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A high order rotorcraft mathematical model is developed and validated against the XV-15 and a Large Civil Tiltrotor (LCTR) concept. Separate modeling of each rotorcraft component in a multi-body like formulation allows for structural flexibility to be included, which is important when modeling large aircraft where structural modes affect the flight dynamics frequency ranges of interest. Details of the formulation of the mathematical model are given, including derivations of structural, aerodynamic, and inertial loads. Flexibility effects are evaluated by looking at the nature of the couplings between rigid-body modes and wing structural modes and vice versa. A proportional-integral feedback on the structural acceleration is used to improve damping and reduce the overall excitation of a structural mode. A model following control architecture is then implemented on full order flexible LCTR models. The impact of structural feedback investigated. A rigid aircraft model has optimistic performance characteristics, and a control system designed for a rigid aircraft could potentially destabilize a flexible one. The various control systems are flown in a fixed-base simulator. Pilot inputs and aircraft performance are recorded and analyzed.