A quadcopter, also called quadrotor or quadrotor helicopter, is a multirotor helicopter that is lifted and propelled by four rotors. Quadcopters are classified as helicopters, unlike fixed-wing aircraft, because their lift is generated by a set of rotors (vertically oriented propellers).
Quadcopters generally use two pairs of identical fixed pitch propellers; two clockwise (CW) and two counterclockwise (CCW). These use an independent variation of the speed of each rotor to achieve control. By changing the speed of each rotor, it is possible to specifically generate a desired total thrust; locate the center of the thrust both laterally and longitudinally; and to create a desired total torque or turning force.
Quadcopters differ from conventional helicopters, which use rotors that can vary the pitch of their blades dynamically as they move around the rotor hub. In the first days of the flight, the quadrocopters (referred to as "quadrotors" or "helicopters") were considered as possible solutions to some of the persistent problems in vertical flight. Torque-induced control problems (as well as efficiency problems that originate in the tail rotor, which does not generate useful lift) can be eliminated by counter-rotation, and relatively short blades are much easier to construct. A series of manned designs appeared in the 1920s and 1930s. These vehicles were among the first most successful vertical take-off and landing vehicles (VTOL) heavier than air. However, the first prototypes suffered from low performance, and the latest prototypes required too much pilot workload, due to a limited increase in stability and limited control authority.
At the end of the 2000s, the advances in electronics allowed the production of light and economic flight controllers, accelerometers (IMU), global positioning system and cameras. This resulted in the quadcopter configuration becoming popular for small unmanned aerial vehicles. With their small size and maneuverability, these quadcopters can be blown both indoors and outdoors.
In a small size, quadcopters are cheaper and more durable than conventional helicopters because of their mechanical simplicity. Its smaller leaves are also advantageous because they have less kinetic energy, which reduces their ability to cause damage. For small-scale quadrographs, this makes vehicles safer for closer interaction. It is also possible to place quadrocopters with protections that enclose the rotors, further reducing the potential for damage. However, as the size increases, fixed-propeller quadrocopters develop disadvantages over conventional helicopters. Increasing the size of the blade increases its momentum. This means that changes in blade speed take more time, which negatively affects control. Helicopters do not experience this problem since increasing the size of the rotor disc does not significantly affect the ability to control the pitch of the blades. Due to its ease of construction and control, quadcopter aircraft are frequently used as amateur model aircraft projects.