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An equivalent orthotropic membrane finite element (EOMFE) procedure is developed to analyze and design stringer-stiffened skin panels of aircraft structures in the advanced design stage. The formulation preserves the relation between force, temperature and panel strains and enforces compatibility between stringer and skin strains. This element has been implemented in a large-scale aeroelastic design optimization program (ADOP) and verified against models with explicit representation of stringers. An analytical procedure has been developed to calculate the bidirectional critical buckling stresses for stringer-stiffened panels. This approach can consider full interaction of all in-plane loads. However, in order to avoid performing a prohibitive number of buckling analyses in the optimization problem, a buckling criteria is proposed by which the problem of evaluating the buckling stress allowables is decomposed into three basic types; longitudinal, lateral and in-plane shear.

A nonlinear transient coupled flap-lag-torsion aeroelastic response and stability analysis of articulated counterrotating Ultra High Bypass fans is presented. Hinged or elastic blade retention systems with arbitrarily oriented axes are allowed. Additional features include pitch control flexibility, arbitrary pretwist, presweep, and precone of the blade root and spanwise distribution of large blade sweep, droop and pretwist angles. The symbolic derivation of the equations of motion avoids explicit algebraic expansions of the velocity and acceleration vectors. The numerical implementation of the symbolic equations, combined with the state variable form of the equations, makes it easy to change geometric features, add new flexible elements, and extend the analysis to the coupled rotor/fuselage case without additional effort. A 15-th order finite-state two dimensional cascade aerodynamic model has been used.