## 5 Conclusions  
The drag reduction potential of riblets deployed on a fixed-wing, low-speed Unmanned Air Vehicle (UAV) has been assessed with RANS simulations, with the aim of determining an optimal coverage policy. While riblets are fully characterised in low-speed flows over plane walls, and studies are available for aeronautical configurations in transonic flow (commercial mid- or long-range passenger aircraft), a low-speed aircraft such as the present one (for which the cruise speed is only 22 m/s) is considered here for the first time. Since the friction component of the aerodynamic drag of the UAV is modest, the effectiveness of riblets in this specific application needs to be assessed. The RANS simulations, which employ a standard OpenFOAM setup, are unable to describe riblets directly. Thus, the presence of riblets is accounted for via a suitable slip condition enforced at the planar wall. The chosen amount of slip is constant in viscous units, and corresponds to riblets that locally possess optimal size in viscous wall units. The slip length model has been validated in the simple flows over a flat plate and around a subsonic airfoil, where results agree with available information. Once applied to the UAV, the simulated riblets have brought out indirect and favourable effects, which go beyond the local reduction of friction drag, and also render the deployment of a friction-reduction device definitely interesting in such a low-speed application. Indeed, riblets significantly change the pressure distribution across the wing of the aircraft, which translates into an additional reduction of form drag, and in a lift increment as well. Although the latter obviously causes an increase in lift-induced drag, the requirement for the aircraft in cruise to fly at a given lift leads to a reduced angle of attack, and thus, to a further contribution to drag reduction. In the end, riblets provide up to 3% reduction of the total drag of the aircraft at cruise speed: a noticeable result, especially when the low-flight Reynolds number of the UAV is considered. Once a cheap computational model is available to reliably compute the global effect of riblets on the aerodynamic drag, varying the riblets coverage policy becomes a computationally affordable task; relatively inexpensive simulations can help determine what drag benefit can be achieved with a given extent and location of the coverage of the aircraft surface. Thanks to the importance of secondary effects on pressure drag reduction induced by riblets, as a consequence of the significant pressure drag component, up to 1.7% of total drag reduction is achieved by placing riblets on the upper surface of the wing only. In this configuration, the total drag reduction is almost 2/3 of the maximum obtained with full coverage, but it is obtained with a coverage of less than 1/3 of the total area. Since riblets costs (for application and maintenance) are directly linked to the amount of riblets-covered surface, the wing-only configuration offers a reduced cost-benefit ratio, and does not touch the UAV fuselage, where systems (sensors, cameras, and transmitters) are designed to be installed. Further analysis can determine the practicality of riblets removal from high-wear areas (e.g., the leading edge), which would further add to the practical appeal of riblets in this application. Such calculations are made possible by the simplicity of the slip-length model, whose validity goes beyond riblets, since it can be used to simulate a generic drag-reducing device which locally reduces the skin friction. Conceptualization and methodology, C.M. and M.Q. ; software and validation, C.B., L.C. and B.M. ; writing--original draft preparation, C.B. and L.C. ; writing--review and editing, F.G. and M.Q. ; data curation, F.G.; supervision, M.Q. ; project administration, C.M. All authors have read and agreed to the published version of the manuscript. This research received no external funding.