Assessment of VRS risk during steep approaches

In 2005, the French Department for Aeronautical Programs and Co-operation (DPAC) tasked ONERA to carry out an assessment of VRS occurrence risk during helicopter steep descents. The realization of this work was possible thanks to a number of previous activities led by the “Office” between 2000 and 2003. These studies supported by experimental investigations on the Dauphin 6075 of CEV (French flight test center), helped to develop two aerodynamic models, one for VRS domain prediction and the other for induced velocities in descent flight. A fast time simulation environment was also developed around Eurocopter’s flight mechanics code HOST (Helicopter Overall Simulation Tool). The assessment of VRS risk was performed, on a generic helicopter (class 2 to 4 tons), using the fast time simulation environment. Simulations were realized with a set of various helicopter and atmospheric parameters (mass, T°, altitude). These parameters were identified, through a sensitivity study, as being the primary parameters impacting on VRS domain limits. A first VRS risk assessment chart was established. This paper describes the work performed and shows up how such a chart can be generated for any type of helicopter. The use of the chart is also illustrated through several examples.