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    Re: Gyro sextant
    From: David Pike
    Date: 2019 Jan 24, 15:21 -0800

    I think Bill was asking for a better explanation of why a spinning, bottom-heavy gyro will erect with its spin axis vertical without using vectors.  I’ve been thinking about this for three years now.  So far I’ve got as far as ‘by magic!’  However, Bill’s post has got me working on it again.  Perhaps success is just around the corner.  I think gyro sextants worked after a fashion, because as I understand it, both the Luftwaffe and the Kriegsmarine used them, and post WW2, Russian copies were made.  There were a sufficient number captured after WW2 for testing on both sides of the Atlantic.  The British Admiralty assessment was “OK, but it’ll never replace the marine sextant”.  See: ADMIRALTY RESEARCH LABORATORY Examination of German Gyro-Sextant SKS – 3D. 1945   http://fer3.com/arc/imgx/arl-report-gyro-sextant-plath.pdf

    For American attempts to produce a gyro sextant see:

    http://www.artefactsconsortium.org/Publications/PDFfiles/Vol5Mil/5.07.Military-Warner,SextantsGr75ppiFFFWEBF%20-%20Copy.pdf  .

    Bill  I’ve got one question and one slight worry about your description in ‘The Sextant Book’.  Looking at the diagram, there doesn’t seem to be a lot of clearance between the spinning gyro and the casing.  Is there anything to stop the spinning gyro coming into contact with the case as a result of not being able to carry the sextant carefully enough, and if it does, what happens next?

    Also you say "When a bubble sextant is subject to an acceleration, all the fluid in the spirit level is affected. As you will see when I describe the gyro unit, when the Kreiselsextant is subject to an acceleration, the only connection between the sextant and the gyro is via its low friction, small area bearing.  Effectively, the gyro is almost detached from the sextant and retains the direction of its axis of spin in space."

    This seems to suggest that the gyro sextant should be more accurate than the bubble, whereas it’s been shown to be about the same. I don’t find it easy to accept that the gyro sextant should be free from acceleration errors.  If the case is subject to longitudinal or lateral accelerations, then the saphire bearing cup is going to move slightly with respect to the steel point.  At the very least, this will cause the gyro to rise slightly in its cup, and this effective pushing of a wedge under the gryro is bound to apply ever so slight a torque about the gyro's CG.  Perhaps one of our applied mathematicians might like like coment on this. DaveP

       
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