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coaxial

Coaxial helis

I sorta got into "flying" robots recently and I have found answers concerning most of my questions, but there is a particular thing I don't understand about helicopters; how do the coaxial system work (I mean, how can I make one from scratch). I understand that each rotor cancels the other's torque, but how does all this mechanicaly work? Are the rotors dependant on each other for speed (I suppose that's not true because the heli turns probably by increasing torque generated by one rotor and decreasing torque on the other).