Evaluation of the impacts of the Objective Function definition in Aircraft Conceptual Design
CHALLENGE - It is widely known that for a given set of mission requirements the reduction of fuel weight (Wfuel) and maximum takeoff weight (MTOW) have a favorable effect on flight performance and on operating expenses. Minimum fuel and empty weight are both desirable properties in aircraft design, but since they are conflicting objectives, they cannot be achieved in the same design and hence, designers have focused on minimizing the MTOW as the objective function for aircraft conceptual design.
SOLUTION - A conceptual design framework integrating technical and financial disciplines was employed to optimize a case study of a mid-range business jet. For this reference case, four objective functions were explored in three different optimization problem formulations. Three figures of merit were chosen for evaluation: empty weight, block fuel and maximum takeoff weight. Simplified equations correlating these objective functions were derived. Then a number of assumptions were determined to derive equations that try to correlate technical figures of merit to economical parameters such as industrial cost and financial return to the airframe manufacturer. The financial return based on value-driven design approach was added to the objective functions list for evaluation. Analysis of the results obtained by the simultaneous minimization of block fuel and empty weight endorses that these are conflicting objectives and therefore multiobjective optimization is chosen using optimization platform modeFRONTIER.
BENEFITS - The lowest cost design is not the one with the lowest empty weight nor the lowest thrust, but a combination of both which has a good agreement with the minimum MTOW design. Analysis reveals that the designs that would have been obtained by optimization using empty weight, block fuel or maximum takeoff weight as objective functions need a 1600m field length to take-off at MTOW, sea-level standard conditions whilst designs with the highest financial return need only around 1320m for the same conditions.