There are magnetic compasses designed for use in magnetic bodies such
as trucks or steel-hulled ships. One installs the compass, and then
makes a number of maneuvers so the compass can calibrate itself.
I have one in my car (came with the car), and it indicates 8 directions
of the compass.
If the candidate compass does not come with calibration instructions,
look for better choices.
You don't read well, Joe. He said "so the compass can calibrate
itself". That's a flux gate. Compensating magnets are required on
"magnetic" compasses when used in or near large metal/magnetic objects
like ships, airplanes, and vehicles.. There is a big difference
between "flux gate" and "magnetic" compasses.
One thing you may experience that compensating magnets don't help is
magnetic deviation from heavy intermittent current draws - like
running the heater blower or WSwipers if the high current wires run
too close to the compass. Sometimes when starting an airplane the
compass will "spin" as well due to the magnetic feild around the
Back when guys were running "boots" on their CB radios, some
experienced their compass spinning when they keyed the mic too.
As Joe saw they can calibrate out the effect of the vehicle, although
I suspect there is a limit.
I had a compass in a Camry years ago, My trip to work took me across a
1/2 mile bridge. As I crossed this bridge the compass would make a 45*
swing and then come back to normal.
firstname.lastname@example.org (Edward A. Falk) fired this volley in
As it will any time there's a substantial current flowing in a line close
to the compass.
Magnetic compasses are tricky bastids, but they can be got a lot closer
than one-two-three-deviation adjustment procedures... just depends on how
much time (and fuel or trolly-hauling) you want to do.
Flux-gate compasses can also be fooled by mains busses. They don't
measure attraction, but fields... but they do measure ANY field, and even
can be made in error by 'bending' of clean fields by large masses of
On Tuesday, February 24, 2015 at 8:41:31 PM UTC-5, Lloyd E. Sponenburgh wro
Retired from it now since last year, but I did many compass calibrations on
airliners. We had a compass rose on the field where I worked, and ships u
sed to be scheduled through for compass cals. Had to be done when you repl
aced the whiskey compass if the calibration card had been lost. If I rememb
er it correctly, there were certain electrical loads that had to be drawing
current before the cal. If it was a cal. for a gyro compass, always must
check to see if somebody had put a steel screw on the cover of the flux val
ve at the wingtips. That was a common problem. You could align one gyro com
pass to the other system, but the compass system had to be cal. checked on
a compass rose within 30 days. Gyro compasses a thing of the past now, with
the Inertial ref. systems. Pretty much useless information nowadays.
Again, not to belabor the point, but to make it clear. All you're doing
with those _nearby_ lumps of iron is making the attraction load of more
distant 'lumps' of iron equal in all directions. Then the only thing
left is the attraction of the magnetic pole.
It's not magic, but might be non-intuitive to someone who's never
actually calibrated a compass.
On Thu, 26 Feb 2015 17:34:29 -0600, "Lloyd E. Sponenburgh"
And on an aluminum plane you don't have magnetized steel structure
like in a rag and tube, which may require degaussing - not a trivial
endeavour. A magnetized fuselage is more likely with a mig welded
fuselage than with a gas welded one. (particularly if welded with a DC
email@example.com fired this volley in
Eh... magnetization has little to do with it. This method was perfected
in the day of riveted iron vessels, not welded.
The simple matter is that the compass pointer itself is THE magnet,
attracted to whatever iron masses may be near enough.
Make those _apparent_ masses equal in all directions, and the pointer
would be equally attracted in all directions, leaving only the earth's
magnetic pole to control its pointing.
And... where have you found a _powered_ aluminum aircraft that did not
have sizable masses of ferrous metal inside and around the engine(s)
(aluminum blocks/heads nothwithstanding)? And, at least in single-engine
planes, that those masses weren't pretty close (relatively) to the
dashboard, console, or windshield where the compass was mounted?
On Thu, 26 Feb 2015 19:19:07 -0600, "Lloyd E. Sponenburgh"
And hot rivets are magnetized too. The proximity of iron, magnetized
or not, will have some effect on the compass, but magnetized iron or
steel has a whole lot more effect.
And the magnetized roll cage on an RV8 causes the compass to be off by
20 to 60 degrees. Degaussing the hoop knocks the compass back to
within 2 dgrees - easily compensated for with the compensating
Google " degauss roll cage "
The attraction of the compass needle to unmagnetized ferrous metals is
insignifficant compared to the attraction/repulsion of a magnetized
ferrous metalfuselage (or even a small portion of the fuselage, as in
the RV8 windsheild hoop, or roll cage.
Recrational Aircraft Association of Canada (RAA)
firstname.lastname@example.org fired this volley in
Ok, so, talking to a pilot, now -- you're claiming that the engine mass
has no significant effect on an aircraft's magnetic compass. That only
steel tube welded fuse frames are to fault. Right?
And, you're saying that aluminum fuselages with iron-bearing engines
don't suffer from that, because they're not 'magnetic' in character?
And, that any of this has anything whatsoever to do with using a magnetic
compass in a semi truck-trailer combo?
Are you sure you don't work for the US FAA? <G>
(I know of a Super Viking owner who might argue your points, unless
spruce and mahogany have become magnetic during my inattention)
On Thu, 26 Feb 2015 21:42:34 -0600, "Lloyd E. Sponenburgh"
That is not what I said. I said the the attraction of the iron in the
nose mounted engine - if not magnetized, is insignificant compared to
the effect of magnetized metal in the fuselage.
I didn't say that.
You remind me of the story of two brothers.Bill and Bob. Bill was a
scoundrel. He had been in just about any kind of trouble you could
imagine, leaving bedlam in his wake - ex wives and illigitamate
offspring and a criminal record as long as your arm.
His brother Bob went to the pastor who was in charge of the funeral
service and offered him $1000 extra if he told everyone his brother
was a saint.
Small-town pastors are not terribly well paid - he figured he could
really use the money.
At the funeral service, the pastor said "As you all know, Bill was a
scoundrel, a theif, and a womanizer,There really isn't a whole lot of
good I can say about him, but compared to his brother Bob, Bill was a
We don't know what kind of Semi he's talking about installing it in.
Aluminum cab, steel cab, or mostly fiberglass. A set of non-sheilded
stereo speakers in the dash of an aluminm cabbed Pete are going to
cause more havoc than a 400 Hp Paccar up front, compass-wise
Emeraude and Jodel too. The compass can be compensated to within a
degree or two with no problem Starter current can spin the compass on
Can't compensate for a magnetized loop. Particularly if the compass is
mounted to one side or other of the panel.
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