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Khand-e

Bash the J-20 thread.

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Unstable is, so far as I can tell, aerodynamically just better always.  It's also one of the few examples of anything in aerodynamics where performance can be improved across the board without compromises.  A designer would be stupid not to make a combat aircraft unstable.  Hell; even airliners are going in that direction, albeit slowly.  The 777 has relaxed stability (but not instability) on its yaw axis because it makes the tail lighter and improves its cruise economy slightly.

 

If it's stable it's a good reason to think it's just a demonstrator.  Boeing's Bird of Prey was stable to save on development time and cost.

 

But if the J-20 is just a tech demonstrator, why are they making so many of them?

 

I'm pretty dubious of the claim, but I can't actually find any video that disproves it.

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Paralay has made one hell of a claim:

 

0awvndx.jpg

 

See how the CoG is in front of the CoL?  Yep; that's right, Paralay thinks the J-20 is stable.

 

There is a way we can test this.  If there is footage of the J-20 taking off or maneuvering, clear enough that we can see what the canards are doing during the initiation of the maneuver and afterwards, then we'll know.

Placement of the weapons bay seems to argue that it is stable while loaded and (perhaps) unstable once ordinance is expended.

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None that I know of that give a very clear view of the take off and landing.

 

This is the closest you'll find on the western web, and the guy risked his ass jumping a fence to get the footage so it's not very good. (the original, not the channel who reuploaded it.)

 

https://www.youtube.com/watch?v=M0dYJJuV6RM

 

 

Yeah, you can't tell from that video.  The canards are basically blurred out by the jet wash at the moment of truth.

 

We'd be looking for something like this:

 

 

If you go to 4:04, you can see the canards deflect.  But as soon as the nose starts to come off the ground, the canards immediately return to the neutral position.

 

Now, compare that to a MiG-29, which is stable:

 

 

From 0:15 onward, you can see that the elevators stay deflected the entire time the plane is pitching up.

 

(and holy hell fulcrums have a high thrust to weight ratio)

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Well, one interesting thing is that, aside from his personal QQ number, he seemed to have left a group QQ number at the end of the video that's literally dedicated to the J-20.

 

Should I ask around?

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Placement of the weapons bay seems to argue that it is stable while loaded and (perhaps) unstable once ordinance is expended.

 

That would be, to my mind, a completely weird way to design a fighter.  It has all the development snags and trouble of designing a full fly by wire system, but it can only exploit the advantages of it once it runs itself out of bullets?

 

The SU-27 is stable when it's fully tanked up with gas, mostly because of the big tank right behind the cockpit.  But the mission planners could reasonably plan to have that fuel expended by the time the flanker reaches the combat zone.  Or they could just fill it up halfway if it's not needed.

 

With missiles that doesn't work as well.

 

Well, one interesting thing is that, aside from his personal QQ number, he seemed to have left a group QQ number at the end of the video that's literally dedicated to the J-20.

 

Should I ask around?

 

I'm genuinely curious.  If anyone knows anything, I'd love to hear it.

 

But please, let's not get anyone killed over this by jumping fences.

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That would be, to my mind, a completely weird way to design a fighter.  It has all the development snags and trouble of designing a full fly by wire system, but it can only exploit the advantages of it once it runs itself out of bullets?

 

The SU-27 is stable when it's fully tanked up with gas, mostly because of the big tank right behind the cockpit.  But the mission planners could reasonably plan to have that fuel expended by the time the flanker reaches the combat zone.  Or they could just fill it up halfway if it's not needed.

 

With missiles that doesn't work as well.

You can rough-estimate the effect that weapons have on CoG.

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You can rough-estimate the effect that weapons have on CoG.

 

True.

 

I'm seeing four PL-12s, (rough AIM-120 equivalents) which are each 180 kg, and two PL-10s (rough AIM-9X/ASRAAM equivalent) at 90 kg each.

 

(F-22 raptor is still the king of A2A weapons capacity at six AIM-120Ds and two AIM-9Xs.  Even Paralay's drawings of the PAK-FA only show it with 4x K-77M and 2x K-74M2)

 

So that's 900 kg of missiles hanging approximately 1.5 meters in front of the supposed CoG.  J-20 is supposed to weigh something like 19 metric tons, so the CoG would shift back something like 7cm as a result of firing all the missiles.

 

 

 

I'll have to consult my books on aircraft design, but I don't think Paralay is doing any of his calculations correctly.  Look at the shaded portions of his drawing; he's showing the reference wing area as extending into the fuselage (which is correct), but he's not showing the same thing for the canards.  Also, I think stable aircraft tend to have the CoL waayyyy behind the CoG, while unstable aircraft have the CoL only a little bit in front of the CoG.  Stable aircraft that have the CoL only slightly behind the CoG are only marginally stable, and thus twitchy enough that they would need computer assistance to reasonably fly anyway.

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True.

 

I'm seeing four PL-12s, (rough AIM-120 equivalents) which are each 180 kg, and two PL-10s (rough AIM-9X/ASRAAM equivalent) at 90 kg each.

 

(F-22 raptor is still the king of A2A weapons capacity at six AIM-120Ds and two AIM-9Xs.  Even Paralay's drawings of the PAK-FA only show it with 4x K-77M and 2x K-74M2)

 

So that's 900 kg of missiles hanging approximately 1.5 meters in front of the supposed CoG.  J-20 is supposed to weigh something like 19 metric tons, so the CoG would shift back something like 7cm as a result of firing all the missiles.

 

 

 

I'll have to consult my books on aircraft design, but I don't think Paralay is doing any of his calculations correctly.  Look at the shaded portions of his drawing; he's showing the reference wing area as extending into the fuselage (which is correct), but he's not showing the same thing for the canards.  Also, I think stable aircraft tend to have the CoL waayyyy behind the CoG, while unstable aircraft have the CoL only a little bit in front of the CoG.  Stable aircraft that have the CoL only slightly behind the CoG are only marginally stable, and thus twitchy enough that they would need computer assistance to reasonably fly anyway.

 

To be fair, unless they changed something from the prototype models to the pre production models, that may not be the strict layout for the J-20s internal weapons bay. (It's possible they did, but I've never seen a photo of the weapons bays of 2101-2103) as, while there are photos if it using 4 non-folders (explain this in a second), there's also different configurations seen.

 

 

YQr3z.jpg(early official mockup)

 

hqdefault.jpg(4 is the side PL-10 bay)

 

nfTLScJ.jpg

 

(2002 on a test flight using both bays instead of one like the most famous photo of it.)

 

 

 

 

 

 

 

 

127iKdJ.jpg  Note the missile on the 5 line is still a PL-12, the only differences from the latest variant are that it's fins fold inward, this was done specifically to better fit the internal weapons bays of the J-20 and J-31. (The plane in the top right is obviously an F-22A Raptor that's simply there for size reference, interestingly some of the first few photos of the J-20's mounting racks inside the bay were actually taken from the F-22A, but they changed it to this later.)

 

So, while Paralay isn't wrong or lying about weapon bays, because there actually are some photos of the J-20 using it's main bay for missiles in that configuration, I'm not certain that's the strictly correct loadout since it's been seen with 6 on multiple occasions or how that would affect it's inflight stability (thats more your area of expertise). I could post more if you wish but I think this got the point.

 

I'm also not sure if they intend to use 100% PL-12 "folding fin" variants as combat loadings since these have proven to save alot of space actually for production aircraft or some finned some not like we've seen in photos, mainly because it's never been officially commented on that I know of.

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Also, I'm very retarded and mistakenly referred to the PL-12 as the AIM-120, but to be fair I just got up on no sleep after a long night and seeing Colli reference the AIM-120 just made it stick in my head or some shit.

 

I fixed it now.

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Everybody knows the PL-12 is just a stolen copy of the AIM-120 anyway :^)

 

(if you actually believe this you're an imbecile)

 

I was actually going to discuss this, though, being I know more about ground and naval vehicle related topics, I'm not sure how much validity this theory has. (for any of the aero experts here, feel free to tell me very bluntly if this is a dumb theory, I don't mind, Jet aircraft are probably my least experienced area military hardware wise.)

 

I think one reason the J-20 is so large isn't because it's intended as a bomber, (or at least not dedicated one, maybe a multi role, we'll have to wait and see) compared to the F-22A (It's about the same size as an F-14D, which is fucking massive for a fighter) is actually because of the Typhoon effect, only in this case with the PL-12 vs the AIM-120.

 

Basically, the reason the Typhoon class of submarines were made so large is the preconception that it was necessary for that multi pressure hull design they have, this is partially true, but the biggest reason is, at the time the Typhoons were built, the Soviets demanded a Submarine that could match the Ohio Class and it's rather ridiculous 24 SLBM payload, the problem was, Soviet SLBMs that could match the Trident missile in performance were also fucking massive (the RSM-52 is over double the weight of the Trident I and significantly larger size wise) meaning the Submarine design had to be made larger to fit the larger missiles while reaching the desired 24 missile payload, and even then the designers ultimately had to convince high command to settle for a 20 missile setup because... well, if you've ever seen a size comparison of a Typhoon, you'll see that even the 20 missile versions that were actually built are obviously very large and intimidating looking.

 

the PL-12 is kind of the same way, it's in the same class as the AIM-120, and has about the same performance in most aspects (range at given altitudes, top speed, guidance, fuzing, fire and forget capabilities, etc) however, one thing is the PL-12 is also physically larger then it's counterpart, at the following.

 

AIM-120D

 

Length: 12 ft (Yes I'm using landed on moon measurements.)

 

Diameter: 7 inches

 

Wingspan: 20.7 inches

 

Weight/Warhead Weight: 335lb/40lbs (reduced from 50lb from earlier variants)

 

PL-12B (non folding fin variant, folding fin is C variant)

 

Length: 12.36-12.50 ft.(?)

 

Diameter: 8 inches.

 

Wingspan: 26.4 inches.

 

Weight/Warhead Weight: 400lb/Unknown.

 

So, that being said, while it's not quite the difference between the Trident I and the RSM-52, Aircraft tend to be much smaller and more fickle to slight changes then Boomers, so it seems plausible that even a rather "slight" size difference could have bigger consequences if you're aiming for the same payload as something you see as your biggest rival much like the Typhoon vs Ohio.

 

Be honest, is there any credence to my pondering here? or is this retarded bullshit, I really need to be criticized to become better.

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True.

 

I'm seeing four PL-12s, (rough AIM-120 equivalents) which are each 180 kg, and two PL-10s (rough AIM-9X/ASRAAM equivalent) at 90 kg each.

 

(F-22 raptor is still the king of A2A weapons capacity at six AIM-120Ds and two AIM-9Xs.  Even Paralay's drawings of the PAK-FA only show it with 4x K-77M and 2x K-74M2)

 

So that's 900 kg of missiles hanging approximately 1.5 meters in front of the supposed CoG.  J-20 is supposed to weigh something like 19 metric tons, so the CoG would shift back something like 7cm as a result of firing all the missiles.

 

 

 

I'll have to consult my books on aircraft design, but I don't think Paralay is doing any of his calculations correctly.  Look at the shaded portions of his drawing; he's showing the reference wing area as extending into the fuselage (which is correct), but he's not showing the same thing for the canards.  Also, I think stable aircraft tend to have the CoL waayyyy behind the CoG, while unstable aircraft have the CoL only a little bit in front of the CoG.  Stable aircraft that have the CoL only slightly behind the CoG are only marginally stable, and thus twitchy enough that they would need computer assistance to reasonably fly anyway.

The general rule (which I'm not sure applies to canards) is that the CoG should be 25% behind the wing leading edge.

 

Here is a model calculator that could be used:

 

http://adamone.rchomepage.com/cg_canard.htm

 

Edit: I'm also kind of surprised with how light the weapons load is. Wasn't J-20 supposed to be a stealthy strike fighter?

 

Edit 2: model forums seem to believe that a quick-n-dirty rule for canards is to have the CoG slightly forward of the CoL. Which would place it in the right spot for stable flight per your picture. My guess here is that he is right about CoP (which you can infer from pictures) and wrong about CoG (which needs some educated guesses regarding structure/component mass and placement). Given that it would be easy to shift CoG rearwards even from his estimates, I'd have no problem believing that the J-20 can be trimmed to be marginally stable or unstable.

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The general rule (which I'm not sure applies to canards) is that the CoG should be 25% behind the wing leading edge.

 

Here is a model calculator that could be used:

 

http://adamone.rchomepage.com/cg_canard.htm

 

Edit: I'm also kind of surprised with how light the weapons load is. Wasn't J-20 supposed to be a stealthy strike fighter?

 

Edit 2: model forums seem to believe that a quick-n-dirty rule for canards is to have the CoG slightly forward of the CoL. Which would place it in the right spot for stable flight per your picture. My guess here is that he is right about CoP (which you can infer from pictures) and wrong about CoG (which needs some educated guesses regarding structure/component mass and placement). Given that it would be easy to shift CoG rearwards even from his estimates, I'd have no problem believing that the J-20 can be trimmed to be marginally stable or unstable.

 

 

Honestly, outside of Chengdu and some higher ups in the PLAAF and the Government, no one really knows what the J-20 is actually supposed to be in the end or knows that much about it.

 

Also, as posted before, it likely holds up to 6 PL-12s as opposed to 4 considering mockups and even in flight photos of it show it carrying that many.

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Apparently, calculating the center of lift on canard designs is tricky because the canards actually change the CL of the main wings as well change as the spanwise lift distribution of the main wings.

 

On the J-20 though, the canards and wings are pretty far apart, so the effect should be smaller.

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A few more sources are confirming that at least some J-20s that will be at the airshow have long awaited WS-15B engine, the performance figures look good but, I'd need our resident clay bird to confirm this.

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Could be almost anything.  Datalink seems as reasonable a guess as any.

 

My first thought was a DAS clone, but I think the apertures are too small and cloudy for that.

 

But on a VLO aircraft, all the airspeed, AOA sensors and whatnot need to be flush with the fuselage so they don't catch radar.  That might be some of what these are.  Others might be for the RWR, and some might even be empty; just provisional slots that will be filled with various toys later on.

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Multifunction Advanced Data Link, it's like high tech walkie talkies for stealth birds.

 

Well, I knew what MADL is, but I was more questioning if it was feasible since I was under the impression that it's a rather hard tech feat to pull off as even the F-22A was to have it but had it cancelled back in 2010 (though, Colli told me alot of programs got axed at the same time for the F-22A so maybe I was just misinformed,)

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