The flow field around a helicopter is dominated by strong coherent vortices originating from the blade tip as a result of the pressure difference between suction and pressure side and a high radial velocity at the tip (Leishman, 2001; Green and Acosta, 1991). These vortices have a strong impact on the rotorcraft acoustics and performance due to its interactions with the rotor blades and the helicopter fuselage (Conlisk, 2001). Detailed knowledge of the vortex development, in particular of the formation process, and the scaling of characteristic parameters, e.g. vortex size and strength, are necessary for the design and the optimisation of vortex control measures. Due to the complexity and the inherent unsteady character of the vortex dynamics, this can only be achieved by time-resolved measurements of the velocity and density field simultaneously.