There's information here on a single motor quadcopter that uses constant speed rotors with variable pitch controlling yaw.
https://www.youtube.com/watch?v=fkSx3fSz0tE
Also, diagrams of the MantaRay here
http://www.dronetrest.com/t/mantaray...adcopter/175/2. Definitely all rotors are the same speed. It's the independent variable pitch.
Dave