## 4 Results
### Partial Coverage  
Perhaps the most interesting consequence of the availability of a simple yet accurate boundary condition to model riblets within RANS simulations is the ability to carry out quick numerical studies to address practical problems related to their use. For example, since riblets produce limited benefits and imply costs and penalties, an elementary cost/benefit analysis should start by addressing the simple question of which area of the aircraft surface would yield the largest benefits after riblets installation. To this aim, we have designed a further set of simulations to explore partial coverage of the aircraft surface with riblets. The amount of coverage is quantified by the ratio \(\beta\) between the riblets-covered area and the total area, with \(\beta=1\) indicating total coverage. In these simulations, the full aircraft is considered, but riblets coverage varies according to Table 2, where case I is the full-coverage case described above. Outcomes of the simulations are shown in Table 3 and graphically represented in Figure 15. Figure 16 schematically illustrates where riblets are applied on the surface of the UAV. \begin{table}
\begin{tabular}{c c c} \hline \hline  & **Riblets Deployment** & \(\beta\) \\ \hline I & Full coverage & 1.000 \\ II & No wing TE & 0.953 \\ III & No booms & 0.935 \\ IV & Only wing & 0.524 \\ V & Only wing, suction side & 0.289 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Coverage configurations. Figure 14: Drag breakdown (**left**) and decomposition of profile drag (**right**). Solid lines with square markers refer to clean configuration; dashed lines with triangular markers refer to riblets configuration. Figure 13: Drag reduction rate vs. angle of attack. The largest drag reduction is achieved in cruise condition. Since, at the trailing edge of the wing, riblets do not provide significant reductions in skin friction (Figure 11) while locally enforcing a substantial change from the optimal size, in configuration II riblets are removed from the trailing edge of the entire wing. The reduction of the riblets-covered surface is minimal (less than 5%) but, as expected, there is no appreciable decrease in terms of performance. Configuration III has riblets removed from the booms that connect the wing to the tail. Again, the overall drag reduction is essentially unchanged, with 6.5% savings in covered areas: pressure drag reduction remains unchanged since the boom is not an aerodynamic body, whereas friction reduction decreases, but minimally so, because the surface of the boom is small. Together, cases II and III suggest that removing riblets from both the trailing edge and the booms would avoid difficult areas, and save over 10% of application surface without incurring insignificant performance degradation. Configuration IV has riblets applied on the wing only, and is motivated by the observation that, in this application, pressure drag is approximately 2/3 of the whole drag, and that riblets placed on the wing produce pressure drag reduction in addition to friction drag reduction. With configuration IV, performance indeed degrades from 3% to 2%, but the saving in coverage area is more than proportional, with riblets surface shrinking down to one half at \(\beta=0.524\). As expected, pressure drag reduction remains almost unchanged at  
Figure 16: Schematic drawing of various riblets coverage configurations, cases II-V.  
Figure 15: Drag reduction contributions for different configurations of riblets coverage from highest (I-full coverage) to lowest (V-wing only, suction side) coverage.