A water tunnel investigation of a small scale rotor operating in the vortex ring state

Abstract

Motivation to expand the understanding of a helicopter rotor descending into the vortex ring state (VRS) stems from the aircraft mishaps that have plagued the helicopter community. The V-22 has become the most recent victim of encounters with VRS. The onset of VRS is associated with the collapse of the helical vortex wake in the plane of the rotor. The resulting wake disturbances develop an irregular and aperiodic flow. Rotor blade interaction with the disturbed vortices causes large variations in the blade spanwise aerodynamic load distribution. Harmonic analysis of the loading indicates that higher harmonic content becomes prevalent in this state. The dynamic flow similarities achieved in a water tunnel are used to explore flow visualization and conduct vibration analysis of a rotor system operating in the VRS. A scaled rotor system was operated in the NPS Aeronautical Engineering Department's water tunnel. Sensors were used to gather thrust and vibration power spectrum data when operating in VRS. Experimental results correlate with full scale flight data and show a significant increase in the vibration levels of the even multiples of the blade passage frequency. The relative strength of these higher harmonics can be used as an indicator of impending VRS encounters.