text: the 
  rotor is shown in figure 6.19 for the conditions 
  of normal hovering and power settling. Part 
  A of figure 6.19 illustrates the typical down- 
  wash distribution 
  If 
  sufficient power were not available to hover 
  at this condition, the helicopter would begin 
  to settle at some rate of descent depending on 
  the deficiency of power. This rate of descent 
  would effectively decrease the downwash 
  throughout the rotor and result in a redistri- 
  to Part B of 
  bution of downwash similar 
  figure 6.19. At 
  the 
  rotor disc, the local induced downwash veloc- 
  ity 
  is greater than the rate of descent and 
  downflow exists. At the center of the rotor 
  disc, the rate of descent is greater than the 
  induced downwash velocity and the 
  local 
  resultant flow is upward. This flow condition 
  results in the rotary “vortex ring” state. By 
  reference to the basic momentum theory it is 
  apparent that the rotor will produce no thrust 
  in this condition if the net mass flow of air 
  through the rotor is zero. 
  It is important to 
  note that the main lifting part of the rotor is 
  not stalled. The rotor roughness and loss of 