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Old 10-01-2011, 12:14 PM   #1
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Location: Greenville, TX
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Default compass deviation

During the last year, my soon to be installed compass has been pointing at the airframe from across the room. You might say the frame has more than a “little” residual magnetism from the welding process.

I spent about an hour yesterday degaussing the airframe with a bulk video tape eraser. It’s working pretty well. I have the compass deviation down from 180 degrees out of whack to within 10 or 15 degrees. I’ll make a few more passes with the buzz box and the compass should settle down to an acceptable error. When I get it to the airport I can use the compensation screws to fine tune the deviation and create a correction card.

I noticed in the “show us your panel” postings most builders mount the compass top center with a few installing it off to one side. There are also a few panels with no compass visible so I wonder where it is hiding. I've read only a few negative posts on compass location, so I’m assuming once you get the magnetism out just about any location will work.

I suppose I could pull the MD-80 stunt and mount it behind the right seat with a mirror on the glare shield to view it. Yep, that’s really how it is mounted in that aircraft. What was the FAA thinking when they bought off on that???

I’m planning on the usual location of glare shield top center… boring, but visible, and far enough from steel tubes. It matches the décor of my steam gauge layout, too.

Just to pass on a magnet in the cockpit story… My dear departed Mother-in-law asked me one year what I’d like for a Christmas gift. I said I’d like a red color, double-D cell, MagLite® flashlight for my aviation kit bag. I wrote down the info to make sure she knew the exact model, size and color. On Christmas morning I opened a package to discover a red plastic Ever-Ready flashlight with a “big-ole-honkin” magnet riveted to the side. Mom asked if it was the right one. I told her it was perfect. (It’s still riding around in my pick-up tool box.) Thanks again, Mom... it’s perfect for the truck.

John Pitkin
Greenville, TX
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Old 10-01-2011, 03:42 PM   #2
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Default Re: compass deviation

As one of the panels that shows no compass the magnometer is in the wing tip and reads on the Dynon unit. Works well and keeps it as far away from the steel frame as possible. Calibration is done in the software once everything is finished.
Dorsal ~~^~~
Series 7 - Tri-Gear
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Old 10-01-2011, 07:33 PM   #3
Dave S
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Location: St Paul, MN
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Default Re: compass deviation

Hi John,

I am one of the guys with my compass on the left side of the glare shield.

I found my airframe possessed with magnetic demons.

Originally the compass of choice was a vertical card compass. - Got a used one that wasn't all that old and I could not adjust it to get it to work where I wanted it - in the middle top of the windscreen.

I spent some time moving it around the cockpit and found the locations of minimal magnetic interference were right and left of the glare shield, however, the vertical card compass was so large (in other words - in my way) and difficult to mount that I decided to go find a smaller compass.

John Mc Bean suggested the SIRS compass which is smaller unit, easy to mount with a swivel base and probably the best compensation system on the market for a traditional on the glare shield mount.

I'll admit it - I am not all that good a exorcising magnetic demons and I never tried; you will probably be better off ofr doing so, but the SIRS compass rolled in with such as small error that it almos irrelevant - compass card has no correections ober 4 or 5 degrees. I am happy with it.

The reason for the location on the side of the glare shield is entirely a strategy to avoid the magnetic demons as much as possible.


Dave S
KF7 Trigear
912ULS warp
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