Riblets are micro-grooves capable of decreasing skin-friction drag, but recent work suggests that additional benefits are possible for other components of aerodynamic drag. The effect of riblets on a fixed-wing, low-speed Unmanned Aerial Vehicle (UAV) on the total aerodynamic drag are assessed here for the first time by means of RANS simulations. Since the microscopic scale of riblets precludes their direct representation in the geometric model of the UAV, we model riblets via a homogenised boundary condition applied on the smooth wall. The boundary condition consists in a suitably tuned partial slip, which assumes riblets to be locally aligned with the flow velocity, and to possess optimal size. Several configurations of riblets coverage are considered to extract the potential for drag reduction of different parts of the aircraft surface. Installing riblets with optimal size over the complete surface of the UAV leads to a reduction of 3% for the drag coefficient of the aircraft. In addition to friction reduction, analysis shows a significant additional form of drag reduction localised on the wing. By installing riblets only on the upper surface of the wing, total drag reduction remains at 1.7%, with a surface coverage that is only 29%, thus yielding a significant improvement in the cost–benefit ratio.