1440. Measuring Moment of Inertia Through Test Part CG When CG Location is Unknown

Paper

Abstract

When measuring the moment of inertia of a projectile or other flight object, it is usually necessary to
first locate the CG of the object, so that the object can be fixtured with its CG coincident with the axis
of oscillation of the torsion pendulum (moment of inertia instrument). Two instruments and two distinct
measurements are usually required, although spherical gas bearing instruments are available which com-
bine the two measurements in one instrument (for example, the Space Electronics Model KGR instruments).
This paper describes an alternate method of measuring moment of inertia through the CG of an object.
With this method only a torsion pendulum is required; CG Location is calculated from the change in
moment of inertia which occurs when the test object is moved a known distance. This method requires:
(1) A high accuracy moment of inertia instrument which can measure test parts whose CG is not
coincident with the measurement axis (this rules out ‘hanging wire’ type instruments).
(2) Means of moving the part from one precisely known location to another precisely known location.
(3) A means of measuring the weight of the part.
CG location is calculated from moment of inertia data taken at two or more positions by making use of the
parallel axis theorem. The moment of inertia of the test object is then decreased by the translation term
d2M to yield the moment of inertia through the CG.
Computer controlled instruments have been manufactured at Space Electronics using this technique. These
instruments acquire the data, perform all calculations, and print out the desired moment of inertia
measurements.
It should be emphasized that this method does not determine CG with sufficient accuracy to be used for
any purpose other than the correction of moment of inertia values. If accurate CG data is desired, then
a spherical bearing instrument or other moment -type CG instrument is recommended.