Performance Assessment of a Boundary Layer Ingesting Distributed Propulsion System at Off-Design

As research on boundary layer ingesting aircraft concepts progresses, it becomes important to develop methods that may be used to model such propulsion systems not only at design point, but also over the full flight envelope. This research presents a methodology and framework for simulating the performance of boundary layer ingesting propulsion systems at off-design conditions. The method is intended for use as a preliminary design tool that may be used to explore the design space and identify design challenges or potential optimum configurations. The method presented in this research enables the rapid analysis of novel BLI configurations at a preliminary design stage. The method was applied to a case study of NASA’s N3-X aircraft, a blended wing body concept with a distributed propulsor array ingesting the airframe boundary layer. The performance of two propulsor in the array was compare, one at the airframe centreline and one at the extreme edge of the array. Due to difference in flow conditions, the centreline propulsor was shown to be more efficient at off-design than the end propulsor. However, this difference in efficiency disappeared at sea level static where the boundary layer thickness is negligible and mass flow ratio is high. Difference in thrust produce by the two propulsors was instead due their different sizes. Performance of the propulsor array as a whole was also presented both independently and including a link to a pair of turbogenerators to provide power. At off design, it was found that there was a discrepancy between the maximum power available from the turbogenerators at off-design operating points and that demanded by the propulsor array operating at 100% fan rotational speed. This discrepancy means that the propulsor array’s performance is limited by the turbogenerators at off-design, particularly for low speed, low altitude operation.

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