Boundary layer ingesting inlet design for a silent aircraft

Abstract

(cont.) common nacelle, L/D ratios between 2.5 and 3.0, fan face to throat area ratios above 1.06, and offsets lower than 11%. Curvature ahead of the inlet should be avoided as well as bifurcations inside the duct. Inlet performance with an evolved version of the airframe decreased, mainly due to a thicker boundary layer. Although further tailoring of the geometry is needed, the above guidelines should provide both direction and rationale for these alterations.

Engine cycle trade studies were conducted to determine how a propulsion system should be integrated with an airframe of a "Silent Aircraft", i.e. an aircraft designed with noise as the first consideration. Embedded, boundary layer ingesting, ultra-high bypass ratio engines were found to be the most appropriate configuration. Based on the results of the study, inlets for twelve, eight and four engine configurations were designed and assessed. The inlets ranged from standard S-ducts to unconventional mail-slotted inlets. Circumferential pressure distortion and pressure recovery were used as figures of merit and were determined from 3D Navier-Stokes simulations. Four and eight engine inlet configurations were found that met the target criteria. The former had the lowest distortion level, a result of lower boundary layer thickness to inlet height ratios ([delta]/H [approximately equal to] 0.3). The eight-engine inlets ingested more of the boundary layer, implying a lower wake momentum deficit from the airframe and thus a potential for fuel burn savings of up to 3% compared to a non-boundary layer ingesting engine. The results of the computations have led to the development of some general guidelines for these types of inlets. The most important parameters are L/D, centerline offset (which has a large impact on boundary layer growth), and inlet throat to fan area ratio. The last variable determines the external compression and the diffusion inside the duct. There is a trade between the reduction of loss and distortion level in that higher fan face Mach numbers increase the former but reduce the latter. The control of peaks in the duct Mach number is essential in reducing friction losses. For the airframe examined, the parameter regime of best performance has inlets integrated