Shrouded Tail RotorsModern helicopters with shrouded tail rotors are essentially employing the ring protection concept, but only with the tail rotor. The benefits are:
Less chance of the tail rotor hitting something
Implementing a protective ring on the main rotors is akin to proposing an airplane without wings to protect against hitting the wings on something. It might work on a different kind of aircraft (UFO-shaped with propellers embedded in the airframe) but is not practical for the existing helicopter design concept.