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You should be able to flatten this case--we can flatten lofted features, but you should create this in Sheetmetal using the Lofted Flange command. Just create 2 arcs separated by the required distance, then loft between them. Pre-ST2 you had to ensure the arc lenghts were the same. If you are using Solid Edge ST2, there is a lot more flexability in how you define the two end cross sections.