Tip: Degaussing the Rollbar

[Guy Prevost]

I've been asked this question several times, so I thought I would write up a quick FAQ.

I really wanted to put my Vertical Card Compass in the upper left corner of my RV-8A. However, when I placed my compass there, it swung approximately 60?. Some experimentation with a handheld hiking compass indicated that my rollbar had a definite magnetic field. I tried a few tricks to locate the exact location of the problem. As it turns out it doesn't matter--we're going to degauss the whole rollbar here.

Here's a couple of photos of my rollbar prior to degaussing.

First photo--The plane is facing ~North.


Here's what the compass does when near the roll bar. Notice the needle always points to the rollbar:




All right. I definitely need to degauss the roll bar if I want to put a compass here. The little Television screen degaussing tools you can buy, or even a soldering gun don't have nearly the oomph to degauss something as massive as this weldment. We need power here. After a bit of research, I discovered the growler.



Basically a growler is a big coil of wire with a stack of iron plates going through it. The alternating current (regular household current) going through the coil creates a magnetic field. The iron plates are there to direct the magnetic field. The plates end in a V-shaped notch which is where the field is strongest. That?s the part you move back and forth over the item you want to demagnetize. In the old days, you would have put the armature from your car or tractor there to test it. The magnetic field generated by the growler would induce a current in the armature which was usually connected to a light bulb. If the light bulb came on?the armature was good. If not, there was a broken wire in the armature somewhere. You could then use the growler and a probe to find the break.

Here?s how it works for demagnetization. If your rollbar (or other item) is magnetized, it means that many of the iron particles in it are aligned with each other. This usually happens during welding. The metal gets melted, allowing the particles to align with the earth?s magnetic field. Once the weld pool cools, they?re pretty much stuck that way. Over time they can move, but it?s a slow process. The magnetic field the growler produces switches directions 60 times a second, because that?s how often your household current switches directions. By passing this field from the growler over a magnetized piece of steel, you make the orientation of the magnetic particles random again. They're trying to align with a field that's changing directions so fast it's basically random.

I padded the jaws of the growler with duct tape to protect the powder coating of the rollbar. I removed all sensitive items from my panel, especially the compass.

Take your watch off, and if you have any medical devices that might be affected by a strong magnetic field--Don't do this!

When the growler is powered up, it will emit an ominous 60Hz hum. It draws 4 amps, so it's turning ~480 Watts of household current into a magnetic field (OK, some of that is going to heat...). Pass the growler slowly and continuously over the entire structure you want to demagnetize. Be patient--it takes time. Eventually the growler will become too warm to comfortably handle. Just shut if off, let it cool, and come back after a bit. I want to emphasize that this takes a bit of time. The more powerful your growler is, the less time it will take. It took me about 3- 10 minute sessions to do my rollbar.

Here's a photo of the handheld compass after the process:


It's now pointing North instead of toward the rollbar. My vertical card compass, located in the left 2.25" hole, went from pointing 60? off of heading to pointing within 2? of the proper heading. I can take that out with its built in adjustment.

Cheers,
Guy

[Bevan]

Can this be done on a finished RV with the windscreen in place or do you need access to all the surfaces all around the roll bar?

Bevan

[Steve Ashby]

Guy,
How can we get access to on of these growlers? Is it possible to rent one? I would hate to have to spend big bucks to purchase one for just one use.

[Guy Prevost]

Originally Posted by Bevan

Can this be done on a finished RV with the windscreen in place or do you need access to all the surfaces all around the roll bar?

Bevan

You should be able to do it with the windscreen in place. There is no need to circle the roll bar. You would want to pad the windscreen carefully to protect your it. The growler has a tendency to jump around a bit during the process, it really wants to latch on to the roll bar--it just can't decide which jaw it wants to latch with since the magnetic field is going back and forth. Also, you would need to remove your compass an any other instruments you think might not like a strong magnetic field.

Originally Posted by Steve Ashby

Guy,
How can we get access to on of these growlers? Is it possible to rent one? I would hate to have to spend big bucks to purchase one for just one use.

I found mine on CraigsList. They're also on Ebay fairly often. Search for "armature tester" and "growler". Ideally it won't have the test feature built in--they get bulky if they do.

If you know a mechanic that's been in the business for a while (30 years?), you might be able to borrow one.

If you know an RV builder that has one and you don't mind paying shipping for a 10 pound object, it may be worthwhile to PM or email that person....

Guy

[Top_prop]

I love it when folks here take the time to post stuff like this!

[Norman CYYJ]

If you want to de-magntize your roll bar before installing the windscreen use your soldering gun. Remove the soldering tip and replace it with some number 14 to number 10 copper wire. Wrap a couple of turns of the wire around the roll bar connect the wire to the soldering gun then starting at one side of the roll bar turn on the gun and move it to the opposite side. Then turn the gun off. Please use bare wire, the melting plastic and fumes might do you in.

[Andrew M]

I had a welding project on a Maule to do, and degaussing was an issue. Our FAA guy said to wrap the assembly with a shop light cord and plug it in, turn it on, and walk away. Leave it over night and your done. The higher the watt light the better.

Andrew
-4 started and sold
11AC Chief flying
dreaming again

[Terry Lutz]

There is another way to degauss the RV-8 rollbar, which was done for my airplane by Lou Farhood, a machinist from Michigan. Here is his description of how to do it: Any machine shop with a surface grinder will have a magnetic table or another name is a magnetic chuck. This table has a three way switch. ON, OFF, and DE-MAG.
You just lay the canopy bow on the table and turn the switch to DE-MAG and a green light will start to blink. With each blink the current will cycle on and off. with each cycle the green light will get dimmer and dimmer. You can hear the current start and stop as the light blinks. When the green light dims to the point that it does not blink with the current cycle any longer, it is finished.

Assuming you can find a machine shop with this equipment, it should take very little time to do it.

Terry Lutz
RV-8 N8TL Flying

[G-force]

Any machine shop that does Magna Flux inspection will usualy also have a "demag" machine. At my work, its basicaly a large coil about 20" in diameter. After we mag cranks or rods or cams, we pass them through the demag machine to degause them.

[RScott]

Your degausser should work better if as you use it you pull it away from the roll bar. With AC current, as the current alternates, the "domains" (what they call the magnetic fields of the atoms) alternate, first parallel one way, then parallel the other. As the AC current diminishes as you pull your degausser away, some domains don't flip back and they get left scrambled. In theory if you just shut it off, many of them will be parallel, which creates the magnetism. The explanation may not be quite technically correct, but that's how I remember it from 24 years ago.

One other thing: Iron and steel, depending on the alloy, will become magnetized over time if left sitting in the same orientation in relation to the earth's magnetic field. As a rough guide, mild steel won't hold magnetism, but a lot of steel used in tools will. So if the roll bar is an alloy that will hold magnetism well and if you park the plane in the same place all the time with the roll bar oriented north-south, it could gradually become magnetized.