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A multirotor or multicopter is a 
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 which cancels out rotational torque (one facing up and one facing down).
Also possible is the addition of horizontal propellers These types of rotorcraft (which have just 4 vertical propellers) are called quadplanes.
A multirotor or multicopter is a rotorcraft
A rotorcraft or rotary-wing aircraft is a heavier-than-air aircraft with rotary wings or rotor blades, which generate lift by rotating around a vertical mast. Several rotor blades mounted on a single mast are referred to as a rotor. The Internat ...
with more than two lift-generating rotor
Rotor may refer to:
Science and technology
Engineering
* Rotor (electric), the non-stationary part of an alternator or electric motor, operating with a stationary element so called the stator
*Helicopter rotor, the rotary wing(s) of a rotorcraft ...
s. An advantage of multirotor aircraft is the simpler rotor mechanics required for flight control. Unlike single- and double-rotor helicopter
A helicopter is a type of rotorcraft in which lift and thrust are supplied by horizontally spinning rotors. This allows the helicopter to take off and land vertically, to hover, and to fly forward, backward and laterally. These attribu ...
s 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 unmanned aerial vehicle (UAV) (drone) projects in which the names tricopter, quadcopter
A quadcopter or quadrotor is a type of helicopter with four rotors.
Although quadrotor helicopters and convertiplanes have long been flown experimentally, the configuration remained a curiosity until the arrival of the modern UAV or drone. T ...
, hexacopter and octocopter are frequently used to refer to 3-, 4-, 6- and 8-rotor rotorcraft, respectively. There is also the X8 (also called ''octo-quad'') configuration that is similar to the quadracopter design, except that it has eight rotors; the lower of which have a reversed rotation direction.
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 which cancels out rotational torque (one facing up and one facing down).
Also possible is the addition of horizontal propellers These types of rotorcraft (which have just 4 vertical propellers) are called quadplanes.
Rotor Configurations
* Coaxial rotors * Cyclorotor *Intermeshing rotors
Intermeshing rotors on a helicopter are a set of two rotors turning in opposite directions, with each rotor mast mounted with a slight angle to the other, in a transversely symmetrical manner, so that the blades intermesh without colliding. ...
* Tail rotor
* Tandem rotors
* Transverse rotors
Transverse rotor aircraft have two large horizontal rotor assemblies mounted side by side.
Single-rotor helicopters (unicopters) need an additional tail rotor or tail exhaust to neutralize the reactional angular momentum produced by the ...
Examples
* de Bothezat helicopter - an American four-rotor helicopter that first flew on December 18, 1922. * Cierva Air Horse - a British three-rotor "heavy lift" helicopter first flying in 1948. Three rotors were used to give a large lift without compromising rotor strength. * Volocopter designs - a series of German prototype electric multicopters with 18 rotors, the first electric multicopter in the world to achieve crewed flight. The large number of low-cost motors make it economical, quiet and provide redundancy with ability to maintain control with up to four failed motors.
References
Multirotor helicopters {{Component-aircraft-stub