A quad copter, also called a quad rotor helicopter or quad rotor, is a multi-rotor helicopter that is lifted and propelled by four rotors. Quad copters are classified as rotorcraft, as opposed to fixed-wing aircraft, because their lift is generated by a set of rotors (vertically oriented propellers). Quad copters generally use two pairs of identical fixed pitched propellers; two clockwise and two counter-clockwise. These use 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; to locate for the centre of thrust both laterally and longitudinally and to create a desired total torque, or turning force.

Quad copters differ from conventional helicopters which use rotors which are able to vary the pitch of their blades dynamically as they move around the rotor hub. In the early days of flight, quad copters (then referred to as 'quad rotors') were seen as possible solutions to some of the persistent problems in vertical flight; torque-induced control issues (as well as efficiency issues originating from the tail rotor, which generates no useful lift) can be eliminated 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 successful heavier-than-air vertical take-off and landing (VTOL) vehicles .However, early prototypes suffered from poor performance, and latter prototypes required too much pilot work load, due to poor stability augmentation and limited control authority. At a small size, quad copters are cheaper and more durable than conventional helicopters due to their mechanical simplicity. Their smaller blades are more advantageous because they possess more kinetic energy, reducing their ability to cause damage. However as size increases fixed propeller quad copters develop disadvantages over conventional helicopters.

Increase blade size increase their momentum. This means that changes in blade speed take longer, which negatively impact control. On the other hand blade size increases, improves efficiency as it takes less energy. Quad copters are moderately shoddy, accessible in assortment of sizes and their basic mechanical configuration implies that they can be worked by beginners. Due to the multi-disciplinary nature of working a quad copter, scholastics from number of fields need to cooperate so as to make critical change to the way quad copters perform. Quad copter ventures are regularly coordinated efforts between software engineering , electrical designing and mechanical building pro.