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One Noisy Mag

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stearman456

Joined: 14 Aug 2010
Posts: 49

Posted: Fri Aug 12, 2011 6:10 am Post subject: One Noisy Mag

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 Aviationâ€™s 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 case on the respective mag.  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.  Just for fun I even tried disconnecting the shielding on the left hand lead and, though it didn't improve, it didn't seem to make it any worse either.  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 shouldn't 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(at)shaw.ca

nuckolls.bob(at)aeroelect
Guest

Posted: Fri Aug 12, 2011 9:05 am Post subject: 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 Aviationâ€™s 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 . . .

stearman456

Joined: 14 Aug 2010
Posts: 49

Posted: Fri Aug 12, 2011 1:58 pm Post 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(at)shaw.ca

[/i]

stearman456

Joined: 14 Aug 2010
Posts: 49

Posted: Fri Aug 12, 2011 9:04 pm Post 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.  Two new filters are on order.  Thanks for the help - sure appreciate it.



Dan

warbirds(at)shaw.ca

nuckolls.bob(at)aeroelect
Guest

Posted: Sat Aug 13, 2011 10:00 am Post subject: One Noisy Mag

At 12:04 AM 8/13/2011, you wrote:

 	  Quote: 	
		  



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.


	


   Pleased to be of service. You were getting close

   by yourself. The detective work was on point.

   Bob . . .

user9253

Joined: 28 Mar 2008
Posts: 1908
Location: Riley TWP Michigan

Posted: Sat Aug 13, 2011 6:44 pm Post subject: Re: One Noisy Mag

 	  Quote: 	
		  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 	


Some magneto switches have a terminal labeled GND to which the shields are attached.  This GND terminal is not an actual airframe ground.  The shields need to be connected to this terminal in order for the switch to short out the P leads.  The mag switch operation should be verified to be sure there is not an always hot mag.

Joe

_________________
Joe Gores

doramsey(at)gmail.com
Guest

Posted: Sun Aug 14, 2011 1:05 pm Post subject: One Noisy Mag

Bob, should all aircraft have these magneto filters?  What is the criteria and what is the recommended choice?



 On Fri, Aug 12, 2011 at 12:59 PM, Robert L. Nuckolls, III <nuckolls.bob(at)aeroelectric.com (nuckolls.bob(at)aeroelectric.com)> wrote:

 [quote]--> AeroElectric-List message posted by: "Robert L. Nuckolls, III" <nuckolls.bob(at)aeroelectric.com (nuckolls.bob(at)aeroelectric.com)>

 

At 09:10 AM 8/12/2011, you wrote:

--> AeroElectric-List message posted by: "stearman456" <warbirds(at)shaw.ca (warbirds(at)shaw.ca)>



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 Aviation’s 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.

http://www.matronics.com/Navigator================================================

  http://forums.m======================   -Matt Dralle, List Admin.

 ================================



 [b]

nuckolls.bob(at)aeroelect
Guest

Posted: Sun Aug 14, 2011 1:32 pm Post subject: One Noisy Mag

At 04:01 PM 8/14/2011, you wrote:

  	  Quote: 	
		  Bob, should all aircraft have these magneto filters?  What is the criteria and what is the recommended choice? 	


   If you got an ignition system noise you don't

   like you may need a filter. It depends on the

   propagation mode of the particular installation.

   But if you have no noises, then there's no

   value in adding a filter.



   Nearly all aircraft flying ADF or Loran will have

   magneto p-lead filters. A portion of other aircraft

   will have them for reasons of demonstrated value.



   Many are flying happily without them.



   



 

        Bob . . .

                    //// 

                   (o o) 

    ===========o00o=(_)=o00o========= 

    < Go ahead, make my day . . .   > 

    < show me where I'm wrong.      > 

    =================================



 [/b]    [quote][b]

raymondj(at)frontiernet.n
Guest

Posted: Sun Aug 14, 2011 3:31 pm Post subject: One Noisy Mag

Bob,

Sorry to say, you won't find any aircraft flying LORAN these days. 

do not archive

Raymond Julian

Kettle River, MN

"And you know that I could have me a million more friends,

and all I'd have to lose is my point of view." - John Prine

On 08/14/2011 04:30 PM, Robert L. Nuckolls, III wrote:

 	  Quote: 	
		   At 04:01 PM 8/14/2011, you wrote:

> Bob, should all aircraft have these magneto filters? What is the

> criteria and what is the recommended choice?

 If you got an ignition system noise you don't

 like you may need a filter. It depends on the

 propagation mode of the particular installation.

 But if you have no noises, then there's no

 value in adding a filter.

 Nearly all aircraft flying ADF or Loran will have

 magneto p-lead filters. A portion of other aircraft

 will have them for reasons of demonstrated value.

 Many are flying happily without them.

 Bob . . .

 ////

 (o o)

 ===========o00o=(_)=o00o=========

 < Go ahead, make my day . . . >

 < show me where I'm wrong. >

 =================================

 *

 *

nuckolls.bob(at)aeroelect
Guest

Posted: Sun Aug 14, 2011 6:37 pm Post subject: One Noisy Mag

At 06:27 PM 8/14/2011, you wrote:

 	  Quote: 	
		  



Bob,



Sorry to say, you won't find any aircraft flying LORAN these days. 


	


   Yeah, too bad. A single receiver set up to receive

   both LORAN and GPS along with the appropriate filter

   software could have provided the ultimate navigation

   reference. GPS works good when there are thunderstorms

   and no sunspots, LORAN works good when GPS is degraded.

   Further, it was harder to jam and a whole lot less expensive

   to build and maintain. But then, folks who know more about

   airplanes than we do must know best.

   Bob . . .

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