Will a magnetic compass work well in a semi truck?

On Thu, 26 Feb 2015 19:19:07 -0600, "Lloyd E. Sponenburgh"


At one point I worked as a mechanic for a comp[any that had contracted to provide primary flight training for the Air force and had been supplied a fleet of about a hundred T-6 aircraft - recep engine, retracting gear and all maneuvers rated. see: http://tinyurl.com/psccosl for a picha
Anyway, I changed the magnetic compass of one of my planes and had to swing the compass and fill out a deviation card.
As no one else seemed to know how to do it I had to read up on the procedure... the Air Force has maintenance books that tell you how to do this, and I became sort of the air base unofficial compass swinger :-)
The point is every one of the airplanes I swung the compass on had some deviation and a different deviation from any other airplane although they were all the same aluminum airframes and wings and had an identical engine installed.
Being made of aluminum and having the engine mounted way out there does not result in zero deviation.
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John B.
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On Friday, February 27, 2015 at 6:05:02 AM UTC-5, John B. Slocomb wrote:

e

That is an accurate representation of every compass swing I did with Boeing , Airbus, Fokker, and MD equipment. The company did finally wise up and sta rt keeping scans of compass cards. Don't know why it took them so long to f igure that out. If one was lost, they sent the scan from maintenance contro l.
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Ig, everyone hit around it, but didn't give you the details.
A GOOD magnetic compass, like an aircraft compass or one made for a real ship-at-sea will have compensation magnets in it... often as many as eight. Sometimes those magnets are movable around the rose although often they're fixed in place, but allowed to be tilted so one or the other pole points to or away from the center as desired. They're also usually pretty weak magnets, intentionally.
They serve not to 'shield' but to make the magnetic influences around the dial more-or-less equal, compensating for masses of metal around. Once the compass needle's attractions to those metal masses have been neutralized by forming attractions equal and opposite to the offending ones, the only thing left is the earth's field to influence it.
So yes; a good magnetic compass could be made to work, and well.
However, flux gate compasses suffer much less from those effects, because they measures fields, not attraction -- and they're electronically adjusted (or software adjusted), so there's not a lot of trial and error. You just accurately point the vehicle to as many as eight points of the rose, telling the system when each position is stable, and it compensates automatically.
Lloyd
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On Sun, 22 Feb 2015 18:19:42 -0600, Ignoramus20074

Yes, magnetic compasses are used on steel ships :-) And given that you really want to know only generally in what direction the truck is heading, in other words you aren't charting a course to, say 1 degree of accuracy, it won't need any adjustment at all. At least the one I have in my pickup didn't :-)
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Is your engine made of iron or aluminum?
The problem is that the needle is attracted to lumps of metal. So, depending on its position, the lump of metal near it (the engine), causes a varying deviation. I would expect that it is difficult to compensate for.
i
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On Sun, 22 Feb 2015 20:55:47 -0600, Ignoramus20074

Cheap car compasses sometimes come with suction cups to stick to the windshield. My experience with them is that they're not too bad. I've checked them against good compasses away from the car, but I haven't used one for 20 years.
They used to cost about $3 and they've kept me from going in the opposite direction a couple of times. d8-)
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It's a solved problem, as of ~1784, when it was an important problem (meaning that the King or Queen cared.) As I recall, Lord Kelvin worked out the math of this problem. The solution in that day was a bunch of nearby iron spheres that compensated for the mass of the ship. Now days, we use math - less bulk, less weight.
Joe Gwinn
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wrote:

We still use compensation to get aircraft compasses close - then we fill out the deviation card so we can navigate accurately. A degree or two off course can add up to missing your feild by quite a distance.
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Ignoramus20074 wrote:

Actually it's pretty easy to counter the presence of large magnetic lumps with small magnets . They still have magnetic compasses on naval vessels , and they HAVE to be accurate . That ship has a whole lot more steel in it than your truck .
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On Sun, 22 Feb 2015 20:55:47 -0600, Ignoramus20074

My engine is cast iron and I can assure you that for what you want it to do a simple magnetic compass will work.
If you planned on crossing the Atlantic where 1 degree error would mean you miss your landfall by 30 miles and hit a reef, then I'd suggest compensation but in your case all you need to know is whether the road that you are on is heading North or East.
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On Mon, 23 Feb 2015 18:27:58 +0700, John B. Slocomb

Unless he's heading across the Sahara - or other trackless desert - which I highly doubt.
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On Tue, 24 Feb 2015 00:19:14 -0500, snipped-for-privacy@snyder.on.ca wrote:

From all my reading I get the idea that people don't shoot off in sundry directions in the Sahara. They stay on the beaten path :-) I got the compass in my truck after I turned right instead of left at an unmarked intersection and didn't discover my mistake until I got to the next town. It is sort of embarrassing to ask someone where Bangkok is and they point back the way you came :-)
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The Sahara once held the most isolated tree on earth, 400 km from any other tree.
In 1973 a truck hit it. http://en.wikipedia.org/wiki/Arbre_du_T%C3%A9n%C3%A9r%C3%A9
-jsw
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On Tue, 24 Feb 2015 06:58:00 -0500, "Jim Wilkins"

I'm an excellent lumberjack. Did I ever tell you my story about the Sahara Forest? <wink,wink,nudge,nudge,knowwhatImean>
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I collect fighter pilots' memoirs. You haven't a prayer of matching them.
(Amazon.com product link shortened) The elephant's side of the story is priceless.
-jsw
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On Tue, 24 Feb 2015 12:45:21 -0500, "Jim Wilkins"

You're supposed to say "What forest? The Sahara is a desert." to which I reply "See what I mean?" My dad and a friend of his wrote a book with WWII fighter and bomber pilot stories, including Dad's drop into France and the German Stalag resort. _To Rule The Sky_ if you're interested. Louis Jaques, Jr. and Bill Leet.

Ever see "The Gods Must Be Crazy"? The sequel was almost as good, too.
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On Tue, 24 Feb 2015 17:39:44 +0700, John B. Slocomb

Have you ever driven in the "trakless desert"? Where the track the last guy made 5 hours ago is now under 5 inches of blown sand???
Trust me, you need a GOOD compass, or better yet a GPS or you may miss the only water stop or fuel cache for 100 milles by several miles and not even know it.

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On Sun, 22 Feb 2015 18:19:42 -0600, Ignoramus20074

Why ask? Point the truck due North according to the compass outside the truck, then mount it or set it in the truck. If it isn't reading the same afterward, you have a problem. (Just don't mount it on a steel bracket. ;)
I doubt there would be. Compasses are installed in most new cars and trucks now. My Tundra rear view mirror has one.
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On Sun, 22 Feb 2015 19:49:35 -0800, Larry Jaques

(a "self calibrating" Flux Gate compass in the Body Control Module which reads out on your rear view mirror)
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On 2/22/2015 6:19 PM, Ignoramus20074 wrote:

If you think of compasses in cars and other things used to take trips - They have calibration screws that move a magnet that counters the field of the car...
I've had magnetic units in oil and electronic ones and they all worked that way.
The electronic ones had to point the car to true north and calibrate and then due east or west and calibrate.
Martin
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