From: petri@venus.iucf.indiana.edu (HOWARD PETRI)
Subject: Earth Induction Compass Pt II
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Reply-To: petri@venus.iucf.indiana.edu
Organization: INDIANA U. CYCLOTRON FACILITY, BLOOMINGTON, IN
Distribution:  world
Date: 27 JUL 92 23:46:54    

     In the Log of Air Navigation there is a picture of the components of
the 1920's Pioneer induction compass. There is a sealed unit with a short 
boom and small propeller attached. This must have been a wind generator that
provided enough power to excite the coils in the compass. There are two
indicators in the picture that are attached to the unit. One looks
like it indicates heading, the other maybe an ammeter to indicate
generator operation. I now believe that the coil doesn't spin, they
are simply energized by the generator. An EMF is produced which will
eventually be used to turn an indicator. 

Further info follows:

In:
Aircraft Instrument Systems
IAP, Inc.
1985
pp 42-43

Begin Quote:
D. Slaved Gyro Compasses
     The floating magnet compass has so many inherent problems that a
totally different concept of direction measurement has been developed.
The principle of the Earth Induction compass has made possible many of
the record breaking long distance flights of the 1920's and has been
updated and improved upon as techniques of electronics and gyros have 
advanced.

    (Here they have drawings of something they call a flux valve.
I can't reproduce it here. Imagine a donut shaped iron core with
three slits completely through--120 deg apart. Each section has an iron
arm which attaches to the others in the center of the donut.)

     A coil wound around the center of the flux valve is excited by 400
Hz AC whose field periodically saturates the arms of the frame.

    (This coil is shown wound the three inner legs.)

In Fig 68 (sorry again) the airplane at A is headed North and the lines
of flux from the earth's field are intercepted by the frame. All pass
through leg A and part of them pass through leg B and part through leg C.
When the airplane turns to a heading of west, the flux lines will change
in the three legs of the frame. Pickup coils are wound around each leg of
the frame, and during part of the excitation cycle when the frame is not
saturated, lines of flux from the earth cut across the coil and induce
a voltage. During that part of the cycle when the frame is saturated, the
lines of flux are rejected. This alternate acceptance and rejection of the
earth's flux generates a voltage in the three windings that differ with
each heading.
End Quote.

     The above is a more modern version of the Earth Inductor compass.
They undoubtedly went to 400 Hz because these high frequency systems
are smaller than a 60 Hz system would be. They eventually use these
voltages to drive a slaved gyro and what they call an Autosyn system.
What the technical details of a 1927 system are I don't know. It
might have run at a different frequency and certainly didn't have
a sophisticated system for indicating the actual heading. The basic
concept of course is the same. We really need a old time engineer
/technician who worked on these systems to give us more detailed info. 
I'm sorry, I don't seem to have anything else on the subject.

Howard Petri
Pegasus N599JH  (EJ) 

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