ABSTRACT
When a helicopter is descending, it can easily enter a state called “Vortex-ring State” (VRS). This is a condition of powered flight where the helicopter settles into its own downwash. This unsteady flight condition can be characterized by severe thrust fluctuations, vibrations, rapid rates of descent, sluggishness and vibrations of the controls, and can result in a temporary loss of helicopter control. The flow pattern of helicopter’s main rotor in VRS is shown in Figure 1.
