[kach22i]

The YF-23’s Air Inlet Design Was Its Most Exotic Feature You Never Heard Of
https://www.thedrive.com/the-war-zon...never-heard-of

So instead of creating separate intake structures with splitter gaps underneath the YF-23, Northrop installed 'gauzing panels' atop and ahead of where the fuselage meets the leading edge of the air intake. These panels had small holes drilled all over them and would 'suck-up' the boundary layer air 'sticking to' to the fuselage before it entered the air intake. This air was then vented out of a flush aperture and a pair of small doors on the YF-23's upper surface. In effect, it acted like an invisible splitter plate of sorts but instead of separating the air it removed it. The system was called the Boundary Layer Control System and worked automatically.

Having holes drilled into splitter-plates wasn't new. It had been done many times in the past to remove the boundary air that began to manifest itself on the plate itself before entering the engine. For instance, the Eurofighter EF2000 has this feature on its upper splitter plate that is easy to spot. The Super Hornet also uses it on the inside of its intakes. Older aircraft, like the F-4, also used perforations on their splitter plates to remove air clinging to the intake's surface. But integrating a more refined version into the jet's fuselage itself and eliminating the splitter plate or intake offset altogether was new.