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  #1  
Old 10-01-2018, 01:36 PM
asw20c asw20c is offline
 
Join Date: Jun 2010
Location: Edgewood, NM
Posts: 155
Default Magnetometer Interference

I, like many on this forum plan to install a full suite of sensors to feed the future G3X system that I plan for my build. I know now that the magnetometer is to go in the first bay outboard of the wing-walk ribs in the left wing. Yup, the same wing as the heated pitot, which I had been thinking would be a real problem for the magnetometer because there is no way to locate the wiring harness 18" or more away as recommended by Garmin, assuming you use the magnetometer install kit from Vans. I dutifully twisted the wiring harness in an effort to mitigate any leaking magnetic field, and had an electrical engineer come have a look at my work. The surprising feedback from him was that despite the heavy current that could potentially be drawn by the heated pitot (if it is cold enough) it probably would not be a problem since once on the field is stable it would not affect the magnetometer. It was the wiring harnesses for the strobe and landing lights that are the real problem because they aren't shielded, and since both will be pulsed, that means an electromagnetic field that will be repeatedly growing and collapsing which will have a far greater effect on the nearby magnetometer. He recommended using mu metal (yeah, I hadn't heard of it either) to wrap all of the wiring harnesses in the vicinity to help mitigate the field near the magnetometer.
So, my first question to those who are now flying and have both heated pitot and wig wag landing lights with their wiring harnesses running near the magnetometer, does it indeed impact the functioning of the magnetometer? And secondly, has anyone used mu metal to shield any of their wiring harnesses? Was it effective?
Thanks,
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Last edited by asw20c : 10-01-2018 at 01:39 PM.
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  #2  
Old 10-01-2018, 01:59 PM
RV-14E's Avatar
RV-14E RV-14E is offline
 
Join Date: Jun 2018
Location: Colorado
Posts: 154
Default

Quote:
Originally Posted by asw20c View Post
I, like many on this forum plan to install a full suite of sensors to feed the future G3X system that I plan for my build. I know now that the magnetometer is to go in the first bay outboard of the wing-walk ribs in the left wing. Yup, the same wing as the heated pitot, which I had been thinking would be a real problem for the magnetometer because there is no way to locate the wiring harness 18" or more away as recommended by Garmin, assuming you use the magnetometer install kit from Vans. I dutifully twisted the wiring harness in an effort to mitigate any leaking magnetic field, and had an electrical engineer come have a look at my work. The surprising feedback from him was that despite the heavy current that could potentially be drawn by the heated pitot (if it is cold enough) it probably would not be a problem since once on the field is stable it would not affect the magnetometer. It was the wiring harnesses for the strobe and landing lights that are the real problem because they aren't shielded, and since both will be pulsed, that means an electromagnetic field that will be repeatedly growing and collapsing which will have a far greater effect on the nearby magnetometer. He recommended using mu metal (yeah, I hadn't heard of it either) to wrap all of the wiring harnesses in the vicinity to help mitigate the field near the magnetometer.
So, my first question to those who are now flying and have both heated pitot and wig wag landing lights with their wiring harnesses running near the magnetometer, does it indeed impact the functioning of the magnetometer? And secondly, has anyone used mu metal to shield any of their wiring harnesses? Was it effective?
Thanks,
Greetings.

I can't comment on that particular magnetometer location as mine is located in the tail.

However, your EE friend is a bit misinformed: When charges (electrons) move, they create an associated magnetic field. A DC current (a humorously redundant term...direct current current) will create a constant magnetic field that could cause the magnetometer to have a constant induced error (hold a compass near a wire carrying DC to see this). An AC current (e.g., sinusoidal, square, etc.) or other changing current (e.g., impulsive like that in a strobe light), given its non-constant nature, can create a varying induced error.

GRT offers this advice, which you may find helpful no matter the manufacturer you use:

"A location can be tested using an app in a smartphone called “Magnetmeter”, or “GPS Status”. These apps display the magnetic field strength, and the direction of the magnetic field. Use the app to measure the earth magnetic field strength when far from any possible sources of magnetic interference. Then place the phone in the proposed location of the magnetometer, and verify the in this location is unchanged. Next, move the flight controls, turn on power to everything in the airplane, and observe that the of the field does not change."
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Last edited by RV-14E : 10-01-2018 at 02:08 PM.
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  #3  
Old 10-01-2018, 02:03 PM
Carl Froehlich's Avatar
Carl Froehlich Carl Froehlich is offline
 
Join Date: Dec 2007
Location: Dogwood Airpark (VA42)
Posts: 1,783
Default

I note on the standard Vanís wiring kit they are very light on shielded wire.

Instead of trying to mitigate the problem, I recommend you strive to eliminate it. For your LED strobes run three #20 conductor shielded wire. Iíd also replace the wire for your magnetometer to shielded cable. If you do wigwag landing lights it would not hurt running shielded wire for them as well.

I mounted both SkyView ADHARS modules in the same wing location. No issues.

The pitot heat will not be an issue either way.

Carl
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  #4  
Old 10-01-2018, 05:37 PM
gen1313 gen1313 is offline
 
Join Date: Sep 2014
Location: Forbes, NSW, Australia
Posts: 41
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My magnetometer is in the left wing with my heated pitot.

I'm fairly sure that I used sheilded wire for the pitot and have no problems. I initially had interference from the wingtip strobes using the standard wiring when the instructions had the nav/strobes lights grounded at the wingtip. Vans advised me to ground these lights right back to the fuse and that fixed the problem. I think the plans have been revised.
170 hrs and no further problems.
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  #5  
Old 10-02-2018, 08:31 AM
KeithB KeithB is offline
 
Join Date: Mar 2014
Location: Granbury, TX
Posts: 184
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I installed standard Vanís harnesses in the wing and fuselage, with fuselage ground per the revised plans. Magnetometer installed in the left wing per plans. No interference from any sources per the Garmin test protocol except pitot heat, with rerouted power and ground wire (twisted).
See this thread:
http://www.vansairforce.com/communit...ter+pitot+heat
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  #6  
Old 10-02-2018, 09:28 PM
wookiemail wookiemail is offline
 
Join Date: Dec 2017
Location: Loveland CO
Posts: 27
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The reason you use shielded wire -- and you are careful not to ground the appliance like a light socket at the socket -- is that the DC current flows down the center conductor, thru the light, then back thru the shield which is grounded somewhere in the fuselage.

That way the unavoidable magnetic fields from the wires carrying currents exactly cancel. A twisted wire pair is almost as good.

The shielded wire must have the shield insulated from everything. If it touches the skin anywhere, the currents no longer flow concentrically and the fields don't cancel. You should check this with an ohmmeter before you tie the supply end of the shield to ground.

Learned this from EE Field Theory 101. When installing Garmin magnetometers in the wingtips of production airplanes, the Nav Lights must have their grounds isolated and be fed with shielded cable as described above or the thing will never pass its checks.

Hate to say, but Mu Metal isn't so good for shielding DC fields.

Bill Hale RV Admirer & paid up subscriber.

Last edited by wookiemail : 10-02-2018 at 09:30 PM. Reason: thought of something else
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  #7  
Old 10-02-2018, 11:25 PM
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jdeas jdeas is offline
 
Join Date: Mar 2006
Location: SoCal
Posts: 608
Default Need a local return

I had the same problem until I isolated the ground return. Make sure every amp of power flowing past the mag returns on a ground line directly against the sending wire pack.

I went from over 10deg change with a full load (heater and landing lights wth skin return) to less than a 1 deg change with the isolated return.

BTW you don't have to go further than past the mag. My gndRet ends in the floor inboard of the wing.
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  #8  
Old 10-03-2018, 12:50 AM
scsmith scsmith is offline
 
Join Date: Jan 2008
Location: Ashland, OR
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You mentioned putting the heated pitot tube in the first rib bay outboard of the wing-walk ribs. It sounds to me that location would be in the propwash.

It is hard to find a place to put a pitot tube where it will read incorrectly, but the one sure-fire place where it will read incorrectly is if it is within the propwash.

Put it half-way out on the wing, or more, and you will be fine.
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  #9  
Old 10-03-2018, 07:14 AM
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Walt Walt is offline
 
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Location: Dallas/Ft Worth, TX
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I have been putting the GMU11 in the right wing tip of most other RV's with good success, this does 2 things: keeps it away from the high current draw of the pitot heat wires and allows the CAN bus to be extended from the roll servo to the GMU with minimum wire length.
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  #10  
Old 10-03-2018, 08:07 AM
Nova RV Nova RV is offline
 
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Location: Leesburg, VA
Posts: 264
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I just mounted my heated pitot yesterday and ran the magnetometer interference test (G3X). No issues and passed the test but I ran a separate shielded cable to the lights (FlyLeds) in the wire bundle with the harness wires and a separate shielded cable for the pitot that was not run in the same bundle as the rest of the wing wire. All grounds were run back to the forest of tabs on the firewall and no local grounding.
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