Visualization of flow inside the Fenestron

Abstract

Flight tests with DLR's EC135-ACT/FHS helicopter were conducted to visualize the flow inside and around the ducted tail rotor (Fenestron) for different combinations of airspeed and side slip. Tufts were fitted on the tail surfaces, inside the diffusor, on the rotor hub and on the stator blades. A second helicopter was flown in close formation with the EC135-ACT/FHS and served as a camera platform to capture the orientation and motion of the tufts. The images were processed with both conventional image processing methods and novel techniques based on deep learning. The results indicate four different flow regimes depending on the combination of airspeed and side slip. An upper/lower asymmetry of the flow inside the duct is observed for positive side slip. Clear reverse flow is only observed inside the duct for low speed and large negative side slip. For other flight conditions the reverse flow is observed indirectly by a flow separation on the inlet lip. The results serve to better understand the complex flow in and around the Fenestron for different flight conditions and can be used for the validation of numerical simulations. Furthermore, the experience gained can be used to apply more complex flow visualization and measurement techniques.