order to allow more power and stability at reduced weight, a quadcopter, like any other multirotor can employ a coaxial rotor configuration. In this case, each arm has two motors running in opposite directions (one facing up and one facing down). Operations Autonomous flight The quadcopter configuration is relatively simple to program for autonomous flight. This has allowed experiments with complex swarming behaviour based on basic sensing of the adjacent drones. Endurance The longest flight time achieved by a battery-powered quadcopter was 2 hours, 31 minutes and 30 seconds. The record was set by Ferdinand Kickinger of Germany in 2016. In setting the record, Kickinger used low discharge-rate, high-capacity lithium-ion batteries and stripped the airframe of non-essential weight to reduce power draw and extend endurance. Alternative power sources like hydrogen fuel cells and hybrid gas-electric generators have been used to dramatically extend endurance because of the increased energy density of both hydrogen and gasoline, respectively. History Pioneers The first heavier-than-air aerodyne to take off vertically was a four-rotor helicopter designed by Louis Breguet. It was tested only in tethered flight and to an altitude of a few feet. In 1908 it was reported as having flown 'several times', although details are sparse. Etienne Oehmichen experimented with rotorcraft designs in the 1920s. Among the designs he tried, his helicopter No.2 had four rotors and eight propellers, all driven by a single engine. The Oehmichen No.2 used a steel-tube frame, with two-bladed rotors at the ends of the four arms. The angle of these blades could be varied by warping. Five of the propellers, spinning in the horizontal plane, stabilized the machine laterally. Another propeller was mounted at the nose for steering. The remaining pair of propellers functioned as its forward propulsion. The aircraft exhibited a considerable degree of stability and increase in control-accuracy for its time, and made over a thousand test flights during the middle 1920s. By 1923 it was able to remain airborne for several minutes at a time, and on April 14, 1924 it established the first-ever FAI distance record for helicopters of . It demonstrated the ability to complete a circular course and later, it completed the first closed-circuit flight by a rotorcraft. Dr. George de Bothezat and Ivan Jerome developed the de Bothezat helicopter, with six-bladed rotors at the end of an X-shaped structure. Two small propellers with variable pitch were used for thrust and yaw