Small multirotor unmanned aerial vehicles (UAVs) are suitable for surveillance or inspection operations, such as disaster site observations or building inspections, as they can hover. Therefore, engineers and associations plan to use them for civilian applications. However, the payload capacity of current UAVs is too small to carry heavy sensors or batteries. Thus, improving payload capacity by increasing the number of rotors has been considered to achieve an increase in the thrust for a limited body size. Although, most rotor arrangements cause rotor flow interaction, which degrade the total thrust. To design a rotor arrangement on small multirotor UAVs, it is necessary to first evaluate the thrust from an aerodynamic perspective. In this study, we evaluated the effect of rotor flow interactions on thrust for three two-rotor configurations. The investigation showed that the thrust of rotors in wake flow is degraded. Thus, we proposed a new octorotor UAV configuration based on the results of our evaluation of the rotor flow interaction and verified the thrust improvement compared to a coaxial octorotor UAV configuration. This investigation demonstrated a thrust improvement of 24% over the total thrust of the coaxial octorotor UAV.