Description
In forward flight, there is no upward flow (upflow) of air in the hub area. As forward airspeed decreases and vertical descent rates increase, an upflow begins because there are no airfoil surfaces in the mast and blade grip area. As volume of upflow increases, the induced flow (air pulled or "induced" down through the rotor system) of the inner blade sections is overcome and the blades begin to stall near the hub. As the inner blade sections stall, a second set of vortices, similar to the rotor tip vortices, form in the center of the rotor system. The inner set of vortices decreases the amount of lift being produced and causes an increase in sink rate. In an accelerated condition, the inner and outer vortices begin to feed each other to the point where any increase in rotor blade pitch angle increases the interaction between the vortices and increases the rate of descent. In this state, the helicopter is operating in its own downwash, descending through descending air. The failure of a helicopter pilot to recognize and react to the condition can lead to high descent rates and impact with terrain.
Read more about this topic: Vortex Ring State
Famous quotes containing the word description:
“He hath achieved a maid
That paragons description and wild fame;
One that excels the quirks of blazoning pens.”
—William Shakespeare (1564–1616)
“An intentional object is given by a word or a phrase which gives a description under which.”
—Gertrude Elizabeth Margaret Anscombe (b. 1919)
“Once a child has demonstrated his capacity for independent functioning in any area, his lapses into dependent behavior, even though temporary, make the mother feel that she is being taken advantage of....What only yesterday was a description of the child’s stage in life has become an indictment, a judgment.”
—Elaine Heffner (20th century)