nuckollsr(at)cox.net
Guest
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 Posted: Wed Sep 06, 2006 6:04 am Post subject: Old Cessna filters/capacitors/avionics |
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At 11:49 PM 9/5/2006 -0400, you wrote:
| Quote: | My 1968 Cardinal has a whiskey flask sized "filter" in the middle of the
alternator output wire, a metal capacitator on each magneto, a 2 inch
square metal box filter that ties into the Zeftronics voltage regulator
and a large like a half a roll of quarters paper shell capacitator between
the turn coordinator wires.
I work under the supervision of a licensed ap mechanic.
1. what are the various filters for? I have been told that the flask
size in the middle of the alternator line is only needed if using adf or
loran. Because to filter out the ac would require a much bigger filter.
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That is correct. When that airplane rolled off the assembly
line, DO-160 testing was but a gleam in the eye of some
seasoned veteran of the Systems Integration Wars. Interestingly
enough, while aviation readily thumbed its nose at "automotive"
technologies, in may respects we lagged far behind the car guys
in understanding and implementation of rudimentary science. In
this picture . . .
http://www.aeroelectric.com/Pictures/Misc/Screen_Room_circa_1935.jpg
. . . we see a screen-room used in the early '30s to identify
and mitigate noise issues on cars. In 1968 our primary source
of test data was from the test pilot's squawk sheets. Pilots who
wore headphones could be counted on to hear and complain about
noises that were never heard through the cabin speaker.
Under this environment, many things were done to mitigate
complaints without benefit of knowing the exact nature of
a noise source, propagation mode and victim vulnerabilities.
A byproduct of this ignorance is manifested in what you see
on your airplane. Alternator filters and shielding of alternator
wiring were added to mitigate perceived noises in ADF receivers.
Turns out that shielding had no beneficial effects but the
experiments were not differential with respect to filter capacitors
and shielding. They went in together, the pilot smiled, they
stayed on the drawings for decades hence.
| Quote: |
2. Would it be wise to remove them one by one and see what noises are
introduced by their removal? I could always put them back on. Will I
hurt anything if I run the aircraft and avionics sans all filters?
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Excellent idea! Do the science. There are no risks to
any hardware for having removed noise filters.
| Quote: |
3. Would it be a good idea (noise wise) to run my avionics bus directly
from the battery (feed and ground).?
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No. Doing what you suggest might have been considered and even
tried by someone in some factory in 1965. It's unlikely that
an action of that type would have made a test pilot any
happier; we don't see airplane production drawings that
call for that particular noise mitigation technique.
| Quote: |
4. Am I correct in assuming these filters only are effective while the
engine is running?
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Yes . . . for alternator and magneto filters.
Suggest you review the chapter in the 'Connection on noise
and then mount an orderly exploratory activity to deduce
effectiveness (and ultimate necessity) for any of the filters
you have on board. Remove one at a time and survey the
performance of your intercom and radios. Certainly listening
with headphones as opposed to cabin speaker. If you don't
hear any deleterious effects for having removed a filter,
you may rationally consider removing it permanently.
Your evaluation in 2006 is just as valid (if not more) than
the opinion of the good fellow who flew that airplane for
the first time in 1968.
Please get back to us and share your findings. Also, if you
DO identify a noise issue that begs attention, let us work
with you to try some things with a goal of increasing
the working knowledge of all the List members.
Bob . . .
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