! I had one builder try to ground his battery to a
longeron behind the seat. During a cold morning and hard
crank, he burned a big hole in longeron and adjacent skin!
In recent years, the ground systems I've developed for
composites have been nicely adapted for use in metal
ships too. I like to put a big brass bolt through the
firewall and use it to mount Fast-On tab ground blocks
for each side. A short jumper braid goes from this bolt
to crankcase. Battery (-) lead is WIRED to the cabin side
of bolt.
In this arrangement, cranking currents do not flow in the
airframe. All panel mounted equipment grounds to firewall
where the bolt penetrates it. Only a few items spread out
on the airplane ground locally . . . landing light, nav
lights, strobe supply, pitot heat. Just about every thing
else goes to the single point on the firewall. Makes for
a VERY quiet system.
Rag and tube airplanes benefit too . . . about once a year
I help some guy de-magnetize parts of his fuselage structure
so that his compass can be swung . . . keeping large current
flows out of steel structures is a good idea. I'm working on
an STC for total replacement of electrical system in a
Tri-Pacer using all of nifty techniques we've developed on
amateur built ships. The single point ground system I've just
described is part of that design.
/But on the other hand, priming the aluminum can insulate
/two adjoining pieces of the airframe, but on the other
/hand, the rivets electrically connect them, but on the
/other hand, some rivets have a coating on them (anodizing?,
/alodine?) that might insulate ...
RIVET is the keyword here. A properly set rivet swells so
tightly into drilled holes that no insulating quality
of any coating is left. I've never seen a case where
riveted pieces of aluminum wouldn't pass electrical bond
tests in spite of primer in holes before rivets were installed.
But even when the airplane as a whole is pretty well bonded
together, there's still the problem of good attachment.
/Bottom line, does one ignore the resistance (voltage drop)
/on the return path when the return path is the airframe or
/is it significant?
It's major significance is loss in the cranking pathways
and noises that must exist in the alternator/battery
charging pathways. The single point ground system eliminates
these concerns in all types of construction. Once the starter,
battery and alternator effects are in control, you can hook
just about everything else to a local ground with no concerns for
ground pathway resistance.
/Any comments on the trick of running a strip of copper foil
/up the windshield for a com antenna? How well does it work?
/Any health issues or is the radiated power not significant?
I've been watching that thread and considering a response. . .
. . . and since you asked . . . .
It's difficult to quantify how well any given antenna arrangement
works without a lot of testing on an antenna test range with lots
of expensive equipment. There are plenty of folk who will attest
to the worth or worthlessness of any given antenna based on
their personal experiences. These anecdotes are never well
quantified. A wet string hung out the window may well be a
perfectly fine antenna for someone who never talks to anyone outside the
pattern of his local airport!
A foil strip on the plexiglass has the potential for being a decent
radiator. It's important to remember that the high current
area of an antenna is where radiation occurs. The first fewinches
of the antenna's length should route from structure straightaway.
Once clear of structure, the far end can be folded back, zig-zaged,
bent off to one side at angle, etc. and have little or no effect on
antenna performance.
An antenna analyzer (see MFJ Corporation ads in QST magazine or rent
mine) should be used to optimize the antenna's length which may or
may not be the popularized 23" value. Surrounding structure and
width of conductor all have the effect of electrically lengthening
an antenna. The REAL optimum value can be as short as 19 inches or
so.
With respect to health issues, there are millions of folk,including
yours truly, who have used VHF and UHF handie talkies for over 20
years where they're looking right at an antenna inches in front of
their face. Both practical experience and watt-density analysis of
this scenario suggest that there are no concerns for putting a comm
antenna on your canopy.
Regards,
Bob . . .
AeroElectric Connection
////
(o o)
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