A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: David Pike
Date: 2019 Jan 25, 01:21 -0800
More thoughts on the Plath Gyro Sextant
There are many attempts in books and on the ‘net’ to provide a ‘simple’ explanation of why a spinning child’s top doesn’t fall over, and a fair few attempt to explain why the spin axis has to move as if around the surface of a cone. However, few if any attempt to explain why the axis eventually becomes vertical, if it ever does. Oh for a child’s top to experiment with. The subtle difference between a child’s top and the Plath gyro is that whereas the CG of a child’s top is necessarily well above the pivot point, the CG of the Plath gyro examined by the Admiralty Research Laboratory was found to be ever so slightly below its pivot point. The relevant dimensions were:
Weight = 0.40lb. CG = 0.14” below pivot point. M of I about spin axis = 0.43 pounds inch squared. M of I about horizontal axis through spindle point = 0.42 pounds inch squared.
So, does this simply mean that the gravity couple will cause the spin axis to gyrate around the cone surface in the opposite direction to a child’s top, or is there a bit more to it than this so far as levelling the gyro is concerned?
A further thought; could Plath have used an aircraft style gimballed gyro horizon, which would have allowed electrical drive? DaveP