HeliUM 2 is the name given to a rotorcraft flight dynamic simulation developed over the last three decades as a teaching and research tool by faculty and students at the University of Maryland. The paper summarizes the history of the development of the simulation. The general architecture of the simulation is described next. The model is primarily composed of a computational kernel in the form of a system of nonlinear ordinary differential equations (ODEs), although it is possible to include components not in ODE form. Solution algorithms are described for trim, linearized analyses, time marching response, and optimization. Illustrative results are presented for several aircraft configurations.