315. Precision Static Center of Gravity Measurement

Paper

Abstract

This paper was presented at the Twenty-first Annual National Conference of the Society of Aeronautical Weight Engineers at Seattle, Washington, May 14-17, 1962. The weighing and location of the Center of Gravity of rocket motors, reentry vehicles, and various other bodies involved in aero space programs has received ever increasing attention during the past few years. During this period we have seen considerable improvement in the instrumentation involved in these measurements.
Ancillary equipment used in conjunction with the basic measuring equipment has become a large part of some weighing and CG locating systems. Recording and computing equipment together with associated analog to digital converters in some cases may become a quite sizeable dollar portion of the overall system.
This paper points out the importance of the placement configuration of load transducers in the reaction measurement method of CG determination and suggests two other methods. These are a single beam system and a repositioning system, both of which are inherently more accurate than any of the reaction measurement methods. Three methods of reaction measurement, the single beam system and the repositioning system are investigated and discussed. A comparison is made of three reaction measurement configurations. The single beam system is primarily for small bodies; therefore a direct comparison between it and the other methods is not made. The repositioning system is not directly compared with the reaction methods because with its basic simplicity no detailed analysis is required to show its accuracy advantage.
It may be concluded that, whenever possible, a system which repositions the test body so that its CG is placed in the plane of the support pivot axis, is always superior to a reaction measurement system. In some instances the size of the body or required method of handling may preclude the use of such a system. If a repositioning system cannot be used, a reaction measurement system which disposes the nominal CG over, or very near to, one reaction point should be used. Such an arrangement should always yield better results than a conventional configuration which disposes the nominal CG midway between widely spaced reaction points.