Multidisciplinary design optimization of blended-wing-body transport aircraft with distributed propulsion

The idea of using distributed propulsion has been suggested with the objective of reducing aircraft noise. This paper investigates the effects of such a system on aircraft performance and weight. The distributed propulsion concept considered here involves replacing a small number of large engines with a moderate number of small engines and ducting part of the engine exhaust to exit out along the trailing edge of the wing. Models to describe the effects of this distributed propulsion concept were formulated and integrated into a multidisciplinary design optimization formulation. The most important effect modeled is the impact on the propulsive efficiency when there is blowing out of the trailing edge of a wing. An increase in propulsive efficiency is attainable with this arrangement as the trailing edge jet ‘fills in’ the wake behind the body, improving the overall aerodynamic/propulsion system, resulting in an increased propulsive efficiency. Other models formulated include the effect of the trailing edge jet on the induced drag, longitudinal control through thrust vectoring of the trailing edge jet, increased weight due to the ducts, and thrust losses within the ducts. The Blended-Wing-Body (BWB) aircraft was used as a testbed in this study. Two different BWB configurations were optimized, a conventional propulsion BWB with four pylon mounted engines and a distributed propulsion BWB with eight boundary layer ingestion inlet engines, for a mission of 7750 nm at Mach 0.85 carrying 478 passengers. The results show that the optimum BWB designs have comparable planform shapes and TOGW   of approximately 860,000 lb, but have different weight distributions. The distributed propulsion BWB has a heavier propulsion system (+17.5%+17.5%) and a lighter wing (−11.5%−11.5%) than the conventional propulsion BWB. Although the distributed propulsion BWB has a 1.6% higher lift-to-drag ratio than the conventional propulsion BWB, the fuel weight is still 1.2% higher, mainly due to 3.8% higher specific fuel consumption associated with smaller turbofan engines. The results furthermore show that more than two-thirds of the possible savings due to filling in the wake will be required to obtain this optimum design. Achieving such high savings by filling in the wake will be challenging. However, by developing more efficient small turbofan engines and reducing the distributed propulsion system weight, the necessary savings by filling in the wake will be achieved.