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From: mmallory@netcom.com (Mark Mallory)
Subject: Re: Compass adjusting
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Date: Fri, 26 Feb 1999 04:43:47 GMT
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Rod Farlee (rodfarlee@aol.com) wrote:
: There are (rare) reports of the engine crankshaft itself being magnetized.
: (Can this be caused by improperly performing the magnaflux inspection 
: during overhaul?)  The engine mount is also steel.  On older 172s with
: O-300 engines, the generator is at the top rear of the engine, not that
: far from the compass.  It should be easy to detect the source of any 
: problem by hunting around the engine compartment and instrument 
: panel with a compass in hand.
: - Rod Farlee

On my '182 the alternator is mounted at the rear on the engine.  
Switching the (nominal) 2 amp field current on and off produces a 10 degree 
change in the compass reading on certain headings.  Because the field 
current will change with changes in the electrical load and RPM, this 
will result in a *varying* compass error.

I'm not quite sure how to solve this problem.  I don't believe it's 
practical to shield the alternator, since such would interfere with the 
cooling airflow.  I'm wondering if a small "compensating coil" mounted a 
few inches from the compass (connected in series with the alt field and 
producing an *opposing* magnetic field) might possibly work?

