The aim of this project is to gain new basic knowledge on the dynamics and instabilities of helical vortex systems, with relevance to applications involving flows around rotors. These include the wake generated by a helicopter and the flow behind a horizontal-axis wind turbine. In both cases, important fluid mechanical issues exist, related to either safety or efficiency of operation: the helicopter vortex wake is known to undergo a hazardous transition to a so-called Vortex Ring State in situations of steep descent, and the spatial evolution of a wind turbine wake has a direct influence on the performance of a second turbine placed downstream, which is today a common configuration. Despite a large amount of accumulated data, these phenomena are still poorly understood today, preventing major advances in these domains. The HELIX project focuses on the study of simplified generic vortex configurations to gain the missing physical understanding of helical vortex dynamics, which will complement the existing, mostly empirical knowledge. An original approach based on the intense use of theoretical modelling, combined with dedicated experiments and numerical simulations, will be developed in order to identify the main parameters governing the instabilities and transitions of helical vortex systems. The same strategy has already been applied with success by the project partners in the framework of previous European collaborations on aircraft trailing vortices, where significant new results on fundamental mechanisms of transition were obtained. The project also involves collaborations with the helicopter manufacturer EUROCOPTER and the Fluid Mechanics Group at the Technical University of Denmark, leading experts in wind turbine aerodynamics. They will assist the partners of the HELIX project in relating the fundamental results obtained using generic models to the full-scale applications. It is expected that this exchange will lead to new ideas and concepts for identifying and controlling rotor wake behaviour, which could help improve the safety of helicopter flight and the efficiency and lifetime of wind turbines.