Multirotor unmanned aerial vehicles (UAVs) form a specific class of aircraft in which lift is generated directly by rotor thrust rather than by fixed wings. This principle enables vertical take-off and landing, hovering and precise low-speed positioning, but it also increases energy demand and makes the flight dynamics strongly coupled. This paper analyses the physical principles that govern multirotor UAV behaviour, with emphasis on the distribution of forces, moments and energy consumption. Hovering, vertical motion, translational motion through aircraft tilt and yaw control by counter-rotating rotors are considered from the perspective of flight control. The energy model is interpreted through disk loading, induced velocity and ideal hover power. A reference numerical example for a 4 kg UAV is used to compare propeller diameters and quadrotor, hexarotor and octorotor configurations. The results show that larger total rotor-disk area reduces induced power and can extend hover endurance, while additional rotors improve redundancy at the cost of mass, structural complexity and control allocation requirements. The analysis provides a compact basis for engineering optimisation of multirotor UAV systems.