## 4 Results
### The UAV  
The complete UAV is now considered in the configuration described above and shown in Figure 1. Consistently with the rest of this study, riblets are assumed to be locally optimal,  
Figure 8: Pressure coefficient on the pressure and suction sides of the isolated UAV wing at \(2y/b=0.52\), at \(\alpha=4^{\circ}\), for the clean case (black line) and difference with the riblets case (red dashed line). Figure 7: Drag breakdown for the isolated UAV wing (**left**), and focus on the profile drag (**right**). Solid lines with square markers indicate the clean configuration; dashed lines with triangular markers indicate the configuration with riblets. with \(l_{g}^{+}=10.5\) and unitary slip length \(\lambda^{+}=1\). The spatial distribution of the optimal riblet size, i.e., \(l_{g}\) (which, for a given cross-sectional shape, leads immediately to the geometric dimensions of the riblets) is retrieved as a result of the simulations. It should be remarked, however, that previous work [10] indicates how the size of locally optimal riblets does not vary much, so that the drag reduction obtained adopting riblets with constant physical size is quite near to the maximum drag reduction. A series of simulations with/without riblets is carried out to provide data points to build the polar of the aircraft (Figure 10). Owing to the already highlighted lift increase provided by riblets, the angle of attack necessary to provide the required lift in cruise conditions slightly decreases from \(\alpha=2.85^{\circ}\) to \(2.81^{\circ}\). The drag reduction obtained for the entire aircraft is an interesting 3%, which derives from a combination of a (less important) friction drag reduced by 6.1% and a (more important) pressure drag reduced by 1.5%. Figure 11 helps determining where the largest percentage changes of the skin friction take place over the surface of the aircraft. \(\Delta C_{f}/C_{f,0}\) is about 6% almost everywhere, roughly  
Figure 10: \(C_{D}(\alpha)\) and polar curves of the UAV, in clean/riblets configurations. Figure 9: Friction coefficient for the isolated UAV wing, at spanwise location \(2y/b=0.52\) and four angles of attack. as expected for a flat plate at this value of \(Re\), except for the region near the trailing edge and for the aft part of the fuselage: here, the absolute value of \(C_{f}\) approaches zero, and its percentage variations become less meaningful. Figure 12 shows the computed height distribution for the locally optimal riblets, by assuming that the cross-sectional riblet shape is a standard V groove, for which \(s^{+}=h^{+}=\sqrt{2}l_{g}^{+}\). The optimal riblets height is about 0.2 mm nearly everywhere, except for the trailing edge of the wing, and for the aft part of the fuselage. This provides graphical evidence for the previous statement that riblets of properly chosen constant physical height would provide drag reduction that is very close to the maximum. Riblets are then tested in off-design situations, i.e., at various incidences different from the cruise angle of attack, to check for robustness and to verify that riblets do not cause unwanted effects on the UAV aerodynamics during manoeuvrers or the climb/descent phases of a typical mission. As already noted for the UAV wing, Figure 13 shows that, although drag reduction is maximum in cruise, the performance degrades only mildly when the angle of attack differs from the cruise value. Again, it is confirmed that friction drag reduction remains nearly constant when \(\alpha\) ranges from \(-2^{\circ}\) to \(5^{\circ}\). Finally, the aerodynamic drag is broken down into profile drag and induced drag in the left plot of Figure 14, while the right plot decomposes further profile drag into friction and form drag. The most obvious difference between clean and riblets configurations is the reduced profile drag, which derives from a sizable reduction in the friction component, joint with a comparable contribution from the form drag.