## 4 Results  
### The Isolated UAV Wing  
The UAV wing is considered first, to focus on the presence of indirect drag reduction effects in three dimensions, but without the geometrical complexities implied by the interaction between wing and fuselage. The UAV finite isolated wing is considered at the cruise flight condition of \(Re_{\infty}=5\times 10^{5}\). As always, locally optimal riblets with \(l_{S}^{+}=10.5\) are placed over the entire wing surface. Figure 6 shows how drag reduction induced by riblets changes with the angle of attack. The friction component of the total drag reduction is nearly constant at 6.3%, whereas pressure and total drag change with \(\alpha\). At \(\alpha=0^{\circ}\), the total drag reduction rate is 3.7%, and diminishes at larger incidences. Clearly, the diminished total drag reduction goes hand in hand with the diminished pressure drag reduction. As already observed for the NACA 0012 airfoil in Section 3.2, riblets tend to modify the pressure distribution in such a way that lift is increased; this is confirmed here for the UAV wing. This phenomenon causes an increase in the lift-induced drag, which is not seen in two dimensions. This should not be regarded as a negative effect of riblets, since the aircraft has to achieve the same lift, and increased aerodynamic efficiency is always beneficial. In fact, riblets' performance should be measured by adjusting \(\alpha\) in such a way that the lift coefficient is unchanged. In Table 1, we compare the clean case and the riblets case at the same angle of attack, and at the same lift coefficient as well. Two configurations are considered, at a nominal angle of attack of \(\alpha=0^{\circ}\) and \(\alpha=4^{\circ}\). Total drag is split into friction \(C_{D,f}\) and pressure \(C_{D,p}\) drag, as well as induced \(C_{D,i}\) and profile \(C_{D,pr}\) drag. As expected, comparing at the same \(C_{L}\) provides larger drag reduction than comparing at the same \(\alpha\). At the same angle of attack, riblets produce a larger lift coefficient, and hence, a larger induced drag. It is worth noticing that the decrease in \(C_{D,pr}\) is almost the same for the cases at constant \(\alpha\) and constant \(C_{L}\), whereas the induced drag is larger when compared at the same \(\alpha\). \begin{table}
\begin{tabular}{l l l l l l l} \hline  & \multicolumn{1}{c}{**Clean**} & \multicolumn{1}{c}{**Riblets**} & \multicolumn{1}{c}{**Clean**} & \multicolumn{1}{c}{**Riblets**} \\ \cline{2-7}  & \(\alpha=0^{\circ}\) & \(\alpha=0^{\circ}\) & \(\alpha=-0.626^{\circ}\) & \(\alpha=4^{\circ}\) & \(\alpha=4^{\circ}\) & \(\alpha=3.885^{\circ}\) \\ \hline \(C_{L}\) & 0.4996 & 0.5055 (+1.2\%) & 0.4996 (-) & 0.8719 & 0.8828 (+1.8\%) & 0.8719 (-) \\ \(C_{D}\) & 0.0227 & 0.0219 (\(-3.7\%\)) & 0.0217 (\(-4.5\%\)) & 0.0386 & 0.0380 (\(-1.8\%\)) & 0.0374 (\(-3.3\%\)) \\ \(C_{D,p}\) & 0.0121 & 0.0119 (\(-1.4\%\)) & 0.0118 (\(-2.8\%\)) & 0.0280 & 0.0280 (-) & 0.027 (\(-2.2\%\)) \\ \(C_{D,f}\) & 0.0106 & 0.0099 (\(-6.4\%\)) & 0.0099 (\(-6.4\%\)) & 0.0106 & 0.0100 (\(-6.3\%\)) & 0.0100 (\(-6.3\%\)) \\ \(C_{D,i}\) & 0.0077 & 0.0079 (\(+2.4\%\)) & 0.0077 (-) & 0.0235 & 0.0241 (\(+2.5\%\)) & 0.0235 (-) \\ \(C_{D,pr}\) & 0.0150 & 0.0140 (\(-6.8\%\)) & 0.0140 (\(-6.8\%\)) & 0.0151 & 0.0138 (\(-8.4\%\)) & 0.0138 (\(-8.5\%\)) \\ \hline \end{tabular}
\end{table} 
Table 1: Aerodynamic coefficients for the isolated UAV wing. Comparison between clean and riblets configurations is carried out at the same angle of attack and at the same lift coefficient for nominal angle of attack of \(\alpha=0^{\circ}\) and \(\alpha=4^{\circ}\). Figure 6: Riblets drag reduction vs. angle of attack for the UAV wing. Drag breakdown is graphically shown at various \(\alpha\) in Figure 7: the focus is on the total, induced, and profile drag on the left panel, and on the contributions to profile drag on the right panel. From the left panel, riblets are seen to mainly act on the profile drag, while the lift-induced drag is essentially unchanged.