305. Predicting the Accuracy of Mass Property Calculations


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W H Ahl: 305. Predicting the Accuracy of Mass Property Calculations. 1962.



This paper was presented at the Twenty-First National Conference of the Society of Aeronautical Weight Engineers at Seattle, Washington, May 14 – 17, 1962. This paper contains the recommendation that weight engineers should predict, well in advance of construction, by rational and practical means, the probable mass property calculation errors, manufacturing variations, and measuring tolerances of aircraft, missiles, spacecraft and any other products where mass property tolerances are of concern. Such predictions will serve to show more clearly the weight engineer what his mass property control problems will be during the design phase. They will provide more precise tolerance data than is otherwise available, thus facilitating optimum design or indicating discrepancies in tolerance requirements. These predictions also will serve to explain and justify the varying mass property values resulting from successive measurements of a series of similar products, as well as point out an erroneous measurement or faulty calculation.
Equations, using the root sum square (RSS) principle, are presented, providing a solution to probable three-sigma variations (assuming a normal distribution) in mass property calculations and manufacturing. Simplified approaches are discussed along with the relative effects of the various parameters contributing to the mass properties. The benefits of subdivision of assemblies to increased accuracy are pointed out. Types and magnitudes of errors or variations are mentioned.
The contents of this paper are directed to those weight engineers who are concerned with knowing before production, as well as after, what probably variations of mass property values will exist, and who will use this knowledge for a more tightly controlled design and improved overall performance for the product.


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