Dynamic Stall on a Fully Equipped Helicopter Model

Three-dimensional dynamic stall was observed on the rotor of a fully equipped sub-scale helicopter model in a wind tunnel. Results of stereoscopic particle image velocimetry measurements on the retreating blade clearly revealed the large-scale dynamic stall vortex at 50% and 60% blade radius. The location of the axes of small-scale shear layer vortices that constitute the dynamic stall vortex were extracted. Based on the spatial distribution of these small-scale vortices, the characteristic largescale structure was found to be unexpectedly compact. Along with an apparent spanwise flexion of the dynamic stall vortex, these results suggested the rotational motion to have a stabilising effect on the formation and convection of this predominant feature. The velocity field information which is now available is also believed to be valuable for validation of recent computational fluid dynamics investigations of three-dimensional dynamic stall