Summary

The Vortex Ring State is a particularly hazardous operating regime that a rotorcraft can enter into during the critical descent and landing phases of flight.

It involves the collapse of the wake of one or more of the vehicle's rotors from its usual regular, ordered state into a highly disordered, re-circulating flow. The pilot usually experiences the onset of the Vortex Ring State as a relatively sudden reduction in the lifting ability of the rotor that can be very difficult to counteract. Although the Vortex Ring State was first characterised in the earliest days of helicopter flight, the physical mechanisms that govern the phenomenon are complex, and the link between the design of the aircraft and its behaviour in the Vortex Ring State is still only partially understood.

During this talk, our current understanding of the Vortex Ring State is examined, and the story is told of how data from laboratory and numerical experiments were combined to change our understanding of the basic mechanisms at the heart of the phenomenon.

As an illustration of how mathematical abstraction can lend deep insights into practical engineering problems, we will see how a theory emerged that is now used to help aircraft designers to understand the susceptibility of their creations to this potentially dangerous aerodynamic phenomenon. The specific case of the V-22 Osprey tilt-rotor will be covered in detail, and some generalisations to the next generation of rotorcraft that are on the drawing boards will be presented.