A quadcopter, also called quadrotor helicopter or quadrotor, is a multitorre helicopter that is lifted and propelled by four rotors. Quadcopters are classified as helicopters, unlike fixed wing aircraft, because their elevation is generated by a set of rotors (vertically oriented propellers).
Quadcopters generally use two pairs of fixed fixed propellers identical; two clockwise (CW) and two anticlockwise (CCW). These use variation independent 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; to locate the thrust center both laterally and longitudinally; and to create a desired total torque, or turning force.
Quadcopters differ from conventional helicopters, which use rotors that are able to vary the pitch of their blades dynamically as they move around the rotor shaft. In the first days of flight, the quadruplets (so-called "four-month" or "helicopters") were considered possible solutions to some of the persistent problems in vertical flight. Torque-induced control problems (as well as efficiency issues arising from the tail rotor, which do not generate useful lift) can be removed by counter-rotation and the relatively short blades are much easier to construct. A number of manned designs appeared in the 1920s and 1930s. These vehicles were among the first vertical lift and landing vehicles (VTOL) with more weight than air. However, the early prototypes suffered poor performance, and the latest prototypes required too much workload from the pilots due to poor stability and limited control authority.
In the late 2000s, advances in electronics allowed the production of cheap light flight controllers, accelerometers (IMU), global positioning system and cameras. This resulted in the configuration of the quadcopter which became popular for small unmanned aerial vehicles. With their small size and maneuverability, these quadcopters can fly both indoors and outdoors.
In a small size, quadcopters are cheaper and more durable than conventional helicopters because of their mechanical simplicity. Their smaller leaves are also advantageous because they possess less kinetic energy, reducing their ability to cause damage. For small-scale quadcopters, this makes vehicles safer for close interaction. It is also possible to install quadcopters with protectors that lock the rotors, further reducing the potential for damage. However, as size increases, fixed propeller shafts have disadvantages over conventional helicopters. Increasing the size of the blade increases its momentum. This means that changes in blade speed take longer, which adversely affects the control. Helicopters do not experience this problem, as increasing the size of the rotor disc does not significantly affect the ability to control the pitch of the blades.