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B&C SD-20 and magnetic interference

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zk-vii(at)rvproject.gen.n
Guest

Posted: Mon Mar 24, 2008 12:51 pm Post subject: B&C SD-20 and magnetic interference

Hi,



We have been running for about 20 hours now chasing different problems.  One

outstanding one is magnetic interference from the SD-20 / LR3 combination.

We have a TSO'd vertical card compass (and yes it is a southern hemisphere

model) mounted in the middle of the panel area (center/center).  We are dual

alt / dual bus configuration.



When the field wire is connected to the SD-20 (lower connection) we get up

to 40 degree swing on the compass - it does vary in amount (from 20-40

degrees) based on direction - south seems the worst.  Pulling the field

breaker causes it swings back about 30 degrees, but the regulator / alt are

still have an affect.



B&C are saying some swing is 'normal' - but 40 degrees - I'm struggling!  I

can't calibrate that much out (best effort so far is +- 15 degrees).



I've tried changing the location / orientation of the regulator - no

difference.  I've tried moving another wet compass around the panel /

cockpit - it looks like same problem (although a bit hard to tell).



I've tried running some grounded braid around the firewall forward side of

the field wire - no change.  Same symptoms on ground, air, zero RPM and full

power.  There may be a small movement (< 3 degrees) based on current draw

from SD-20.  With SD-20 field disconnected and X-Feed in - no movement.



Questions:

	a) Have others got this / solved this?

	b) Any suggested methods of screening the SD-20?



Thanks,



Carl

--

Carl Morgan - ZK-VII - RV 7A

http://www.rvproject.gen.nz/

Checked by AVG.

15:03

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nuckolls.bob(at)cox.net
Guest

Posted: Tue Mar 25, 2008 3:17 am Post subject: B&C SD-20 and magnetic interference

At 09:44 AM 3/25/2008 +1300, you wrote:



 	  Quote: 	
		  

<zk-vii(at)rvproject.gen.nz>



Hi,



We have been running for about 20 hours now chasing different problems.  One

outstanding one is magnetic interference from the SD-20 / LR3 combination.

We have a TSO'd vertical card compass (and yes it is a southern hemisphere

model) mounted in the middle of the panel area (center/center).  We are dual

alt / dual bus configuration.



When the field wire is connected to the SD-20 (lower connection) we get up

to 40 degree swing on the compass - it does vary in amount (from 20-40

degrees) based on direction - south seems the worst.  Pulling the field

breaker causes it swings back about 30 degrees, but the regulator / alt are

still have an affect.



B&C are saying some swing is 'normal' - but 40 degrees - I'm struggling!  I

can't calibrate that much out (best effort so far is +- 15 degrees).



I've tried changing the location / orientation of the regulator - no

difference.  I've tried moving another wet compass around the panel /

cockpit - it looks like same problem (although a bit hard to tell).



I've tried running some grounded braid around the firewall forward side of

the field wire - no change.  Same symptoms on ground, air, zero RPM and full

power.  There may be a small movement (< 3 degrees) based on current draw

from SD-20.  With SD-20 field disconnected and X-Feed in - no movement.



Questions:

         a) Have others got this / solved this?

         b) Any suggested methods of screening the SD-20?



Thanks,



Carl


	




    The problem is magnetic, not electrical.

    Had this problem on the Bonanza too. The SD-20

    (as do most alternators) has a considerable

    leakage of magnetic flux from the rotor winding.

    This isn't a problem for most installations

    where the alternator mounts on the front of

    the engine.



    Alternators hung on the rear of the engine

    are much closer to the instrument panel and

    more likely to be an issue with a panel

    mounted compass.



    In the Bonanza, we moved the compass off the

    glareshield to a location on higher on the

    windshield. Adding a magnetic shield around

    the SD-20 is a pretty busy task. The shield

    material would have to be magnetic and cover

    the rear and perhaps the circumference of the

    alternator. This has some issues with cooling,

    wiring egress and labor to implement it.



    The easiest fix is to increase distance between

    the compass and the alternator. Magnetic

    'radiation' is a weak propagator . . . a little

    distance really helps.



    Alternatively, an electronic compass with

    a remote sensor out in a wing is the best

    way to achieve the highest performance

    in a magnetic direction indicator. The

    case can be made that since you have

    3 sources of DC power, reliability of

    the electronic compass is assured. I would

    also hope that you carry a hand-held GPS

    in the flight bag too.



    I'm not sure I'd bother to install a

    whisky compass any more. Likelihood of

    ever needing one these days is about nil.

    Same thing happened to buggy whips. We

    were fortunate not to be plagued by the

    FHA (Federal Horsecart Administration)

    when the transition occurred. Hmmmm . . .

    maybe they didn't really go away . . .

    they just morphed into radio antennas.



    Let's be really clear about this condition.

    This is NOT a downside checkmark on the blackboard

    for the B&C SD-20. ALL alternators have a steel

    shaft that runs through the device parallel

    to the magnetic lines of force generated in

    the field windings. See:



http://www.aeroelectric.com/Pictures/Alternators/Alternator_w_Internal_Fan2.gif



    This mechanical feature insures that EVERY

    alternator will present a strong external

    magnetic field. It's a system integration problem

    that seldom presents but has multiple solutions

    for maintaining flight-system reliability. The

    REAL problem is getting those-who-know-more-

    about-airplanes-than-we-do to join us in

    the 21st century.



    Bob . . .

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zk-vii(at)rvproject.gen.n
Guest

Posted: Tue Mar 25, 2008 12:45 pm Post subject: B&C SD-20 and magnetic interference

Hi Bob,



Removing the 'whiskey compass' could / will result in my aircraft being

grounded.  I have been round the loop with the NZ CAA and the 'need' for a

traditional compass, I already have a GRT EFIS with magnetometer, IFR GPS

and still not enough.  At the moment it is going to be an up cliff paperwork

struggle.



Removing the SD-20 is a 'better' option from their perspective, but with a

full FADEC I'm not comfortable with just dual battery, single alternator -

and having got to this point where the engine is running great (or too well)

removing the SD-20 isn't an option either.  Distance changing is going to be

in the order of 1" difference, unless I mounted it in the baggage

compartment.  What options can be used to reduce the magnetic impact - is it

a luck of the draw with the SD-20 - ie. they all vary abit?  We are only

taking 3-4 amps under normal operations.  I understand some interference

coming out, but what is 'too much'?



WRT:

 	  Quote: 	
		       This is NOT a downside checkmark on the blackboard

     for the B&C SD-20. ALL alternators have a steel

     shaft that runs through the device parallel

     to the magnetic lines of force generated in

     the field windings.


	


My biggest complaint is there is NO indication or warnings about this

anywhere in the instruction / website information - I would have

reconsidered the panel layout with this information, as it is I'm fixed

without major rework and it is affecting my Autopilot backup and one of the

few instruments the regulator is specifically interested in....



How can I reduce this interference at the source - hopefully a small

reduction will be enough to get calibration within spec.



Thanks,



Carl

--

Carl Morgan - ZK-VII - RV 7A

http://www.rvproject.gen.nz/



[quote] --

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mprather(at)spro.net
Guest

Posted: Tue Mar 25, 2008 3:07 pm Post subject: B&C SD-20 and magnetic interference

Are _you_ concerned about the function of the whiskey compass?  If not

(given the other equipment on board), do your best to come up with a

calibration card for it and call it good.



Side question..  I got the impression that your compass reads accurately

when the field is turned off on the SD-20.  Can the SD-20 be used only

when your main alternator is off line?  If so, the only time your compass

will have a problem is if the main alternator fails.  I could live with

that..



Also, what's the wire routing of the output and field of the SD-20 (with

respect to the compass?  Is it possible that the wire routing is

contributing to the B-field generated by the SD-20?  Is the ground

following a different route than the field and output wires?  I'm pretty

sure Bob's assessment of the cause of your headache was correct, but I

wonder if there might be additional factors...



In a different direction, you can get a remote compass from Century

Instruments:



http://www.trade-a-plane.com/dispads/0000769074.pdf



Kinda spendy at $500 for the kit..  And, I don't know if it actually

satisfies the requirement.

Regards,



Matt-



[quote] 

 <zk-vii(at)rvproject.gen.nz>



 Hi Bob,



 Removing the 'whiskey compass' could / will result in my aircraft being

 grounded.  I have been round the loop with the NZ CAA and the 'need' for a

 traditional compass, I already have a GRT EFIS with magnetometer, IFR GPS

 and still not enough.  At the moment it is going to be an up cliff

 paperwork

 struggle.



 Removing the SD-20 is a 'better' option from their perspective, but with a

 full FADEC I'm not comfortable with just dual battery, single alternator -

 and having got to this point where the engine is running great (or too

 well)

 removing the SD-20 isn't an option either.  Distance changing is going to

 be

 in the order of 1" difference, unless I mounted it in the baggage

 compartment.  What options can be used to reduce the magnetic impact - is

 it

 a luck of the draw with the SD-20 - ie. they all vary abit?  We are only

 taking 3-4 amps under normal operations.  I understand some interference

 coming out, but what is 'too much'?



 WRT:

>     This is NOT a downside checkmark on the blackboard

>     for the B&C SD-20. ALL alternators have a steel

>     shaft that runs through the device parallel

>     to the magnetic lines of force generated in

>     the field windings.



 My biggest complaint is there is NO indication or warnings about this

 anywhere in the instruction / website information - I would have

 reconsidered the panel layout with this information, as it is I'm fixed

 without major rework and it is affecting my Autopilot backup and one of

 the

 few instruments the regulator is specifically interested in....



 How can I reduce this interference at the source - hopefully a small

 reduction will be enough to get calibration within spec.



 Thanks,



 Carl

 --

 Carl Morgan - ZK-VII - RV 7A

 http://www.rvproject.gen.nz/



> --

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zk-vii(at)rvproject.gen.n
Guest

Posted: Tue Mar 25, 2008 3:54 pm Post subject: B&C SD-20 and magnetic interference

Hi Matt,



Thanks for the ideas, I'm really not that concerned about the whiskey

compass functionality - if I've lost all my other mag direction indicators -

the engine has stopped, so I don't really care about direction   However

the CAA so far is and the calibration card per regs has to be within 10

degrees .



SD-20 off / disconnected - yes I think the compass reads right - but we are

dual bus (two independent active buses), not endurance / essential bus - so

both alts need to be running / working.



Wire routing - the B lead and field goes aft from the SD-20 to the firewall,

left across the top of the firewall, aft again through the firewall and back

to a mid location for the regulator which is on a tray between the forward

panel ribs (RV 7A Tipup).



Remote compass - it could yet be the option - but I don't like to even think

about the routing of the cables through the tunnel / spar / wing.....



Regards,



Carl

--

Carl Morgan - ZK-VII - RV 7A

http://www.rvproject.gen.nz/



[quote] --

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khorton01(at)rogers.com
Guest

Posted: Tue Mar 25, 2008 5:21 pm Post subject: B&C SD-20 and magnetic interference

There are type certificated aircraft where the whiskey compass is  

only accurate in a very particular electrical system configuration.   

For example, on the Cessna C550 (Citation II business jet), the  

whiskey compass is only useable if both generators are OFF.  This is  

quite acceptable to the FAA, Transport Canada, and I presume the NZ CAA.



Given that you have an EFIS with a remote flux valve that acts as  

your primary source of magnetic heading, you don't need the whiskey  

compass to read accurately most of the time.  I wonder if it might be  

acceptable to put a placard by the whiskey compass that said "SD-20  

Alternator must be OFF to use compass".  Then make a compass  

correction card that is only valid if the SD-20 is OFF.



Kevin Horton

On 25 Mar 2008, at 19:51, Carl Morgan wrote:

[quote] 

 vii(at)rvproject.gen.nz>



 Hi Matt,



 Thanks for the ideas, I'm really not that concerned about the whiskey

 compass functionality - if I've lost all my other mag direction  

 indicators -

 the engine has stopped, so I don't really care about direction    

 However

 the CAA so far is and the calibration card per regs has to be  

 within 10

 degrees .



 SD-20 off / disconnected - yes I think the compass reads right -  

 but we are

 dual bus (two independent active buses), not endurance / essential  

 bus - so

 both alts need to be running / working.



 Wire routing - the B lead and field goes aft from the SD-20 to the  

 firewall,

 left across the top of the firewall, aft again through the firewall  

 and back

 to a mid location for the regulator which is on a tray between the  

 forward

 panel ribs (RV 7A Tipup).



 Remote compass - it could yet be the option - but I don't like to  

 even think

 about the routing of the cables through the tunnel / spar / wing.....



 Regards,



 Carl

 --

 Carl Morgan - ZK-VII - RV 7A

 http://www.rvproject.gen.nz/



> --

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zk-vii(at)rvproject.gen.n
Guest

Posted: Tue Mar 25, 2008 6:06 pm Post subject: B&C SD-20 and magnetic interference

Very interesting - Thanks Kevin....  This could be a short term solution for

the paperwork battle....



Carl



PS: I wonder how close a C550 is to a glider with both generators OFF 



[quote] --

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nuckolls.bob(at)cox.net
Guest

Posted: Wed Mar 26, 2008 3:33 am Post subject: B&C SD-20 and magnetic interference

At 12:51 PM 3/26/2008 +1300, you wrote:



 	  Quote: 	
		  

<zk-vii(at)rvproject.gen.nz>



Hi Matt,



Thanks for the ideas, I'm really not that concerned about the whiskey

compass functionality - if I've lost all my other mag direction indicators -

the engine has stopped, so I don't really care about direction   However

the CAA so far is and the calibration card per regs has to be within 10

degrees .



SD-20 off / disconnected - yes I think the compass reads right - but we are

dual bus (two independent active buses), not endurance / essential bus - so

both alts need to be running / working.


	


   If you have a cross-feed, then just about every individual who knows-

   more-about-airplanes-than-we-do will accept a fall back position for

   dealing with failures that might present other limits as well.

   Assuming that all other "normal" methods for pointing the nose in

   the right direction are no longer available, then shutting the SD-20

   OFF, closing the cross-feed contactor, reducing total loads to that

   of the main alternator and flying with a now acceptable performance

   from the antiquated pointing device is a perfectly valid 'plan-B'.

 	  Quote: 	
		  Wire routing - the B lead and field goes aft from the SD-20 to the firewall,

left across the top of the firewall, aft again through the firewall and back

to a mid location for the regulator which is on a tray between the forward

panel ribs (RV 7A Tipup).


	


   The B-lead comes into the cockpit? Where are your batteries? The

   B-lead can and should be well away from the instrument panel and

   tie to fat-wires through a small ANL or MAXI fuse. I presume that

   you're seeing maximum effect with the engine at idle while doing

   the Mexican hat-dance at the compass rose. These are the conditions

   where current in the b-lead is least . . . perhaps even zero while

   current through the field is maxed out. The regulator is hollering

   down to the engine room for "more power Mr. Scott . . . I need

   MORE power". Of course, at low engine RPM, the regulator isn't

   going to get what it wants to see and field current and associated

   external effects are at their maximums.

 	  Quote: 	
		  Remote compass - it could yet be the option - but I don't like to even think

about the routing of the cables through the tunnel / spar / wing.....


	


   Then consider the special ops protocols for getting the whisky

   compass to live in the real world of a highly capable, failure

   tolerant electrical system. Perhaps some day, those-who-know-

   more-about-airplanes-than-we-do will allow us to relegate the

   once venerable magnetic direction indicator to the function of

   hood ornaments.



   Bob . . .

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nuckolls.bob(at)cox.net
Guest

Posted: Wed Mar 26, 2008 3:52 am Post subject: B&C SD-20 and magnetic interference

. . . a luck of the draw with the SD-20 - ie. they all vary abit?  We

  are only taking 3-4 amps under normal operations.  I understand some

  interference coming out, but what is 'too much'?



   WRT:

       "This is NOT a downside checkmark on the blackboard

       for the B&C SD-20. ALL alternators have a steel

       shaft that runs through the device parallel

       to the magnetic lines of force generated in

       the field windings."



  My biggest complaint is there is NO indication or warnings about

  this anywhere in the instruction / website information - I would have

  reconsidered the panel layout with this information, as it is I'm

  fixed without major rework and it is affecting my Autopilot backup and

  one of the few instruments the regulator is specifically interested in....



  How can I reduce this interference at the source - hopefully a small

  reduction will be enough to get calibration within spec.



     It wouldn't hurt to drop B&C a note suggesting that their

     install literature be appropriately enhanced. But in their

     defense, this is not a common occurrence . . . you COULD

     be having a similar problem by routing some fat wire too

     close to the compass . . . or some other system integration

     problem like installing one of the older DC motor driven

     turn indicators too close and having interference from

     its internal magnets.



     It's unfortunate that after all the work you've put

     into the airplane (and being so VERY close to finished)

     that this problem should become the longest pole in your

     tent. Welcome to the real world of aviation products design

     and qualification! 95% of my services to clients on the

     TC side of the house right now involve chasing $100

     rats from multi-million dollar airplanes . . . with

     those-who-know-more-about-airplanes-than-we-do holding

     the type certification hostage until the flight system

     is pest-free.



     A well considered and easily demonstrated low-risk

     plan-B for making the compass readings fall within specs

     is the no-hardware, no hammers-n-saws approach to

     getting your machine blessed until it's legal to move

     the "hood ornament" out onto the cowl.



     Interestingly enough, I recall sometime in the past

     seeing a compass installation that was on a bracket

     just outside the windscreen. I think it was put there

     to get outside the cage of a day-vfr, rag-n-tube

     machine that was too badly magnetized to allow

     calibration of the compass when installed in it's

     normal position within the cockpit.



     Bob . . .

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