Hypersizer V5.7.x can not read a FEM with discrete stiffeners (Shell elements representing stiffeners). The standard analysis methodology is to create stiffened panel components on the wing skin surface using smeared orthotropic properties. The user defines initial parameters for the skin and stiffener lay-up and Hypersizer determines the optimal solution for the panel parameters supplied.
When exporting stiffened panel wing properties to a FEM, Hypersizer creates MAT2 orthotropic smeared properties representing both the stiffener and skin.
Issue 1: Internal Loads
Wing FEM standard practice is to have the nodes at the OML. A typical wing box depth is in the range of 7 to 10 inches. The CG of the stiffeners and skin combined (smeared property) is approximately 0.50 inches inboard of the skin OML. Nastran determines shell internal loads based on the GRID locations. If Hypersizer smeared properties are placed at the OML, when running a Nastran solution the internal loads in the skin and stringers were proven to be correct.
Issue 2: Stiffness
Per the discussion above the wing bending stiffness EI recovered from a Hypersizer smeared property model is accurate.
The torsional stiffness requires the skin properties to be at the OML. The ideal would be to have the GRIDS at the OML and the skin offset by half the thickness. The co-cured stiffener flanges are assumed to be included in the torsional stiffness. In this regard the HyperSizer formulation is also accurate.
