Calculation vs Measurement
The MOI of simple objects can be calculated fairly accurately using well-known formulas. To calculate the MOI for more complicated objects, you would need to model the object as a collection of simple shapes, calculate the individual MOI’s for each, and add the MOI values to obtain an aggregate value. However, when you consider the amount of uncertainty in dimension and uniformity of density, you begin to realize that the calculation approach is likely to result in significant errors. It is more practical and far more accurate to measure MOI directly. Direct measurement takes less time than doing the calculations, and it removes all uncertainties that come from variations in shape, dimension, and density.
Measuring MOI directly has these advantages:
Greater Accuracy – Typical errors in calculated MOI can range to over 30% due to simplifying the part shape or making assumptions about average density. Do you anticipate any variations in manufacturing? Does your object have wire bundles? How sure are you about dimensions? Remember, dimension has a square effect on MOI. The typical accuracy you can obtain by direct measurement is better than 1%. Space Electronics offers instruments that can measure MOI to better than 0.1%.
Cost Savings – Measurements can generally be made in a small fraction of the time required for MOI calculations. Cost savings in engineering time alone can quickly pay for the instrument.
Quality Assurance – Military and industrial specifications frequently set limits on MOI where these parameters are critical to the performance of a flight vehicle. The specified MOI needs to be verified to ensure proper flight characteristics are achieved. Direct measurement is the only way to be sure of the actual mass properties.
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