J-20... The New Generation Fighter III

Inst said:

Just remember, though. The J-20 is more optimized for speed and range. The key criteria of the J-20, compared to the F-22 and PAK-FA, is high speed super-cruise and long range.

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A heavy assumption, and almost certainly a wrong one imho.
From translated articles of reliable "insiders" great emphasis has been placed on manoverability as well. Not sure why people keep like to say somehow J-20 isn't planned for ACM. Is it because it looks longer than F-22 (not by much) or is it one of those unconscious prejudices manifesting itself?

This has to be achievable even if the WS-15 program hits a snag and the final engines are only WS-10Gs or even WS-10Bs. For these reasons, the J-20 has put less emphasis on maneuverability; it only needs to meet the F-22, not exceed it. If it has to fight the PAK-FA, it will attempt to win in BVR, and if it cannot, it'll just flee from the battlefield.

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... Honestly I somehow doubt the PLAAF planned a fighter that can't outfly similar generation planes of what the US and russians when J-XX was being planned. Especially when PLAAF aren't exactly in a vietnam BVR era of A2A. They've been playing with ACM for quite a while and recognize its importance in A2A. The idea they'll go back a few decades in doctrine and make their new frontline fighter an interceptor unable to fight close in is ridiculous.
In fact it's been said the canard/delta configuration was chosen over conventional, because it offered greater manoverability while keeping 5th gen speeds, with lower thrust engines if the indigenous WS-15 couldn't meet teh criteria.

siegecrossbow said:

To add to that (and go off topic and rant for a bit) I would just comment on how amazing it is for sinodefence to bring people with different political, ethnic, and religious background and essentially let everyone interact in a constructive manner.

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I couldn't agree more siege, and I can't have these kind of discussions anywhere else, there are a few folks in Central Illinios that fly, but since my Dad passed in Sept of 02, I haven't really had anyone to talk to, I still have a friend who gets me some Air Force magazines, thats really been my primer with well written and honest articles, lots of great editorials through the years, as well as the Air Force Times paper. My Dad was a great patriot, worked out of the US Embassy in Saigon, and was there during TET, and actually had about 65 hours in the Combat Talon 1, with her 400 lbs of radar absorbing paint, not your average C-130. So this has been a real treat, from the noobs to the old heads, everyone seems to have a passion for defense and the tools of the trade, and a respect for all that entails. Keep up the good work gentlemen.

The reason most people assume the J-20 is some kind of interceptor is because the J-20 has a low aspect ratio, and the wing area is, if I recall, 660 square feet (including body lift!) compared to the 780 of the F-22, while being a larger airframe. The canard-lerx-delta configuration is interesting, same as the Rafale, but to me, as far as I understand it, it's a compromise, allowing the J-20 to have low aspect ratios while retaining maneuverability.

If the J-20 had a higher aspect ratio, it would perform better in WVR combat maneuvers, but it would also require much more powerful engines for supercruise.

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With TVC, I would expect the airframe to at least match the Raptor, but against the PAK-FA, which makes less compromises for engine issues, it would probably be inferior in maneuverability.

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Strictly speaking, I am not saying that the J-20 is an interceptor or a strike plane. It's not. If it can keep pace with the F-22, it can't possibly be an interceptor. But I am saying that the plane is less optimized for maneuverability than it is for speed and range, and that if the J-20 team felt that they had a surfeit of engine power to work with, the aspect ratio would be higher and the plane would be more maneuverable.

One thing to notice, though, is that the plane will end up having Mach limitations either way; above Mach 2, the friction caused by air results in greatly increased operation costs. The YF-22 flew with a Mach 1.4 Mach 1.5 supercruise speed, while the YF-23 had a Mach 1.8 supercruise speed. The modern F-22 has a Mach 1.8 supercruise speed. If the F-23 were to be improved by the same margin, it would have a supercruise speed of Mach 2.1, which would dramatically increase the maintenance costs from the air friction.

Given the aerodynamics of the J-20, it would be quite possible for it to achieve a similar supercruise speed if all the kinks were worked out and the engines were fully up to par.

^ I agree with most of that, thanks for clearing it up. Although I wonder about the wing area (including body?) number, as J-20 has a good deal more body surface area simply by having a longer fuselage.

I wonder if the J-20's strakes/ventral fins have anything to do with the relatively low aspect ratio. On the F-16 strakes apparently allowed for the smaller, lower aspect ratio wing. But I don't have much more to add to aerodynamics discussions beyond just that unfortunately. The only expertise I have in this area is that statements about unknown fighters like J-20 and PAK FA derived from visual observation should be taken with massive doses of salt.

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Seeing as we're already here, I'll speak my view again. I feel J-20 is effectively meant to be an F-22 with longer range and endurance. It's a plane meant to be as good or slightly better than the F-22 kinematically, avionics (taking advantage of new systems like HMDs and EO DAS), and be able to survive and come out ahead in engagements against the F-22 for air superiority. That is almost certainly it's main mission which ended up determining much of its overall design features -- the PLAAF knew and know the F-22 is the greatest air threat, and while they have missiles able to target air bases which F-22s are based at in westpac, they also need to have a plane that can match it and beat it in the air. But it's also obviously a larger, longer plane and most of that volume would be used for fuel, for greater range/endurance. Obviously the PLAAF will not have a massive fleet of large tankers to support their fighters for the forseeable future so they'll need onboard fuel to keep them going -- not to mention the vulnerability of tankers from enemy aircraft which coudl make your super duper 5th gen planes fall out of the sky without tanker support. The F-22's more moderate internal fuel also played a factor into the PLAAF's thinking too I imagine. They clearly plan for the J-20 to fly long endurance missions.

Bltizo said:

A heavy assumption, and almost certainly a wrong one imho.

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I like your reasoning/speculation. But over on the Chinese forums, there're also people in agreement with Inst. They figure that the J-20 has a relatively high wing loading, etc. which indicates that its designers don't see future dogfight the way it used to be, hence no need for, say, Mig-29's idea of the Pak Fa type of manoverability (hence indeed less manoverable than Pak Fa).

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Bltizo said:

^ I agree with most of that, thanks for clearing it up. Although I wonder about the wing area (including body?) number, as J-20 has a good deal more body surface area simply by having a longer fuselage.

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Speaking of which, I recall that early on last year, some big shrimp commented that the J-20 is very high on body (and nose) lifting... I don't recall ever seeing a single photo in which its canards are not in an angle that pushes down its nose...

Player99 said:

I like your reasoning/speculation. But over on the Chinese forums, there're also people in agreement with Inst. They figure that the J-20 has a relatively high wing loading, etc. which indicates that its designers don't see future dogfight the way it used to be, hence no need for, say, Mig-29's idea of the Pak Fa type of manoverability (hence indeed less manoverable than Pak Fa).

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Speaking of which, I recall that early on last year, some big shrimp commented that the J-20 is very high on body (and nose) lifting... I don't recall ever seeing a single photo in which its canards are not in an angle that pushes down its nose...

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The problem I have with assuming the wing loading of the J-20 is that I don't think it actually has a smaller wing area than the F-22...That said I haven't seen anyone do a comprehensive measurement of the J-20 in comparison to any other plane. Most measurement arguments have revolved around its length.

Player99 said:

I like your reasoning/speculation. But over on the Chinese forums, there're also people in agreement with Inst. They figure that the J-20 has a relatively high wing loading, etc. which indicates that its designers don't see future dogfight the way it used to be, hence no need for, say, Mig-29's idea of the Pak Fa type of manoverability (hence indeed less manoverable than Pak Fa).

---------- Post added at 02:57 AM ---------- Previous post was at 02:45 AM ----------



Speaking of which, I recall that early on last year, some big shrimp commented that the J-20 is very high on body (and nose) lifting... I don't recall ever seeing a single photo in which its canards are not in an angle that pushes down its nose...

Click to expand...

From what I've come to understand,the J-20 chooses a different route for maneuvrebility. Besides of the consideration of wing loading, its canard delta configuration is designed for flying at a high max of AOA to maximize the required lift. All fighters probably do high alpha for maneuovrebility but the design of the J-20 specifically try to enhance the aspect of high alpha.

I agree with the raptor with long range/speed POV. Some sacrifices had to be made to accommodate for supercruise and Song's paper addressed it as well.

I know little about supersonic aerodynamics but I can present a few considerations.
In subsonic flight induced drag is very important and you want a large wing span.
In supersonic flight wave drag is way more important and so you want a long aircraft with modest wingspan. The fuselage fully contributes to the lift. Also in supersonic flight the center of lift is aft of that in subsonic flight. So for equilibrium the canards will be producing a little lift in supersonic flight but have to produce negative lift in subsonic flight. That's just what we see in J-20.

Addenda: we know where the center of gravity is. It is about 15% of the distance between the nose wheel and the main wheels in front of the main wheels. That means that about the whole of the wing is behind the center of gravity. That shows how important the lift contribution by the fuselage is.

The method used for the F-22 involves completing the triangles of the wings to include body lift.

Using 13.3 as the wingspan, and assuming this picture is to scale, we get a wing area of 74.4 m^2. Probably off by +-2m^2.

337m^2 on 25 tons, 405m^2 on 30 tons. It's okay, and could be better, but it could also be worse.

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If the wingspan is only 12.3, on the other hand, then wing area is 63.6 m^2. We do need to get a reliable estimate of the wingspan.