Recently, a resurgence of interest in the coaxial rotor helicopter configuration has been prompted by its potential to achieve higher thrusts and higher forward speeds than has traditionally been possible with conventional single-rotor platforms. Accurate prediction of the performance of such systems is extremely difficult because of the strong aerodynamic interaction between the upper and lower rotors. The Vorticity Transport Model (VTM) is a comprehensive rotorcraft analysis code based on the solution of the time dependent Navier-Stokes equation in vorticity-velo city form. The high resolution of the wake modelling technique used in the VTM makes it particularly well suited to capturing the complex pattern of interacting vortical structures within the wake of coaxial systems. This dissertation demonstrates that the VTM is able to capture accurately the highly interactive aerodynamic environment associated with coaxial rotor systems. The aerodynamic performance and acoustic characteristics of a coaxial rotor are contrasted with those of an equivalent single rotor. The coaxial rotor is shown to consume less induced power than the single rotor and the aerodynamic origin of the differences in the performance are highlighted. Increasing the flapwise stiffness of the coaxial system reduces its induced power consumption further. Additional savings in power can be achieved, particularly at high speeds, if the system is augmented in thrust using an auxiliary device. Aerodynamic interactions between the sub-components of a thrust- compounded helicopter with a rigid coaxial rotor are identified as the sources of acoustic focusing and unsteady loading on the aircraft. These results suggest that state-of- the-art numerical models such as the VTM may have developed to the point where they can lend useful insights into the detailed aerodynamic characteristics of modern, complex helicopter configurations.