734. Symmetric 3 Sigma Weight and Center of Gravity Program


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C E Belknap, A K Johnson, G Nagy, L A Prince: 734. Symmetric 3 Sigma Weight and Center of Gravity Program. 1969.



A computer program has been developed that provides a fast and accurate means of assuring that 99.74 percent of all weights and centers of gravity of a production aerospace vehicle will fall within the limits determined by the program. The program can also be used during the proposal phase to establish a realistic tolerance on the weight and center of gravity estimates and can serve as a useful tool in establishing specification guarantees.
The program determines the tolerance of a weight and cg of an assembly by either or both of two methods: (1) By inputting the component weight and cg and their associated deviations and (2) by calculating the component weight, cg, and deviation by inputting component parameters and their deviations. Fifty geometric shapes or ‘Building Blocks’ and associated equations for component parts have been programmed for computer calculation. With the data transmittal forms shown, the weight engineer can input data directly or select geometric shapes interchangeably, add them together to make an assembly, and add the assemblies to obtain the weight, cg and deviations of a total vehicle.
The printed output of the program lists each component weight and cg with their deviations, each assembly weight, cg and 3 sigma deviations and the total vehicle weight, cg and 3 sigma deviations.
This paper describes a sample computer run with instructions and gives the basic equations formulating the computer program. Changes can be made with little effort and tolerance data can be kept current with little effort. An average run takes from 3 to 4 min. on an IBM 360 Model 40 computer. Component weight, inputted or calculated, can be subtracted.
The program was developed for small aerospace vehicles and can be expanded by adding geometric shapes and associated equations as needed. It is a logical extension to the program described in SAWE Paper No. 663, titled ‘Mass Properties by Computerized Building Blocks’.


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