Why did they not design an S-duct into this fighter. Does it mean the fan blocker is definitely effective? Western commentary on this S-duct issue seems to suggest that Russian engines are not able to deal with the "turbulence" from S-ducts or they just simply can't do it. This is obviously hard to believe given how many other fighters in Soviet and Russian arsenal had S-ducts, not to mention the J-10 and J-20 both fly with S-ducts while using AL-31s.
Seems like a real oversight but there's gotta be more to it. Radar blockers? Offset fan surfaces? If it's possible to design stealthy bomber and fighter airfoils, it's possible to design stealthy fan surfaces. Personally I think given the slightly offset angle of the engines and the position they occupy from almost all frontal angles, the blades are designed to bounce energy off some internal sections which either absorbs and/or reflects the energy further away from source.
Because an s-duct did not fulfill their requirements as well as a short, straight duct with a radar blocker.
The notions that Russia would be unaware of the fundamentally important requirement to deal with engine face radar signature or incapable of designing an s-duct for lack of money are indeed risible. As you say, a number of Soviet/Russian aircraft have curved inlet ducts and even if in most of these the reason isn't RCS reduction (so you could be tempted argue there is some "secret sauce" they lack), that is not true for the Su-47. In a document on LO work performed at Sukhoi, its s-ducts are not only mentioned, but specifically described as a deliberate RCS reduction measure - they were certainly no accident. So there is institutional experience with s-ducts in the company, and in terms of engine matching it was a success too, with the Su-47 flying at high angles of attack during air shows (despite its D-30F6 engines which were hardly intended for such antics).
So how does/will the Su-57 solve this issue? All indications are that it will use a blocker of some type - there is a Sukhoi patent on inlet blockers but it is (probably deliberately) vague and describes a multitude of possible configurations. For the most part it only serves a further good clue that this is the kind of solution which will be adopted - although it can be interpreted as hinting that the blocker will not be integrated with the engine fan IGVs. That's a bit of a surprise given the rather exotic shape of the Izd.30 fan IGVs (photos of which were leaked a while ago) that are reminiscent of the GE YF120 configuration for the YF-23, which despite s-ducts had incomplete engine face masking. Possibly the separate blocker is only intended for the interim Izd.117 engine, so it retains maximum commonality with the 117S on the Su-35S?
Anyway, this begs the question of how an s-duct is inferior, given its widespread adoption elsewhere. While it's definitely an elegant solution there are certain drawbacks:
- Introducing sharp bends into an air duct inevitably increases flow resistance, i.e. causes pressure loss. Careful design can minimize but never eliminate this effect.
- One solution is to make the duct very long, so complete engine masking can be achieved even with gentle, large radius bends. Longer ducts increase pressure loss from wall friction, however.
- The offset between intake aperture and engine face may increase maximum cross sectional area which can cause drag issues from poor cross sectional area distribution and increased wetted area.
- Particularly if the duct is long, it takes up a lot of internal volume that is already at a premium in stealthy aircraft due to internal weapons bays and high internal fuel fraction (to avoid drop tanks). This can drive up weight.
None of these are show stoppers and by careful packaging the F-22 and J-20 have dealt really well with the third point in particular, but it gives an indication how a short, straight duct with a blocker could be an attractive alternative. If the pressure loss penalty from the blocker can be kept low enough (for example by integrating it with the fan IGVs) to ensure competitive engine performance it might well yield a handsome weight saving at the aircraft level. So why has nobody else done it? Well, they have - the Super Hornet is operational with a blocker, the YF-23 would likely have required one for complete engine masking, the Boeing ATF entrant and JSF design certainly had one (the latter beating MDD's s-ducted design)...
All we know is that Sukhoi did its homework on the subject and apparently came away convinced that they were better off without an s-duct.
