Flying-wing aircraft are known for their high aerodynamic efficiency and are often utilized in solar-powered aircraft. However, they generally exhibit poor stability and controllability. Unswept-wing flying-wing aircraft, also called “flying planks”, possess aerodynamic characteristics that differ from swept-wing flying-wing designs. This study utilizes the vortex lattice method to calculate the aerodynamic coefficients and derivatives of a lightweight, small-scale, unswept-wing flying-wing unmanned aerial vehicle (UAV). The study analyzes the longitudinal static stability, longitudinal and lateral-directional dynamic stability under varying angles of attack (AoAs) and climb angles as well as the effects of the centre of gravity (CG) position below the wing on the stability of the UAV.An aeromodel is constructed and test-flown to validate the findings. Results show that the longitudinal stability of the unswept-wing flying-wing aircraft differs from that of conventional configurations. These aircraft show almost no tendency for dutch-roll, and maintain lateral-directional stability without stability augmentation. Lowering the CG position can alter the trimmed AoAs, and enhance the pitch stability at positive AoAs and deteriorates the pitch stability at negative AoAs or large climbing angles; it also narrows the flight envelope. Unswept-wing flying wings possess various unconventional characteristics during flight and need further investigation.