Small scale unmanned aerial vehicles using multirotor propulsion systems have received considerable attention for a wide range of military and commercial applications in recent years. In the multirotor configuration, the rotor interaction phenomenon occurs severely because the rotors are located in close proximity to each other. Therefore, the separation distance between the adjacent rotor tips has a strong effect on the wake structures and flow fields, which consequently play an important role in determining the aerodynamic performance and noise level of the multirotor vehicle. In the present study, numerical simulations of a quadcopter under hover flight conditions are conducted to investigate the mutual rotor-to-rotor interactional effects on the aerodynamic performance, wake structures, and sound pressure level using the nonlinear vortex lattice method with the vortex particle method and acoustic analogy based on Farassat's formulation 1A. Calculations for the multirotor configurations with different separation distances show that the average thrust force decreases significantly and force fluctuation is found to increase dramatically as the rotor spacing gets smaller. In addition, the wake geometry and induced flow structure behind the rotor tend to be radially dragged down toward the center of the vehicle due to the existence of the adjoining rotor, which consequently results in strong wake-to-wake interaction and the formation of asymmetric wake structures although the multirotor operates under the hovering condition. It is also observed that unsteady loading introduced by rotor interaction leads to a considerable increase in the sound pressure level, particularly the normal direction of the rotor plane.