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    Re: The Bygrave
    From: Gary LaPook
    Date: 2009 Jul 05, 07:00 -0700

    It appears that Bygrave also came up with the idea for the "slaved gyro"
    which is in common use in almost all planes with more than about six
    seats. It consists of a compass element mounted out by the wing tip,
    away from ferrous metals and electrical influences, connected to the
    directional gyro (DG). In simpler instruments, the DG is free running
    and precesses over time due to friction in the bearings. In addition to
    this actual precession there is "apparent precession" due to the plane
    moving over the surface of the earth. In addition, there is "earth
    transport precession" caused by the earth rotating away from the
    original plane of reference. So in an airplane sitting on the ground
    with a frictionless perfect gyro the heading will appear to drift at a
    rate of 15.04 degrees per hour times the sine of the latitude just like
    a Foucault pendulum since they both maintain their orientations in
    inertial space. So pilots periodically read the compass in the cockpit
    and reset the DG. A compass in a plane can only be read in straight and
    level unaccelerated flight. If the airplane is banked or if it speeds up
    or slows down the compass card is moved away from horizontal and is then
    caused to turn by the vertical component of the earth's magnetic field
    giving erroneous readings. The slaved gyro's remote compass constantly
    resets the DG while in level flight but is disconnected during turns and
    accelerations and the gyroscope maintains its direction during those
    short periods.
    douglas.denny@btopenworld.com wrote:
    > source:
    > The Journal of Navigation‎ - Page 136
    > Institute of Navigation (Great Britain) - 1955
    > article:
    > "Automatic Dead Reckoning and Navigation Instruments for Aircraft" by H.C. Pritchard
    > extract:
    > ==============
    > In fact it was published in 1956 in Vol. IX  of 'The Journal Of the Institute of Navigation'
    > Here is the first page.
    > Douglas Denny.
    > Chichester.  England.
    > >
    > ------------------------------------------------------------------------
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