Today's Message Index:
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1. 07:15 AM - One Noisy Mag (stearman456)
2. 10:05 AM - Re: One Noisy Mag (Robert L. Nuckolls, III)
3. 03:02 PM - Re: One Noisy Mag (stearman456)
4. 10:07 PM - Re: One Noisy Mag (stearman456)
Message 1
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This past week I decided to tackle some radio noise in my '49 Aeronca Champ. Although
I can transmit just fine (Narco Comm 111) there was always some background
ignition noise when receiving (you need to be about only 15 miles away to
hear an ATIS clearly). But when you switch over to just the right mag (Bendix
mags) the noise goes away. The noise is ignition noise, it varies directly
with the rpm, and I believe it's radiated noise as it varies with the radio volume
control.
If you disconnect the P lead from the left mag's filter (Lone Star Aviations Magneto
filters) the noise goes away completely and the radio is perfect. The P
leads were shielded wire, probably 16 gauge, and the shields were grounded to
the mag cases. Checking the mag switch (AAF A-7 type) I found that the shields
were also grounded at the switch. "Aha..." says I. "exactly what Bob said
never to do." So I reached in with a pair of side cutters and cut the connections
to the shields. Rolled her back outside, started her up and... no change
- still a noisy left mag.
Deciding that the P lead itself must be chafed or the shielding was open somewhere
I removed both leads and made up two new ones from 18 gauge shielded wire,
and carefully added a second wire (not just a pigtail) to connect the shielding
to the mag cases on the respective mags. Installed them last night, rolled
her back outside and... no change! Same quiet right mag/noisy left mag. I couldn't
see it being the switch itself but even so I swapped the leads on the
switch and the noise followed the lead, so it's not the switch. The only difference
I can come up with between the mags is the left mag has the impulse coupling
on it (re-read your article about mag switches in the EAA magazine, Bob.
Lots of good info there - thanks) but that should have anything at all to do
with it.
I'm stumped. Should I perhaps use heavier wire for the P leads, or is this an
actual mag problem?
Thanks in advance.
Dan
warbirds@shaw.ca
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=349246#349246
Message 2
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Subject: | Re: One Noisy Mag |
At 09:10 AM 8/12/2011, you wrote:
This past week I decided to tackle some radio
noise in my '49 Aeronca Champ. Although I can
transmit just fine (Narco Comm 111) there was
always some background ignition noise when
receiving (you need to be about only 15 miles
away to hear an ATIS clearly). But when you
switch over to just the right mag (Bendix mags)
the noise goes away. The noise is ignition
noise, it varies directly with the rpm, and I
believe it's radiated noise as it varies with the radio volume control.
good point
If you disconnect the P lead from the left mag's
filter (Lone Star Aviations Magneto filters)
the noise goes away completely and the radio is perfect.
If I read this experiment correctly, the left mag filter is still
wired to the p-connection on the mag itself . . . but the
wire going to the magneto switch is disconnected from the
other end?
The P leads were shielded wire, probably 16
gauge, and the shields were grounded to the mag
cases. Checking the mag switch (AAF A-7 type) I
found that the shields were also grounded at the
switch. "Aha..." says I. "exactly what Bob said never to do."
Not so much a problem for noise. But it provides a second
ground path between crankcase and airframe that parallels
all other grounds . . . there is a potential for burning your
p-lead wire as described in the 'Connection.
So I reached in with a pair of side cutters and
cut the connections to the shields. Rolled her
back outside, started her up and... no change - still a noisy left mag.
So this experiment has the shielded wire grounded
a the magneto (probably to the filter mounting screw)
at one end and to the GRD terminal of the magneto
switch at the other end?
Deciding that the P lead itself must be chafed or
the shielding was open somewhere I removed both
leads and made up two new ones from 18 gauge
shielded wire, and carefully added a second wire
(not just a pigtail) to connect the shielding to
the mag cases on the respective mags.
Okay
Installed them last night, rolled her back
outside and... no change! Same quiet right
mag/noisy left mag. I couldn't see it being the
switch itself but even so I swapped the leads on
the switch and the noise followed the lead, so
it's not the switch. The only difference I can
come up with between the mags is the left mag has
the impulse coupling on it (re-read your article
about mag switches in the EAA magazine,
Bob. Lots of good info there - thanks) but that
should have anything at all to do with it.
Good deduction
I'm stumped. Should I perhaps use heavier wire
for the P leads, or is this an actual mag problem?
No, actual size of the wires is unimportant.
24AWG would 'function'. The general rule of thumb
for operational wires (not instrumentation) under
the cowl is 20AWG or heavier . . . for mechanical
robustness.
What we may be witnessing is the innate perversity
of shielded wires . . . at least in the eyes of
many builders. Shielding is 99.9% effective in
breaking the electro-static coupling mode and about
1% effective in breaking a radiated coupling mode
depending on the frequencies involved.
EXCEPTION: Plug wires with shields connected to
ground at both ends behave more like transmission
lines . . . like your coax from antenna to radio.
In this case, there is considerable attenuation
of the electro-static coupling of a fast-rise high-
voltage spark pulse. Further, they break the
wire's ability to be an efficient antenna. Modern
plug wires will have resistance wire as the center
conductor which has little effect on spark energy
but really whacks the wire's performance as an antenna.
This is the ONE place where shielding is indicated for
both coupling modes . . . if you put an effective
RFI filter on a plug wire, the plug wouldn't fire!
The shields behave more as an 'enclosure' since the
wires are short and the shields grounded a very
low-impedance ground (crankcase).
The filters on both p-leads that break the radiated
coupling mode before the trash gets out onto the p-
leads. So if your noise goes away when the engine is
run without a p-lead connected to the filter . . .
then a reasonable deduction is that the filter is not
doing its job.
The fact that noise goes away with a p-lead
disconnected also says your plug wires are good.
I've never personally seen a filter go bad but
they're a spares item in most inventories so it
must happen now and then. Try swapping the filters
between mags . . . If this makes a difference as
to which mag is noisy, you might want to replace
both. One tired ol' filter going t-u may be a
portent of things to come for the other one as
well.
Good detective work!
Bob . . .
Message 3
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Subject: | Re: One Noisy Mag |
Hi Bob,
Thanks for the help.
If I read this experiment correctly, the left mag filter is still
wired to the p-connection on the mag itself . . . but the
wire going to the magneto switch is disconnected from the
other end?
Yes, the P lead was disconnected from the filter, so when the engine was running
with no P lead to the switch the filter was still connected to the mag itself.
So this experiment has the shielded wire grounded
a the magneto (probably to the filter mounting screw)
at one end and to the GRD terminal of the magneto
switch at the other end?
The P lead wire itself is grounded at the mag switch (the switch itself has a ground
wire to the fuselage frame) but the shielding for the P lead is only grounded
at the mag case to one of the screws.
I'll swap the filters around tonight and let you know how that goes. Really appreciate
the help.
Dan
warbirds@shaw.ca
[/i]
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=349285#349285
Message 4
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Subject: | Re: One Noisy Mag |
Hi Bob,
That was it - I swapped the mag filters and now I have a noisy right hand mag and
a quiet left hand one. Don't know why I didn't think of that, though they're
not all that old. Must have had a dud right out of the box because it's always
been like that and I installed those filters about five years ago, but...
that happens sometimes. Thanks for the help - sure appreciate it.
Dan
warbirds@shaw.ca
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=349300#349300
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