A quadrotor, also called a quadrotor helicopter, is an aircraft that is lifted and propelled by four rotors. Quadrotors are classified as rotorcraft, as opposed to fixed-wing aircraft, because their lift is derived from four rotors. They can also be classified as helicopters, though unlike standard helicopter, quadrotors are able to use fixed-pitch blades, whose angle of attack does not vary as the blades rotate. Control of vehicle motion can be achieved by varying the relative speed of each rotor to change the thrust and torque produced by each.
There exist two generations of quadrotor designs. The first generation quadrotors were designed to carry one or more passengers. 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.
The more recent generation of quadrotors are commonly designed to be unmanned aerial vehicles (UAVs). These vehicles use an electronic control system and electronic sensors to stabilize the aircraft. With their small size and agile maneuverability, these quadrotors can be flown indoors as well as outdoors.
The advantages of the current generation of quadrotors, versus comparably scale helicopters, are as follows. First, quadrotors do not require mechanical linkages to vary rotor angle of attack as they spin. This simplifies the design of the vehicle, and reduces maintenance time and cost. Second, the use of four rotors allows each individual rotor to have a smaller diameter than the equivalent helicopter rotor, for a given vehicle size, allowing them to store less kinetic energy during flight. This reduces the damage caused should the rotors hit any objects. For small scale UAVs, this makes the vehicles safer to interact with in close proximity. Finally, by enclosing the rotors within a frame, the rotors can be protected during collisions, permitting flights indoors and in obstacle-dense environments, with low risk of damaging the vehicle, its operators, or its surroundings.
Each rotor produces both a thrust and torque about its center of rotation, as well as a drag force opposite to the vehicle's direction of flight. If all rotors are spinning at the same angular velocity, with rotors one and three rotating clockwise and rotors two and four counterclockwise, the net aerodynamic torque, and hence the angular acceleration about the yaw axis is exactly zero, which implies that the yaw stabilizing rotor of conventional helicopters is not needed. Yaw is induced by mismatching the balance in aerodynamic torques (i.e. by offsetting the cumulative thrust commands between the counter-rotating blade pairs).
Angular accelerations about the pitch and roll axes can be caused separately without impacting the yaw axis. Each pair of blades rotating in the same direction controls one axis, either roll or pitch, and increasing thrust for one rotor while decreasing thrust for the other will maintain the torque balance needed for yaw stability and induce a net torque about the roll or pitch axes. This way, fixed rotor blades can be made to maneuver the quad rotor vehicle in all dimensions. Translational acceleration is achieved by maintaining a non-zero pitch or roll angle.