Aerodynamics thread
- Thread starter siegecrossbow
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From the Boeing X-36 example, we can see that a canard delta tailfinless can be absurdly maneuverable, almost ideal for a fighter type. Of course, as you say, the human body can only sustain 9Gs, but when it comes to instantaneous turn rate the human body can survive a maximum of 45Gs, enough for an aircraft to outmaneuver a low-G missile and delegate the aircraft to drone control with the pilot blinded mid-term and having passed out.
IMO, the problem with arguing that canards by themselves are enough is that the ideal canard fighter is the Rafale, with the canards coupled closely to the wing, and placed directly above the main wing. Neither the J-20, the J-10, nor the Eurofighter have this configuration, and because of this, they don't experience the full benefits of a canard-delta planeform. The J-10 and J-20 try to compensate with large canards, and the J-20's long-coupled canards, as opposed to the J-10's mid-coupled canards, try to compensate by using LERX to recarry the ensuing vortex. But the J-20 also has the canards roughly on the same plane as the main wing, using an anhedral / dihedral configuration to enhance stealth as well as to simulate canards above the main wing.
This is why, I suspect, the J-20 won't hit the same limits as the Rafale can, or a J-20 with a close-coupled lifted canards, stealth be damned. It is the difference in maneuverability that gives TVC a role; the J-20 has already done everything it can to fit within the space of compromises through its planform, and the only way to compensate for what is lacking is to staple TVC to the rear of the aircraft.
And now that China has almost-mature TVC technology, it is viable.
IMO, the apprehension about TVC technology is a Chinese version of "wasn't built here" syndrome. China, up until recently, was not a leader in TVC technology; that was the reserve of the Americans and the Russians. Both had experimented with canard fighters, but ditched them, seeing TVC as a better compromise in the name of drag and stealth, with the Russians having gone further, eventually developing a LEVCON. But now that China possesses TVC technology, will we see it fit to every Chinese fighter?
As I mentioned before, a J-20 with TVC is oh-so-close to the X-36 configuration, a highly-stealthy, fast, and maneuverable aerodynamic formula. It simply needs the tailfins stripped off; the total number of control surfaces becomes the same as a J-20 without TVC. Yaw control is slightly more difficult, as the X-36 uses the B-2's rudder alignment, but the J-20's anhedral/dihedral wing set-up allows it to compensate somewhat as its ailerons can mimic, to some extent, the action of V-tails. Here, the kitchen sink approach promises to be an advantage.
Moreover, the easiest way to experiment with tailfinless is to use canard-delta-tailfinless designs. The additional control surfaces, especially with the anhedral/dihedral set-up, allows the J-20 to regain control even if the TVC fails, making it much less risky to test-fly. It is an advantage the Chinese have that neither the Americans nor the Russians possess; the Russians are likely going to be strapped for funding in the coming years, and neither have the same extensive experience with canard fighters as China does, with its successful J-10 and J-20 programs.
Hence, the advantage of the Chinese kitchen sink approach. The only thing they haven't tried is the canted tail of the 25DMU, and such an approach will increase drag and RCS. By holding everything else, though, they are in a position to integrate the best technologies and techniques the world has to offer and produce superior aircraft that no other state can rival.
IMO, the problem with arguing that canards by themselves are enough is that the ideal canard fighter is the Rafale, with the canards coupled closely to the wing, and placed directly above the main wing. Neither the J-20, the J-10, nor the Eurofighter have this configuration, and because of this, they don't experience the full benefits of a canard-delta planeform. The J-10 and J-20 try to compensate with large canards, and the J-20's long-coupled canards, as opposed to the J-10's mid-coupled canards, try to compensate by using LERX to recarry the ensuing vortex. But the J-20 also has the canards roughly on the same plane as the main wing, using an anhedral / dihedral configuration to enhance stealth as well as to simulate canards above the main wing.
This is why, I suspect, the J-20 won't hit the same limits as the Rafale can, or a J-20 with a close-coupled lifted canards, stealth be damned. It is the difference in maneuverability that gives TVC a role; the J-20 has already done everything it can to fit within the space of compromises through its planform, and the only way to compensate for what is lacking is to staple TVC to the rear of the aircraft.
And now that China has almost-mature TVC technology, it is viable.
IMO, the apprehension about TVC technology is a Chinese version of "wasn't built here" syndrome. China, up until recently, was not a leader in TVC technology; that was the reserve of the Americans and the Russians. Both had experimented with canard fighters, but ditched them, seeing TVC as a better compromise in the name of drag and stealth, with the Russians having gone further, eventually developing a LEVCON. But now that China possesses TVC technology, will we see it fit to every Chinese fighter?
As I mentioned before, a J-20 with TVC is oh-so-close to the X-36 configuration, a highly-stealthy, fast, and maneuverable aerodynamic formula. It simply needs the tailfins stripped off; the total number of control surfaces becomes the same as a J-20 without TVC. Yaw control is slightly more difficult, as the X-36 uses the B-2's rudder alignment, but the J-20's anhedral/dihedral wing set-up allows it to compensate somewhat as its ailerons can mimic, to some extent, the action of V-tails. Here, the kitchen sink approach promises to be an advantage.
Moreover, the easiest way to experiment with tailfinless is to use canard-delta-tailfinless designs. The additional control surfaces, especially with the anhedral/dihedral set-up, allows the J-20 to regain control even if the TVC fails, making it much less risky to test-fly. It is an advantage the Chinese have that neither the Americans nor the Russians possess; the Russians are likely going to be strapped for funding in the coming years, and neither have the same extensive experience with canard fighters as China does, with its successful J-10 and J-20 programs.
Hence, the advantage of the Chinese kitchen sink approach. The only thing they haven't tried is the canted tail of the 25DMU, and such an approach will increase drag and RCS. By holding everything else, though, they are in a position to integrate the best technologies and techniques the world has to offer and produce superior aircraft that no other state can rival.
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Internet sources claim that it's unknown. Still, the fact that the nozzles are integral to the engine suggests that the rated figure is for the F119 post-thrust reduction. I'm a bit flabbergasted that no one thinks the same as I do; that the F-22's F119, without the flat nozzles, would have had 190 kN, on par with the F135. In response to Air Force Brat, that is what I'm implying with the flat nozzles statement, but it's not totally on the US's side; it implies that F135 technology, retrofitted to the F119, would only provide a moderate increase in max thrust.
Engineer
Major
You are right in that LEVCONs provide less control compared to canard, hence why PAK-FA also needs tail plane and thrust vectoring. The so call "reduction" in drag and RCS just gets add back in other parts of the plane. Don't be fooled.
J-20's configuration is a continuation on a design that existed way before J-10 did. I believe it is more correct to say J-10 branch off of J-20 aerodynamic principles. Certainly, CAC engineers would have applied what they learned over the course of J-10 development back to the J-20 as well.
TVC is not an an aerodynamic control surface. In fact, the entire point of TVC is not to interact with aerodynamics so as to not be constrained by aerodynamics. As for manoeuvrability, TVC allows an aircraft to manoeuvre in a post-stall situation, but an aircraft should not be in stall to begin with. In a dog fight situation, TVC actually decreases manoeuvrability.
From Dr. Song's paper about J-20's configuration, a primary design aim is to prevent the aircraft from going into stall in the first place, rather than to rectify the situation later using non-aerodynamic means.
This is something off of marketing brochure which gets repeated very often. Moving control surfaces impact stealth but moving nozzles won't? Think about that. Another thing you should remember is that the entire frontal sector of a stealth aircraft is stealthy, such that pointing a few degrees away from the enemy does not break stealthiness. A few degrees in control surface deflection makes even less difference.
Trimming isn't as much of an issue for canard configuration as in traditional configuration. Aircraft like J-20 has canard and trailing edge flaps for trimming, whereas aircraft with traditional configuration only have tail-plane available.
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That depends on which flight envelope you're talking about. Both speed and altitude matter, since flight control surfaces aren't consistently as effective at every flight regime.
Which is why I talk about the "not invented here" problem, no?
TVC nozzles can produce less drag, especially when compared to the J-20's large canards. They may be slightly larger in form than non-TVC nozzles, but it's negligable compared to canards. Also, the advantage of TVC nozzles vs canards is that TVC nozzles are in the rear of the aircraft; a problem if you're concerned about rear stealth, but otherwise, they're hidden from frontal or frontal-side view in a way that canards are not.
About the late Song Wencong's paper, most aircraft's aerodynamic regime is limited by their stall envelope. If it's as you say, that the J-20 is designed to minimize its stall envelope, when it comes to instantaneous turn rate, the J-20 can exit its stall envelope.
Also, when you're considering the TVC as a control surface, it may not be properly a control surface, but it remains a mechanism of control, a way to temporarily trade off thrust for acceleration by diverting the thrust force.
Lastly, when you're talking about the nozzles, as I've mentioned, nozzles are at the rear. You're impacting rear stealth instead of frontal / frontal stealth; the full body of the aircraft is blocking almost the entirety of the nozzle.
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Right now, the Chinese do have almost mature TVC technology. I'm not trying to bash your vaunted 5th gen, I'm just saying that with the ability available, the aircraft will benefit from the addition of TVC. 3D TVC has a minimal weight penalty compared to 2D TVC, and 3D TVC does not reduce thrust in the same way as 2D TVC. The number one problem with TVC is not cost, weight, or benefit, but the MTBO. The Russian TVCs on the Su-30MKIs almost crippled the fighters as the Russians did not design for high MTBO.
However, with the recent claim of significant improvements to Chinese TVC longevity, it will be practical to start throwing TVC around willy-nilly with an increased TVC MTBO.
Most of what you've said is true, but irrelevant and thus sophistic.
TVC nozzles can produce less drag, especially when compared to the J-20's large canards. They may be slightly larger in form than non-TVC nozzles, but it's negligable compared to canards. Also, the advantage of TVC nozzles vs canards is that TVC nozzles are in the rear of the aircraft; a problem if you're concerned about rear stealth, but otherwise, they're hidden from frontal or frontal-side view in a way that canards are not.
About the late Song Wencong's paper, most aircraft's aerodynamic regime is limited by their stall envelope. If it's as you say, that the J-20 is designed to minimize its stall envelope, when it comes to instantaneous turn rate, the J-20 can exit its stall envelope.
Also, when you're considering the TVC as a control surface, it may not be properly a control surface, but it remains a mechanism of control, a way to temporarily trade off thrust for acceleration by diverting the thrust force.
Lastly, when you're talking about the nozzles, as I've mentioned, nozzles are at the rear. You're impacting rear stealth instead of frontal / frontal stealth; the full body of the aircraft is blocking almost the entirety of the nozzle.
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Right now, the Chinese do have almost mature TVC technology. I'm not trying to bash your vaunted 5th gen, I'm just saying that with the ability available, the aircraft will benefit from the addition of TVC. 3D TVC has a minimal weight penalty compared to 2D TVC, and 3D TVC does not reduce thrust in the same way as 2D TVC. The number one problem with TVC is not cost, weight, or benefit, but the MTBO. The Russian TVCs on the Su-30MKIs almost crippled the fighters as the Russians did not design for high MTBO.
However, with the recent claim of significant improvements to Chinese TVC longevity, it will be practical to start throwing TVC around willy-nilly with an increased TVC MTBO.
Most of what you've said is true, but irrelevant and thus sophistic.
One issue here is that conventional control surfaces tend to lose effectiveness at high speeds. This is one of the reasons that the J-20 uses long-coupled canards; canards further away from the center of gravity allow the canards to achieve high leverage and thus high authority. The other way to do it, though, is to do it through TVC. The maneuverability benefit might be relatively slight at subsonic speeds, but at supersonic speeds the combination of canards and TVC will likely be significant.
One thing I have noted is that subsonically, many fighters can hit or exceed the 9G limit for sustained human acceleration. Once you get to supersonic regimes, though, it starts to fall off, and if you envision the J-20 as a transsonic or supersonic fighter, it can obtain a significant maneuverability advantage BVR.
plawolf
Lieutenant General
Once someone looses consciousness from extreme G lock, there is no reliable way to know when, or even if, they will regain it. Might be a few seconds, or hours, or never. It's almost never a good idea to pull such extreme manoeuvres.
That is an exceptionally contentious claim. Without meaning to engage in THAT infamous, never-ending debate, the facts speak for themselves, with half of the world's top fighter companies betting on close coupled canards, with the other half favours long coupled.
Take the near identical performances of the Typhoon vs Rafale as proof that there is no magical perfect canard layout, and how well a design works depends on how well all its design features are integrated and works together.
Actually, it's only the Russians who are super keen on TVC. The Europeans are showing little interest, the Americans only applied it to one operational fighter, despite extensive studies, and the Chinese remain largely uninterested despite the Russians making the technology available long ago.
Again, I think the key is general aerodynamic.
Everyone else was able to advance their own aerodynamics research and study to the point where they can achieve combat aircraft agility levels well in excess of human physical tolerances.
OTOH, when everyone else was making these important transitions from 3rd gen to 4th gen aerodynamics advancements, the then Soviets were kneecapped by the calamity of the collapse of the USSR and the long, messy aftermath.
Russian aerodynamics research effectively ground to a holt from the late 80s for well over a decade. They were stuck making incremental improvements to the Flanker design until the PAKFA, and even that could be uncharitably described as an extreme evolution of the baseline Flanker design as opposed to a clean slate new design.
That's why the Russians were turning so heavily to TVC - it's the cheapest, and only effective way they can evolve their baseline 3rd gen Flanker design to keep up with the new generation aerodynamics of canard deltas.
But given the inherently better agility of delta canards, TVC offers limited benefits to those designs because of human G limits.
The Americans went their own way with stealth over aerodynamics as the chief priority for the F22, and probably opted to use TVC for different reasons - stealth, and to give the conventional layout Raptor more of an edge over conventional 4th gens that prioritised agility above all else.
The purely experimental X planes were never meant to be operationally viable. They are one-trick ponies designed and built to test out one area of significant interest.
Just because an X-plane was so designed is no indication it's a good idea for your operational fighters to adopt the same overall design.
That makes for an interesting case for China to maybe build their own tailless X plane based on the J20, but it's a far cry from a convincing argument that that would be the ideal lay out for future generation combat aircraft.
Indeed, that has been one of the most striking, and best characteristics of Chinese aviation (and general technological) advancement in the last few decades - the Chinese does not care, or hold any biases towards the source of an idea or concept. The only thing they care about is whether it works and works for their needs. If it does, then they will take it and use it and make it their own.
Given the hunger of China for advanced tech of all sources, it's exceptionally out of character for them to be rejecting an open offer of technology they do not yet possess. As such, I think it is infinitely more likely that the Chinese made a calculated decision not to pursue Russian TVC tech, because they judged the tech itself to be of limited utility given the costs and performance penaltiesand what their baseline designs can already do, as opposed to because of some odd sense of national pride.
Even if, indeed especially if, they were making massive strides in secret TVC engine research that they have been able to keep totally under wraps, it would have made sense for them to take the Russians up on their offer of TVC engines, especially for the J20 flight test programme.
If they wanted to fit the J20s with TVC eventually, it would have been a no brainer to take the Russian TVC engines on offer and use them to conduct flight testing, rather than the non-TVC Russian engines they are using, if they ultimately wanted to use their own TVC engines on production aircraft.
@plawolf
Regarding the usefulness of an emergency maneuver, if the alternative is losing both the plane and the pilot, it's worth having the pilot red-out. As I mentioned before, the pilot will be at least temporarily blinded, and thus useless, so an automated emergency maneuver button, similar to the ejection trigger, could allow the plane to hit 40-45G, then autopilot home to base, saving precious resources, both human and material.
About the close-coupled vs long-coupled, you are incorrect when you claim that half the world's leading aircraft companies bet on close-coupled and the other half on long-coupled. If you look at the field between the Russians, the Americans, the Chinese, and the Europeans, it's the Chinese and the Europeans that have pursued canards most aggressively. The Russians tried with the Su-30, then ditched it for TVC (and as I mentioned before, I'm not saying that TVC is better than canards, it's only that the Su-30 wasn't designed with canards in mind and thus benefited more from TVC than from canards). The Americans did have their X-planes and the naval ATF, but please do note that Northrup's concept involved canards above the main wing, which the J-20 doesn't have, and that while the F-35 had canards at some stage of its design, the Lockmart decided to ditch them during the design process. We can say that's because of their inexperience with canard designs.
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The funny thing here, is that you guys are turning this into yet another TVC vs canards fight. It's ridiculous, because at this point in the game Chengdu can have both; a privilege no other aircraft manufacturer can boast.
Also, regarding the X-plane, consider that Boeing is using its planeform for its attempt at the F/A-XX project, and for a while the F-35 looked comparable to the X-36, with the addition of tailfins.
Yet another thing, about the Chinese using the J-20 as a basis for an X-plane, I wouldn't at all disagree with it. The J-20 is close enough for the test, but the funny thing is, if the project is successful, it can be fed back into the J-20 project itself. If the X-plane has enough advantages over the base J-20, why keep the tailfins?
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Regarding building the J-20 for TVC; well, consider this, perhaps the Chinese wanted to produce the J-20 from completely indigenous parts. Going to TVC, a technology that China had not yet mastered at the time, and something, that we all agree, the J-20 doesn't need to be effective, would just make the Chinese even more beholden to the Russians. Moreover, Russian TVC has a bad reputation for MTBO, effectively translating to the Chinese needing to buy a ridiculous number of Russian TVC engines and/or nozzles to keep their planes in the air.
The PLAAF made the right decision when they chose the J-20 airframe. Compare, say, the ATD-X or the F-35. The J-20 is an aircraft that can be effective even without the advanced technologies necessary to make the aircraft superlative; it is reasonably fast, can supercruise, has absurd range, and has reasonable maneuverability. The ATD-X, on the other hand, is quasi-vaporware and probably will be hobbled by its engines as well. The F-35 turned out to be a flying turkey, a miracle to be sure, but an awfully unaerodynamic one. That's because they're both dependent on technologies that are as of yet immature, unlike the J-20.
But when you add the next generation technologies, like the intended 180kN engines, or GaN AESA, or TVC, the J-20 can be something special.
Regarding the usefulness of an emergency maneuver, if the alternative is losing both the plane and the pilot, it's worth having the pilot red-out. As I mentioned before, the pilot will be at least temporarily blinded, and thus useless, so an automated emergency maneuver button, similar to the ejection trigger, could allow the plane to hit 40-45G, then autopilot home to base, saving precious resources, both human and material.
About the close-coupled vs long-coupled, you are incorrect when you claim that half the world's leading aircraft companies bet on close-coupled and the other half on long-coupled. If you look at the field between the Russians, the Americans, the Chinese, and the Europeans, it's the Chinese and the Europeans that have pursued canards most aggressively. The Russians tried with the Su-30, then ditched it for TVC (and as I mentioned before, I'm not saying that TVC is better than canards, it's only that the Su-30 wasn't designed with canards in mind and thus benefited more from TVC than from canards). The Americans did have their X-planes and the naval ATF, but please do note that Northrup's concept involved canards above the main wing, which the J-20 doesn't have, and that while the F-35 had canards at some stage of its design, the Lockmart decided to ditch them during the design process. We can say that's because of their inexperience with canard designs.
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The funny thing here, is that you guys are turning this into yet another TVC vs canards fight. It's ridiculous, because at this point in the game Chengdu can have both; a privilege no other aircraft manufacturer can boast.
Also, regarding the X-plane, consider that Boeing is using its planeform for its attempt at the F/A-XX project, and for a while the F-35 looked comparable to the X-36, with the addition of tailfins.
Yet another thing, about the Chinese using the J-20 as a basis for an X-plane, I wouldn't at all disagree with it. The J-20 is close enough for the test, but the funny thing is, if the project is successful, it can be fed back into the J-20 project itself. If the X-plane has enough advantages over the base J-20, why keep the tailfins?
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Regarding building the J-20 for TVC; well, consider this, perhaps the Chinese wanted to produce the J-20 from completely indigenous parts. Going to TVC, a technology that China had not yet mastered at the time, and something, that we all agree, the J-20 doesn't need to be effective, would just make the Chinese even more beholden to the Russians. Moreover, Russian TVC has a bad reputation for MTBO, effectively translating to the Chinese needing to buy a ridiculous number of Russian TVC engines and/or nozzles to keep their planes in the air.
The PLAAF made the right decision when they chose the J-20 airframe. Compare, say, the ATD-X or the F-35. The J-20 is an aircraft that can be effective even without the advanced technologies necessary to make the aircraft superlative; it is reasonably fast, can supercruise, has absurd range, and has reasonable maneuverability. The ATD-X, on the other hand, is quasi-vaporware and probably will be hobbled by its engines as well. The F-35 turned out to be a flying turkey, a miracle to be sure, but an awfully unaerodynamic one. That's because they're both dependent on technologies that are as of yet immature, unlike the J-20.
But when you add the next generation technologies, like the intended 180kN engines, or GaN AESA, or TVC, the J-20 can be something special.