Multirotors are UAVs that use more than two rotors with fixed-pitch spinning blades that generate lift. By changing the speed of the rotors so that the thrust generated is greater than, equal to or less than the forces of gravity and drag acting on the aircraft, the drone can be made to ascend, hover or descend. By varying the speeds of particular rotors, it is also possible to make the drone turn or move in a horizontal direction.
Multirotor UAVs are typically found with three, four, six or eight rotors, and aircraft with these configurations are typically known respectively as tricopters, quadcopters, hexacopters and octocopters. Quadcopters are the most common design, having a less complex stabilising mechanism than tricopters and fewer parts (and therefore less manufacturing cost) than hexacopters or octocopters.
The more rotors a multirotor drone has, the more thrust it can generate and thus the greater a payload it can lift. Heavy lift multirotor drones that lift very heavy industrial cameras or delivery payloads will often be hexacopters or octocopters. Drones with more than four rotors also have a degree of redundancy, allowing them to still make a gradual descent in the case of an individual rotor failure.
The tradeoff is that a greater number of rotors requires a higher current draw, and therefore hexacopters and octocopters will need to carry a greater weight in batteries to achieve the same flight endurance as a quadcopter of similar size. Larger multicopter drones may have the ability to carry hydrogen fuel cells, which can provide a greater energy output to mass ratio than conventional battery technology.
Multirotor UAVs, due to their VTOL operation, can operate in a wider variety of environments than fixed-wing drones, as they do not require any extra space to take off and land. Their ability to hover in place makes them ideal for surveillance missions and certain imaging applications.
Most multicopters, especially smaller ones, are limited to batteries as a power source, which puts limits on their flight endurance. While fuel cells can be fitted to some multirotors to improve their flight times, internal combustion engines are too bulky, as well as too unresponsive for multicopter drone operation, which relies on very fast adjustments to rotor speeds.
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