[rootusrootus]

My lay understanding is that more rotors are more efficient, but also more unstable due to a concept known as disc loading (the ratio of weight vs swept area of the blades). I'd guess the main reason we don't see more than two rotors on traditional helicopters is partly due to the complexity of transmitting power to multiple locations from a single engine (or having multiple engines), and partly because making such a system responsive enough for stable flight is hard. Electric motors neatly solve both problems.