Flying on one x 
I notice that BillCJ rightly removed an assertion that it can't fly on one engine, it can. However the way I read  its helicopter controls to work flying with one proprotor assembly would be almost impossible. Not only do the two rotors counter torque forces, but they appear to oppose each other while maneuvering (ie to yaw right, the right prop rotor moves back, the left one forward). Could this be what some people mean by one one engine, even if it's technically incorrect?