Control of a helicopter is complex, and includes cross coupling of forces and balancing of forces. While sophisticated and advanced controllers can achieve stable control, good mechanical design can reduce the problems and hence make tuning the control loops easier. In this paper, we argue that a coaxial-rotor design solves a number of problems that make control of a four-rotor helicopter difficult. First, we examine several coaxial helicopters and discuss the concepts behind coaxial propulsion. Then we develop a dynamic model of the Lama coaxial helicopter. Finally, we compare this model to a model of the Dragonflyer four-rotor helicopter to show the difference in dynamics and how they impact the control.