If I'm not wrong, the research on DSI started in the early 1990s.
The airframe of the F-22 was designed in the 80s and design selection for the ATF was made in 91. You can't just tack on some new research you made in the mid 90s when the aircraft is already beginning production, particularly for a stealth design where particular shaping is crucial to meeting performance requirements.
Don't get me wrong. The DSI intake obviously has a lot of advantages over the variable intake, like lower frontal RCS (compared to a variable intake) and weight saving.
My opinion is that the DSI intake's main function is to control inlet airflow at different speeds, and not to directly improve stealth but reducing radar reflection. (Woudn't having nothing in the way of the radar wave be the best option? - assuming no direct line of sight to the engine)
Why can't a DSI be implemented for more than one reason? Besides, it makes no sense to implement it primarily or solely for controlling inlet airflow when you already have a variable intake that does that job just fine. The only reason to switch to a DSI would be if it offers additional benefits. In addition there is no such thing as "nothing" in front of the radar wave. Eventually that radar will hit something, more specifically the interior of the inlet, and maybe even the engine fan itself. If I'm not wrong, when the radar does it will bounce off the interior surfaces, making it much harder to control the radar rellection (unless you have reentrant shaped structures inside). Think of it this way, does your voice echo more inside or outside a box (even if that box is s-shaped
)?
The fact is it's not possible to control inlet airflow without the control surfaces (of either the DSI or the variable intake) that would contribute to radar reflections, however tolerable it may be in most scan angle and operational circumstances by careful design of the DSI inlet with stealth in mind. On the other hand, the F-22 is able to handle this inlet flow problem differently.
There is no such thing as a surface that does not add to radar reflection. Radar reflects on all surfaces (even to a degree on RAM I think). It's more about reflecting the radar in a way that guarantees it doesn't come back to the source, and S-shaped inlets are one way to control for this. However, it's less relevant what additional surfaces you have as much as how you've oriented and shaped those surfaces in relation to the overall structure to control the direction in which the radar bounces off on, so a DSI can either help or hurt a plane's stealth characteristics depending on how its shaped and oriented (that is to say type of inlet has nothing to do with how stealthy a plane is, but the design of the inlet does). However, one main benefit the DSI offers over a variable intake is that there are no moving parts, so the geometry of your plane isn't changing, and so you're guaranteed to have a certain unchanging radar characteristic (think of how when weapons bays open stealth can be compromised).
How the F-22 handles radar reflection probably has to do with how the inlets are structured internally. It's also unclear how the F-22 guarantees performance at higher speeds without some method of controlling the airflow at different speeds, but that probably has a lot to do with engine design as much as it has to do with inlet design. Nonetheless, the type of intake has nothing to do with how well stealth can be achieved. It's about how the intakes are placed and shaped. Just because the F-22 can achieve high performance and stealth with a fixed inlet does not mean that other designs can't reach or exceed the same level of stealth and performance.
