J-XX stealth fighter

[shingen]

Read the Chinese docs that get released. They talk about hitting high value targets. The J-20 could be used against AWACS, carriers, and airbases.

[cola]

How does that image show anything of the stability?

Really simple, if you know what you're looking at...
So, the plane is obviously in a fast turn, judging by the wing cambering, which is configured for a best turning performance with flat trailing edge and dropped leading edge.
Vertices from intakes, canards and slats' tooth give a suggestion of alpha, the plane is in atm, meaning positive g turn.
Canards trimming nose down, means that the center of pressure is in front of the center of gravity, which again, means the plane is unstable.

No, the Russian analyst is absolutely right.

No he's not and I doubt he's either a pilot or an engineer, but more likely a peddler.

This photo shows beyond any shadow of a doubt that it is stable: http://q-zon-fighterplanes.com/wp-conte ... u-J-20.jpg

No, it doesn't, since it was taken at landing (most likely) and you can't tell the alpha the plane is at, atm.

Here's a picture of Rafale landing in the same manner and the plane is unstable beyond doubt > http://www.european-security.com/imgbiblio/04-CDG-531-CA-23-MP-PLOUVIE-s.jpg
Even the Eurofighter which is probably the most unstable fighter today still applies some positive lift from canards at landing, as per this photo > http://www.flickr.com/photos/nigelblake/2535016943/

I'll skip the part where you describe stability, because it's correct.
Conclusions you draw are wrong, though.

...However, since the canards are smaller and will stall before the wings, that limits its potential rate of pitch.

Well, canards set to stall before wings is a setup from when canards were fixed, mainly to avoid uncommanded dynamic instability (oscillations)...today, with slab canards you don't do that anymore and FBW can stall them whenever you want.

So, the J20 is a high performance airframe design, which is usually tied to air superiority, because it does put the most strain on the machine.

Hope this helps clearing this issue.

[count_to_10]

river_otter: that evaluation of stability sounds reasonable enough, but there is the issue of how much lift the body itself generates. I'm not qualified to rate that kind of thing, but it looks to me that the J-20 has fairly small wings compared to the area of it's body, and that could move it's center of lift up.

[river_otter]

How does that image show anything of the stability?

Really simple, if you know what you're looking at...

Which, if you saw the recent video of the J-20 flying, and realize this is still an experimental aircraft, you would know it's still only going relatively slow and making relatively lazy turns.

To me it looked like they were just trying to slightly one-up the recent PAK-FA performance and didn't want to stress the prototype any more than needed for that. But that's just impression.

So, the plane is obviously in a fast turn, judging by the wing cambering, which is configured for a best turning performance with flat trailing edge and dropped leading edge.

Best doesn't mean fast. It means fast relative to what it's capable of. A lot of WWII prop planes had slats. Airliners have slats. They can't all turn like an F-16.

Vertices from intakes, canards and slats' tooth give a suggestion of alpha, the plane is in atm, meaning positive g turn.

Correct. I have no doubt it's pulling some positive g, and has a positive alpha. But how many g? And what alpha? If it's giving its all at 4.5g and 25 degrees alpha, that's not very impressive for a fighter. Though it's not bad for a bomber.

Canards trimming nose down, means that the center of pressure is in front of the center of gravity, which again, means the plane is unstable.

No, it doesn't mean anything like that because you don't know what alpha it's pulling other than it's a positive alpha. (Actually, if it was flying straight and level, this would prove it's unstable, but you've already said, correctly, that it's in positive alpha.) But it could be a relatively high positive alpha for a relatively low g turn. Canards have an alpha independent of the alpha of the main wings/fuselage. If the wings are at an alpha of a(w) and the canards are at an alpha of a(w)+a(c), there's a possibility that the canards would stall and cease to generate lift. The canards lack the wings' slats. They are smaller than the wings. If you also increase their alpha to more than the alpha of the wings, they will stall first. To counter that, the canards' additional a(c) can be negative. Relative to the direction of flight, they still have a positive alpha and generate lift.

Or maybe they snapped a shot of the canards being rapidly brought down because the plane was starting to depart from controlled flight.

Far from showing the plane is unstable, this could be taken either way. It is the structure of the plane that proves it is stable, not a snapshot of one moment of the flight controls' position.

This photo shows beyond any shadow of a doubt that it is stable: http://q-zon-fighterplanes.com/wp-conte ... u-J-20.jpg

No, it doesn't, since it was taken at landing (most likely) and you can't tell the alpha the plane is at, atm.

Nothing I said has anything to do with landing or the alpha of the canards. It has to do only with where the parts of the plane are attached to the fuselage. Wheels are located ahead of wings, so center of gravity must be located ahead of center of lift. End of story. The attachment points of of the gear and wings to the fuselage don't change when it lands, unless it's actually made by Hasbro rather than Chengdu, and takes its orders from Megatron rather than the PLAAF.

[cola]

@river otter,
here's the sequence of photos taken in the same turn (http://defensetech.org/2012/03/10/awesome-new-pics-of-chinas-stealth-fighter/).
As you can see, canards trim the nose down through all 8 pictures in that quarter of the circle.
The rest of your "analysis" is, well...a joke and I can't see why you keep pushing it.

Finally, the position of wheels has nothing to do with position of Cp in relation to position of Cg.
That wheels are this forward only means that the plane gets a large portion of lift from body, which isn't strange considering its aerodynamic layout.
Why is it so difficult to understand?

Far from showing the plane is unstable, this could be taken either way. It is the structure of the plane that proves it is stable, not a snapshot of one moment of the flight controls' position.

I don't think you know what does the "unstable" actually mean...
Stable aircraft have their Cp behind their Cg, all the time and so they don't require FBW.
However, I can tell you some older Mig21s were uncontrollable once they spent too much fuel and the Cp came dangerously close to Cg, particularly at landing where it usually went ahead and pilot had to land on ejection seat, instead of wings.

[river_otter]

@river otter,
here's the sequence of photos taken in the same turn (http://defensetech.org/2012/03/10/awesome-new-pics-of-chinas-stealth-fighter/).
As you can see, canards trim the nose down through all 8 pictures in that quarter of the circle.

Ha! Those photos prove I'm right. The plane is definitely pulling nose up, yes? And yet the canards are at negative alpha relative to the plane the whole time. While the nose goes up. Meaning the plane would have to lower the canards even more to merely maintain level flight instead of pitching the nose up. What kind of imbecile would design a plane where the canards have to be deflected farther down than that just to maintain level flight? The drag would be ridiculous.

More definitively, here's a J-20 in level flight: http://www.jeffhead.com/redseadragon/j20-03a.jpg Canards are flat. And another. http://4.bp.blogspot.com/__A2VG2LiIPk/T ... ht-111.jpg Canards are up, not down. Here's a video: http://www.youtube.com/watch?v=2FfeLIVSV9Q Around 3:20, the plane is in a shallow turn; no downward deflection of the canards visible. Around 3:40, the plane makes a level pass much closer. Canards are flat. Another video: http://www.youtube.com/watch?v=pJSVmpV1Kb0 Level pass at 4:35. Canards are flat. The J-20 does not need to deflect the canards down to push the nose down to maintain level flight. It therefore cannot need to deflect the canards downward further to push the nose down harder in order to let the nose come up. The canards must be deflected downwards in those loop photos of yours because they would stall if their alpha was any higher. Watch the second video again; your photos are from 3:56 onwards. The canards are pointed slightly upwards from the direction of aircraft motion. They're not pushing the nose down. They're generating lift to keep the nose pitching upwards into the loop. Check and mate. It is stable.

Finally, the position of wheels has nothing to do with position of Cp in relation to position of Cg.
That wheels are this forward only means that the plane gets a large portion of lift from body, which isn't strange considering its aerodynamic layout.

There are no vortices above the cockpit chines in the loop photos. It's not getting nose lift. The canards eclipse the sides of the engine nacelles. It's CAN'T get body lift there. There are large vortices on the LERXes, and onto the wings. It's getting lift mostly from the wings. There are vortices on top of the canards, which means the canards are generating lift not pushing the nose down. Check and mate again, stable design.

I don't think you know what does the "unstable" actually mean...
Stable aircraft have their Cp behind their Cg, all the time and so they don't require FBW.

What you think doesn't really amount to much concern to me. Static stability is determined by where the plane generates lift, and where the plane's mass is. It is the structure of the plane that determines both. The momentary deflections of control surfaces do not. Whether or not the plane has FBW controls does not control whether it is stable or unstable (though unstable aircraft must have FBW). Many stable aircraft also have FBW. Airbus passenger liners have FBW controls, and I think the 787 is supposed to as well. Neither is unstable. (And if you actually understood relaxed static stability, you wouldn't have said "all the time." The center of lift moves backwards as speed increases. Unstable designs become stable at higher speeds.)

[haavarla]

Guys, almost all, if not all carnards fighter out there use the carnards upon take-off to create a positive forward lift. This means the carnards will actuall point a little upwards, but only for a few sec. The whole point is just to help lift the nose up or in a better term 'help rotate' the aircraft.

The T-50 design have a high-lift body.
Still, In one of the vids, one can clearly see this on the T-50, upon take-off the LECONS are slightly pointing upwards.

All modern fighter use advanced FBW or FCS(Flight Control System) to do this. And that include the J-xx. It doesn't matter how unstable/stable the airframe is.

There is nothing usually about this.
But as in manuvereing in flight, the Carnards are mostly used to aid air vortices over the main wings.. which means when doing a high alpha, the carnards are very much pointing down.

Adding Carnards does help on other Aerodynamic area, but this is more as a subsitude effect.
Strike aircaft design to fly fast and low or Aircraft with a slightly heavy nose section(See Su-34). That would be the part of CG/CP though.

[river_otter]

river_otter: that evaluation of stability sounds reasonable enough, but there is the issue of how much lift the body itself generates. I'm not qualified to rate that kind of thing, but it looks to me that the J-20 has fairly small wings compared to the area of it's body, and that could move it's center of lift up.

A reasonable speculation. See my post just above this why I think it's not relying on body lift.

In addition, if it had high nose lift already, why add the canards and wreck its stealth profile in the first place? You already have lift up front; you don't need another set of little wings to make more. And why have the canards push down against the lift you're trying to generate? You'd get more efficient flight just using your tail control surfaces to raise the lift at the back slightly, like the F-16, F-22, and F-35 do. Not needing to push down against existing lift is one of the key advantages of an unstable design.

[cola]

What kind of imbecile would design a plane where the canards have to be deflected farther down than that just to maintain level flight? The drag would be ridiculous.

The kind of an imbecile that designs an unstable plane?
(besides, you're hardly in position to call anyone an imbecile, from what I saw from you, so far)
Look, you're obviously lost on the subject, so I think it's best to just drop it.
I don't mind you thinking whatever you like, but don't quote and particularly argue on things you're not sure about.

Whether or not the plane has FBW controls does not control whether it is stable or unstable (though unstable aircraft must have FBW). Many stable aircraft also have FBW.

Well it does if one plane must have FBW to keep it in the air and the other doesn't.
This is why I said I thought you don't get the difference and now I'm sure.

Here's an F16 in level subsonic flight, with flat tailplane (perhaps even trimming the nose up a bit) http://www.marketwatch.pk/blog/wp-content/uploads/2010/04/f16-19-pakistan.jpg
It doesn't mean it's a stable aircraft.

[shingen]

Check out the Typhoon and Rafale, they are unstable canards. There is trim drag while subsonic from the canards but they decided it was worth it to get the benefits of an unstable design.

[river_otter]

Check out the Typhoon and Rafale, they are unstable canards. There is trim drag while subsonic from the canards but they decided it was worth it to get the benefits of an unstable design.

You're correct, but it's a matter of degree. Typhoon and Rafale are unstable, but they're unstable with neutral canards. http://www.youtube.com/watch?v=_veA7eJiY6s shows Rafales flying level with a few degrees downward deflection of the canards. In level flight the canards do trim down a few degrees to reduce nose lift (reduce nose lift, not push the nose down), and also to trim the canards properly into the airflow. Nothing ridiculous about a few degrees of control surface trim. They also use the aft control surfaces to lift the tail along with the nose. But they definitely don't deflect the canards down 20+ degrees in order to lift the nose into a loop, which would mean having to deflect down even more than that to maintain level flight. The deflections seen on the J-20 are too extreme by far to be for nose trim, especially when they actually want the nose to pitch up into a loop. The drag for level flight would be ridiculous if they deflected more than that to actually prevent the nose from pitching up. And the videos of the J-20 also show the canards are not trimmed down in level flight, they're trimmed neutral to up. Opposite the Rafale. They're clearly deflected in the loop for maintaining high but unstalled canard alpha relative to the airstream, not to push the nose down while they want it to pitch up. That's the major difference. And look at how close the wing root of the Typhoon and Rafale are to the nose; they get a significant amount of wing lift ahead of the center of mass. In the J-20, only the canards are ahead of the center of mass. That's another difference.

The Russian analyst is right, this has to be a stable design from its construction and flight characteristics, and it is reliant on the canards for nose lift in level flight or maneuvering.

[mor10]

Could it be that the J-20 will have a different center of gravity when the new engines are installed, forcing them to use aerodynamics to control it at this point with a temporary engine?

[exorcet]

It's hard to tell when just looking. In all the photos posted, we have to make assumptions and from that comes error. The photos with the F-16's are a good example. They look pretty flat, but the picture might be tilted such that we can't see the actual alpha, and the tails looked like they are trimmed for downforce.

It would be nice if we had a video of the J-20 in flight with an overlay of the HUD, but I wouldn't wait on that.

As for the issue being different cg brought about by the wrong engines, they could probably fix that by adding weight or shifting the current engines around.

[shingen]

Check out the Typhoon and Rafale, they are unstable canards. There is trim drag while subsonic from the canards but they decided it was worth it to get the benefits of an unstable design.

You're correct, but it's a matter of degree. Typhoon and Rafale are unstable, but they're unstable with neutral canards. http://www.youtube.com/watch?v=_veA7eJiY6s shows Rafales flying level with a few degrees downward deflection of the canards. In level flight the canards do trim down a few degrees to reduce nose lift (reduce nose lift, not push the nose down), and also to trim the canards properly into the airflow. Nothing ridiculous about a few degrees of control surface trim. They also use the aft control surfaces to lift the tail along with the nose. But they definitely don't deflect the canards down 20+ degrees in order to lift the nose into a loop, which would mean having to deflect down even more than that to maintain level flight. The deflections seen on the J-20 are too extreme by far to be for nose trim, especially when they actually want the nose to pitch up into a loop. The drag for level flight would be ridiculous if they deflected more than that to actually prevent the nose from pitching up. And the videos of the J-20 also show the canards are not trimmed down in level flight, they're trimmed neutral to up. Opposite the Rafale. They're clearly deflected in the loop for maintaining high but unstalled canard alpha relative to the airstream, not to push the nose down while they want it to pitch up. That's the major difference. And look at how close the wing root of the Typhoon and Rafale are to the nose; they get a significant amount of wing lift ahead of the center of mass. In the J-20, only the canards are ahead of the center of mass. That's another difference.

The Russian analyst is right, this has to be a stable design from its construction and flight characteristics, and it is reliant on the canards for nose lift in level flight or maneuvering.

What about the combination of LEX and canards on the double delta wing? That seems strange. IIRC the double delta has a problem with pitch up. Is that why the J-20 has both?

[river_otter]

What about the combination of LEX and canards on the double delta wing? That seems strange. IIRC the double delta has a problem with pitch up. Is that why the J-20 has both?

I agree that's a distinct possibility. Certainly both probably do (LEX almost certainly) add more lifting power to the wings and aft fuselage. That would oppose sudden pitch-ups. That lift enhancement might also explain why the wings can be so small. Highly-loaded wings can make for a fast, turbulence-resistant, and efficient strike platform. As I said much earlier in the thread, I think it's an excellent design.