503. Selection of Techniques for Measurement of Moment of Inertia
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Paper
Abstract
The MOI (moment of inertia) of a body is a relatively difficult mass distribution property to measure even though it may be found in a number of different ways. Before any test method is considered the need for the measurement should be clearly established. In some cases the MOI of the object may be determined more accurately and at less expense by means other than testing.
If testing is necessary it is very important that the axis about which the MOI is to be measured be properly defined. A measurement of MOI is generally of most significance when it occurs about an axis through the cg (center of gravity) of the test specimen. The easiest methods of measuring MOI are based on a particular type of oscillating system where the test article is supported in such a manner that it may oscillate about the defined axis of rotation with angular harmonic motion. Angular harmonic motion occurs when the restoring torque is proportional to the angular displacement. The MOI of the test object is deduced from the natural period of the oscillating system and from a system constant. The system constant depends on how the restoring torque is produced.
Consideration of various MOI measuring methods, where angular harmonic motion is utilized, shows that each technique generally has its own unique advantages and disadvantages. However, there is usually one technique most suitable to a given situation. The selection is influenced by such factors as: 1) the characteristics of the particular test specimen, including means of attachment; 2) the axis about which the MOI is required; 3) ground handling considerations; 4) other mass properties to be measured; 5) required accuracy of the measurements; 6) available test facilities and experience. It is not believed that any one technique is always superior to all others.
The weight and balance test engineer cannot select the most appropriate technique for each situation unless he has a working knowledge of a variety of methods. Such knowledge is, however, very difficult to acquire except through practical experience combined with R&D (research and development) testing. It is recommended that the aerospace industries, the customer (federal agencies) and the professional society unite in a common and coordinated effort to develop more efficient mass properties testing methods and to provide detailed design criteria for this kind of test equipment.
