local induced downwash velocity and the 
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