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

Tirdent

Junior Member
Registered Member
LOL! You keep conjuring up with "certain requirement" which only exists within your imagination.

I've been very clear on which requirements those are, if you somehow manage to miss (or deliberately ignore) that regardless I can't be held accountable for it.

Have you considered the "certain requirement" where variable geometry wing doesn't apply at all? That's "certain requirement" is reality at the moment.

Sure, in the very quote which you're ranting about there:

Modern fighters strongly prefer LERX because roles and development priorities have shifted. Due to weight, VG wings were never very popular on classical fighters (MiG-23 and F-14 are practically the only exceptions, and the Tomcat has a strong air defence bias) and once for cost reasons such types took on the jobs formerly performed by dedicated strike & air defence platforms, they went away altogether. Stealth later added another powerful incentive to prefer fixed wings due planform alignment (nonetheless, the stealthy NATF & A/F-X were designed in the 1990s with VG wings for patrol endurance!) and removed the need for low altitude penetration in the strike role.

Do you even read what I write?

Modern fighters use LERX as opposed to variable geometry wing, that's what matter.

Not quite - what matters is also the *reason* why that is this way. Is there actually a better solution to the problem or has it simply diminished in importance relative to other issues, as with the needs which VG wings address (which never were all that relevant in the fighter role to begin with)?

It just isn't as simple as "aircraft X and Y don't use solution A, so aircraft Z which adopts solution B instead is automatically wrong to do so", because that assumes aircraft Z is designed to meet identical requirements to X and Y which is liable to be a flawed premise. Russia and the US went with conventional horizontal stabilizers while China settled on canards - China and the US selected DSI while Russia prefers a variable caret. Who are you to say any of those countries is wrong on any of those choices?

I assume you know this already? Apparently not, since you act like you have discovered a new continent or something. The conflicts between subsonic and supersonic cruise has been addressed by relaxed stability and vortex lift. This is the basis for J-20's aerodynamics layout, as described in
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While the J-20 paper is very interesting, it is irrelevant to the subject of VG wings. Like most fighters (recent or even legacy), the requirements it must meet are of a nature that makes VG wings a non-starter from the outset, as outlined earlier. You might as well post a paper on the Z-10 for all it would be worth in proving anything on this issue - the vast majority of the contents is devoted to high-lift/high-AoA maneuvering flight conditions while subsonic loiter isn't mentioned at all (even subsonic cruise is only referred to once in passing). Just read the figure captions - max lift and high AoA all over the place.

Pretty good indication of where the priorities lie and why it has no bearing on the issue of VG wings - thanks for making my point, I suppose :rolleyes:
 
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Engineer

Major
I've been very clear on which requirements those are, if you somehow manage to miss (or deliberately ignore) that regardless I can't be held accountable for it.

Sure, in the very quote which you're ranting about there:

Do you even read what I write?
I have been very clear that your requirements have no bearing with those found on modern fighter aircraft.

Not quite - what matters is also the *reason* why that is this way. Is there actually a better solution to the problem or has it simply diminished in importance relative to other issues, as with the needs which VG wings address (which never were all that relevant in the fighter role to begin with)?

It just isn't as simple as "aircraft X and Y don't use solution A, so aircraft Z which adopts solution B instead is automatically wrong to do so", because that assumes aircraft Z is designed to meet identical requirements to X and Y which is liable to be a flawed premise. Russia and the US went with conventional horizontal stabilizers while China settled on canards - China and the US selected DSI while Russia prefers a variable caret. Who are you to say any of those countries is wrong on any of those choices?
No, the reason does not matter, because in both cases variable geometry wing doesn't meet requirements as well as alternatives. This is simply a case of variable geometry wing being obsoleted.

While the J-20 paper is very interesting, it is irrelevant to the subject of VG wings. Like the vast majority of fighters (recent or even legacy), the requirements it must meet are of a nature that makes VG wings a non-starter from the outset, as outlined earlier. You might as well post a paper on the Z-10 for all it would be worth in proving anything on this issue - the vast majority of the contents is devoted to high-lift/high-AoA maneuvering flight conditions while subsonic loiter isn't mentioned at all (even subsonic cruise is only referred to once in passing). Just read the figure captions - max lift and high AoA all over the place.

Pretty good indication of where the priorities lie and why it has no bearing on the issue of VG wings - thanks for making my point, I suppose :rolleyes:
Actually, the J-20 paper is very relevant because it addressed the very points you have raised. The paper devoted the entire section 3 talking about transonic lift-to-drag, with the following opening:
Trans-sonic lift to drag characteristics determine an aircraft's maximum range and sustained turn capability. The future fighter's demands for these characteristics will exceed those of modern 4th gen. fighters. Modern fighters employ the strategies of relaxing longitudinal stability, adapting wings with medium sweep and aspect ratio, twisting the wing, and adding wing-bending mechanisms to greatly improve the lift-to-drag characteristics...

Section 5 deals with supersonic drag characteristics.

The paper is very comprehensive, and will probably address all the points you will ever raise on the subject of variable wing. The benefits associate with variable wing have been provided with alternatives, proving my point exactly.
 
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Gloire_bb

Captain
Registered Member
and the F-35, while named the JSF, turns out to have all the F-22 aerodynamic tweaks,
But at the same time:
1)it's a much smaller aircraft , but size of it's bays is actually larger and much deeper.
2)On top of that, internal fuel carriage is huge.
3)While having a single engine helps alot, it is still not enough.
In the end, aircraft clearly sacrifices air specs for a2g mission, trying to get as much as possible back through other means, as well as relying on overall US Air superiority in all regards(organization, support assets, experience, interservice cooperation and so on).
 

Engineer

Major
Russia and the US went with conventional horizontal stabilizers while China settled on canards - China and the US selected DSI while Russia prefers a variable caret. Who are you to say any of those countries is wrong on any of those choices?

Pretty good indication of where the priorities lie and why it has no bearing on the issue of VG wings
China and US both have access to variable geometry inlet for their latest aircraft, and both made the conscious choice to use DSI. China could use a traditional configuration for J-20 like she has done so for J-31, but made the conscious choice not to. Russia has no DSI, so there isn't a choice but to settle with variable geometry inlet. Russia lacks inlet alternatives, so there is nothing to prioritize over, and that means the claim of having different priorities is an excuse.

No one said Russia's use of variable geometry inlet is wrong, so stop your strawman. Frankly, I think it is the right decision for Russia given it is the lowest risk approach, as Russia simply doesn't have resources to fool around with new technologies. The problem is that someone on this forum has an assumption that Russia made decisions base purely on technical constraints, when financial constraints have much bigger influence.
 

Tirdent

Junior Member
Registered Member
I have been very clear that your requirements have no bearing with those found on modern fighter aircraft.

Bravo - some progress perhaps? VG wings are no longer used because the requirements they address are not important on modern fighters (doesn't change the fact that they still solve said requirements better than any other solution developed to date). Exactly!

Actually, the J-20 paper is very relevant because it addressed the very points you have raised. The paper devoted the entire section 3 talking about transonic lift-to-drag, with the following opening:

The paper is very comprehensive, and will probably address all the points you will ever raise on the subject of variable wing. The benefits associate with variable wing have been provided with alternatives, proving my point exactly.

Ok, I was too optimistic :rolleyes: Congrats on finding the ONE reference to subsonic cruise in the entire paper, as I mentioned - your emphasis there is extremely misleading, because in all other instances where it deals with the transonic/subsonic regimes, it's other capabilities which are discussed.

Your highlighting gives disproportionate weight to a very peripheral aspect of the paper (which doesn't even have a single figure, or indeed more than that one cursory part of a sentence, devoted to it) over pretty much all the rest, which studies subsonic and supersonic *maneuvering* issues. It's just that subsonic loiter is of far lower concern in a fighter like the J-20 than other stuff - things which pretty much preclude VG wings right off the bat (you seemed to have understood that alright in the first quote above - why do you then go on to contradict yourself?). Let's have a another, less skewed look at section 3:

... Trans-sonic lift to drag characteristics determine an aircraft's maximum range and sustained turn capability. The future fighter's demands for these characteristics will exceed those of modern 4th gen. fighters. Modern fighters employ the strategies of relaxing longitudinal stability, adapting wings with medium sweep and aspect ratio, twisting the wing, and adding wing-bending mechanisms to greatly improve the lift-to-drag characteristics. Due to the future fighter's requirement for supercruise, supersonic drag characteristic is a critical design point and designers must avoid using aerodynamic measures that may potentially increase supersonic drag. As a result, the wing shape and wing twist coefficient can't be selected based on trans-sonic lift to drag characteristics alone. It is necessary to employ wing-bending mechanisms but its aerodynamic efficiency has already been exhausted.

Further decreasing the aircraft's longitudinal relaxed stability is an excellent solution to this problem. Diagram 1 shows how the variation tendency of trim-drag coefficients against longitudinal instability of a conventional fighter aircraft in a tight, sustained turn....

Similarly, section 5 first narrows down to a bunch of wing planform options according to supersonic factors and then settles on the one which does best at Mach 0.2 (ever seen a post 1930 fighter cruise or loiter at that speed?). And by does best, it apparently means performance at AoA greater than 20 degrees, to the extent of accepting a penalty at AoAs below that. Not very cruise- or loiter-oriented priorities...

No problem though - it's simply not very important to the job the J-20 and its counterparts are supposed to do and the paper therefore is irrelevant to the subject of VG wings. In a fighter where patrol endurance for naval fleet defence was a crucial criterion, for example, you might have seen VG wings even on a LO platform (NATF, A/F-X), however. Requirements, requirements, requirements.

Frankly, I think it is the right decision for Russia given it is the lowest risk approach, as Russia simply doesn't have resources to fool around with new technologies. The problem is that someone on this forum has an assumption that Russia made decisions base purely on technical constraints, when financial constraints have much bigger influence.

Lowest-risk? Here I thought the Russians were supposed to be reckless :rolleyes:

More seriously, the fact that they adopted a configuration which is likely to be the most unstable in yaw to date shows they took on a challenge far more difficult than an inlet configuration they are known to have studied 20 years ago (and for which they have all the development tools in the bag). But realizing that would require you to have a sound understanding of the basics behind DSI, which you've shown yourself to be incapable of even when spoon-fed with sources. Just because you can't seem to wrap your mind round it doesn't mean Sukhoi can't...
 

Engineer

Major
Bravo - some progress perhaps? VG wings are no longer used because the requirements they address are not important on modern fighters (doesn't change the fact that they still solve said requirements better than any other solution developed to date). Exactly!
Nope. Variable geometry wing is no longer used proves it isn't better than other solutions. It has already been shown that actual requirements addressed by variable geometry wing can be better address by alternatives. As for the "requirements" that you never defined but claim only variable geometry wing can solve, those exist only in your imagination with no relation to real world. It is that simple.

Ok, I was too optimistic :rolleyes: Congrats on finding the ONE reference to subsonic cruise in the entire paper, as I mentioned - your emphasis there is extremely misleading, because in all other instances where it deals with the transonic/subsonic regimes, it's other capabilities which are discussed.

Your highlighting gives disproportionate weight to a very peripheral aspect of the paper (which doesn't even have a single figure, or indeed more than that one cursory part of a sentence, devoted to it) over pretty much all the rest, which studies subsonic and supersonic *maneuvering* issues. It's just that subsonic loiter is of far lower concern in a fighter like the J-20 than other stuff - things which pretty much preclude VG wings right off the bat (you seemed to have understood that alright in the first quote above - why do you then go on to contradict yourself?). Let's have a another, less skewed look at section 3:

Similarly, section 5 first narrows down to a bunch of wing planform options according to supersonic factors and then settles on the one which does best at Mach 0.2 (ever seen a post 1930 fighter cruise or loiter at that speed?). And by does best, it apparently means performance at AoA greater than 20 degrees, to the extent of accepting a penalty at AoAs below that. Not very cruise- or loiter-oriented priorities...

No problem though - it's simply not very important to the job the J-20 and its counterparts are supposed to do and the paper therefore is irrelevant to the subject of VG wings. In a fighter where patrol endurance for naval fleet defence was a crucial criterion, for example, you might have seen VG wings even on a LO platform (NATF, A/F-X), however. Requirements, requirements, requirements.
The paper talks about other maneuvering issues just means the paper is comprehensive. It doesn't diminishing the fact that your point regarding loitering duration has been addressed. Let us look at opening of Section 3 again, this time with different emphasis:
Trans-sonic lift to drag characteristics determine an aircraft's maximum range and sustained turn capability. The future fighter's demands for these characteristics will exceed those of modern 4th gen. fighters. Modern fighters employ the strategies of relaxing longitudinal stability, adapting wings with medium sweep and aspect ratio, twisting the wing, and adding wing-bending mechanisms to greatly improve the lift-to-drag characteristics...

It has clearly been shown that what variable geometry wing can solve can also be solved with other solutions. Now you are avoiding the issue by setting up another strawman, where you try to assign the author your notion of priorities. :rolleyes:

Lowest-risk? Here I thought the Russians were supposed to be reckless :rolleyes:
Low risk as in project flop. :rolleyes:

More seriously, the fact that they adopted a configuration which is likely to be the most unstable in yaw to date shows they took on a challenge far more difficult than an inlet configuration they are known to have studied 20 years ago (and for which they have all the development tools in the bag). But realizing that would require you to have a sound understanding of the basics behind DSI, which you've shown yourself to be incapable of even when spoon-fed with sources. Just because you can't seem to wrap your mind round it doesn't mean Sukhoi can't...
LOL! This coming from the person who:
  • makes bold claims about variable geometry wing while not knowing the design conflicts that variable geometry was invented to solve,
  • sees "streamline traced" get mentioned and automatically assumes that is an inlet category,
  • doesn't understand why the technical term variable geometry inlet exists and insists on using the term movable ramp inlet instead,
  • treats inlet and engine as the same thing,
You are also using weasel word again. "Likely" to be the most unstable in yaw? Let's see some numbers on stability margin compare to F-22, F-35 and the J-20. Adopting a configuration with yaw instability is not a worthy accomplishment when compared to other 5-th generation designs, as the J-20 configuration is also unstable in yaw.
7. A comprehensive study of a design example

The design team made a future fighter proposal based on the points raised by this article. The proposal employs lift-body LERX canard configuration. It is unstable in both the lateral and yaw directions.
Remember how I said the paper
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will addresses your points raised in the future? :rolleyes:
 

Tirdent

Junior Member
Registered Member
The paper talks about other maneuvering issues just means the paper is comprehensive. It doesn't diminishing the fact that your point regarding loitering duration has been addressed.

It doesn't just talk about other issues also, it is - that one sentence apart which doesn't even exclusively deal with the subject - almost completely about other issues. Loitering/endurance meanwhile isn't touched upon *anywhere* in it - stop lying.

All in all, there are 10 figures - how many relate to subsonic cruise or loiter? That's right, zero. From start to finish, the paper consists of some 140 sentences - how many refer to subsonic cruise? One. Must be of absolutely critical importance for sure, if 99.3% (95% if we're very generous and count half of section 3 as applicable to this rather than to turning, or 100% if we consider loiter) of the text and 100% of the diagrams have nothing to do with it.

LOL! This coming from the person who:
makes bold claims about variable geometry wing while not knowing the design conflicts that variable geometry was invented to solve,

The only one here who has proven he doesn't know that is you, even though there were nice, concise explanations staring you in the face.

sees "streamline traced" get mentioned and automatically assumes that is an inlet category,

Streamline tracing is a method of deriving aerodynamic shapes (including intakes). What's wrong with categorizing intake types which are based on this technique together? I mean, "category" quite literally means simply a collection of items that share a characteristic of some kind.

Oh, I get it - this was an attempt to start another terminology debate based on the contrived assertion of yours that it is not "technically" allowable to do so unless there is some mysterious reference which says so :rolleyes: Meanwhile you'll blithely ignore manifold perfectly technical counter-examples and eventually get so confused by your own logical gyrations that you begin to contradict yourself at every turn.

Let's spare everyone another futile exercise of that sort.

doesn't understand why the technical term variable geometry inlet exists and insists on using the term movable ramp inlet instead,

Between the two of us, I'm actually the one who didn't insist on anything regarding intake terminology. I merely played along with your hair-splitting for a while to expose how absurd it was and watch you get increasingly tangled up in your own web of semantic deception.

I'd much rather discuss these gems which I missed earlier, if you insist on a mud slinging match:

Look at your first source, and check out the patent filing date. The patent was filed in 2006, whereas DSI first flew in December 1996. DSI exists long before your so called "category" does.

What about the references in several of those sources to the 1985 work by Seddon and Goldsmith that you are so conveniently ignoring? But I can do even better than that, here's a patent which explains what your so-called 1950s "bump theory" is in fact all about:

754zedrhc.png

574weht.png

6845rjt.png

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It proposes a mixed compression inlet combining the bump with an internal supersonic compression diffusor (DSI fighter intakes are external compression only for stable subcritical operation, AFAIK), but clearly the approach for constructing the bump geometry is the same. So streamline tracing existed as far back as that, and as did the basic principle behind what today is called DSI.

Not that I expect you to accept this, since you've demonstrated time and again that you are incapable recognizing a concept if it's not called by the name familiar to you personally, which then means it's not the "technical term" :rolleyes:

Also, look at your own source to see what is a streamline. Streamline is basically the path traced by a particle. That sounds just like CFD.

So according to Tirdent, anything that involves analysis of streamline falls under a category of "streamline traced design". Modern automobile — streamline traced automobile category. Modern buildings — streamline traced building category. LOL!

LOL, indeed. This has to rank as one of the most stupid things I've ever read from somebody claiming to have a knowledge of fluid dynamics.

A streamline is NOT generally the path traced by a particle. It can be under certain circumstances, and in flow fields which are to be used for generating geometry by streamline tracing it even must be, but this is NOT the case per se.

CFD does NOT equate to streamline tracing - which is why the distinction above isn't mere sophistry, since CFD is widely used to analyse flow phenomena where streamlines most certainly do not correspond to particle trajectories (very important in building aerodynamics, for example).

You could (barely) have gotten away with equating streamlines and particle paths, since for the purpose of creating shapes from streamline tracing this has to be the case, but going on to say CFD is the same as streamline tracing is just embarrassing.

You are also using weasel word again. "Likely" to be the most unstable in yaw?

Not weasel words - honesty and rigour (alien concepts to you, I know). I don't have comprehensive data on yaw stability margins, so I can't be absolutely certain and I reflect that in my choice of words.

Let's see some numbers on stability margin compare to F-22, F-35 and the J-20. Adopting a configuration with yaw instability is not a worthy accomplishment when compared to other 5-th generation designs, as the J-20 configuration is also unstable in yaw.

By all means, let's see numbers. Do you have any? Or are you just bluffing and hiding behind an element of doubt which however applies to your own position *at least* equally? Something which you, unlike me, refuse to even acknowledge though - and then even have the gall to accuse me of using weasel words when I'm simply being more honest.

Remember how I said the paper A Study of Low Aspect-Ratio High-Lift Aerodynamics Layout will addresses your points raised in the future?

As you've already admitted yourself, a certain degree of yaw instability is common - the issue is how much. The presence of ventral strakes on the J-20 and the fact that even without them the tails on the Su-57 are still about 20% smaller in area make it quite likely that the latter is more unstable. So not only does the paper not even begin to address the points I made about VG wings (which is what you originally said), it is unable to prove me wrong on yaw instability as well.

BTW, we do have approximate numbers on *longitudinal* instability - the J-20 paper gives a desired goal of going to "around 10%" from low single-digits in legacy designs and cautions against higher values due to flight control challenges while the Su-57 is reportedly (Butowski) in the 15% ball park. Apparently Sukhoi is ready to tackle greater flight control system challenges, which makes it rather plausible that they'd be able to achieve a higher yaw instability margin.
 
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Engineer

Major
It doesn't just talk about other issues also, it is - that one sentence apart which doesn't even exclusively deal with the subject - almost completely about other issues. Loitering/endurance meanwhile isn't touched upon *anywhere* in it - stop lying.

All in all, there are 10 figures - how many relate to subsonic cruise or loiter? That's right, zero. From start to finish, the paper consists of some 140 sentences - how many refer to subsonic cruise? One. Must be of absolutely critical importance for sure, if 99.3% (95% if we're very generous and count half of section 3 as applicable to this rather than to turning, or 100% if we consider loiter) of the text and 100% of the diagrams have nothing to do with it.


The only one here who has proven he doesn't know that is you, even though there were nice, concise explanations staring you in the face.
Loitering time/endurance is related to lift-to-drag ratio. So by touching upon the ratio, the paper has touched upon loitering time/endurance. Section 3 references Diagram 1, which you clearly don't understand. So the one who doesn't know the subject is you.


Streamline tracing is a method of deriving aerodynamic shapes (including intakes). What's wrong with categorizing intake types which are based on this technique together? I mean, "category" quite literally means simply a collection of items that share a characteristic of some kind.

Oh, I get it - this was an attempt to start another terminology debate based on the contrived assertion of yours that it is not "technically" allowable to do so unless there is some mysterious reference which says so :rolleyes: Meanwhile you'll blithely ignore manifold perfectly technical counter-examples and eventually get so confused by your own logical gyrations that you begin to contradict yourself at every turn.

Let's spare everyone another futile exercise of that sort.

Between the two of us, I'm actually the one who didn't insist on anything regarding intake terminology. I merely played along with your hair-splitting for a while to expose how absurd it was and watch you get increasingly tangled up in your own web of semantic deception.
Category of inlets is related to functional and physical characteristics of the inlet: external/internal/mixed compression, fixed/variable geometry, shape, diverter/diverterless. Clearly, someone who proclaims to be knowledgeable about inlets should at least knows about the fundamentals. Of course, it is no surprise that you avoid technical terms (and logic) like plaques, because you won't have the freedom to make stuffs up. :rolleyes:

I'd much rather discuss these gems which I missed earlier, if you insist on a mud slinging match:

What about the references in several of those sources to the 1985 work by Seddon and Goldsmith that you are so conveniently ignoring?

But I can do even better than that, here's a patent which explains what your so-called 1950s "bump theory" is in fact all about:

View attachment 44962

View attachment 44961

View attachment 44963

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It proposes a mixed compression inlet combining the bump with an internal supersonic compression diffusor (DSI fighter intakes are external compression only for stable subcritical operation, AFAIK), but clearly the approach for constructing the bump geometry is the same. So streamline tracing existed as far back as that, and as did the basic principle behind what today is called DSI.

Not that I expect you to accept this, since you've demonstrated time and again that you are incapable recognizing a concept if it's not called by the name familiar to you personally, which then means it's not the "technical term" :rolleyes:
Where do they categorize DSI as a "streamline traced inlet"? You know, the title which you so proudly highlighted in the 2007 patent which you thought refers to a category. Let me jolt your memory, but with my highlighting instead:
rIA3CbM.png


The title of your own source actually categorizes DSI as "external compression inlet". Mixed compression is a category, yes. Streamline tracing is by your own admission a design method. :rolleyes:


LOL, indeed. This has to rank as one of the most stupid things I've ever read from somebody claiming to have a knowledge of fluid dynamics.

A streamline is NOT generally the path traced by a particle. It can be under certain circumstances, and in flow fields which are to be used for generating geometry by streamline tracing it even must be, but this is NOT the case per se.
So in the context of DSI, it is. We have no disagreement then.

CFD does NOT equate to streamline tracing - which is why the distinction above isn't mere sophistry, since CFD is widely used to analyse flow phenomena where streamlines most certainly do not correspond to particle trajectories (very important in building aerodynamics, for example).

You could (barely) have gotten away with equating streamlines and particle paths, since for the purpose of creating shapes from streamline tracing this has to be the case, but going on to say CFD is the same as streamline tracing is just embarrassing.
What a smack of hypocrisy. You talk about terminologies when you find it convenient. At least I am consistent. :rolleyes: No one claims CFD equates to streamline tracing, but CFD is involved in streamline tracing. The bump isn't the only thing of the inlet. I let you ramble on but all you talked about is streamline tracing, completely grossing over the important detail of cowling's effects on flow, which requires the use of CFD first before the bump can be defined. Not only does your explanation of DSI design method not present the whole picture, it isn't even "theory behind DSI":
Meanwhile, we're still waiting for your explanation about the theory behind DSI of course.

But it's okay. We both know the true purpose of your talk about design is so that you can distract from a fact — the fact that DSI is chosen by China and US on their top-of-the-line fighters despite both countries can design variable geometry inlet.

Not weasel words - honesty and rigour (alien concepts to you, I know). I don't have comprehensive data on yaw stability margins, so I can't be absolutely certain and I reflect that in my choice of words.

By all means, let's see numbers. Do you have any? Or are you just bluffing and hiding behind an element of doubt which however applies to your own position *at least* equally? Something which you, unlike me, refuse to even acknowledge though - and then even have the gall to accuse me of using weasel words when I'm simply being more honest.

As you've already admitted yourself, a certain degree of yaw instability is common - the issue is how much. The presence of ventral strakes on the J-20 and the fact that even without them the tails on the Su-57 are still about 20% smaller in area make it quite likely that the latter is more unstable. So not only does the paper not even begin to address the points I made about VG wings (which is what you originally said), it is unable to prove me wrong on yaw instability as well.

BTW, we do have approximate numbers on *longitudinal* instability - the J-20 paper gives a desired goal of going to "around 10%" from low single-digits in legacy designs and cautions against higher values due to flight control challenges while the Su-57 is reportedly (Butowski) in the 15% ball park. Apparently Sukhoi is ready to tackle greater flight control system challenges, which makes it rather plausible that they'd be able to achieve a higher yaw instability margin.
So no numbers then, and you try to get around this by introducing weasel word "plausible". Rigorous, you are not. Your denial of facts and misrepresentation of others' position with strawman fallacies show you are anything but honest.
 

Tirdent

Junior Member
Registered Member
Loitering time/endurance is related to lift-to-drag ratio. So by touching upon the ratio, the paper has touched upon loitering time/endurance. Section 3 references Diagram 1, which you clearly don't understand. So the one who doesn't know the subject is you.

L/D ratio affects many things and varies with flight condition, so you have to pay attention to the way in which L/D-ratio is considered. For optimizing loiter, you'd look at L/D ratio in 1g level flight at a Mach slightly below optimum long range cruise. Now let's take a closer look at figure 1, which supposedly I don't understand:

jFrZG.jpg

It deals with a sustained turn (i.e. non-trivial g-load and therefore too high a lift coefficient for loiter) at Mach 0.85 (somewhat too fast for optimum loiter). So the investigation which diagram 1 illustrates isn't about loiter or range at all - it is a study on transonic sustained turn performance (just like most of the text in section 3). But plainly you know better and I have no idea what I'm talking about :rolleyes:

Where do they categorize DSI as a "streamline traced inlet"? You know, the title which you so proudly highlighted in the 2007 patent which you thought refers to a category. Let me jolt your memory, but with my highlighting instead:

As predicted - getting hung up on my use of the word "category" and failing to recognize that the principle described in the 1955 patent is the same as that still used today in designing DSI bumps.

Listen, I did not claim those references "categorize" (as in, use that word to denote) DSI as a streamline traced intake, only that they confirm my assertion that DSI is based on streamline tracing methods. *I* grouped (cue hysterical discourse about my use of the word "group" next :rolleyes:) DSI in the category of intake designs based on that method, in the same way as I can categorize both leaves and limes as "green in colour". This is completely fine for me to do, since logically it doesn't require a source which includes the word "category", it's sufficient for the items I'm talking about to confirmedly have the characteristic I am grouping them by.

You (of all people!) don't get to decide on what characteristics can or cannot be used to categorize intakes.

Mixed compression is a category, yes. Streamline tracing is by your own admission a design method. :rolleyes:

Arguable - in that case the title would have to read "Method for streamline tracing an external compression inlet", very stricly speaking. It doesn't though, and that can be interpreted as DSI belonging in two categories: "external compression inlets" and "streamline traced inlets". In the same way that limes are both "green in colour" and "fruit". Can we stop this dumb nit-picking through? Whether there exists a source that explicitly defines a "category of streamline traced inlets" or not is actually immaterial to the validity of my argument.

What a smack of hypocrisy. You talk about terminologies when you find it convenient. At least I am consistent. No one claims CFD equates to streamline tracing, but CFD is involved in streamline tracing. The bump isn't the only thing of the inlet.

Hypocrisy indeed, since YOU are in fact the one who claimed just that:

So according to Tirdent, anything that involves analysis of streamline falls under a category of "streamline traced design". Modern automobile — streamline traced automobile category. Modern buildings — streamline traced building category. LOL!

The above accusation is founded on the belief that streamline tracing = CFD, and that hence by saying DSI is based on streamline tracing I was somehow implying all objects designed with the help of CFD were streamline traced :rolleyes:

But it's okay. We both know the true purpose of your talk about design is so that you can distract from a fact — the fact that DSI is chosen by China and US on their top-of-the-line fighters despite both countries can design variable geometry inlet.

Requirements. We've been through this SO often now.
 
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Engineer

Major
L/D ratio affects many things and varies with flight condition, so you have to pay attention to the way in which L/D-ratio is considered. For optimizing loiter, you'd look at L/D ratio in 1g level flight at a Mach slightly below optimum long range cruise. Now let's take a closer look at figure 1, which supposedly I don't understand:

View attachment 44969

It deals with a sustained turn (i.e. non-trivial g-load and therefore too high a lift coefficient for loiter) at Mach 0.85 (somewhat too fast for optimum loiter). So the investigation which diagram 1 illustrates isn't about loiter or range at all - it is a study on transonic sustained turn performance (just like most of the text in section 3). But plainly you know better and I have no idea what I'm talking about :rolleyes:

Diagram 1 is an example that illustrates author's point about reduction of drag through relaxing stability. The point holds regardless of flight condition as long as flight is in equilibrium. Sustained turn is in equilibrium just as level flight is. High-aspect ratio wing provides good loitering range because of higher lift-to-drag ratio as compared to higher swept wing, so loitering range is addressed by addressing lift-to-drag ratio.

As predicted - getting hung up on my use of the word "category" and failing to recognize that the principle described in the 1955 patent is the same as that still used today in designing DSI bumps.

Listen, I did not claim those references "categorize" (as in, use that word to denote) DSI as a streamline traced intake, only that they confirm my assertion that DSI is based on streamline tracing methods. *I* grouped (cue hysterical discourse about my use of the word "group" next :rolleyes:) DSI in the category of intake designs based on that method, in the same way as I can categorize both leaves and limes as "green in colour". This is completely fine for me to do, since logically it doesn't require a source which includes the word "category", it's sufficient for the items I'm talking about to confirmedly have the characteristic I am grouping them by.

You (of all people!) don't get to decide on what characteristics can or cannot be used to categorize intakes.

Arguable - in that case the title would have to read "Method for streamline tracing an external compression inlet", very stricly speaking. It doesn't though, and that can be interpreted as DSI belonging in two categories: "external compression inlets" and "streamline traced inlets". In the same way that limes are both "green in colour" and "fruit". Can we stop this dumb nit-picking through?

Whether there exists a source that explicitly defines a "category of streamline traced inlets" or not is actually immaterial to the validity of my argument.

Hypocrisy indeed, since YOU are in fact the one who claimed just that:

The above accusation is founded on the belief that streamline tracing = CFD, and that hence by saying DSI is based on streamline tracing I was somehow implying all objects designed with the help of CFD were streamline traced :rolleyes:
"It is streamline traced inlet as long as Tirdent says so", and that is your argument? Your own source is quite explicit as to the inlet category of DSI:
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Also, the method of streamline tracing is what being patented, if the title wasn't obvious to you already. So of course streamline tracing would be described.
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Also, how DSI is designed has no bearing on whether DSI is better/worse as compared to variable geometry inlet.

Requirements. We've been through this SO often now.
Yes, we have. China and US both consciously choose DSI means variable geometry inlet is not better at meeting those requirements. Russian didn't have a choice, and they were not stupid to go with a technology which they have not test flown themselves.
 
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