EXPERIMENTAL AND THEORETICAL INVESTIGATIONS TO DEVELOP A MODEL OF ROTOR AERODYNAMICS ADAPTED TO STEEP DESCENTS

In cooperation with the French Flight Test Center (CEV), ONERA is carrying on an experimental and theoretical investigation in order to develop a model of rotor aerodynamics for steep descents. The activity, which is still ongoing, has already provided promising results in the general understanding of helicopter dynamics during a steep descent. This paper describes the work performed up to now. A review of literature combined to theoretical investigations helped define a model of induced velocities in descent and a criterion of vortex-ring state prediction. In parallel, two flight test programs have been organized with CEV, in order to gather a sufficient database. First, the experimental Dauphin 6075 has been used to analyze the flight dynamics within the vortex-ring state. The second part of the flight program was dedicated to main rotor flow measurements. A specific device was designed to measure the flow airspeed below the rotor. First analyses of vertical flights are available in this paper. The model developed during the theoretical investigations was compared to these flights. The comparisons highlight the promising characteristics of this model. The phenomena of Vortex-Ring State and power settling could be demonstrated.