1066. Mass Properties Measurements of Large Objects


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H H Hawks: 1066. Mass Properties Measurements of Large Objects. In: 34th Annual Conference, Seattle, Washington, May 5-7, pp. 28, Society of Allied Weight Engineers, Inc., Seattle, Washington, 1975.



The measuring and testing of mass properties is not new to the weights engineer. Numerous papers have been presented
on methods, procedures, facilities, and analyses of various mass properties tests. However, when confronted with the
task of measuring a heavy, outsized object, it is found that most previous experience is with smaller aerospace
components. Even so, the theory of moment-of-inertia testing remains constant; it should be simply a matter of
applying the same proven test procedure to a larger item, but as will be shown in this paper, there were other
The method of moment-of-inertia testing selected was the bifilar pendulum. The mechanics of the bifilar method seemed
most workable and the measured parameters relatively easy to obtain. The task was to determine the weight, center of
gravity, and three-axis (yaw, pitch, and roll) moment of inertia of two similar Boeing747 engine pod buildups, each
weighing 4950 kilograms (11040 pounds) in a space envelope about 2.5 meters (8 feet) in diameter by 6 meters (20 feet)
in length. Solving the logistics and ground handling problems of such a large and expensive object proved interesting
but uneventful. However, during the first round of tests it became evident that we were at the low end of a learning
curve in applying the bifilar pendulum method techniques.
In determining the moment of inertia via the bifilar pendulum method, four parameters were measured: filar length,
filar radius, system weight, and period of swing. Other considerations in determining the values of these parameters
were the effect of load cells installed within the filars; filar end connections; filar elongation, flexibility, and
energy-absorbing properties; system calibration; alignment of fixture and object center of gravity; and the effect of
air damping.
During the tests many problems were encountered and solutions determined. When testing was completed, not only did we
have a fair determination of the moments of inertia, but also a good idea of what to avoid on future tests.


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