Nonlinear modeling of coaxial microhelicopters is studied. All equations are derived using a Lagrangian approach and simplified aerodynamics assumptions so that all parameters have a physical meaning; there is no “black box.” The model is constructed with a view toward control design and real-time simulation. The state-space size is kept minimal, and the numerical conditioning of the resulting differential equations is critical. A set of coordinate transformations is proposed that allows averaging and reducing the number of system equations, effectively suppressing stiffness and tremendously accelerating numerical integration.