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\chapter{Interface Definition and N$^2$ Charts}  \label{cha:N2}  %Please do not forget to add new symbols to the nomenclature e.g.: \nomenclature{$t_{element}$}{Thickness coefficient of plate \nomunit{[$-$]}}  In this chapter an overview is given of the interfaces between the various subsystems that have been identified by means of an N$^2$ chart. The N$^2$ chart serves as a tool in the design process \cite{Shroff2015}. The N$^2$ chart is given in Figure~\ref{fig:N2}.  The N$^2$ chart is structured such that the outputs of subsystems are written as horizontal elements in the matrix and the inputs for subsystems are written as vertical components, with the subsystems listed on the diagonal. By so doing, the N$^2$ chart creates a straightforward overview of the correlation between two subsystems or likewise of one subsystems with all other subsystems. This way it becomes very clear how changes or iterations to a particular subsystem impact other subsystems. With the design process being a continuous iterative process, the N$^2$ chart can be used as a tool to ensure that the impact of changes to one subsystem on another subsystem is not overlooked. Furthermore, the N$^2$ chart can serve as a tool to assist in the trade-offs of various design options.  Not all correlations will be elaborated upon here in the text, but a few will be mentioned for clarification and to serve as an example for the other relations.  \begin{itemize}  \item The landing gear increases the drag experienced in flight (output of the landing gear), consequently more power will be required to propel the aircraft (input for the power system). This correlation has to be considered in the trade-off for a fixed or retractable landing gear.  \item The position of the power system will impact the location of the center of gravity (cg) (output of power system), consequently it will impact the longitudinal position of the wing (input for the wing) and size and position of the empennage (input empennage) in order to obtain desired longitudinal stability and control characteristics. Hence, the weight and allocation of the power system in the fuselage has to be carefully considered in both the design option trade-offs as well as during the positioning of the various subsystems in the fuselage.   \item The efficiency of the propeller and motor (output of the propulsion unit) impact the required power from the power unit (input). Thus, the N$^2$ chart helps identify that the choose for a particular propeller and motor might require more or less overall power.   \end{itemize}  \begin{figure}[htbp]  \begin{sideways}  \includegraphics[width = \textheight]{Figures/N2_Fig.png}  \end{sideways}  \centering  \caption{An overview of the interfaces between the various subsystems given in an N2 chart.}  \label{fig:N2}  \end{figure}