1066. Mass Properties Measurements of Large Objects

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Paper Title: 
1066. Mass Properties Measurements of Large Objects
Year: 
1975
Paper Number: 
1066
Abstract: 

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 considerations. The method of moment-of-inertia testing selected was the bifilar pendulum. The mechanics of the bifilar method seemedmost 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 interestingbut 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 ofair damping. During the tests many problems were encountered and solutions determined. When testing was completed, not only did wehave a fair determination of the moments of inertia, but also a good idea of what to avoid on future tests.

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Pages: 
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