# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Re: Artificial horizon: gyro vs bubble?
From: David Pike
Date: 2020 Feb 24, 14:07 -0800

Bill Morris you wrote:  You can find an account of a gyro sextant on my web site here: https://sextantbook.com/?s=gyro

Bill

I’ve studied your paper and the Admiralty and USN papers at the end of WW2 re the German WW2 sextant, and agree it ought to be tilt free, so long as handling it doesn’t become so difficult that the gyro crashes against its casing.  However, I cannot agree that it ought to be free from acceleration error as well.  The counter argument is the cg being 3.5mm below the pivot point and the concavity of the bearing.  I’m not sure if the quoted radius of the bearing of 3.7mm is the radius of curvature of its concavity, or just half its physical diameter, but it doesn’t matter.  It must have some radius of concavity.  Therefore, if the whole caboodle sextant plus gyro is accelerated in the horizontal plane, the gyro pivot is going to ride up the concave bearing like a ‘wall of death’ rider, and there’ll be a couple on the gyro causing it to precess like a ‘witches hat’ .  Now how fast this precession is going to be I’ve no idea, and how the tilt is going to coincide with alignment of the light path with the viewing hole on top of the gyro I’ve also got no idea, but I would imagine the effect on the Plath gyro graticule might be similar to the moving up and down of the bubble or pendulous reference graticule of a Smiths-Kelvin-Hughes when observing laterally from an aircraft subject to Dutch roll or longitudinally from an aircraft subject to the phugoid.  Another way of looking at it is to say that if a pendulously suspended spinning gyro eventually erects with its spin axis inline with gravity, then if gravity is interfered with by inertia forces, the gyro will eventually align itself with its spin axis aligned with the direction of the vector sum of gravity and the applied acceleration, which I hope says roughly the same thing.  This is obviously a problem for Bill Lionheart’s Year I students decide.  DaveP

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