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. The analysis allows the study of the effects of transient and/or nonuniform axial and tangential flows, blade dissimilarities and harmonic pitch control inputs.