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Thread: Saw a magnetic compass spin for an hour.

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    Member Float Pilot's Avatar
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    Default Saw a magnetic compass spin for an hour.

    A local guy bought a beat-up C-172 and we went out for his first lesson. Somebody had replaced the old magnetic compass with one of the $50 models that appears like a compass card. All was well for the first 30 minutes, and then it started to spin. Eventually it was going along at about 30-40 rpm. I tried turning off the lights and radios, but it kept on going. Weird but entertaining.
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    Were you flying in a circle really fast?!?

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    Member Barnstormer's Avatar
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    What's a compass?
    ;-)
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    I would tell the student that your instruction is so good the world really does rotate around you!!!!
    DENNY

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    The offending magic compass. I guess it stopped after she was tied down for awhile....
    I spent a bunch of time looking for hidden wires or a battery box. I figured it was a gag.....
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    I saw a GPS point 180 degrees backward on the north slope a few years ago. I thought it was an issue with the GPS until the next year I was in the same area and had a compass go 180 degrees backwards in the same area. Note to self some odd stuff up there.

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    That compass has a handful of interlocking gears, a gimbal for the gyro, and other necessary goodies. Could possibly be a gear whose teeth have been stripped by an earlier adjustment . . . ?

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    Member Float Pilot's Avatar
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    It is the magnetic compass, so no gyro. Just demonic possession.
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    Most unusual. What dampens it?

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    Member Float Pilot's Avatar
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    My new theory is that some sort of electrical short or ground is generating a magnetic field. After I sat down and thought about it, I remembered that it seemed to be working ( well it was 20 degrees off) until I put the plane into a few stalls. It has an electric switch type stall horn. The first two or tree times there was no horn, then finally it started to buzz away when I made the nose drop pretty hard. After recovery the compass started to slowly wander. Then as time went on it increased in speed until it looks like a 33 rpm record. I tried different light switches and such, but it was the owners first lesson and I did not want to become distracted from the task at hand. After we shut it down and tied up it stopped moving.

    Or maybe there was an inter-dimensional worm-hole somewhere near Anchor Point that day.
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    Supporting Member iofthetaiga's Avatar
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    Quote Originally Posted by Float Pilot View Post
    My new theory is that some sort of electrical short or ground is generating a magnetic field.
    That's along the lines of my first thought. I was going to suggest a shielding (lack thereof) issue somewhere.
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    Electric fields outside the aircraft can also spin a regular wet compass. Alcoa had an electric furnace near the downwind leg of TYS airport
    which could spin a compass from pattern altitude. The rate of spin would blur the instrument face. It was not a good place for times turns to a magnetic heading!

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    I was watching a movie and the compass in the car started to spin when a flying saucer was over the car? I think aliens thought you were in trouble and came to your rescue and before they could beam you up, you pulled out of the dive. LOL

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    Quote Originally Posted by Float Pilot View Post
    It is the magnetic compass, so no gyro. Just demonic possession.

    I'm a little confused (obviously). Where is it mounted, and how thick is it?

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    Quote Originally Posted by Float Pilot View Post
    It is the magnetic compass, so no gyro. Just demonic possession.

    I'm still a little confused (obviously). Where and how is the compass mounted? Windshield or panel? And, more important, how thick is it?

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    Could be harmonic between prop imbalance brought on by power reduction at entry into stall and a overly rigid mount of the vertical card compass?
    Most common vertical card compass, the PAI 700 unit, install instructions warn against a overly rigid mount as a "resonant vibration transmitted directly to the compass case may cause undue magnet and dial card movement." Now I'm only guessing here as to the extent of undue movement implied by the install instructions. The UFO theory could be valid too. Change in soundtrack is a dead give away for UFO interference. That $215, sometimes TSO, unit also warns against panel mounting which may be the direction Grizzly 2 is taking. Install instructions with PAI 700 call for loose mounting.

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    Quote Originally Posted by upstreamV View Post
    Could be harmonic between prop imbalance brought on by power reduction at entry into stall and a overly rigid mount of the vertical card compass?
    Most common vertical card compass, the PAI 700 unit, install instructions warn against a overly rigid mount as a "resonant vibration transmitted directly to the compass case may cause undue magnet and dial card movement." Now I'm only guessing here as to the extent of undue movement implied by the install instructions. The UFO theory could be valid too. Change in soundtrack is a dead give away for UFO interference. That $215, sometimes TSO, unit also warns against panel mounting which may be the direction Grizzly 2 is taking. Install instructions with PAI 700 call for loose mounting.
    I'm sure the PAI 700 has the gyro, gimbel, and gears I mentioned . . .

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    Member Float Pilot's Avatar
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    It is mounted up on the windshield. No wires going to it. Not even for lighting. I posted a photo in here someplace or another. Not my plane..
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    Oops, I missed that. Sure looks like a standard setup capable of dampening stray harmonics. Must be those UFOs after all.

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    Here's another theory:

    Some of you might remember the workings of the old (pre-electronic) automobile speedometer. A small rotating magnet interacted with an aluminum cup. The faster the magnet moved, the faster the cup would rotate. However, the cup was prevented from rotating by a spring. The faster the magnet rotated the more force was induced into the cup and it would move farther, carrying the pointer. Without the spring, the cup would have rotated, pulled along by the magnet.

    Maybe this is a reverse situation. The large aluminum mass (the prop) is interacting with the magnet in the compass, causing the compass to rotate. All it took was a flight maneuver that got the ball rolling.

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