Final Trade-Off of the Selected Design Configurations

\label{cha:trade_31} The three design configurations determined as shown in Chapter \ref{cha:trade_83} were defined further by trading off the respective fitting subsystems, shown in Chapter \ref{cha:trade_subsystems}. These three more detailed designs are traded off again to determine the best configuration that will be used for all further design steps.

In this chapter the trade-off of the three concepts is presented. The criteria and weights are presented first, then the choices that were made are justified and the final ranking is shown. The matrices with the pairwise comparisons of the criteria and the options are shown in Appendix \ref{app:AHP_results2}.

Trade-Off Criteria and Weights

For this trade-off, the same criteria as in the trade-off in Chapter \ref{cha:trade_83} are used. The respective relations between the criteria remains the same. Maneuverability is split up into “Longitudinal maneuverability”, “Lateral maneuverability” and “Stall characteristics”. Additionally, to get a more detailed trade-off, “Visibility” and “Safety” are added as criteria, since it is expected that the designs will score differently and thus make the result of the trade-off more clear. To perform the trade-off, the same approach as before is taken, the analytical hierarchy process. The criteria and weights are shown in Table \ref{tab:criteria_finalconfig}.

L/D Ratio The L/D ratio defines how well the aircraft can perform in a race situation. By keeping a low drag the aircraft can fly fast and use its lift efficiently. Furthermore, a high L/D ratio allows to perform tight turns. Generally, this criterion is an indicator for the aerodynamic efficiency.

Complexity This criterion is selected for the same reasons as in the trade-off performed in Chapter \ref{sec:trade_criteria}.

Maneuverability This criterion is redefined and split up in sub-criteria to allow for a better judgement of the performance as it is intended in the race. The sub-criteria are “Pull up and Turn” and the “ability to roll”. Thereby, “Pull up and Turn” is rated higher than “ability to roll”, since all aircraft are assumed to be able to perform to roll rates currently shown by aircraft in the RBAR. A higher roll rate is not assumed to yield clear benefits in the race. Also, the differences in inertia that influence roll behaviour, shown in Chapter \ref{sec:concept_analysis}, show that the canard and the biplaned have a lower rolling inertia, thus allowing for even faster turns. On the other side, an aircraft that can perform faster and sharper pull ups and turns is assumed to be able to finish the race track faster than other planes.

Propulsive Efficiency The propulsive efficiency criterion was defined in Chapter \ref{sec:trade_criteria}.

Visibility Visibility is particularly important for the pilot. In order to judge attitude, orientation and position, the pilot should at all times be able to see the wings, canard and the ground as well as be able to judge his respective orientation.

Safety Safety for the pilot and the spectators is an important criterion for racing operation. Hereby, it is crucial that the pilot is safe from internal systems of the plane that could harm him. Also, the pilot has to be able to evacuate the plane in a fast and safe way.

Take Off and Landing This criterion judges the take off and landing performance of an aircraft. It incorporates the take off and landing distance as well as the range of possible take off and approach angles. Thus, design configurations with small clearance angles perform worse in this criterion. Also, it is judged how well the configuration can perform on which type of surface.

Uncertainty of design This criterion is important for the designers. Since the aircraft under consideration is a newly developed concept but should be able to perform with or even outperform conventional aircraft in races, in order to prove the competitiveness of the design, it has to meet high standards. Thus, when selecting one configuration out of the three remaining ones, there is always the risk that the final design will not live up to the expectations because of the new approach that was taken.

\label{tab:criteria_finalconfig}

Trade-Off Configurations

The factors allocated to the designs in the pairwise comparison can be seen in the tables in Appendix \ref{app:AHP_results2}. The result of the trade-off is shown in Table \ref{tab:last_trade_ranking}.

\label{tab:last_trade_ranking}

The trade-off showed that the canard is the best of the considered options. This can be explained by considering the weights for the different criteria and investigating the pair wise relation. Particularly the relation between the conventional design and the canard has to be considered, since they only differ by 3 % in the trade-off result. The biplane scored considerably low.

The reasons why the canard scores highest in the trade-off:

• The importance of L/D Since the canard configuration has two lifting surfaces and its drag is considered to be similar to the one of the conventional configuration it can have a higher L/D ratio. Also, a shorter wing area, for the same reason, can even possibly decrease drag. This is the main criteria where the biplane scores lowest.

• The importance of maneuverability The canard configuration scores highly in both categories. The roll maneuverability was judged quantitatively, using the roll inertia determined in \ref{sec:MOI_initial}. Because of the decreased wing area the canard scores higher than the conventional design. The canard also excels in the high G turns since it has two lifting surfaces pulling it into the direction of the turn. Thus, no force “out of the turn” is created.

• Complexity is not the most important criterion Even though the structural complexity of the canard is expected to be higher than the others, and the complexity of the aerodynamic analysis is higher as well, this does not cause the canard to score lower overall in the trade-off. However, this is the main reason why the canard and the conventional design are very close in the results.

• The high propulsive efficiency The canard configuration works with a contra-rotating propeller that uses energy more efficiently.

• Uncertainty of design is considered a solvable problem Since canards have been proven to be working designs and even though they are not used in air races yet, this criterion is deemed not as severely as the others. It is expected that the canard can possibly even outperform current designs.

For these reasons, the canard is a challenging but promising choice.