J-20... The New Generation Fighter II


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Engineer

Major
What exactly are stable and unstable canards? Did you mean control canards and lifting canards?

Of the examples of "stable" canards you've lifted which one is capable of the same deflection featured on the J-20. If the canards on the J-20 are simply used to provide lift why not fix them and use control rudders, as in the case of the Viggen?
Longitudinal stability is determined by the relative position between center-of-gravity and aerodynamic center. When aerodynamic center is ahead of the c.g., the aircraft is unstable in the longitudinal axis, and vice-versa when the c.g. is ahead of the aerodynamic center. This is the weathercock effect.

Think of a pendulum. When the center-of-mass is higher than the hinge, it's unstable because the mass will always go away from that position. When the center-of-mass sits directly below the hinge, the system is stable because the mass stays at that position. On an aircraft, the center-of-mass is replaced by the aerodynamic center, while the hinge is replaced by the center-of-mass.

Relaxed stability is same as stable, except that the distance between center-of-mass and aerodynamic center is shorter than normal, lowering the stability of the aircraft.
 
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siegecrossbow

Brigadier
Staff member
Super Moderator
The Viggen canards are not fixed they have hinges or flaps in other words, the use of foreplanes is not a synonim of relaxed stability, there are not stable or unstable canards just relaxed stability aircraft
Dude you were the one who used the term stable/unstable canards in the first place. I was just trying to point that out since I was confused by the term.
 

Engineer

Major
Whether an aircraft with canards is stable or unstable depends on how far the canards shift the aerodynamic center forward. When the aerodynamic center is aft of the center-of-gravity, the aircraft is stable. When the two overlaps, the aircraft has neutral stability. When aerodynamic-center is ahead of center-of-gravity, the aircraft is unstable.

The issue here is that Mr. MiG-29 keeps on saying that the wings is aft of the landing gears, evidently he only sees the aerodynamic center of the wings and ignore the influence of canards, so that he can assert J-20 is positively stable. The problem being that one cannot claim whether the plane is stable or unstable unless you absolutely know the location of the aerodynamic center in relation to the center-of-gravity.
 

Engineer

Major
incorrect the center of gravity remain the same unless you use artificial stability like in the Su-27, also canard are swiveling on a fixed point that is the real fulcrum of the lever arm so you are just claiming irrealities
Whoops, my brain crossed wires there.

Increase in speed means the air exerts more force for a given control surface deflection, and greater pitching moment is provided. The canards do not necessary have to deflect more to compensate for the shift in aerodynamic center.
 

johnqh

Junior Member
Why are we discussing canards vs conventional layout at all?

Yes, it is interesting.....but both layouts are used for 3G fighters and it is safe to say both have their merits. I don't think we need to discuss the details....maybe 20 years ago it is an interesting topic.

Typhoon and Rafael are as good as any conventional layout 3G/3.5G fighters. Even against F-22, Rafael and Typhoon hold their own on WVR dogfights in exercises. F-22's maneuverability comes from its super duper engines and TVC, which nobody else can match. However, its aerodynamic is no better than 3G fighters like F-15 or F/A-18.

Chinese doesn't have the engine technology as US. However, according to the research paper, J-20's aerodynamic is the more different from any other fighters (3G or 4G). It is not just a canard/delta. It combines canard, lex, lifting body and delta. The paper (written in 80's? before J-20 was designed, based on extensive wind tunnel testing) showed this combination offers 80% more lift than delta only, way more than canard/delta. You can find this paper online (in Chinese language only). It also said the additional lift is at the front edge of the main wing.

The landing gear position proved the conclusion of this paper. Actually, before J-20 has its first flight, I thought the center of gravity was very strange and there is no way it could fly, but it did.

Don't tell me the additional lift is from canards only. Take a look at J-10, Typhoon and Rafael. All of them have the landing gear at about 40% of the main wing. Viggen is a little bit more to the front (25%?) due to its lifting canard, but J-20's canards are not lifting canards. They a re small, and take a look at the in-flight photos. The canards are between 0 degree to 3 degree.

So, F-22 gets its maneuverability from its engines. J-20 gets it from aerodynamic designs. Let's just leave it here.
 

MiG-29

Banned Idiot
No one claims that all aircraft with canards are unstable. I am questioning your claim that J-20 is stable. Also, Concorde does not have canards.


In level flight, canards can provide lift by simply deflecting upward.


Nice try at confusing the issues. Distance between canards and center-of-gravity has no relationship with the vortices' ability to re-energizing the airflow above the wing. That is affected by the distance between the canards and the wings.
Vortices shed by canards or LERXs do burst,


If you position the canards farther from the wing the ability of increasing lift reduces, plus buffeting increases, on the Eurofighter after the canard you can see a strake just behind the cockpit canopy to try to fix that


On the J-20 the wing has a LERX to do the same fuction.

The F-22 avoids that by using thrust vectoring
 
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MiG-29

Banned Idiot
Why are we discussing canards vs conventional layout at all?

Yes, it is interesting.....but both layouts are used for 3G fighters and it is safe to say both have their merits. I don't think we need to discuss the details....maybe 20 years ago it is an interesting topic.

Typhoon and Rafael are as good as any conventional layout 3G/3.5G fighters. Even against F-22, Rafael and Typhoon hold their own on WVR dogfights in exercises. F-22's maneuverability comes from its super duper engines and TVC, which nobody else can match. However, its aerodynamic is no better than 3G fighters like F-15 or F/A-18.

Chinese doesn't have the engine technology as US. However, according to the research paper, J-20's aerodynamic is the more different from any other fighters (3G or 4G). It is not just a canard/delta. It combines canard, lex, lifting body and delta. The paper (written in 80's? before J-20 was designed, based on extensive wind tunnel testing) showed this combination offers 80% more lift than delta only, way more than canard/delta. You can find this paper online (in Chinese language only). It also said the additional lift is at the front edge of the main wing.

The landing gear position proved the conclusion of this paper. Actually, before J-20 has its first flight, I thought the center of gravity was very strange and there is no way it could fly, but it did.

Don't tell me the additional lift is from canards only. Take a look at J-10, Typhoon and Rafael. All of them have the landing gear at about 40% of the main wing. Viggen is a little bit more to the front (25%?) due to its lifting canard, but J-20's canards are not lifting canards. They a re small, and take a look at the in-flight photos. The canards are between 0 degree to 3 degree.

So, F-22 gets its maneuverability from its engines. J-20 gets it from aerodynamic designs. Let's just leave it here.
Interesting but subsonic aircraft have the upper camber near the leading edge so their aerodynamic centers are near the leading edge, however to be true, then the J-20`s wing profile must be quit fat, not a very good wing for a supersonic jet, the J-20`s wing profile is supersonic then its wing camber can not be so close as you claim because supersonic wing cambers are rather thin without too much camber.
The F-22 `s wing has a tip twist to increase lift and leading edge flaps.
The Lifting bodies are also different concepts too they are basicly work like kites, so the J-20 also does not work like that, the J-20 is not unique in having a LERX and canards the Rafale does it too and a detail you missed the Rafale vectors a compressed vortex generated by its inlet and forebody plus it has a better positioned canard.


The J-20 has like the F-22 a semi blended fuselage-wing but its design is not unique.
 
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kyanges

Junior Member
Interesting but subsonic aircraft have the upper camber near the leading edge so their aerodynamic centers are near the leading edge, however to be true, then the J-20`s wing profile must be quit fat, not a very good wing for a supersonic jet, the J-20`s wing profile is supersonic then its wing camber can not be so close as you claim.
The F-22 `s wing has a tip twist to increase lift and leading edge flaps.
The Lifting bodies are also different concepts too they are basicly work like kites, so the J-20 also does not work like that, the J-20 is not unique in having a LERX and canards the Rafale does it too and a detail you missed the Rafale vectors a compressed vortex generated by its inlet and forebody plus it has a better positioned canard.


The J-20 has like the F-22 a semi blended fuselage-wing but its design is not unique.

I just want to be clear that I'm not trying to stand on any side before I ask this. Just asking because I'm interested in the discussion.

Okay, so that said, neither the Rafale or the F-22 have all of those traits together like the J-20 though right? So doesn't that mean saying that the Rafale has this, and the F-22 has that, doesn't really refute that the J-20 is unique from either?
 

delft

Brigadier
J-20 has smaller vertical dorsal tail than the F-22:true
the F-22 has no ventral tails while the J-20 does:true
the J-20 vertical dorsal tails and ventral tails total area is close to the F-22`s vertical tail area:true
then it can not have much less drag.

does the J-20 have a longer fuselage and larger cross section than the F-22?:yes it does
does the F-22 and J-20`s cross sections have similar size and shape? :yes they do

was the F-22 designed using super computers?: yes it was

Is it possible the F-22 has a design more optimized to the air dominance role?: yes it is possible

conclusion: it is more likely the J-20 is designed as an interceptor and strike aircraft that will use HMS and advanced missiles in a similar fashion to the F-35 and not to the F-22.
it will be less stealthy due to canards and size than the F-35 but will probably carry much more ordenance and longer range missiles than the F-35
F-22 was designed 20 years earlier than J-20. What happens with the strength of supercomputer hard and software in twenty years?
 

johnqh

Junior Member
I just want to be clear that I'm not trying to stand on any side before I ask this. Just asking because I'm interested in the discussion.

Okay, so that said, neither the Rafale or the F-22 have all of those traits together like the J-20 though right? So doesn't that mean saying that the Rafale has this, and the F-22 has that, doesn't really refute that the J-20 is unique from either?
Actually,

No other plane has lerx/delta combination.
No other canard/delta has them on the same level.

We don't need to look far to see the uniqueness.

And it is reflected in it's center of lift. No other plane has it as close to the front edge of the main wing as J20.

It's aerodynamic design is not something we (armchair designers) can talk out of thin air. Even real designers will have to build models to test in wind tunnels to give a judgement. That's why I see no point discussing this.
 
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