## 4 Conclusions  
In this study, the optimal segment control of the active twist rotor is investigated for the power reduction in forward flight. The UH-60A rotor at four selected flight conditions is considered to be the baseline. A numerical model to predict the isolated rotor power in steady-level flights is deployed and validated with experimental data that were obtained from literatures. The optimal schedules and segment layouts of the segment twist control for power reduction are obtained while using a GA optimization framework. When compared with single-harmonic control, multi-harmonic control and segment twist control achieve a further reduction of rotor power. The following are the key conclusions of this paper:  
1. The active twist configurations improve performance by reconciling the distribution of rotor disk loads. The active twist actuations reduce the negative negative-lift area at the tip region on the advancing side, which results in the decrease of the overall induced power. 2. The established optimization algorithm can effectively obtain the optimal solutions of control parameters and segment layouts for uniform multi-harmonic twist control and segment twist control. 3. The effects on rotor power mitigation are limited while using the single-harmonic twist control. In contrast, the multi-harmonic twist control obtains more power reduction due to the refined control schedule along the circumferential direction. 4. The segment twist control reduces more rotor power than the uniform twist control. The load distribution of the rotor disk has been improved in both circumferential and spanwise directions. The segment point of the optimal 2-seg control is at 0.6 L, and divergent control schedules are applied to the inner and outer segments. The optimal 3-seg control includes an individual control segment at the tip region with a schedule that is refined for the wide dynamic pressure range. The 4-seg control and 5-seg control yield limited benefits and they may be penalized with an increase in system complexity. Finally, the optimal segment twist control configuration with a saturation limit is related to the aerodynamic shape of the initial blade. In this study, the optimal segment layouts of the segment twist control configurations are similar to the airfoil layout of the baseline UH-60A rotor blade. The optimal control configuration and performance improvement levels may vary for a rotor with different platforms, airfoils, pre-twists, etc. Moreover, the aerodynamic shape of the baseline blade is a result of the multi-objective optimization of the conventional rotor system, and it is not optimal for the active twist rotor. In future study, an optimization design of the aerodynamic shape that is targeted for the active twist blade may yield further benefits. Conceptualization, X.Z., Z.W. and D.Y. ; methodology, X.Z. and Z.W. ; software, X.Z. ; validation, X.Z. ; formal analysis, X.Z. ; investigation, X.Z. ; resources, X.Z. ; data curation, X.Z. ; writing--original draft preparation, X.Z. and Z.W. ; writing--review and editing, X.Z. and Z.W. ; visualization, X.Z. ; supervision, X.Z. ; project administration, X.Z. and D.Y. ; funding acquisition, Z.W. and D.Y. All authors have read and agreed to the published version of the manuscript. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{
\begin{tabular}{c} **Active Twist** \\ **Configuration** \\ \end{tabular} } & \multirow{2}{*}{**Deployment**} & \multirow{2}{*}{\(C_{W}\) = 0.0065**} & \multicolumn{2}{c}{**Power Reduction**} & \multirow{2}{*}{\(C_{W}\) = 0.0083**} & \multirow{2}{*}{\(C_{W}\) = 0.0083 \\  & & \(\mu\) = 0.25 & & \(\mu\) = 0.35 \\ \hline Uniform & Single-Harmonic & 1.76\% & 4.01\% & 2.27\% & 5.07\% \\ Uniform & Multi-Harmonic & 3.59\% & 6.86\% & 5.84\% & 9.89\% \\ Segment & 2-seg & 4.72\% & 8.95\% & 6.54\% & 11.52\% \\ Segment & 3-seg & 5.47\% & 9.49\% & 6.90\% & 11.91\% \\ Segment & 4-seg & 5.85\% & 9.52\% & 7.05\% & 12.20\% \\ Segment & 5-seg & 6.39\% & 9.68\% & 7.14\% & 12.42\% \\ \hline \hline \end{tabular}
\end{table}