Trade-Off Criteria

\label{sec:trade_criteria} In order to perform a trade-off and thus find the ideal configuration and subsystems, criteria have to be defined that are deemed favourable for the respective components. Therefore, the requirements as well as performance and stakeholder needs are considered.

This section describes the criteria that are important in the configuration trade-off process. Firstly, the criteria for the entire system and thus the configurations that were defined beforehand are outlined. Thereafter, the separate criteria for the individual subsystems are named. To keep the trade-off practical for the top-level configuration, only four main criteria are considered.

Criteria for the Complete Configurations

For the entire aircraft, the following criteria are particularly important:

• L/D The aircraft should produce lift efficiently, since cruise and climb have to be performed with minimum power. This is important since the aircraft is newly developed and electric and power consumption should be kept at a minimum. Also, the drag should be kept low. To judge the efficiency and many flight performance characteristics, the lift per drag ratio should be investigated.

• Complexity A more complex design introduces additional costs and efforts to realize the design. In order to stay within the proclaimed boundaries, a less complex design is favourable. Also, increased complexity will make production more difficult and may lead to the need for subcontractors.

• Maneuverability During the race, the aircraft has to perform demanding and complex maneuvers. To allow for a fast lap time and safe operation, the pilot has to be fully in control at all times. Since many maneuvers, such as knife-edge-turns, require a fast roll rate, but only fewer maneuvers take place in vertical direction, the maneuverability in lateral direction was deemed more important than in longitudinal direction. This was considered in the criteria comparison.

• Propulsive Efficiency Propulsive efficiency defines the use of the available resources, and thus allows to judge the performance in terms of demand. Also, for the same origin of electricity, a better propulsive efficiency can yield information on the expected sustainability of a system. Since the race is short and demands full power over its time, propulsive efficiency was deemed less important compared to the other criteria. It would have a far higher value when considering a long range or long time mission.

Criteria for the Subsystems

Since the subsystems perform different tasks, different criteria are important. Weight, complexity and performance, however, are always crucial, whereby performance is to be considered with respect to the individual subsystems too, for example, either deliver as much power per energy consumed (propulsion unit) or provide structural integrity and shield the pilot (fuselage). The trade-off criteria are shown in Table \ref{tab:subsyscriteria}. The cost of a subsystem is often corresponding to the complexity of the design, which is considered in cases where it is complicated to determine cost influences of a certain design, such as for the control system, or where the complexity greatly influences the manufacturability. These aspects are closely related. Also, the readiness of technology is considered in subsystems that are likely to make a jump in its development over the next 10 years and or that are not ready nowadays to a point where they are fully applicable. On top of that, safety is considered when the system can potentially directly harm the pilot.

\label{tab:subsyscriteria}