text: control experienced during “power settling” 
  flow on 
  results from the turbulent rotational 
  the blades and the unsteady shifting of the 
  flow in and out spanwise along the blade. 
  There is an area of positive thrust in the outer 
  portion of the rotor as a result of the mass of 
  air accelerated downward and an area of 
  negative thrust at the center of the rotor as a 
  result of the mass of air flowing upward. The 
  rotor 
  is stalled only near the hub but no 
  important effect is contributed because of the 
  low local velocities. 
  
  Operation in the “vortex  ring” 