Aerodynamics thread

[Quickie]

Re: J-20... The New Generation Fighter III

I can see you can not say how does F-22 generate forebody lift, and no wonder, vortices are shed by the forebody once the aircraft is pitched, at high AoA and many jets generate them too

I've already answered it, but you just won't listen. The F-22, and J-20, generate forebody lift by the shape of its forebody. Vortices are just an aid to controlling boundary layer separation. Even commercial airliner generates body lift, otherwise the fuselage weight would be simply too heavy for the wing structure. (I remember this was discussed in another forum)

look you "explanation" simply is not logic, J-20 as Rafale or any other fighter have flaps and both elevons and canards are pitch control, at take off the canard is on a positive angle pushing the nose up while the elevon deflected up pushing the tail down besides these jets apply flaps they have flaps for a reason.

The canards and elevons are used in combination during takeoff because this will produce the required AOA and max lift at the slow takeoff airspeed during which the canards' effect on the overall (mainly the wing) lift has become much less significant than during higher airspeed, so its priority shifts to that of pitch control. See the logic? (But didn't you say the rafale only need to deflect its canards slightly because of its more balanced stabililty? So why the too much deflection now? )

The J-20 do not deflect up its flaps or elevons during takeoff unlike the rafale. Of course all jetfighters use its elevons, they're there for a reason but different aircrafts of different designs use the elevons differently depending on flight conditions. Is that so hard to understand?

Look, I can't give you the answers for every question you come out with. From now on, you'll have to look for your own answers.

 

[MiG-29]

Re: J-20... The New Generation Fighter III

I've already answered it, but you just won't listen. The F-22, and J-20, generate forebody lift by the shape of its forebody. Vortices are just an aid to controlling boundary layer separation. Even commercial airliner generates body lift, otherwise the fuselage weight would be simply too heavy for the wing structure. (I remember this was discussed in another forum)




The canards and elevons are used in combination during takeoff because this will produce the required AOA and max lift at the slow takeoff airspeed during which the canards' effect on the overall (mainly the wing) lift has become much less significant than during higher airspeed, so its priority shifts to that of pitch control. See the logic? (But didn't you say the rafale only need to deflect its canards slightly because of its more balanced stabililty? So why the too much deflection now? )

The J-20 do not deflect up its flaps or elevons during takeoff unlike the rafale. Of course all jetfighters use its elevons, they're there for a reason but different aircrafts of different designs use the elevons differently depending on flight conditions. Is that so hard to understand?

Look, I can't give you the answers for every question you come out with. From now on, you'll have to look for your own answers.

The quoted article says J-20 will have more deflection, but your theory, was it generates so much lift that the J-20 pitches down the nose.

You have claimed canards deflected down can not generate lift, i prove you they can.
You answer then was a excuse, you claimed the article talked about close coupled canards.

I posted you an article where they say aligning the canard at high alpha keeps the canard unstalled even when the main wing is stalled or near stalled.

This picture shows how X-31 keeps the canard aligned to the air flow to reduce drag and keep it unstalled and with some lift and controlability over the nose

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To that you replied no one will use air tunnel data to make aircraft.

Your theory can not explain why not use flaps to push down the tail?
The answer was the canard is deflected down but flaps generate more drag so J-20 only uses canards hahahah this was really a proof of your lack of knowledge, the Gripen breaks at landing by deflecting the canards down
Here is the proof, it deflects down the canards and generate drag and a pitch down force, so your theory that canard deflection does not generate drag at low AoA such at landing shows do not understand canard deflection

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You claimed that flaps generate drag, but in some instances like this Rafale the canard and flap keep a balance, in this picture you see the flap is slightly deflected down generating lift and the canard keeps the nose up

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The article mentions this, J-20 will need higher deflection simply because it has a canard with more limits the J-20 was not flying a Herbst maneuvre, a Cobra or a Kulbit, it was just doing a simple turn.


Other jets do their turns without deflecting too much their canards and flaps, why? simple as the article mentioned the relaxed stability adds the extra pitch up that reduces the need for deflection.

 

[Quickie]

Re: J-20... The New Generation Fighter III

The quoted article says J-20 will have more deflection, but your theory, was it generates so much lift that the J-20 pitches down the nose.

You have claimed canards deflected down can not generate lift, i prove you they can.
You answer then was a excuse, you claimed the article talked about close coupled canards.

I posted you an article where they say aligning the canard at high alpha keeps the canard unstalled even when the main wing is stalled or near stalled.

This picture shows how X-31 keeps the canard aligned to the air flow to reduce drag and keep it unstalled and with some lift and controlability over the nose

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To that you replied no one will use air tunnel data to make aircraft.

Your theory can not explain why not use flaps to push down the tail?
The answer was the canard is deflected down but flaps generate more drag so J-20 only uses canards hahahah this was really a proof of your lack of knowledge, the Gripen breaks at landing by deflecting the canards down
Here is the proof, it deflects down the canards and generate drag and a pitch down force, so your theory that canard deflection does not generate drag at low AoA such at landing shows do not understand canard deflection

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You claimed that flaps generate drag, but in some instances like this Rafale the canard and flap keep a balance, in this picture you see the flap is slightly deflected down generating lift and the canard keeps the nose up

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The article mentions this, J-20 will need higher deflection simply because it has a canard with more limits the J-20 was not flying a Herbst maneuvre, a Cobra or a Kulbit, it was just doing a simple turn.


Other jets do their turns without deflecting too much their canards and flaps, why? simple as the article mentioned the relaxed stability adds the extra pitch up that reduces the need for deflection.

Listen here, I will not be bothered with reading with the rest of your post here after reading the first few lines because you're starting to make up ridiculous stories of what I said and how the discussion went. The only correct line is this: "canards deflected down can not generate lift." although it's not the exact wordings. My claim was that: canards in negative AOA will generate a down force.


Let me just say that if you continue with your ridiculous theory of aerodynamics here, I have no doubt anyone with a serious desire to engage in this thread's topic will even be bothered with joining the discussion here. I'm going to put you in my ignore list and it'll be the first time I've ever done so after so many years in this forum.

 

[MiG-29]

Re: J-20... The New Generation Fighter III

Listen here, I will not be bothered with reading with the rest of your post here after reading the first few lines because you're starting to make up ridiculous stories of what I said and how the discussion went. The only correct line is this: "canards deflected down can not generate lift." although it's not the exact wordings. My claim was that: canards in negative AOA will generate a down force.


Let me just say that if you continue with your ridiculous theory of aerodynamics here, I have no doubt anyone with a serious desire to engage in this thread's topic will even be bothered with joining the discussion here. I'm going to put you in my ignore list and it'll be the first time I've ever done so after so many years in this forum.

it is not my theory as you claim, it is what the papers say, to whom you have no presented any actual study, just i heard in this forum, or things like this

No matter what, the experimental model without the horizontral tail exists only in the wind tunnel to collect experimental data. No one would be crazy enough to actually built one and fly it. This is why they have to proceed further with the testing of the trisurface configuration as quoted below. So, my point still stands, you can't simply extrapolate the experimental data to a very different design configuration like the J-20..

but of course you are welcome to decide what ever you deem is okay and what you wish to believe, but so far you never presented how J-20 generates forebody lift you just said i heard in a forum things like that you never explained me what compromises have stealth fighters

I've already answered it, but you just won't listen. The F-22, and J-20, generate forebody lift by the shape of its forebody. Vortices are just an aid to controlling boundary layer separation. Even commercial airliner generates body lift, otherwise the fuselage weight would be simply too heavy for the wing structure. (I remember this was discussed in another forum)..

but no problem i respect you decision

but see Rafale also deflects the canards negatively as it turns see minute 4:15

[video=youtube;BzYq0xrawEI]http://www.youtube.com/watch?v=BzYq0xrawEI&feature=related[/video]

this shows physics are the same for the canards of J-20 and Rafale, and J-20 is just adjusting the canards for lower AoA while turning and pushing up the nose as Rafale does, but Rafale deflects much less its canards in turns

regards

 

[MiG-29]

Re: J-20... The New Generation Fighter III

The J-20's nose follows a similar design approach with some differences as the F-22. It's shaped to reduce radar cross section, but it is also shaped precisely to reduce nose slice and unintended lateral instability. (The hexagonal shape of the J-20's nose, like the F-22's diamond like cross-section, helps with lateral control)

I have read somewhere that the nose of the J-20 itself is also a lifting body and produces positive lift. Why they would design it to be so and then use canards to force nose down I do not know. But maybe it has to do with shifting centre of lift at different airspeeds?

i think the nose does not generate lift, the rhomboid shape cross section does not seem to be an airfoil, howevere the nacelles, intakes, basicly the section between the end of the canopy and root of the wing might generate lift, the bottom part is flat and the upper section is curved

 

[tch1972]

Re: J-20... The New Generation Fighter III

I am a private pilot license holder and allow me to explain some basic aerodynamic. An aircraft is naturally design to fly unlike helicopters. When the power is off, it starts to drop whereas aircraft will slowly glide down. If an aircraft is trim flying straight and level at a particular airspeed, increasing the thrust will produce a nose up tendency. It has nothing to do with nose producing lift. It a result that the wing is producing extra lift. What the pilots needs to do is to push the elevator or canard downward to counteract this nose up movement. This will convert the excessive vertical lift into horizontal thrust . The end result is the aircraft will fly straight and level at a higher airspeed. Vice versa if the pilot decrease thrust.

Slats and/or flaps are employ under 3 situation, during take-off, landing or slow flying. Depend on the setting, it can increase lift or increase both lift and drag. Full flaps and slat are usually employed during landing to enable the plane to achieve a steeper descending profile at lower airspeed. Partial flaps are lower during take-off to increase lift so as to enable the plane to achieve lower rotational speed.

During slow flying, pilot may employ partial flaps or slat to lower the stalling speed. The end result is the aircraft can fly straight and level at lower airspeed and AOA. Those photos of J20 with it canard deflected downward all have it leading edge slat deployed. Most likely the aircraft is flying straight and level and at low speed. This can't be compare with footage of Rafale flying at higher airspeed ad wing clean. It a different aerodynamic setting and we are not comparing apple with apple. I am still waiting to see the amount of canard deflection of J20 while it doing a high speed turning.

 

[Engineer]

Re: J-20... The New Generation Fighter III

thanks, it is a nice debate, do not take me wrong, the J-20 might be an interceptor (i think it is) however TVC nozzles can transform it in many was see

Flight tests of thrust-vectoring designs began in the early 1990s with airplanes like NASA’s modified F/A-18 and F-15, the Rockwell/MBB X-31, and a modified Air Force F-16. In 1994, the X-31 demonstrator was fitted with what German program managers called a “poor man’s thrust vectoring nozzle”—three paddle-like vanes that pushed into the exhaust stream—and the results were spectacular. Without thrust vectoring, the X-31 lost twice as often as it won against the F/A-18 in mock combat; with it, the X-31 didn’t lose once in 129 matches.

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the advantage is while we look at the videos of Gripen and Rafale turning without deflecting too much the canards, there is the J-20 advantage of no carrying weapons externally, at heavy weights, Rafale loses agility and some supercruise ability.


So if you add TVC nozzles and no external weapons perhaps it can be an air superiority, people sometimes think F-22 is excellent without TVC nozzles but it is not, is not that much better than F-15 or F-16, so who knows with TVC nozzles it might become a more deadly weapon



TVC nozzles might improve the J-20 to the level of perhaps achieve F-22 agility, because i know F-22 expands a lot its AoA with TVC nozzles
see figure 9-15. Role rate performance, showing the effect of thrust vectoring. (Courtesy of LMAS)

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and see Figure 5: Maximum pitch up, pitch down of the YF-22 aircraft compared with F-16 [6].

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Dr. Song's paper has made it clear that the initial concept of J-20 has a target of 10% unstability. This does not mean J-20 actually has 10% unstability, but that it is one of the design aims. The statement in his paper is as follow:

The future fighter could enjoy a significant improvement in lift-to-drag if the longitudinal instability could be increased to a magnitude of around 10%.

Since unstability correlates with manuerability, we at least know that J-20 was designed with manuerability in mind. Authorative statement like the above runs contrary to the opinion that J-20 is sluggish and is nothing more than a stealthied J-8.

In another statement related to TVC, Dr. Song points out that aerodynamic of the aircraft itself is the basis of utilizing TVN. The reason is that when TVN malfunctions, aerodynamics are all the aircraft has:

Although it is possible to solve the problem of post-stall controllability through the use of thrust vectoring nozzles, the aerodynamic configuration itself must provide enough pitch down control capability to guarantee the aircraft to safely recover from post-stall AOA should the thrust vectoring mechanism malfunction. As a result, it is vitally important to study unconventional aerodynamic control mechanisms for high AOA flights.

In other words, to take advantages offered by TVN the aircraft itself must be manuverable. The notion that adding TVN to interceptor such as J-8 and the plane would become F-22 killing platform is unrealistic, and is nothing more than a pretense to claim J-20 as unmanuverable.

Do you think the J-20 will deflect the canards further to turn tighter?
In my opinion J-20 will improve not becuase as you think its aerodynamics are uncompromised, but because once it adds TVC nozzles it will increase high AoA agility

see the advantages

3D Thrust Vectoring Enabling Enhanced Aircraft Flight Performance with Lower Operational Cost
Safety:
Additional control device.
Reduced aircraft loss rate due to low speed departure.
A/C Superior Performance and Handling Qualities:
extended flight envelope.
increased angle of attack (including post stall).
increased rolling rate.
Mission Performance:
Reduced take off & landing distance.
Increased thrust and reduced fuel consumption by means of afterbody drag reduction at supercruise conditions and nozzle exit area optimisation for the whole flight envelope.
Life Cycle Cost Reduction:
Engine life increase.
A/C life increase.
No aircraft or engine structural modifications required, currently studied for the EF2000 aircraft, could not require either airframe or engine structural reinforcement.

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So in my opinion what will transform J-20 will be TVC nozzles, now the jet, seems similar to the predictions done by the paper, but TVC can transform it, if China adds a right engine in the region of 15-17 tonnes of thrust and TVC nozzles

Citing advantages of TVN from a company that is pushing to sell its TVN does not do several things. First, it does not dismiss the disadvantages of using TVN, namely higher complexity, added weight, and higher operating cost. Gaining those so call advantages but taking on a bunch of disadvantages at the same time cannot be referred as gains unless tradeoff analysis is done. Secondly, it does not mean whatever advantages that company claims to offer is applicable to every single TVN out there. Case in point, the "afterbody drag reduction at supercruise conditions and nozzle exit area optimisation for the whole flight envelope" is a result of their innovative three-ring system. Thirdly, it does not say anything about the current dynamics of J-20.

---------- Post added at 03:05 AM ---------- Previous post was at 02:26 AM ----------

The thrust T, lift L, drag D, velocity V, and various angular attitudes associated with and acting upon the aircraft in piloted supernormal flight are best illustrated in FIG. 2C. In such supernormal flight, the aircraft operates at an attackangle .alpha. much greater than the angle of attack for maximum lift so that the fixed wing 15 is either completely or partially stalled while the canard surfaces 19 are deflected in a negative sense through the deflection angle-.delta..sub.c. The absolute deflection magnitude of the canard surfaces 19 is approximately the same as the attack angle .alpha. for the entire aircraft so that such canard surfaces 19 are nearly aligned with the local air flow and are, therefore, unstalled. Thus , the canard surfaces 19 remain effective as lift surfaces in providing the required forces and moments for controlling the entire aircraft.

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This quote of the patent says that unstalling the canard is to keep it an effective control surfaces for pitch control. It does not say that keeping the canard near zero angle-of-attack is to enhance lift to keep the noise up.

Check the J-20 is not flying with thrust vectoring, niether is close to a stalled wing situation the jet is basicly, executing a shallow turn What song`s paper is trying to address is something similar but he metions TVC nozzles, on that video J-20 does not have thrust vectoring niether flying at high AoA, the jet is just simply deflecting the canards like a heavy loaded Rafale would do in order to increase lift.

In my opinion today`s J-20 still is far from being the ideal jet Song`s paper describe because it lacks TVC nozzles and without it the contradictions of stealth and aerodynamics represent a higher performance penalty

You must be looking at a different video than the rest of us then. Check out

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, and you will see that J-20 is flying at high AoA. The canards are being maintained at zero AoA with respects to the oncoming air, which is what your quote of the patent from above is also saying. At the same time, the canards made a large angle with respect to the body, meaning the body is flying at high AoA. Judging by the angle between the canard and body, the aircraft would be flying at about 30 degrees AoA between 4:20 and 4:30.

It is also amusing to see you intentionally distort Dr. Song statements in order to justify your "J-20 needs TVC" premise. Here is his actual statement with regards to TVN:

Although it is possible to solve the problem of post-stall controllability through the use of thrust vectoring nozzles, the aerodynamic configuration itself must provide enough pitch down control capability to guarantee the aircraft to safely recover from post-stall AOA should the thrust vectoring mechanism malfunction. As a result, it is vitally important to study unconventional aerodynamic control mechanisms for high AOA flights.

So, you are correct in claiming Dr. Song mentioned TVN in his paper. What you conviently left out is that he was in fact referring to lack of TVN when talking about the J-20 concept. In fact, the thesis of his paper is to use aerodynamics to meet the design criterias for 4th generation fighter aircraft:

The author believes that in-depth study of fluid dynamics, exploration of the full practical potential of current aerodynamic designs, development of new design concepts, employment of corresponding systematic and control measures, and necessary compromise among numerous design proposals will allow us to achieve our design goals.

---------- Post added at 03:36 AM ---------- Previous post was at 03:05 AM ----------

I will tell you why i do not believe that theory, i have presented you two reasons why canards are deflected negatively, one is because at high AoA the lift and pitch moment is increased, two because at very high AoA canards are used to regain control of a stalled wing.

If the J-20 needs a strong nose down moment, it only needs to deploy flaps why?

The other drawback with the aft-set wing is that it is difficult to use flaps. Flaps almost double the lift produced by the wing, but on a lifting canard this would produce such a strong nose-down pitching moment that only a very large foreplane could overcome it. Consequently, flap size and effectiveness become factors in the thorny issue of canard sizing.

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However you see the J-20 not deploying flaps, but deploying wing leading edge flaps to increase lift and donward deflection of canards to increase lift.

Canards are deflected instead of flaps because flaps go into stall when the wing is already in high AoA. When an airfoil such as the wing is at large AoA, further increase in AoA will cause stall. Deploying flaps essentially increase the AoA of flaps, stalling the control surfaces and rendering them useless. Canards do not run into this stalling issue, because when pitch-down authority is needed, the canards simply need to go into negative AoA to lose lift. This is addressed by Dr. Song's paper:

The requirement for high AOA pitch down control capability is closely related to the longitudinal static instability requirement. The greater the longitudinal static instability, the higher the demands for pitch down control capabilities. As described in chapter 3, the future fighter will hopefully increase its longitudinal static instability to around 10% its average aerodynamic chord length to enhance the trim's lift to drag and lift characteristics. As a result there should be a corresponding improvement in the pitch down control capability. We can categorize two types of control surfaces based on the relative position of the pitch control surfaces with respect to the aircraft's center of mass: positive load pitch down control surface and negative pitch down control surface. Control surfaces placed behind the center of mass, including the vertical stabilizers and trailing edge flaps, generate pitch down control torque by increasing lift. They are considered positive load control surfaces. Control surfaces placed in front of the center of mass, like the canards, are negative load control surfaces. Since the main wing's ability to generate lift tends to saturate under high AOA conditions, the positive load control surfaces' pitch down control capabilities tend to saturate under high AOA as well. Therefore it will be wise to employ negative load control surfaces for pitch down control under high AOA conditions. Figure 7 compares the pitch down control capabilities of the canards and horizontal stabilizers. From the high AOA pitch down control stand point, it will be wise to use canards on the future fighter.

The quote you have used does not say anything about downward deflecting canards genearting lift. Rather, it is saying that flaps deployment requires canard deflection to overcome the nose down moment. We see this when J-20 is taking off:

Notice in the photograph, balancing pitch down moment causes the canards to deflect upward to provide extra lift at the nose, not downward.

Another one of your pseudo-aerodynamic theories debunked.

Rafale does not use too much its canards because trailing edge flaps and relaxed stability are balanced while turning or doing vertical turns

Rafale does not use its canards because it has close couple canards, that are essentially useless as pitch control devices because the moment arm is so short.

from minute 0:55 you can see the Gripen flying high AoA and the canards with negative deflection

the aircraft operates at an attackangle .alpha. much greater than the angle of attack for maximum lift so that the fixed wing 15 is either completely or partially stalled while the canard surfaces 19 are deflected in a negative sense through the deflection angle-.delta..sub.c. The absolute deflection magnitude of the canard surfaces 19 is approximately the same as the attack angle .alpha. for the entire aircraft so that such canard surfaces 19 are nearly aligned with the local air flow and are, therefore, unstalled .

Right. I have taken the liberity to change the underlining in your quote. The canards are deflected downward to maintain zero AoA and remain unstalled, not to provide extra lift. The angle that the canard makes with respects to the aircraft corresponds to the AoA at which the aircraft is flying, meaning J-20 is flying at high AoA when there is large canard deflection. Your own quote disproves your pseudo-aerodynamic theories.

---------- Post added at 03:49 AM ---------- Previous post was at 03:36 AM ----------

I do not think that is the case, and i will tell you why, the J-20 by applying flaps will generate a nose down movement simply like that, tailess fighters like Mirage 2000 or LCA is what they do when they over pitch.

Song`s paper talks about using canards at very high alpha, near poststall, and like the paper or X-31 video i posted you, canards are deflected at the same Angle but with opposite sign of the AoA at post stall, the chinese theory is similar to the European in that they want to use delta wings so they added Lerx, and canard and some wing-fuselage blending, but is not like you are thinking, Song`s paper is anything but new or revolutionary, looking at the Rafale or even Gripen you can find similar aerodynamics, it is nothing new, X-31 is similar in TVC nozzles application at high AoA and coupling with canards deflection, something that both Germans and Americans have already tested in test flights not in theoretical work and tested his theories since the 1990s.

Whether ideas presented in Dr. Song's paper are revolutionary or not is irrelevant to the fact that deflecting canards downward is not to add lift.

In fact JAST JSF were with similar solutions, what really has surprised western analysts is the possibility that China has made advances in avionics and materials and adapted western technologies into an original concept.

Watch this video and see that LCA can control the jet with pure wing trailing flaps and elevons, J-20 can do that, but they do not apply flaps simply because flaps mean a pitch down force, so they do not apply flaps otherwise the nose will go down

J-20 uses canards instead of flaps during high AoA because the flaps would stall, whereas canards wouldn't. Deflecting the canard into negative AoA achieves the same pitch down moment anyway, which is the exact effect Dr. Song wants for pitch authority, and is confirmed by observations from the video.

---------- Post added at 03:56 AM ---------- Previous post was at 03:49 AM ----------

LCA and Mirage 2000 are definitively limited by the use of flaps, but they use a trick, and it is called low wing loading, and high relaxed stability, other fighters with canards like IAI Lavi, Rafale, Gripen or Eurofighter can use the canard to pitch up.

J-20 is not using flaps simply because by using them a strong nose down pitch will be generated, the canard is just having a lower AoA with respect the air flow and it is aligned with it.


If the theory it needs a down force was true, then deploying flaps is a smarter idea, why? by deploying flaps the J-20 achieves higher lift, thus better turn rates are achieveable, however the reality is the J-20 is not deploying them simply because the jet is using the canards to lift up the nose.

At minute 4:00 you can see the Gripen with negative canards deflection, that is just to align the flow with the canards and reduce their AoA

If your theory that canards are being used to lift J-20's nose up as true, then we should see a positive deflection in the canards. This is because AoA positively correlates with amount of lift generated. However, the reality is that not only are the canards not level with the body, they have to be deflected downward to reduce AoA and lift at the front of the aircraft. The canards have their AoA reduce to zero with respects to the airflow, and be readied to go into negative AoA at any moment to reign in pitch-up moment. This is completely opposite to your pseudo-aerodynamic theory.

---------- Post added at 04:16 AM ---------- Previous post was at 03:56 AM ----------

Here in this forum there is the idea, that Song`s theories are revolutionary, new, unknown in the west, J-20 is uncompromised, and so on.

This is called a

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argument. You are unable to back up your idea, so you invented one for the opposite team instead then make arguments against it, giving the illusion that you have retorted when you haven't.

The fact is, no one on this forum claimed Dr. Song's theories are revolutionary, new, or unknown in the West. What's happening here is that Dr. Song's paper debunks your pseudo-aerodynamic theories on J-20, and now you are desperately trying to make the paper seem irrelevant so you can seem more credible.

So what is J-20 and Song`s paper? well is an original chinese design, but the Chinese have based their fighter in knowledge first studied in the west and Russia that has led them to design J-20 as it is today

So, what is J-20 and Dr. Song's paper? An aircraft capable of extreme high AoA flight and manuerability, achieved using aerodynamics and not TVN. Unstability is aimed to approach 10% MAC, compared to 3% for 3rd generation fighter aircraft. Canards are chosen over traditional tails because canards are capable of providing pitch-down authority at high AoA without stalling. Canards are placed far ahead of the wing to provide better moment arm, while effects of close-couple canards are retained through the employment of LERX. Low-aspect wing is used for low drag, while the lift necessary for low-speed handling and maneuvering at high AoA is provided by body-lift, as well as vortices generated by Canards, LERX, and chines.

---------- Post added at 04:41 AM ---------- Previous post was at 04:16 AM ----------

that is okay but here is not my mistake as you claim

Kersh's work indicated that the lift coefficient increased as the fixed-canard
deflection angle was varied from 0 to -7 degrees, at this angle of attack
TABLE 4: ALPHA = 34,

Canard Deflection -7 Degrees
This deflection produced a canard angle of attack of 27 degrees

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As it was mentioned in Volume 2 that small negative deflections had a
beneficial effect on performance at low angles of attack and only minor un-
favorable effects at high angles of attack.
In general, the incremental change in
lift due to either positive or negative deflection is relatively constant
with Mach number at each angle of attack for all configurations

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The thesis considers the effect of canards to the aircraft. The lift that the author refers to is the lift to the total aircraft through canard-wing vortex interaction. He was not referring to lift generated at the canard.

The absolute deflection magnitude of the canard surfaces is approximately the same as the attack angle .alpha. for the entire aircraft so that such canard surfaces are nearly aligned with the local air flow and are, therefore, unstalled. Thus , the canard surfaces remain effective as lift surfaces in providing the required forces and moments for controlling the entire aircraft.

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Aircraft with canards do that, so is not seudo aerodynamics it is simply you can not prove otherwise

No where in the above quote mentioned that canards deflecting in negative direction creates canard lift and results in pitch-up moment. This is your pseudo-aerodynamic theory, completely opposite to actual aerodynamic theory where AoA and lift are positively correlated to forcibly argue that J-20 is nose-heavy.

tell me why this Rafale is turning and deflecting negative its canards?

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According to you, Rafale is nose-heavy and requires canard deflection to lift its nose up.

But you know let us leave it here if you want, as long as you do not that canards are deflected negatively and they can generate lift at high AoA the is no point to discuss specially if you do not understand this

The other drawback with the aft-set wing is that it is difficult to use flaps. Flaps almost double the lift produced by the wing, but on a lifting canard this would produce such a strong nose-down pitching moment that only a very large foreplane could overcome it. Consequently, flap size and effectiveness become factors in the thorny issue of canard sizing.

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This has nothing to do with negative canard deflection on the J-20. The reason, as given in Dr. Song's paper is as follow:

We can categorize two types of control surfaces based on the relative position of the pitch control surfaces with respect to the aircraft's center of mass: positive load pitch down control surface and negative pitch down control surface. Control surfaces placed behind the center of mass, including the vertical stabilizers and trailing edge flaps, generate pitch down control torque by increasing lift. They are considered positive load control surfaces. Control surfaces placed in front of the center of mass, like the canards, are negative load control surfaces. Since the main wing's ability to generate lift tends to saturate under high AOA conditions, the positive load control surfaces' pitch down control capabilities tend to saturate under high AOA as well. Therefore it will be wise to employ negative load control surfaces for pitch down control under high AOA conditions.

---------- Post added at 05:16 AM ---------- Previous post was at 04:41 AM ----------

The quoted article says J-20 will have more deflection, but your theory, was it generates so much lift that the J-20 pitches down the nose.

You have claimed canards deflected down can not generate lift, i prove you they can.

You answer then was a excuse, you claimed the article talked about close coupled canards.

Lift at the wing contributed by the canard is not the same as lift generated at the canard. You are grasping at straw with that particular thesis, because there is nothing remotely close that you can use to back up your pseudo-aerodynamic theories otherwise. However, the fact is that the author never mention anything about canard lift. When he is speaking of lift, he refers to that for the entire aircraft, generated through canard-wing vortex interaction. Thus, the thesis does not support your position that J-20 is deflecting its canards downward to lift the nose.

I posted you an article where they say aligning the canard at high alpha keeps the canard unstalled even when the main wing is stalled or near stalled.

This picture shows how X-31 keeps the canard aligned to the air flow to reduce drag and keep it unstalled and with some lift and controlability over the nose

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Keeping the canard unstall by giving them zero AoA is different from using the canard to generate extra lift in order to lift the nose up.

To that you replied no one will use air tunnel data to make aircraft.

Your theory can not explain why not use flaps to push down the tail?
The answer was the canard is deflected down but flaps generate more drag so J-20 only uses canards hahahah this was really a proof of your lack of knowledge, the Gripen breaks at landing by deflecting the canards down
Here is the proof, it deflects down the canards and generate drag and a pitch down force, so your theory that canard deflection does not generate drag at low AoA such at landing shows do not understand canard deflection

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So, you claim canards deflect downward to generate drag and pitch down force while simultaneously claiming it generates pitch up force for the J-20? Sounds like you have just contradicted yourself.

These sort of contradictions are bounded to happen when certain someone is here to spin everything about the J-20 into bugs or design flaws instead of truly wanting to discuss aerodynamics.

You claimed that flaps generate drag, but in some instances like this Rafale the canard and flap keep a balance, in this picture you see the flap is slightly deflected down generating lift and the canard keeps the nose up

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The J-20 does the

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at slow speed. Canards are deflected upward when lift at the nose is required, not downward as your pseudo-aerodynamic theories claim.

The article mentions this, J-20 will need higher deflection simply because it has a canard with more limits the J-20 was not flying a Herbst maneuvre, a Cobra or a Kulbit, it was just doing a simple turn.

Well, the article is wrong, just like your pseudo-aerodynamic theories are wrong.

With regards to canard deflection, the article is wrong on two accounts. First, J-20 canards can deflect 180 degrees, from vertically down to vertically up. This is confirmed by various videos. The deflection limitation that the article speaks of simply does not exist. Second, what encounter limitation in deflection are the flaps, as they would saturate when their AoA becomes too high. Thus, the authors got the limitation of canards and flaps reversed. So, either the authors are ignorant, or they have an axe to grind with the J-20 just like you do.

The problem of control saturation and why canards are used instead of flaps are addressed by Dr. Song's paper:

The requirement for high AOA pitch down control capability is closely related to the longitudinal static instability requirement. The greater the longitudinal static instability, the higher the demands for pitch down control capabilities. As described in chapter 3, the future fighter will hopefully increase its longitudinal static instability to around 10% its average aerodynamic chord length to enhance the trim's lift to drag and lift characteristics. As a result there should be a corresponding improvement in the pitch down control capability. We can categorize two types of control surfaces based on the relative position of the pitch control surfaces with respect to the aircraft's center of mass: positive load pitch down control surface and negative pitch down control surface. Control surfaces placed behind the center of mass, including the vertical stabilizers and trailing edge flaps, generate pitch down control torque by increasing lift. They are considered positive load control surfaces. Control surfaces placed in front of the center of mass, like the canards, are negative load control surfaces. Since the main wing's ability to generate lift tends to saturate under high AOA conditions, the positive load control surfaces' pitch down control capabilities tend to saturate under high AOA as well. Therefore it will be wise to employ negative load control surfaces for pitch down control under high AOA conditions. Figure 7 compares the pitch down control capabilities of the canards and horizontal stabilizers. From the high AOA pitch down control stand point, it will be wise to use canards on the future fighter.

Of course, you chose to ignore this and repeat your incorrect claims over and over again. However, your claim that J-20 deflect its canards downward to pitch up is still incorrect.

Other jets do their turns without deflecting too much their canards and flaps, why? simple as the article mentioned the relaxed stability adds the extra pitch up that reduces the need for deflection.

False. Other jets do have large deflection in their canards, as in the case of Rafale in this picture:

In the picture, you can see Rafale was performing a high AoA manuever, where the canards are aligned with the oncoming airflow. From the picture, I would gague the AoA of the aircraft at that time to be around 30 degrees. In videos we have seen, J-20 was performing high AoA maneuver, which explains why its canards were deflecting downward at large angle. They are deflected for the exact same purpose -- to maintain zero AoA with respects to oncoming airflow.

 

[MiG-29]

Re: J-20... The New Generation Fighter III

So, you are correct in claiming Dr. Song mentioned TVN in his paper. What you conviently left out is that he was in fact referring to lack of TVN when talking about the J-20 concept. In fact, the thesis of his paper is to use aerodynamics to meet the design criterias for 4th generation fighter aircraft:


Canards are deflected instead of flaps because flaps go into stall when the wing is already in high AoA. When an airfoil such as the wing is at large AoA, further increase in AoA will cause stall. Deploying flaps essentially increase the AoA of flaps, stalling the control surfaces and rendering them useless. Canards do not run into this stalling issue, because when pitch-down authority is needed, the canards simply need to go into negative AoA to lose lift. This is addressed by Dr. Song's paper:


.

Song`s paper does talk about post tall handling, why? simple after the wings stalls, you need someting to bring the nose down.

In this video you can see a F-18 at high AoA using flaps

[video=youtube;zClTYI7bVdA]http://www.youtube.com/watch?v=zClTYI7bVdA&feature=BFa&list=HL1332636210&lf=mh_lolz[/video]

here is a picture of a F-18E doing high AoA and using flaps

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does Su-27 or any flanker use flaps during any high AoA the answer is yes

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Now why is difficult to use flaps on a canard aircraft?



"The greatest disadvantage to canard
airplanes is that it is difficult to put flaps on
the main wing. Remember that the canard
itself is already flapped (the elevator), and is
already working as hard as it can. If you put
flaps on the main wing, the wing would generate
even more lift, which would raise the
tail and lower the nose. After you pull full aft
stick to raise the nose, you'll find that the
airplane is still diving. The reason is that the
canard can't produce enough lift to balance
the airplane when flaps are deflected on the
rear wing. This is not good!"


So why canards are used to control stall?
Aspect ratio
is defined as wingspan squared divided by
the wing ara. You'll recall that aspect ratio is
the largest factor in determining how much
lift you get for each degree angle of attack
(the others were speed and sweep). Remember
that wingtips are leaky, and that aspect
ratio really tells you the ratio of "nonleaking"
wing perimeter to "leaking" wing
perimeter. The lower the aspect ratio, then,
the more "leaky" the wing is, and the greater
the angle of attack will have to be in order
to produce a certain amount of lift. The Concorde's
wing not only has a low aspect ratio,
but also leaks more due to its high sweep
angle.
One way to ensure that the canard stalls
before the wing is to give it a higher aspect
ratio than the wing. This means that the
canard will make more CL for each degree
angle of attack than the wing, and will therefore
reach its maximum CL and stall before
the wing does.
If you study the charts in Theory of Wing
Sections, you'll see that a wing with a deflected
flap stalls at a lower angle of attack
than an unflapped wing. So another way of
making the canard stall first is to put a flap
on it. You can call this flap an elevator, and
use it for pitch control. To get to high angles
of attack, you will raise the nose by deflecting
this elevator trailing edge down, increasing
the canard's lift. Having the elevator deflected
means that you will have lowered the
canard's stalling angle of attack.

See that Rafale, Eurofighter, Gripen have high aspect canards and low aspect wings and most modern fighter jets have that configuration a high aspect canard

So now let us see why canards are used?

"If the canard, which is
in front of the c of g, stalls while the wing
behind the c of g is still flying, then the nose
will lower itself automatically. At a lower
angle of attack the canard will unstall, and
its restored lift will again increase the angle
of attack until it stalls again."


now what happens if the wing stalls first?

"What would happen if the wing stalled but
the canard didn't? In that case, the canard
would raise the nose while the wing let the
tail drop. There would be a sudden increase
in the airplane's angle of attack. This is not
good!"

but okay the text also says you can recover up to a level the aircraft

"It is not true, however, that you can
never let the back wing stall on a canard.
Even on a stalled airplane, there are forces
at work (lift, drag and gravity) - and each of
these forces has a lever arm back to the
center of gravity. It's OK to have both the
canard and the wing stalled, as long as the
pitching moments trying to lower the nose
are greater than those trying to raise the
nose"

So what Song`s paper tries to address is that possibilty

So what he does really say? he says in his opinion the tailplane is not as good compared to a foreplane (canard) to bring the nose down in a post stall situation.


Well, Su-27 brings perfectly the nose down during the cobra after 120 degrees of AoA

X-31 uses TVC nozzles to bring the nose down along side canards

well let us see if Eurofighter or Gripen deflect down the canards as J-20 does?


from minute 1:30 you see the Eurofighter turning tighter than J-20 and Gripen the same, you can see almost no deflection.
[video=youtube;GSkmzjTsl1M]http://www.youtube.com/watch?v=GSkmzjTsl1M[/video]

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The X-31 program demonstrated the value of thrust vectoring (directing engine exhaust flow) coupled with advanced flight control systems, to provide controlled flight during close-in air combat at very high angles of attack. The result of this increased maneuverability is an airplane with a significant advantage over conventional fighters.

"Angle-of-attack" (alpha) is an engineering term to describe the angle of an aircraft body and wings relative to its actual flight path. During maneuvers, pilots often fly at extreme angles of attack -- with the nose pitched up while he aircraft continues in its original direction. This can lead to loss of control and result in the loss of the aircraft, pilot or both.

Three thrust-vectoring paddles made of graphite epoxy mounted on the exhaust nozzle of the X-31 aircraft directed the exhaust flow to provide control in pitch (up and down) and yaw (right and left) to improve control. The paddles can sustain heat of up to 1,500 degrees centigrade for extended periods of time. In addition the X-31 aircraft were configured with movable forward canards and fixed aft strakes. The canards were small wing-like structures set on the wing line between the nose and the leading edge of the wing. The strakes were set on the same line between the trailing edge of the wing and the engine exhaust. Both supplied additional control in tight maneuvering situations.

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[MiG-29]

Re: J-20... The New Generation Fighter III

During slow flying, pilot may employ partial flaps or slat to lower the stalling speed. The end result is the aircraft can fly straight and level at lower airspeed and AOA. Those photos of J20 with it canard deflected downward all have it leading edge slat deployed. Most likely the aircraft is flying straight and level and at low speed. This can't be compare with footage of Rafale flying at higher airspeed ad wing clean. It a different aerodynamic setting and we are not comparing apple with apple. I am still waiting to see the amount of canard deflection of J20 while it doing a high speed turning.

I will disagree just using simple studies of canard deflections in the USA.

Let us start witha simple fact:

Delta wings have low aspect ratio thus they achieve their max lift at higher AoA than higher aspect wings, since J-20 has a delta, it shows how much its struggles with the AoA to achieve lift
see

"Wings with aspect ratios less than 5 typically stall at
higher angles of attack than higher-aspect-ratio wings. The
wing of the model had an aspect ratio of 3. The overall shape
of the CL versus AOA curve is similar to curves for other lowaspect-
ratio wing configurations"


Let us see

"The AOA of
the wing/body was varied from -8 to 50 degrees,and CL varied
from .036 to 1.586. The maximum lift occurred at an AOA of 40
degrees."

Here it is clear the delta wing of the model achieved its max lift at a pretty high AoA

Now
You are claiming the J-20 is flying straight, that is not true it is turning, but why you can explain such as large deflection of the canard?

See the very likely J-20 has not its main engines, therefore it is very likely stil underpowerd.

So what happens with drag at high AoA, well it is also very high so the model needs powerful engines to increase Sustained turn rates

"Values in the high angle-of-attack
regime from 20 to 40 degrees, where an agile-aircraft might be
expected to have an excursion, decrease from about 2.8 to 1.3,
indicating the significant thrust levels required for
supermaneuverability."

simple let us see the Max lift attained by the model at different AoA and at what Angle of canard deflection

See:

"Figure 14 shows the variation of CL with canard deflection
angle at a wing/body AOA of 10 degrees. The maximum CL was
0.698 at 17 degrees."

later it goes

"Figure 16 shows the variation of CL with canard deflection
angle for a wing/body AOA of 22 degrees. The maximum CL was
1.422 at a canard angle of 7 degrees"
later

"Figure 18 shows the variation of CL with canard deflection
angle for a wing/body AOA of 34 degrees. This angle of attack
is in the region midway along the second rise of the baseline
configuration lift curve. The maximum CL was 1.642 at a
canard angle of -7 degrees."



and later

"Figure 20 shows the variation of CL with canard deflection
angle for a wing/body AOA of 40 degrees. This wing/body angle
corresponds to the condition of the maximum CL for the
baseline configuration. The maximum CL was 1.700 at a canard
angle of -15 degrees."

See that if the Jet is trying to achieve higher lift it will need a relatively high negative deflection, and a high AoA if it is underpowered

"CANARD/WING/BODY CONFIGURATION; AOA 48 DEGRZES
Figure 22 shows the variation of CL with canard deflection
angle for a wing/body AOA of 48 degrees. This wing/body angle
is deep in the second post-stall region for the baseline
configuration. The maximum CL was 1.649 at -17 degrees."

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So what is a turn? it is basicly a high AoA so the J-20 is in reality deflecting to achieve higher lift and is not using flaps to avoid pitch down the nose like any delta canard jet will do

 

[Engineer]

Re: J-20... The New Generation Fighter III

Since i know you are the typical i know better regardless of what you say, i will be brief and simple, i know perfectly your style.

That's correct, I do know better than you, which is why your b.s. doesn't work with me.

Song`s paper does talk about post tall handling, why? simple after the wings stalls, you need someting to bring the nose down.

This is a fallacy known as

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. At no time did I claim that Dr. Song paper doesn't talk about post stall handling.

In the paper, Dr. Song explained that canards can bring the nose down by providing negative lift, and called the canards "negative load control surfaces". The exact statement is as follow:

We can categorize two types of control surfaces based on the relative position of the pitch control surfaces with respect to the aircraft's center of mass: positive load pitch down control surface and negative pitch down control surface. Control surfaces placed behind the center of mass, including the vertical stabilizers and trailing edge flaps, generate pitch down control torque by increasing lift. They are considered positive load control surfaces. Control surfaces placed in front of the center of mass, like the canards, are negative load control surfaces.

Continue on, he explained that canards are advantageous over flaps in pitch control. Specifically, at high AoA the wing would be close to stalling, and extending flaps is equivilent to increasing their AoA, causing them to stall and lose control. Canards simply have to go into negative AoA to force the nose down.

Since the main wing's ability to generate lift tends to saturate under high AOA conditions, the positive load control surfaces' pitch down control capabilities tend to saturate under high AOA as well. Therefore it will be wise to employ negative load control surfaces for pitch down control under high AOA conditions. Figure 7 compares the pitch down control capabilities of the canards and horizontal stabilizers.

In this video you can see a F-18 at high AoA using flaps

here is a picture of a F-18E doing high AoA and using flaps

does Su-27 or any flanker use flaps during any high AoA the answer is yes

Another

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. At no time did I claim any of these aircraft does not employ flaps.

Now why is difficult to use flaps on a canard aircraft?

"The greatest disadvantage to canard airplanes is that it is difficult to put flaps on the main wing. Remember that the canard itself is already flapped (the elevator), and is already working as hard as it can. If you put flaps on the main wing, the wing would generate even more lift, which would raise the tail and lower the nose. After you pull full aft
stick to raise the nose, you'll find that the airplane is still diving. The reason is that the canard can't produce enough lift to balance the airplane when flaps are deflected on the rear wing. This is not good!"

"So why canards are used to control stall?
Aspect ratio is defined as wingspan squared divided by the wing ara. You'll recall that aspect ratio is the largest factor in determining how much lift you get for each degree angle of attack (the others were speed and sweep). Remember
that wingtips are leaky, and that aspect ratio really tells you the ratio of "nonleaking" wing perimeter to "leaking" wing
perimeter. The lower the aspect ratio, then, the more "leaky" the wing is, and the greater the angle of attack will have to be in order to produce a certain amount of lift. The Concorde's wing not only has a low aspect ratio, but also leaks more due to its high sweep angle. One way to ensure that the canard stalls before the wing is to give it a higher aspect
ratio than the wing. This means that the canard will make more CL for each degree angle of attack than the wing, and will therefore reach its maximum CL and stall before the wing does. If you study the charts in Theory of Wing
Sections, you'll see that a wing with a deflected flap stalls at a lower angle of attack than an unflapped wing. So another way of making the canard stall first is to put a flap on it. You can call this flap an elevator, and use it for pitch control. To get to high angles of attack, you will raise the nose by deflecting this elevator trailing edge down, increasing
the canard's lift. Having the elevator deflected means that you will have lowered the canard's stalling angle of attack.

See that Rafale, Eurofighter, Gripen have high aspect canards and low aspect wings and most modern fighter jets have that configuration a high aspect canard

So now let us see why canards are used?

"If the canard, which is in front of the c of g, stalls while the wing behind the c of g is still flying, then the nose will lower itself automatically. At a lower angle of attack the canard will unstall, and its restored lift will again increase the angle of attack until it stalls again."


now what happens if the canard stalls first?

"What would happen if the wing stalled but the canard didn't? In that case, the canard would raise the nose while the wing let the tail drop. There would be a sudden increase in the airplane's angle of attack. This is not good!"

but okay the text also says you can recover up to a level the aircraft

"It is not true, however, that you can never let the back wing stall on a canard. Even on a stalled airplane, there are forces at work (lift, drag and gravity) - and each of these forces has a lever arm back to the center of gravity. It's OK to have both the canard and the wing stalled, as long as the pitching moments trying to lower the nose are greater than those trying to raise the nose"

Although your quotes from this

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are informative, your use of it consistutes a fallacy known as

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. Why? Because you are using the quotes to sound technical, but the quotes themselves do not address the point of contention and are not applicable to J-20.

When the article you quoted talks about canards, it is referring to fixed canards. Specifically, the article is talking about the Beechcraft Starship, having fixed canards fitted with trailing flaps. This is why the article says "that the canard itself is already flapped (the elevator)" and why it mentioned the canards stalling. A picture of Beechcraft Starship is shown below:

So what Song`s paper tries to address is that possibilty

No, it doesn't. The canards on J-20 are totally different in that they can be fully articulated, not fixed. These canards have no trailing flaps. The canards are maintained at zero AoA while the aircraft is flying at high AoA, so the issue of canards stalling simply doesn't occur. Your deliberately misinterpetation of his paper doesn't alter what the paper actually trying to address, which is the use of aerodynamics to meet design criterias for a 4th generation fighter.

So what he does really say? he says in his opinion the tailplane is not as good compared to a foreplane (canard) to bring the nose down in a post stall situation.

False. At no point did he says it is his opinion. He made an affirmitive statement pointing out trailing edge pitch control devices (including flaps) ARE inferior to canards, because the former will stall when the aircraft is at high AoA and the latter will not. What is an opinion is your (mis)interpetation of his statements. Observe what Dr. Song said:

Since the main wing's ability to generate lift tends to saturate under high AOA conditions, the positive load control surfaces' pitch down control capabilities tend to saturate under high AOA as well. Therefore it will be wise to employ negative load control surfaces for pitch down control under high AOA conditions.

Well, Su-27 brings perfectly the nose down during the cobra after 120 degrees of AoA

False. Su-27 doesn't "bring its nose down" through active deployment of flaps or deflection of elevators. The Su-27 nose comes down because as the aircraft is flying at over 90 AoA, at which point drag at the tail is not compensated by an equivilent surface at the nose. At such high AoA, the control surfaces of Su-27 are useless. Thus, this little fact of your's doesn't contradict Dr. Song's statement in anyway.

X-31 uses TVC nozzles to bring the nose down along side canards

X-31 TVN can be switched off. The aircraft can be flown perfectly without TVN turned on at all times. At high AoA, the canards are orientated to have zero AoA with respects to the oncoming air.

This means the canards are orientated to provide as little lift as possible. To bring the nose down, the canards are further deflected downward to create a negative lift at the nose, generating a noise-down moment. This is the same pitch control method that Dr. Song talked about in his paper, one that we have already seen the J-20 using in videos

well let us see if Eurofighter or Gripen deflect down the canards as J-20 does?


from minute 1:30 you see the Eurofighter turning tighter than J-20 and Gripen the same, you can see almost no deflection.

And here are photos of Eurofighter undergoing high-G maneuver, showing downward deflection of the canards:

We know that the Eurofighter has relaxed stability, where the aerodynamic center is infront of the center of gravity. Thus, the Eurofighter is not nose-heavy and does not need to use canards to keep its nose up. From the above pictures, we see Eurofighter's canards also deflect downward during a turn. Since the Eurofighter has no requirment of lifting its nose up during a turn, the purpose of canards deflecting downward is clearly not to enhance lift at the nose, thus completely debunk your ridiculous pseudo-aerodynamic theories.

There is an additional reason why the J-20 requires significant downward deflection in the canards. The reason is unstability, and is explained by Dr. Song in his paper:

Further relaxing the longitudinal instability could not only enhance trans-sonic lift to drag characteristics but also improve super sonic lift to drag capabilities, increase take-off and landing characteristics, and maximize low-speed lift characteristics. This is akin to killing three birds with a single stone. Yet a increase in longitudinal instability will also increase the burden on high AOA pitch down control and subsequently increase flight control complexities.

In kiddie's terms, the more unstable aircraft is, the more pitch-down capability the aircraft requires.

Since canards provide that pitch-down capability, it is natural to see more downward deflection from them for a more unstable aircraft. In other words, J-20 requires more canard deflection simply means the aircraft is more unstable than fighter aircraft that are currently in service.