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Multi-rotor drones are the most heavily used drones by the professional or hobbyist for various activities and operations mainly for aerial-photography and surveillance etc. They are further classified based on the number of rotors in a platform. There is a Quadcopter having four-rotors, a Hexacopter with six-rotors, Bicopter with two-rotors, an Octocopter with eight-rotors, and Tricopter with three-rotors. Among these, Quadcopter is the heavily used variant.

Figure 1.1 Fixed-wing UAVs [33].

It is easy to manufacture and having capabilities like vertical take-off, loiter makes it suitable for many applications but like others, it also has some downsides too. The prominent ones being are its speed, limited endurance, and flying time. They are not much suitable for large-scale projects like surveillance or long-distance aerial mapping [9]. The major problem with this type of drone is that majority of their energy is spent in stabilizing themselves in the air and defying gravity. At present, most of the multi-rotor drones are powered from battery sources and are capable of 20–30 min of flying time with some exceptional cases going to 50 min. often with a payload of a lightweight camera.