Russian Su-57 Aircraft Thread (PAK-FA and IAF FGFA)

Tirdent said:

In terms of pressure recovery it does. Nothing you've managed to supply to date contradicts that view.

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Again, conical flow doesn't tell us anything about boundary layer diversion or how boundary layer diversion affects pressure recovery, which is what DSI is all about. Nothing you have managed to supply contradicts that view.

Tirdent said:

The point still stands - you're once more trying to make sources which only deal with a very restricted field prove a more general point that they were never intended (and are unable) to address.

The Su-57 has a variable intake - your source does not concern itself with that at all, clearly referencing fixed intakes exclusively. And even there the statement only holds true in specific cases, not as a general rule like you keep asserting (which is also reflected in the wording: "certain favourable characteristics compared to other fixed intakes" doesn't necessarily refer to *all* other fixed inlets). This is borne out by the two sources I've provided which indicate some conventional types provide better pressure recovery than DSI.

There is not actually any contradiction between what your sources claim and what mine say, none of them is wrong - your generalization of fixed/conventional intakes is.

Exactly - a fixed intake provides optimum pressure recovery only in a narrow set of operating conditions, while a variable intake performs much like an optimized fixed intake over a far wider range. No assumptions, discrepancies or generalizations involved at all.

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The Su-57 uses variable ramp in its intakes, which means it has the problems associated with fixed inlets that were analysed by the sources. The sources don't need to explicitly compare DSI to every other possible inlet design. The sources only need to show the factors giving DSI advantages over conventional fixed inlets, factors such as lower distortion, lower drag, and less performance degradation from boundary layer. These factors are still applicable when comparing DSI to other inlet designs.

Your point has no ground to stand on, because I am not the one who is generalizing. You are. This is a recurring theme: you generalized pressure recovery as the performance metric for inlet; you generalized endurance as the criteria for wing choice. Right now, you are trying to find one exception and generalize that DSI not having superior performance over other fixed inlets, despite multiple sources explicitly state that DSI has better performance.

Tirdent said:

As I said, for the purpose of proving that DSI gets away without bleed whereas a conventional intake doesn't (which is what the paper is about) it's a sensible comparison to make, but for the question of pressure recovery in configurations applicable to real-world aircraft (which is what our discussion is about) it's useless. Fitting a BL bleed to a DSI also adds all of the associated drawbacks back in too, not just the advantages.

DSI does start from a higher pressure recovery base point in these specific sources, but it's due to the more efficient shock system (also available to a conventionally designed inlet in principle), not any magical property unique to DSI. Again, run the numbers - the difference in pressure recovery at Mach 1.6 (design point) between the shock system of the DSI in [6] and a wedge with the same oblique shock angle is a pretty good match to the difference in experimental results seen in figure 16b at Mach 1.53 (and it even predicts the increase in difference at Mach 1.8 off-design pretty well too). Both intake designs obviously suffer virtually identical penalties compared to their respective theoretical potential from incorporating the required means of BL diversion (bump or diverter + bleed) - the offset between the two remains constant in practise.

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You accused the paper of making apple-to-orange comparison, when the paper in fact made an apple-to-apple comparison by looking at both DSI and the fixed inlet without bleed system. From sources [4] and [5], we see DSI's better pressure recovery is due to ingesting less boundary layer, or in other words being less affected by shock-boundary-layer-interactions. This is not magic, but also clearly not related to shock structure.

Advantages available to fixed inlet such as bleed system is available to DSI as well, with the result of DSI still having higher performance. Unlike conventional fixed inlets, DSI can better adapted to non-design-point operating condition, as shown below. So, if we really want to talk about more efficient shock system, what is available to DSI is not actually available for conventionally designed fixed inlets.

Tirdent said:

Yes - having adopted it, the US found its pressure recovery inferior to the best available 4th generation intakes and is researching alternatives. Apparently, pressure recovery comparable to a good 4th generation variable inlet is still relevant to requirements they consider relevant for the future, even though it works well for the F-35.

And again, where are all those super efficient, low-drag (yeah, drag is important, but DSI has no fundamental advantage in this regard - we've been through how this follows from the design process before) DSIs on modern SST concepts? Your attempt to support a generalization with a couple of singular examples just doesn't work - obstinately ignoring the counterexamples doesn't make that reality go away.

No, the J-20 is the anomaly: canards (playing devil's advocate here).

If the Su-57 is an anomaly in terms intake type that's most likely because it is an anomaly with respect to the requirements driving intake design.

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You made it sounds like US blindly adopted DSI and ended up in some big blunder when that's not the case. Keep in mind China adopted DSI as well. The chance of multiple engineering teams in two different countries making the exact same blunder on multiple projects is practically non-existing, which makes your view on DSI the incorrect one. Research is always ongoing for ever better alternatives. This is how progress is made and doesn't reflect DSI in any bad way. One day, DSI will too be obsoleted, but right now it is variable geometry inlet which is outdated. Aside from the Su-57 anomaly, no new fighter aircraft employs variable geometry inlet. The fact that I can point out real-world examples whereas you can only point out a paper airplane (a non-fighter and one that doesn't even use external compression inlet) speaks volume.

Guys each intake has their own merits. The decision to use either is obviously a conscious one. DSI is better than VG on J-20 for its purposes and anticipated missions. VG intakes are better for Su-57 for its own purposes. Chinese engineers are more than capable of designing a VG intake, they chose not to. Intakes are far from the most difficult task when designing a fighter. I doubt Russian engineers will have trouble with DSI.

ougoah said:

Guys each intake has their own merits. The decision to use either is obviously a conscious one. DSI is better than VG on J-20 for its purposes and anticipated missions. VG intakes are better for Su-57 for its own purposes. Chinese engineers are more than capable of designing a VG intake, they chose not to. Intakes are far from the most difficult task when designing a fighter. I doubt Russian engineers will have trouble with DSI.

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American and Chinese engineers can implement variable geometry inlets but deliberately chose not to. The same can't be said for Russian, since they never demonstrated a working DSI or fixed Caret inlet. The weight saving gain from abandoning variable geometry inlet would be equivalent to having more powerful engines. Considering the structural issues encountered by early prototypes, and the plan of using more powerful engines on the Su-57, the Russian clearly value high thrust-to-weight ratio. In other words, the argument about Su-57 having different requirements is just cope, much like typical Russian fanboys explaining Su-57's poor stealth by saying stealth isn't an important requirement.

Engineer said:

American and Chinese engineers can implement variable geometry inlets but deliberately chose not to. The same can't be said for Russian, since they never demonstrated a working DSI or fixed Caret inlet. The weight saving gain from abandoning variable geometry inlet would be equivalent to having more powerful engines. Considering the structural issues encountered by early prototypes, and the plan of using more powerful engines on the Su-57, the Russian clearly value high thrust-to-weight ratio. In other words, the argument about Su-57 having different requirements is just cope, much like typical Russian fanboys explaining Su-57's poor stealth by saying stealth isn't an important requirement.

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How much weight does DSI save over alternative intakes? I believe the reduction in RCS is well worth switching to DSI but beyond that, each have their own advantages. Overall though it would make sense that DSI is a superior choice otherwise the latest three 5th gen fighters including prototypes won't be using this.

Engineer said:

fixed Caret inlet

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Su-34 is a sci-fi plane, or su-24 was?
Come on, it's the simplest one to design. It just doesn't really go beyond M=2.0 very well.
Same with DSI.

the argument about Su-57 having different requirements is just cope, much like typical Russian fanboys explaining Su-57's poor stealth by saying stealth isn't an important requirement.

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Plane isn't designed on forums by fanboys.

ougoah said:

How much weight does DSI save over alternative intakes? I believe the reduction in RCS is well worth switching to DSI but beyond that, each have their own advantages. Overall though it would make sense that DSI is a superior choice otherwise the latest three 5th gen fighters including prototypes won't be using this.

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It is said to be .

Gloire_bb said:

Su-34 is a sci-fi plane, or su-24 was?
Come on, it's the simplest one to design. It just doesn't really go beyond M=2.0 very well.
Same with DSI.

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Su-34 uses fixed ramp inlet, not fixed Caret inlet.

Engineer said:

Another thing, the Su-57 doesn't have S-duct either.

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It doesn't. It is known to be designed with composite blockers, though.

At the same time, it carries several features unfound on other 5th gens.(at least surviving ones)

Engineer said:

Again, conical flow doesn't tell us anything about boundary layer diversion or how boundary layer diversion affects pressure recovery, which is what DSI is all about. Nothing you have managed to supply contradicts that view.

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But that's just the point - providing that both the DSI and the conventional intake are well-designed, according to [6] there is no effect (*relatively* speaking). The inlet with a shock system which has better pressure recovery without regard to BL effects (i.e. DSI) merely maintains that advantage after consideration of BL effects and a means of diversion (bump or diverter + bleed, as the case may be) are introduced - the lead doesn't get bigger than it already is. Both intake types therefore suffer a penalty of the same magnitude compared to their theoretical potential, so for a relative comparison of pressure recovery it's the shock system which matters most, not the method of BL diversion.

Engineer said:

The Su-57 uses variable ramp in its intakes, which means it has the problems associated with fixed inlets that were analysed by the sources. The sources don't need to explicitly compare DSI to every other possible inlet design.

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If that's the blanket statement you intend to make, then yes, sources you submit as proof of it do have to explicitly state DSI is better than every other design or at least lead back to principles that are fundamental enough to apply universally. Else they don't hold as proof of the point you are arguing - that an advantage over a select few particular types of conventional means a superiority across the board is an unproven assumption (in fact, as mentioned below, it's been proven incorrect).

Engineer said:

The sources only need to show the factors giving DSI advantages over conventional fixed inlets, factors such as lower distortion, lower drag, and less performance degradation from boundary layer.

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Compared to some other designs, but plainly not *all* other designs. Or are you genuinely trying to argue NASA and the USAF (and several others, for that matter) are wrong?

See, that's where your argument falls apart, you're assuming the pressure recovery advantages of DSI over certain specific conventional types translate into an advantage over all conventional inlets in general. That assumption is demonstrably wrong.

Engineer said:

This is a recurring theme: you generalized pressure recovery as the performance metric for inlet; you generalized endurance as the criteria for wing choice.

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Absolutely not - believing that requires you to completely mis-read what I've been saying. Pressure recovery is however one area where DSI is inferior to some conventional types of inlet, hence for aircraft requiring very good performance in this regard DSI is not the right choice. Again, I do not generalize at all, I'm merely pointing out that it's the requirements which drive these decisions. As always, there is no one-size-fits-all solution.

Engineer said:

You accused the paper of making apple-to-orange comparison, when the paper in fact made an apple-to-apple comparison by looking at both DSI and the fixed inlet without bleed system.

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Correction: I accused YOU of making an apples-to-oranges comparison. For the point the paper intends to make it's a pertinent device - trouble is, the paper isn't aimed at answering the question for which you're attempting to use it as proof. That isn't a problem with the paper however, it's just you trying to use it as something it cannot possibly be by design.

Engineer said:

From sources [4] and [5], we see DSI's better pressure recovery is due to ingesting less boundary layer, or in other words being less affected by shock-boundary-layer-interactions. This is not magic, but also clearly not related to shock structure.

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Again, for the purpose of proving that DSI can function without a BL bleed whereas a conventional intake cannot it's a sensible approach, but that's not what we're discussing. We're arguing about intakes that are applicable to a realistic aircraft design, and the bleedless 2D intakes from those papers clearly do not meet that criterion - they're merely thought experiments, notional vehicles for illustrating a point very different from the subject of our debate.

Engineer said:

Unlike conventional fixed inlets, DSI can better adapted to non-design-point operating condition, as shown below. So, if we really want to talk about more efficient shock system, what is available to DSI is not actually available for conventionally designed fixed inlets.

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I have had that paper for ages and it nowhere specifies what exact kind of intake that "conventional trend" graph represents (a more recent, very similar Chinese paper went on to copy it without providing more detail either). As we've seen, this would be rather important to know though, because in terms of pressure recovery "conventional intake" is far too broad a category to be generalized like that.

Engineer said:

You made it sounds like US blindly adopted DSI and ended up in some big blunder when that's not the case. Keep in mind China adopted DSI as well. The chance of multiple engineering teams in two different countries making the exact same blunder on multiple projects is practically non-existing, which makes your view on DSI the incorrect one.

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If that's what it sounded like to you, you seriously need to work on your reading comprehension. I explicitly stated in this thread that I believe DSI is the best choice for both the J-20 and F-35 - my point was always that the Su-57 diverging from this pattern is NOT an indication that the others got it wrong, but that the requirements are different. Please stop putting words in my mouth.

ougoah said:

Guys each intake has their own merits. The decision to use either is obviously a conscious one. DSI is better than VG on J-20 for its purposes and anticipated missions. VG intakes are better for Su-57 for its own purposes. Chinese engineers are more than capable of designing a VG intake, they chose not to. Intakes are far from the most difficult task when designing a fighter. I doubt Russian engineers will have trouble with DSI.

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"Guys"? Between Engineer and myself, only one of us has disagreed with what you've stated here and it sure as hell wasn't me In fact, it's what I've been saying all along in a nutshell.

ougoah said:

How much weight does DSI save over alternative intakes? I believe the reduction in RCS is well worth switching to DSI but beyond that, each have their own advantages.

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About 140kg per F-35, though probably quite a bit more for the Su-57 (larger and has movable ramps, but OTOH higher specific thrust engines):



which would have to be weighed against the penalty in pressure recovery, however. It therefore doesn't necessarily translate into a SEP benefit because (depending on flight condition) the same engine will perform better with the variable intake.

Since DSI pressure recovery is unremarkable compared to that achievable with the same shock system in a conventional design and availability of such conventional inlets did not render variable intakes redundant in the past, there apparently must be a net benefit for some performance needs. The question is merely how prominently such requirements figure in the mission you are designing your aircraft for (more prominently on the Su-57 than on its Chinese and American counterparts, apparently).