J-20... The New Generation Fighter II

pugachev_diver said:

this is insanely tiring....But I actually learned a lot. normally when you read posts this long, you tend to skip through it quickly. You jump between sentences. But I would have to understand every sentence to the full in order to properly translate it.

It will be mind ****** to read at first as it is very techinical. But later on it's really a piece of cake.
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p_d I would edit your thread before popeye sees it.

Some suggested changes to these terms:

dissipation rather than diccipation

side curtain extension -> leading edge root extension (LERX)


easy on the natural stablity of the plane -> relaxed stability

large sideskirts -> leading edge or LERX

"In a conventional layout, the surface controlling the pitch is usually at the rear of the vehicle, some even gone extreme to raise level of control, by further extending passing the engine, thinking that as airflow passes a flat plane will not apply further effects on the plane, but canard layouts have its vortex right around its nose, making it much more turbulent."

--> In conventional layout the the tailplane is the last part of aircraft that airflow comes into contact, therefore one does not need to worry about what happens behind it, whereas canard design has to take into account of the fact that airflow past the canard will interact with the rest of the plane behind it.

movable sideskirts -> leading edge control surface (?)

regards

Chapter 3 - The Might and Raq Power of F22

Rafael used close coupling (again, means canards being close to the delta wing), which literally undermines pitch controllability. The two canards provice great vortex lift, boost agility, but its pitch controllability is really bad. It has great subsonic agility and bad supersonic maneouverability. But take notice of the sideskirts, the French did a very good job.

It is well know that the close coupling planes like the Gripen and Rafael skips the problem of pitch controllability, but F22 caters the need for both subsonic agility and especially supersonic maneouverability. It emphasizes on taking good position through its sheer speed, even without turning on afterburners it could supermaneouvre at around Mach 1.5. (I wondered, won't those pilots die maneouvring at high G's while maintain speed of Mach 1.5)!

In supersonic supermaneouvres, supersonic roll maneouvre would be hard for all planes, including F22, T50 and J20. But it's great for escape and catching geat positions in dogfights. But even 5th generations can hardly open their weapon bay during a rolling at supersonic speed to throw out a missile. If fire first during supercruise, then supersonic spiralling ability would be very important, and this is F22's greatest strong point!

Many countries claim to be able to achieve supercruise, like the Mig25, although it's still only high thrust engines with temporarily turning on afterburners to gain momentum to achieve short term super cruise without having afterburner on the whole time. Even the British Lightning can achieve supercruise at Mach 1.1, some even say that J-12 could cruise at Mach 0.94 without afterburners, claiming to be super subsonic cruising.

But all those are fake rumours, none of them are capable of true supercruise, especially that none can perform supermaneouvres while maintaining supersonic speed. F22 could perform exotic maneouvres while flying at Mach 1.5. On the contrary, the famous Mig 25 has very bad pitch controllability, it is very sluggish in supersonic speeds. The Americans nickname it to be first class escape machine, third class attack.

Quoting from expert again, "F22's maneouvrability at supersonic speed is the most focused point on the plane, it is also the generation gap determining symbol. Apart from the supercruise, superaccelerate and climb rate, spiralling rate at supersonic speed is also greatly improved. According to a report, this plane could achieve 6.5G at M1.7. Considering F15 at similar conditions far lag behind, and Su27 could only achieve this in midsky and at speed of M0.9. It is undoubtedly a suprising achievement".

It could achieve such high G-force in supersonic spiralling is because of its insane engine, but also because of its lift-drag ratio and pitch controllability.

Speaking of lift-drag ratio, it is not hard to understand. As longas you can pull of high G-force, the wings will create lift, and drag with come along and grow exponentially (the value of drag coeffecient to wings positive angle of attack is exponentially related, and inversely related to the wings). If the drag coeffecient gets too great and increases too rapidly, it will quickly counteracts and reduces the extra bit of thrust of the engine. Although the plane could still pull off high G maneouvres, the engine will not provide enough thrust to maintain stable flight. The Mirage III's transient hovering performance was good but the stability drastically drops when it tries to spiral steadily. Because of this, to today's aeronautic industry, it is very hard to develop high lift-drag ratio wings or wings possessing good supersonic maneouverability, and it's even harder to merge the two into one in short period of time. This is something that F22 should be proud about!

no_name said:

Some suggested changes to these terms:

dissipation rather than diccipation

side curtain extension -> leading edge root extension (LERX)


easy on the natural stablity of the plane -> relaxed stability

large sideskirts -> leading edge

regards

Click to expand...

thanks, i'll change it now

Usually pit controllability is missed out by people. Wing's high lift is fundamental to high G force maneouvres, but as life increases, so does the back flipping force (seriouslly no clue how to say this, pretty much the force making the plane flipping backwards). If the plane itself cannot provide suffecient counter force, it will easily lose control when the plane flips backwards, or it will get thrown back down by the lift generated by the nose. This happens especially during supersonic flights, as the centre is in teh back of the plane, the nose generates great deal of downward force. So supersonic manevoures require great pitch movement handling ability. The plane famous for supersonic speed, the Mig 25, cannot perform supersonic manevoures because of its balance issues. When it flies straight in supersonic speed, its horizontal tail's ability to move already reaches its limit, there's very little room left for exotic maneouvres when flying supersonic. Although the body can handle much more G forces, it can only perform up to 3G.

In another word, F22 is scary not only because of its stealth, but rather its insane supercruise and supermaneouvre capability. During military exercises, even if F22s carry radar deflectors to lose its stealth, F15s and F16s could still not beat it. F22 could use its 4S to gain perfect ground to launch missile before the other two could, and its sheer speed also add a lot of momentum to its missiles. This means it has a first shot ability, and even if it misses, it could still escape and occupy another good ground, and the 3rd generations wouldn't be able to even follow the beat F22 opeates with.

In those exercises, pilots say that "when we fight F22s, as long as they go into supercruise mode, its game over, even if we turn afterburner to max we were still not able to catch up. If we keep chasing then it's bingo (out of oil) and the exercises would be over. We would have to quickly return to refuel by tanker planes, while F22 still has lots of fuel remaining."

"This thing is born for speed"


One sentence, "pitch control and lift-drag ratio decides for supercruise and supermaneouvres", vortex lift dictates for subsonic agility and lift. BUt in carnard layouts, it is self contradicting. And as for 4th generation plane, it has to be best in both fields".

"Canard layout's advantage only exists on the enemies"

"To solve pitch control problem, one is to increase relaxed stability, move the centre of teh plane to the front. This way, even when the centre shifts towards teh abck, but being close to the centre of gravity, the downward force will be relatively small. But this way, the plane in subsonic maneouvres at high positive angle of attack will still face pitch control problem - this time balancing the nose's upward force. The media overly exaggerate close coupling layout, it's placed closed to centre of gravity, so the pitch control ability will be low, F16 head chief said, the best place for canards are on enemy planes.

So, how do the Chinese ressearchers decast this curse?

To the Chinese engineers, carnard layout's benefits are exremely tempting, but how are those major problems solved?

To be continued...

Thanks for all your effort. It's all going way over my head but i'm enjoying the read still.Keep up the good work.

pugachev_diver said:

Shenyang criticised the flaws in conventional canard design. They also presented the benefits of their deisgn, which was a tri-delta design, similar to that of early Su-33. You can think of it as a F22 with canards. THe canards are a delta, the wing is another delta, the last is the large horizontal stabilizer. They explained how its better for balancing, maneouvering, and lift. But it still has some unavoidable flaws; including tri-delta design would increase RCS signatures more than canards and conventional layouts would, it would create drag which add burden to the engines. Therefore 601 institute has a conventional design as a backup. It was this undecisiveness and hesistation that made Shenyang lost the competition for the contract of heavy stealth fighter.

But, Chengdu's proposal was much more challenging than the conventional layout. As a result, they had a cutthroat promise (engineers will lose their jobs if it's not successful). Now the J-20 program finally started striding towards its fruition."

Canard design literally means adding a pair of canards close to the pilots position. Many nations have studied this layout, it was even studied prior to WWII. The Soviets also went into great depth with the Mig-21 but failed. None of the countries were sucessful in the beginning. This layout was not amazing, but not too bad either. Prior to the 80s, the primitive flight control systems were not powerful enough to support the canard designs. Only until 4-axis control became standard, that's when canard designs began to thrive.

Quoting from an expert, up to the 60s, all planes were designed to keep the airflow and supressed air from diccipating; but as the demand for agility and angle of attack increased, the separation was unavoidable. As close-coupling (literally means canards been super close to the main delta wing), Gripen took practical use of vortex lift, it was able to take control of the separation fo air flow, and vortical flows became widely used until today.

Have to run again, I'll keep translating this afternoon.

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I have made some improvements, mainly using appropiate terminology.



Designing an airplane is the most difficult in the branch of engineering, due to the contradictions in engineering requirements. When one performance of one requirement rises, then the performance in another area will fall. A jet fighter usually pursues to be an all rounded fighter, seeking balance in all requirements.

Just the wing area for an example, large wing area will decrease wing loading, increase agility at subsonic speeds. But at the same time, it will affect speed and climb rate. Delta wings are the best for supersonic flights, but at the same it increases wing loading, making it very diffcult to perform agile manoeuvres. After using those sluggish 2nd generation fighters, like the F4, F5, and F104, those that emphasize on supersonic speeds, the American Air Force could no longer bare it. They believe that when jets are fully loaded, it is very rare that they would go into pure supersonic dog fights. Since the F15, they aggressively adopted the low wing loading and high power engines in their design approach. But to those countries with aviation industry but lacking high thrust engines, it is impossible for them to do the same. When the F22 flew, it's proved to be even more radical and aggressive in utilizing this approach, using highly swept saucuer shaped wings, which maintains supersonic agility while decreases wing loading. It reached perfect balance with those unhumanly powerful F119; without F119, those large saucuer wings would make the plane very sluggish. This design is very elegant, but even so, the high tech materials, and a super complexed flight control systems resulting from stealth requirements still remain an uncrossable obstacle for other nations for the last 2 decades.

We had no decent engines, no decent materials, but we can do theoretical researches. By the time of building a prototype, these problems should already be solved. To design and study a 5th generation stealth fighter, the CHinese researchers and military personnels must decide their main focus:

1. Is it going to be light, medium, or heavy?
Undoubtedly, despite advancements in materials and avoinics, aircraft size grows bigger and bigger. On one hand this is due to bigger combat radius and payload demands; on the other hand this is due to complexed avoincis, where jamming equipments began to be integrated into the body. F-16 and J10s combat radius and agility match those of much heavier counterparts, but their flexibility cannot match those heavy fighters. (I guess he meant the ability to perform multiple type of missions. Since bigger planes can perform more types of missions because they can carry more diversed types of weapons and avoinics due to their sizes.)

The introduction of stealth as a standard for 5th gen jets is a new challenge. Although the Russians proposed the plasma stealth, it is still too far from being practical and operational. The internal bays enormity and the S-like intake will definitely cause increase in size. (No idea what he is trying to say here. Maybe he meant the size of J20 is a challenge in achieving stealth.)

China could totally take a low risk approach and design a medium sized 5th gen jet, with weapon bays similar to those of the silent eagle. But its flexibility and stealth will not match those of a heavy fighter.

China never had experience on heavy fighter development, but this time it's an exception. The loom of 96 Strait Crisis still hang over our head. The ability to attack is a must have criteria.

2. What layout to use?
After 96, it was time to examine the countermeasures we have. Chinese researchers thoroughly analysed F22s design, also S37 and Mig 1.42 when they were revealed. As to our own planes, the researchers had no idea what was going to happen as our military industrial complex was hoplessly backward.

Adopting a conventional layout similar to the F22 is the safest approach, but both China and Russia understand, when being behind in material science and engines, it is impossible to rival a f22 with conventional layout design. Prior to 2000, China and Russia cooperated in those technological fields, but did not reach a full agreement on the 5th gen fighter. But for sure, prior to publishing of Research on Low Wingspan High-Lift Aircraft Layout by Professor Song of the 611 institute (Chengdu), China already did a lot of groundwork on the early stage research for the 5th gen, but a final layout was still not chosen.

According to an expert: "The layout discussed in Research on Low Wingspan High-Lift Aircraft Layout results from 611 Institute studies on aerodynamics of 4th gen aircraft. This design's use of vortex emphasized on the design of aerodynamics coupling of LERX, canards, and wing; the diamond shaped head was adopted due to its stealthiness and sideslip stability, but it was not well integrated into the overall lahout. The vortex from the nose was considered to be undesirable for the canards. The current aerodynamics layout of the J-10 is the least risky from early developments, largely followed the knowledges acquired from J-9s wind tunnel testings. The even more radical double delta wings were dropped due to being too risky to attempt. The use of LERX and canards in one layout as mentioned in the Research on Low Wingspan High-Lift Aircraft Layout was probably a new revision of the earlier sacked double delta canard design.

At the time, Research on Low Wingspan High-Lift Aircraft Layout was still only a candidate proposal. Shenyang also had its own proposal. In the competition, Shenyang criticised the flaws in canard configuration, explained how triple airfoil layout is beneficial to a plane's balance, agility, and lift. But several problems still exist: including tri-delta design would increase RCS signatures more than canards and conventional layouts would, it would increase drag which add burden to the engines. Therefore 601 institute has a conventional design as a backup. It was their undecisiveness and hesistation that made Shenyang lost the competition for the contract of heavy stealth fighter.

But, Chengdu's proposal was much more challenging than the conventional layout. It is rumored that they signed a cutthroat promise (engineers will lose their jobs if it's not successful) as a result. Now the J-20 program finally started striding towards its fruition.

Canard configuration literally means adding a pair of wings near the nose of the aircraft. Many nations have studied this layout, it was even studied prior to WWII. The Soviets also went into great depth with the Mig-21 but failed. None of the countries were sucessful. This layout was not complete trash, neither is it an amazing layout. Prior to the 80s, flight control systems were not advanced enough to migitate problems due to present of canards. Only when digital flight control systems mature did canard designs began to thrive.

pugachev_diver said:

this is insanely tiring....But I actually learned a lot. normally when you read posts this long, you tend to skip through it quickly. You jump between sentences. But I would have to understand every sentence to the full in order to properly translate it.

Click to expand...

Quoting from an expert: "up to the 60s, all planes were designed to maintain laminar flow and prevent it from separating; but as the demand for agility increase so does angle of attack, and separation was unavoidable. As Gripen 's close-coupled canards (literally means canards been super close to the main delta wing) and its use of vortex became practical, it was able to take control of the separation of airflow, and use of vortical flows became widely accepted today. Jet fighter's use of lift from vortex mainly rely on vortex generators (canards, LERX) creation of vortex. The fast spinning vortex decreases the pressure over the wing surface, the vortex's intensity increases with the increase angle, creating large amount of vortex lift, which is nonlinear and highly noticable in the lift coefficent curve. But its benifit also adds immense difficulties to the flight control system. Observing the sophisication in the application of vortex, there seems to be 3 stages of developments. The first is the classic example F15, this early 3rd gen jet doesn't have vortex generators, it doesn't utilize the vortex lift effect. It's layout is positvely stable. Then comes F16, the first true 3rd gen jet, along with Su27. Both have small LERX as vortex generators, and both have relax stablity, and psuedo-fly-by-wire systems to achieve controllability. The third, is the adoption of large LERX, like those of F18 and FC1, the other is canard layout Typhoon, Rafael, Gripen, Lavi and J10. These jets all have unstable stability, and psuedo-fly-by-wire can no longer meet demands, digitial fly-by-wire becomes a standard. In summary, the more aerodynamics benifits, the harder and riskier it is to control. For a fighter with a conventional layout, the elevators are usually situated at the aft of the vehicle, some even gone extreme to increase the moment arm, by further extending the elevators passed the engine nozzles, which assumed airflow passes beyond the plane will not apply further effects on the plane itself; but canard layouts have its pitch control surfaces placed before the wings, with the resulting turbulence still influence the aircraft. Furthermore, the canards doubled as vortex generators, and movement of canards result in changes in vortices above the wings. As a result, canards movements have large and complex influence on the entire aircraft. Conventional layout doesn't mean the designers don't understand the benefits of having vortex generators that doubled as control surfaces, such as movable LERX that is similar to canard. Rather, they are not used due to questions on trade-off and efficiency. However, the more complex the effects, the greater potential. A jet with a set of full-motion canards (like those on J20, almost turning a full circle) demands greater understanding of aerodynamics and control theories than conventional layout, including those 3rd gen aircraft that adopts LERX. This is why these planes (with canards) came out later and have excellenet flight characterisitcs. Some people even called aircraft with full-motion canard layout 3.5 gen. From aerodynamics and flight control systems point of view, this claim is not groundless."

But there are two sides to everything: canard layouts do bring lots of benefits, but it doesn't mean super maneouverability, it instead brings better vortex lift and unstability. To countries with weak engines, this is good news. But it also bears many problems, apart from sophisticated flight control systems, there is another big problem. Canards have two effects: 1. providing control to vortex in lift generation, 2. control of pitching moments. Control of vortex in lift generation has already been mentioned, trimming and pitch control are other effects, and are also the hardest problem to solve on canard layout compared to conventional counterparts.

Any plane would have a centre of gravity and a centre of lift, if those two overlap in the same area, the plane would be perfectly trimmed. Although canards can achieve trimming and pitch control, different placement of canards put different level of emphasis on those two. Even the position of canards is a problem which causes big headache.

"The position of the canards reflect its design emphais. In fact, if ignoring fuel, weapon load and other factors, the centre of gravity is basically fixed. However, centre of lift changes with speed and attitude, thus require elevators (conventional layout) or canards (canard layout) to maintain trimming, this is balancing effect. By purposely offset the balance, it would cause the plane to pitch up and down, and is the purpose of pitch control. Although canards can perform trimming and pitch control, different position of the carnards have different emphasis on the two. Having carnards more closer to the front is called control-canard. Because of the long moment arm, small canards are enough for trimming and pitch control. They are lighter and create less drag, thus more suitable to high speed flights. Its disadvantage is the far distance between canard and wings is diffcult to generate vortex for lift. Canards placed closer to the aft are called close-coupling canards. Close-coupling canards are usually overlap with the wings, with the trailing edge of the canards above the leading edge of the wings. The effect from vortices are larger, and is benificial to improving agility. However, because the moment arm is short, the control on the pitch movement is limited. The surface areas have to be increased to get the same result compared to a control-canard. This adds weight and especially drag. In addition, when canards and wings closely situate to one another, they aerodynamically intefere with one anothers increasing drag'. Square-Square

To put it simply, this contradiction from canards placement have bothered engineers around the world. The US did even more research than Europe in the 80s. Lift generation with LERX and canards have appeared long ago. In 1970, NASA had a high speed maneouvre aircraft technology demonstrator (HIMAT). The 17A proposal uses such layout, but limitation from the aerodynamics and flight control theroies led to its abandonment. In 1997s May, NASA and Boeing jointly developed the X36 carnard layout demonstrator. This plane used LERX and canard layout, where stealth and agility are part of its main testing focus. But the Americans might have the following belief: it doesn't matter if it's a control-canard or close-coupled canard, neither is beneficial to the next generation fighter. This is truly sad for the carnard layout. The phrase "advantages of the canard layout exists only on the enemies" now being used by manys to force the idea of non-stealthiness onto the canard configuration. But China Aviation Inc. back in early 90s already studied thoroughly on the stealthiness of this layout. In fact, the JSF layout and the Gripen proved that canard makes no difference on the stealthiness compared to conventional layouts.

pugachev_diver said:

QUoting from expert again, "canard and conventional don;t make much difference in terms of stealth performance, doesn;t matter if its conventional or canard, all their stealtiness are calculated assuming they are in cruising state, and almost completely neglected when maneouvering or only do minimals, but this doesn't influence its figure (I guess he meant the overall RCS value). Canard layout and conventionals' only difference is that canard has an extra surface in the front compared to conventionals. But the front view of the plane wouldn't change. (like a sword, doesn;t matter how long it is, you can only see a small point when its pointed at you). the canard itself can be made to be stealthy. canard and conventional layouts' difference in stealth performance can only be substantial when the RCS is lowered down to 0.001 and it has to be scanned by very powerful radars with specific wavelengths."

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J-20 design approaches as reflected in researches on canard configurations of 1990's

Quoting from expert again, "canard and conventional layouts don't make much difference in terms of stealth design; stealtiness is calculated assuming the aircraft is in cruising state, with flight control surfaces assumed to be fixed. When maneouvering, stealth is ignored or only minimals migitations are taken, but this doesn't influence its figure (I guess he meant the overall RCS value). The only difference in canard and conventional layout is that canard has an extra surface in the front (of the wings) compared to conventional layout. However, the theory of controlling direction of radar reflection is the same: leading edge of the canards being paralleled to the trailing edge, and internal structure of the canards can be optimized to enhance stealth characteristics. The main difficulty in management of radar reflection lies in the wingroot of the canards, and is as difficult as managing radar reflection of the root of leading edge slats in a conventional layout. The difficulties are the same, and the migitiation techniques are basically identical. Canard and conventional layouts' difference in stealth performance is only noticable when the RCS is lowered down to 0.001 m2. Radar reflection from canards is more difficult to manage because of their small size, thus have poorer performance when shined by radars with middle to long wavelength. Migitation techniques involve combination notch-band absorption and special coating. Such problems can also be found in conventional layout, but the wings can hide the horizontal stabilizers to a certain degree." In conclusion, wings in a canard layout is large, with long wingroot leading to better stealth charcteristics; Vertical stabilizers are far from the canards with little interference; All aspect stealth (of the wings) is slightly better than conventional layout. However, the present of canards require more effort and complicate techniques to migitate, but there are little difference between canard and conventional layout. The difficulties in combining stealth with aerodynamics are about the same.