A multirotor or multicopter is a rotorcraft with more than two rotors. An advantage of multirotor aircraft is the simpler rotor mechanics required for flight control. Unlike single- and double-rotor helicopters which use complex variable pitch rotors whose pitch varies as the blade rotates for flight stability and control, multirotors often use fixed-pitch blades; control of vehicle motion is achieved by varying the relative speed of each rotor to change the thrust and torque produced by each.
Due to their ease of both construction and control, multirotor aircraft are frequently used in radio control aircraft and UAV projects in which the names tricopter, quadcopter, hexacopter and octocopter are frequently used to refer to 3-, 4-, 6- and 8-rotor rotorcraft, respectively.
In order to allow more power and stability at reduced weight, coaxial rotors can be employed, in which each arm has two motors, running in opposite directions (one facing up and one facing down).
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