A multi-rotor aerial vehicle comprising: at least two rotors (14a, 14b, 14c, 14d) which generate aerodynamic thrust on the vehicle, a controller (21, 116), a power supply, an output shaft driven by the power supply, and at least one shaft-driven hydraulic machine (30a, 30b, 30c, 30d) mechanically coupled to the output shaft, at least two rotor-driving hydraulic machines (40a, 40b, 40c, 40d) each of which drives a respective rotor, wherein the at least two rotor-driving hydraulic machines are hydraulically coupled to and thereby driven by the at least one shaft-driven hydraulic machine, at least one of said hydraulic machines being an electronically commutated hydraulic machine, the at least one electronically commutated machine comprising a plurality of working chambers of cyclically changing volume and one or more fluid outlets or inlets, each said fluid outlet or inlet being fluidically connected to a respective group of said working chambers, wherein the displacement of hydraulic fluid through each working chamber is regulated by one or more electronically controllable valves, during each cycle of working chamber volume, in phased relationship to cycles of working chamber volume, to thereby regulate the net displacement of hydraulic fluid through the fluid outlets or inlets, characterised in that, the controller controls the electronically controllable valves of the at least one electronically commutated hydraulic machine to thereby independently control the flow of hydraulic fluid through the respective hydraulic machines to thereby independently control the speed of rotation of the at least two rotors. at least two rotors (14a, 14b, 14c, 14d) which generate aerodynamic thrust on the vehicle, a controller (21, 116), a power supply, an output shaft driven by the power supply, and at least one shaft-driven hydraulic machine (30a, 30b, 30c, 30d) mechanically coupled to the output shaft, at least two rotor-driving hydraulic machines (40a, 40b, 40c, 40d) each of which drives a respective rotor, wherein the at least two rotor-driving hydraulic machines are hydraulically coupled to and thereby driven by the at least one shaft-driven hydraulic machine, at least one of said hydraulic machines being an electronically commutated hydraulic machine, the at least one electronically commutated machine comprising a plurality of working chambers of cyclically changing volume and one or more fluid outlets or inlets, each said fluid outlet or inlet being fluidically connected to a respective group of said working chambers, wherein the displacement of hydraulic fluid through each working chamber is regulated by one or more electronically controllable valves, during each cycle of working chamber volume, in phased relationship to cycles of working chamber volume, to thereby regulate the net displacement of hydraulic fluid through the fluid outlets or inlets, characterised in that, the controller controls the electronically controllable valves of the at least one electronically commutated hydraulic machine to thereby independently control the flow of hydraulic fluid through the respective hydraulic machines to thereby independently control the speed of rotation of the at least two rotors.