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chinesespaceman
Joined: 18 May 2011
Posts: 3
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 Posted: Sun Jan 08, 2012 1:06 pm Post subject: Noise in damp weather |
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Hi,
I am hoping someone can give me some pointers in finding and eliminating a problem.
I have a french built wooden aircraft with 0-200, B&C starter and 30Amp alternator. Narco radio and transponder with intercom. The intercom is wired with shielded wiring as per the manufacturers instructions.
This aircraft uses a negative earth, with a switch that is cable actuated.
The B&C Alternator is wired as per the manufacturers diagram, using a large capacitor on the DC Output.
All of the lighting and as many earths as I could find have been wired independently to a "forest of earth" on the inside of the firewall, connected by a brass bolt to another earth forest on the engine side of the firewall. A heavy earth cable then runs from the 5/16" brass bolt to the engine.
There is a skyflash strobe with strobe heads top and bottom of fuz. On most frequencies there is a sharp crack through the radio as ea ch strobe discharges. Never got to the bottom of this problem, despite various attempts.
The few times, including today that I have been caught in rain or very damp weather and have put the nav and/or landing light on I am getting a loud buzz / shriek through the radio into the headsets. This is normally silent in dry weather.
On finals today, the radio went dead, I thought a fuse had blown, so I switched it off. When landed and clear the active I switched it back on and it worked ok.
I would appreciate any ideas on where to start looking for the source of this noise in the wet weather.
There is never any noise from the mags or the alternator.
Thanks
Gavin
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nuckolls.bob(at)aeroelect
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| Posted: Mon Jan 09, 2012 5:18 am Post subject: Noise in damp weather |
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At 03:06 PM 1/8/2012, you wrote:
| Quote: |
<gavintt(at)hotmail.co.uk>
Hi,
I am hoping someone can give me some pointers in finding and
eliminating a problem.
I have a french built wooden aircraft with 0-200, B&C starter and
30Amp alternator. Narco radio and transponder with intercom. The
intercom is wired with shielded wiring as per the manufacturers instructions.
This aircraft uses a negative earth, with a switch that is cable actuated.
The B&C Alternator is wired as per the manufacturers diagram, using
a large capacitor on the DC Output.
All of the lighting and as many earths as I could find have been
wired independently to a "forest of earth" on the inside of the
firewall, connected by a brass bolt to another earth forest on the
engine side of the firewall. A heavy earth cable then runs from the
5/16" brass bolt to the engine.
There is a skyflash strobe with strobe heads top and bottom of fuz.
On most frequencies there is a sharp crack through the radio as ea
ch strobe discharges. Never got to the bottom of this problem,
despite various attempts.
|
This style of noise is usually RADIATED . . . I.e. comes
directly from the strobe tube and is received through the
receiver antenna. This is an easy thing to confirm but a
TOUGH nut to crack. Check out chapter in the 'Connection on
interference analysis. In this case, I'm pretty sure the
noise you hear is affected by volume control on receiver
which means the noise is coming in through the antenna.
Proximity of the antenna to flash tube can be widened.
Sometimes a fine wire, coarsely woven wire mesh shield
over the flash tube grounded to base helps. This is one
of the hardest noise issues to eliminate.
| Quote: | The few times, including today that I have been caught in rain or
very damp weather and have put the nav and/or landing light on I am
getting a loud buzz / shriek through the radio into the headsets.
This is normally silent in dry weather.
On finals today, the radio went dead, I thought a fuse had blown, so
I switched it off. When landed and clear the active I switched it
back on and it worked ok.
I would appreciate any ideas on where to start looking for the
source of this noise in the wet weather.
There is never any noise from the mags or the alternator.
|
Man! Don't know where to even begin. Intermittent and/or
special-conditions events can be very $hard$ to track down.
One of my most satisfying victories involved tracking down
a pushed back pin in a pressure bulkhead connector that
would only manifest failure at very low outside ambient
temperatures and in-flight. I had to develop an instrumentation
system that we could read in the cabin to make measurements
back in the un-pressurized 'hell-hole' while in flight.
I found that I could tape ribbon-cable to the
side of the fuselage and slip it through the door gaskets
at each end to extend test signals to the cabin. Even then,
we made several test flights to 41,000 feet in an airplane
that cost a LOT of money to operate.
I would have to hear the nature of your noise to even begin
to identify its source. You might carry a hand held radio
to see if the noise can be heard it in as well. Do the
diagnostics steps defined in the chapter on noise. Is the
squeal affected by volume control . . . position of switches
power on intercom (does your intercom do a auto-bypass
when powered down so you can still use the radio?).
This can be a challenge but the problems you've observed
DO have explanations in simple physics.
Bob . . .
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Eric M. Jones
Joined: 10 Jan 2006
Posts: 565
Location: Massachusetts
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| Posted: Mon Jan 09, 2012 5:39 am Post subject: Re: Noise in damp weather |
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Chinesespaceman--
You DO have the ability to dry airplane and then wet one section at a time.
Also, remember that WD-40 is really made for Water Displacement. It works extremely well at driving off water and displacing it with a non-conductive oil.
There is not much downside to spraying electrical connections where leakage to ground or other connections might occur.
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_________________
Eric M. Jones
www.PerihelionDesign.com
113 Brentwood Drive
Southbridge, MA 01550
(508) 764-2072
emjones(at)charter.net |
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aerobubba(at)earthlink.ne
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| Posted: Tue Jan 10, 2012 6:41 am Post subject: Noise in damp weather |
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The part about the shriek with an applied load that is corrected by cycling the master sounds a lot like an experience I once had with a balky reverse current relay. On landing roll out, just when no one would ever be looking at the electrical gauges, the bus voltage would go way low and the avionics would start to howl.
Clearly an RCR isn't the problem here, but perhaps transient very low bus voltage is...
Glen Matejcek
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chinesespaceman
Joined: 18 May 2011
Posts: 3
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| Posted: Sat Jan 14, 2012 1:48 pm Post subject: Re: Noise in damp weather |
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Guys, thanks for the input.
Given that the battery was too low today to crank the motor, I guess the voltage was indeed low at the end of the last flight. I have listed below the checks I made and corrections before flying today. (it was -5.5C this morning here in the UK)
My aircraft has a cable operated master switch that serves to break the negative lead from the battery. It has a 30Amp B&C Alternator, that feeds 3 phase to the regulator rectifier:
Flew today and made a few tests and observations.
I haven't as yet checked the battery.
When I arrived I checked all the terminals and connections related to the alternator.
Tightened the screws that secure the DC + and - through the large capacitor. They weren't hanging off, but not as tight as should have been.
There was a very loose connection from 1 of the output phases into the fuse box. I checked and tightened up all of the terminals of the 3 phase output each side of their fuses.
Battery terminals both secure. Earth lead battery to negative master switch secure. Main earth lead from forest of earth on firewall to engine mount secure and clean.
Checked the negative master switch (cable operated) to ensure it was functioning correctly, all appeared good, with no high resistance when closed.
No continuity through the master switch when open.
Hand to hand start the engine as the battery was a bit low, I expect it hasn't been charging on the return leg of it's last journey due to the loose connection from the alternator.
Flew for total of 1:20 without any electrical issues, turning lights on etc did not cause any noise.
When landed and parked up, I pulled the master switch while the engine was running. Got a severe noise blast through the headsets via the intercom. This got worse if I turned for example the nav lights on. If I switched the alternator off by it's switch then all went quiet.
Battery voltage after flight was 13.7 volts. I measured the voltage between battery + and the - after the master switch and that too was 13.7v, so doesn't seem to be a master switch problem.
I am guessing that when I disconnect the master switch, i.e. the primary earth return, then there still exists an earth return through the panel, cable outers etc. However, even if they make their way back to the engine, the negative master switch should prevent it getting back to the battery? So, the current from the alternator is somehow providing electrical power + and - even with the master switch off?
When I got the aircraft all of the earths ran everywhere, lots through the panel, mostly low current stuff like fuel gauges etc. There are still some of these remaining, although 90% + do run to the forest of earth inside the firewall.
With the engine off and the master switch turned off, there is no current flow.
Maybe I don't have anything to worry about?
Your thoughts appreciated
Gavin
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user9253
Joined: 28 Mar 2008
Posts: 1925
Location: Riley TWP Michigan
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| Posted: Sun Jan 15, 2012 1:18 pm Post subject: Re: Noise in damp weather |
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Gavin,
It seems that you have found the problem with the loose alternator connection. The AC ripple voltage is much larger when one of the alternator wires is not connected. It has been my experience that a threaded fastener that is 1/4 turn loose can cause problems.
It is not a good idea to run the engine and alternator with the battery disconnected. The battery helps to stabilize the alternator output.
You asked, | Quote: | | "Maybe I don't have anything to worry about?" |
Not if everything is working to your satisfaction. The vast majority of electrical problems are due to bad connections. Good job fixing it.
Joe
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