deletions | additions       diff --git a/Chapters/6f.Propulsion.tex b/Chapters/6f.Propulsion.tex  new file mode 100644  index 0000000..ca462e5  --- /dev/null  +++ b/Chapters/6f.Propulsion.tex 
...
\section{Propulsion}  \label{sec:dot_propulsion}  %Please do not forget to add new symbols to the nomenclature e.g.: \nomenclature{$t_{element}$}{Thickness coefficient of plate \nomunit{[$-$]}}  Different options are considered and explored for the propulsion system, as presented in Figure~\ref{fig:prop_dot}. An electrically propelled aircraft can accelerates air and therefore create forward thrust by using an air-screw, as first proposed by Leonardo Da Vinci~\cite{Vinci}, flapping wings or a propeller. Other concepts exist but are not explored further due to the limited development budget. This top-level choice has the most influence on the whole design. For an electrically driven propeller however, still many possible features and design options exist that can have numerous influences on the design and are presented in Figure~\ref{fig:prop_dot}.  \subsection{Design Space and Influences on Aircraft Design}  \begin{itemize}  \item \textbf{Distributed propulsion}:  The choice between distributed propulsion and a single propeller has a high impact on the design of the aircraft as the structure needs to provide mounting points and the aerodynamic layout needs to take the slipstream from the propulsion system into account.   \item \textbf{Position}:  The layout of the propeller and its position have an influence on the center of gravity of the aircraft, and thereby affect the overall configuration and weight distribution.   \item \textbf{Motor}:  The design choices for variable pitch angle, cooling method and the electric motor influence the efficiency and the required power. The design of the energy storage system therefore depends significantly on the design of the propulsion system.  \item \textbf{Gearbox}:  Finally the electric motor and rotor need to be connected by a transmission. Depending on the optimum operating speed of the motor a gearbox might be needed in order to ensure optimum propeller speeds. This might result in a trade-off between a heavy gearbox to allow for a more efficient motor at higher speeds or a less efficient motor and direct transmission.  \end{itemize}  As can be seen the propulsion system has a significant influence on other subsystems as well as the whole aircraft, so that design choices have to be made carefully and trade-off criteria need to include influences on the whole system. The concepts of an air-screw and flapping wings are not further elaborated on as they don't promise the required race performance but are still included in the design options.  \subsection{Consideration Pusher and Puller Configuration}  \textbf{Pusher:} In a pusher configuration the propeller is behind the center of gravity (c.g.)having a thrust vector behind the c.g. can create an undesirable control characteristics in which application of power produces a nose down pitching moment. The main benefit why this option is interesting is the reduction of the skin friction drag. Since the fuselage will fly in undisturbed air. A pusher can be placed on the wing or on the fuselage, if it is placed on the fuselage the wing profile drag is reduced due to the absence of prop wash over any section of the wing. But having the engine on the wing creates bending relief and reduces the structural weight. Another benefit of having a pusher is that the aircraft can have a lower wetted area and thus can have a smaller fuselage. This is favourable for a canard pusher configuration since a canard requires a smaller tail arm.  However there are several drawback having a pusher propeller. The main drawback is that the propeller will be in the disturbed airflow of the wing or fuselage. This decreases the efficiency of the propeller by 10 \% . Also because the propeller will be located behind the fuselage or wing within the disturbed flow, the blades will induce vibrations, inducing fatigue and noise. This can be resolved by increasing the distance between the fuselage or wing and the propeller. The blades also need to be stiffened since it encounters flows from several directions due to the wake. Another disadvantage is that the engines will encounter cooling problems since the air won't be pushed into it compared with a puller aircraft. A pusher aircraft will have ground clearance problems and requires a longer landing gears.Another issue is that the propeller is more likely to be damaged by rocks thrown up by the wheels. The stability decreases when the force is applied behind the c.g, this can be visualized by pushing a stick in water. However when it is pulled (a puller configuration) any disturbance is corrected, thus is more stable.  \textbf{Puller:} Placing the propellers in front tends to shorten the fore body allowing for a smaller tail and an improved stability. Another advantage of having the propeller in front is that it gets clean air increasing the propeller efficiency and being more silent. However this will cause that the fuselage and the wing will be in a disturbed flow, increasing the overall skin friction drag. Since propellers are placed in front of the fuselage or the wing allows for a reduced stream tube inflow distortion and less asymmetric loading, decreasing the stresses and the weight of the blades. A puller propeller pushes the air over the fuselage and the main wing, which means that the aileron receives a more direct flow. In pusher configuration the aileron also receives a direct flow. During take-off the propeller can increase the dynamic pressure by rotating, thus decreasing the take-off length. During landing having a puller propeller will not create ground clearance problems. During deep stall a puller is beneficial since the propeller pushes the air over the wing.  However when having a puller propeller the visibility is reduced and the cabin noise is increased. Since the fuselage is in the wake of the propeller the skin friction drag increases. Another drawback is that the propeller creates prop-washes, which are pressure pulses. These pressure pulses creates structural vibrations, which increases the weight. During landing there is a possibility of ground strike when a hard landing occurs.  \subsection{Non-feasible Options}  \begin{itemize}  \item \textbf{Air screw}:  An air screw with one winding and large diameter (Leonardo Da Vinci\cite{Vinci}) would require a large gear box to allow for low rounds per minute while electric motor rotates at higher speeds for efficiency. A smaller diameter air screw with more windings rotating at higher speeds is not accelerating air very efficiently due to compression and induced vortices. This option is therefore disregarded in the design option tree.  \item \textbf{Flapping wings}:  Although this concept promises maneuverability, it does not allow for high speeds and acceleration of which both are required for an air race. It presents in interesting option for acrobatics but is not well suited for the track and speed layout of the Red Bull Air Races.  \item \textbf{Other propulsion concepts}:  The time and money needed for research and testing of an entirely new propulsion concept is not available for this project. The propulsion concept will therefore be restricted to a propeller, as can be seen in Figure~\ref{fig:prop_dot}.  \item \textbf{Other propeller concepts}:  Fully researching and testing a new propeller concept with more stages or a different setup of stages is not possible with the given budget. The design options for a propeller will therefore be limited to single rotor or contra-rotating propellers.  \end{itemize}  Eliminating the non-feasible options slightly narrows down the design space to a propeller with either a single rotor or a contra-rotating propeller. Within this options however, there still exist many detail design choices as can be seen in Figure~\ref{fig:prop_dot}.  \begin{figure}[htbp]  \centering   \includegraphics[height = 0.95\textheight]{Figures/prop_dot.png}  \caption{Design option tree for the propulsion system}  \label{fig:prop_dot}  \end{figure}