Mean-axis models of flight dynamics for flexible aircraft are being utilized more frequently in recent dynamics and controls research. The nonlinear equations of motion resulting from the mean-axis formulation are frequently developed with Lagrangian mechanics. In addition, the models are typically simplified using assumptions regarding the effects of the elastic deformation. These assumptions involve the inertia tensor of the vehicle and the character of the unrestrained free vibration modes of the structure. In this paper, the equations of motion are derived from first principles utilizing Newtonian, rather than Lagrangian, mechanics. A threelumped-mass idealization of a rolling flexible aircraft is presented as an example of the mean-axis equations of motion. The example is also used to investigate the effects of common simplifying assumptions. The equations of motion are developed without any such assumptions, and simulation results allow for a comparison of the exact and simplified dynamics.