The rotor interaction phenomenon occurs frequently in unmanned aerial vehicles (UAVs) with multi-rotor configuration. In this study, we investigated the rotor interaction effects of a small-size quadcopter UAV on the aerodynamic performance and acoustic characteristics. The numerical simulation of DJI Phantom 2 model in the hover flight condition was conducted using the nonlinear vortex lattice method (NVLM) with the vortex particle method (VPM) and acoustic analogy based on the Farassat-1A formula. NVLM can consider the nonlinear aerodynamic characteristics that are mainly associated with viscous and low Reynolds number effects by incorporating airfoil look-up table and vortex strength correction. VPM is well suitable for simulating complex wake structures of quadcopter UAV because maintaining the connectivity between adjacent vortex particles is not required. Calculations for an isolated rotor were compared against measurements to validate the predicted results, including thrust force and tonal noise at 1st blade passing frequency (BPF). The calculations for a quadcopter configuration show that the average thrust coefficient decreases and thrust coefficient begins to fluctuate dramatically due to severe rotor interaction. In addition, the sound pressure level of the quadcopter is much higher than that of the isolated rotor and a significant difference in noise directivity between the quadcopter and isolated rotor is clearly captured. It is observed that unsteady loading introduced by rotor interaction leads to a considerable increase in noise. This study is a preliminary study to the aeroacoustic problem in multicopter UAV. The discussion in this work gives scope for further research on noise reductions of multicopter UAV by controlling a separation distance between the rotors.