Conceptual Design of Blended Wing Body Airliners Within a Semi-automated Design Framework

Abstract

Blended wing body aircraft represent a paradigm shift in jet transport aircraft design. Stepping away from the conventional tube-and-wing philosophy, they promise benefits over existing or future conventional aircraft. The most significant challenge with the concept is the increased coupling between aircraft design disciplines that has necessitated the development and implementation of multidisciplinary design optimisation routines. A novel conceptual aircraft design program named the Initiator has been developed that is able to design conventional and unconventional passenger transport aircraft, enabling comparisons to be made which are based on the same top level requirements and analysis fidelity. It however lacks the ability to design or analyse the blended wing body. The aim of this thesis is to make comparative studies between the blended-wing-body aircraft and its conventional tube-and-wing counterpart based upon the same design requirements. To this end the work investigates the methods that are required to implement the blended wing body aircraft in a semi-automated design framework such as the Initiator. By developing a novel geometric parametrisation of the blended wing body, the design possibilities have been increased while maintaining straightforward shaping manipulation and robustness. All relevant topics of conceptual aircraft layout are considered, making the resulting aircraft feasible in terms of the integration of its components. Furthermore, methods have been implemented or developed which are capable of analysing the mass, aerodynamic performance and longitudinal stability of the aircraft to a fidelity which is suitable for conceptual design. The mass estimation methods that have been implemented are verified and validated to be within 10% of reference blended wing bodies with a smaller error of 5% being common. There is however significant scatter in reference results, making conclusive statements about accuracy difficult. Drag estimations perform less accurately with drag being overpredicted by approximately 20%. The cause of this over prediction was largely due to empirical corrections for miscellaneous and unaccounted drag sources as is done for conventional aircraft. Wave drag is considerably higher than reference cases (7 versus 1 counts). Considering the applicability of the implemented method to blended wing bodies and the limited specific transonic design that is performed, it is chosen to accept this result as a conservative estimate until higher order validations of the wave drag can be performed. Induced drag was also higher for the test cases but results are inconclusive whether this is an error or a true result of the design choices. Zero-lift drag has however been accurately estimated by the novel implementation of empirical methods. Test case blended wing body and tube and wing aircraft were formed in the 150, 250 and 400 passenger classes. The comparisons of the resulting aircraft show that the blended wing body is feasible at the fidelity level achieved. They have reduced mass, improved aerodynamic efficiency and higher fuel economy. Trends show that the improvements over tube and wing aircraft increase with aircraft size. The qualitative results contained herein should still be treated as provisional since the implementation of the concept is not complete and remaining topics could still have significant effects on the results.