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A vortex ring, also called a toroidal vortex or Hill's vortex, is a region of rotating fluid moving through the same or different fluid where the flow pattern takes on a doughnut shape. The movement of the fluid is about the poloidal or circular axis of the doughnut, in a twisting vortex motion.

One way a vortex ring may be formed is by pushing a spherical mass of fast moving fluid (A) into a mass of stationary fluid (B). A and B may chemically be the same fluid. As B hits the ball of A it pushes the outer layers of A with it. The inner layers are less affected. The main mass of A forms a 'shadow' of lower pressure behind it, and the layer peeled off by B begins to curve round back into the main mass of A. This inward curving flow initiates the vortex, and splits it into a doughnut shape. Now B flows past both the inner and outer circumferences of the doughnut. The greater outer perimeter causes a net rolling the doughnut of A.

The leading edge of a plume, sometimes called the 'starting-plume', usually has a vortex-ring structure, as does a smoke ring. The motion of an isolated vortex ring and the interaction of two or more vortices are discussed in eg Batchelor's text book (ref 1)

For many purposes a ring vortex may be approximated as having a vortex-core of small cross-section. However a simple theoretical solution, called Hill's spherical vortex, is known in which the vorticity is distributed within a sphere (the internal symmetry of the flow is however still annular). Such a structure or an electromagnetic equivalent has been suggested as an explanation for the internal structure of ball lightning.

The curved arrows indicate airflow circulation about the rotor disc. The helicopter shown is the RAH-66 Comanche.

The V-22 Osprey in flight.

In typical flight, the rotor disc directs the airflow downwards, creating lift. A vortex ring state (VRS), though, involves a toroid-shaped path of airflow circumscribing the blade disc, as the airflow moves down through the disc, then outward, and then down through the top again. This re-circulation of flow can negate much of the lifting force and cause a catastrophic loss of altitude.

A helicopter typically induces a vortex ring state by descending into its own downwash. This requires low airspeed and a moderate rate of descent with power applied, and can lead to an undesirable phase of flight known as settling with power. This condition can be corrected by lowering the collective, which controls the pitch angle of the rotor blade, slightly pitching nose down, and establishing forward flight. The aircraft will fly into "clean air", and will be able to regain lift.

A clear understanding of this condition is essential for helicopter pilots to avoid danger.

• On a fast descent, no vortex will form because the vertical airspeed is faster than the recirculation speed - although rapid descent through one's own downwash is itself a highly dangerous manoeuvre.
• With high airspeed, no vortex will form because the translational airflow is faster than the recirculation speed.

In testing of the V-22 Osprey, the April 8, 2000 crash which killed 19 was attributed to VRS. The specific cause was officially determined to be due to a rate of descent of over 2000 feet per minute (600 m/min) of the aircraft while at slow horizontal speeds of around 30 knots (56 km/h). In addition, there were two planes descending in tandem, a possible risk factor for VRS. The military claims that subsequent testing has shown that the Osprey, and the tiltrotor in general, is less susceptible to VRS, that the conditions are easily recognized by and presented to the pilots, that recovery from VRS requires a more natural action by the pilot than for helicopters, and that the altitude loss is significantly less than for helicopters. They claim that with sufficient altitude (2000 feet or more), VRS recovery is relatively easy.[1] They also claim that it is easy to train new pilots in the recognition of and recovery from VRS.

• Aeronautical engineering
• Autorotation
• Bubble ring
• Ground effect in aircraft
• Helicopter flight controls
• Helicopter pilotage
• Helicopter rotor
• Mushroom cloud
• Smoke ring
• Vortex ring toys

• Free-Vortex Wake Calculations of Helicopter Rotors and Tilt-Rotors Operating-In and Transitioning Through the Vortex Ring State

Batchelor, G. K., (1967), An Introduction to Fluid Dynamics, Cambridge UP (reprinted 2000)