Vortex Ring
In the right conditions, a helicoptor can create a vortex ring, known
as the verticle ring state. If the helicoptor is flying at a speed
lower than that which would provide translational lift, is rapidly
descending, and is using most of the availible power already.
image from http://en.wikipedia.org/wiki/File:Vortex_ring_helicopter.jpg
Vortex rings are also found around nature. Under water, dolphins
and whales have blown bubble rings, and in some cases, will play
with the bubble rings. Volcanoes have also blown smoke rings, as
gases escape. A great place for these to occur are at Mt. Etna.
image from http://news.bbc.co.uk/2/hi/science/nature/696953.stm
When there is a spherical mass of fluid moving hrough a stationary
mass of fluid, the burst pushes fluid forward, and the low pressure
created behind of the quickly moving sphere pulls that fluid back
into a doughnut shape. A simple way to demonstrate this would be to
let a drop of dye fall into a glass of water. The drop of dye can
be considered the quickly moving spherical shape, and the water is
the stationary mass of fluid. As the dye hits the water, it moves
in on itself, creating the vortex ring. You can see the direction
of the flow as it breaks down due to the interacting viscous forces.