Helicopter behaviour in the vortex-ring conditions

Abstract

A region of roughness, associated with the airflow conditions of the vortex-ring state, occurs when a helicopter is operating in vertical or near vertical descents in the range of about 500 and 1,500 ft/min. The turbulent circulating air and the rapid changes in local velocities in this flow pattern can cause serious helicopter handling difficulties. This could cause concern in a slow steep approach, particularly under instrument-flying conditions. This report describes flight experience in the vortex-ring conditions with the Sikorsky R-4B, R-6 and S-51, Bell 47 and Bristol 171 helicopters. It is shown that the helicopter behaviour varies from a mild wallowing on the best type to a complete loss of control on the worst case. These effects are due to the turbulent-flow changes in the vortex ring and the loss of control is thought to be caused by large changes in pitching moments on the fuselage with small displacements of the helicopter relative to the unusual flow pattern.