J-31 Normal Weight

[Inst]

I'm bringing this to WAB because if you bring this up elsewhere, fanboys will tell you about this new-fangled Chinese radar system that can explode the American President's brain from over 3,000 nm away like something out of Sayonara Gangsters, which is to say, you'll get a bullshit response.

According to an Aviationweek article, the J-31 is stated to have a normal take-off weight of 17,500 kg, with length of 16.9m and with a 11.5m wingspan, alongside a 1,250km combat range.

If you compare it to its original, however, which is the F-35, it has similar combat range, while being both larger and weighing less.

What's going on? Are the Chinese faking their normal take-off weight by using 50% fuel weight? With the J-31, stealth coatings are external like with the F-22, so are they selling it without the stealth coating? Or they counting on their new-fangled 3D printers to increase titanium use and reduce weight?

[Skywatcher]

I think the J-31/21 (designation changes according to who you ask) has a lot less internal volume than the F-35 since the former is a twin engine configuration.

Incidentally, where did Aviationweek get those figures for the J-31?

And yes, the Chinese have heavily used 3D printing to get around their deficiencies in machined titanium parts (though that would probably only save 10%, tops).

[Inst]

Shenyang J-21/31/F-60/AMF thread part 1 [Archive] - Page 2 - Key Publishing Ltd Aviation Forums

If you look at the transition from the YF-22 to the F-22, it gained around 5 tons and lost fuel capacity from prototype to final version. It's possible that the numbers are also referring to an F-60 without stealth coatings.

[Josh]

Where did they get that?

Shenyang J-21/31/F-60/AMF thread part 1 [Archive] - Page 2 - Key Publishing Ltd Aviation Forums

If you look at the transition from the YF-22 to the F-22, it gained around 5 tons and lost fuel capacity from prototype to final version. It's possible that the numbers are also referring to an F-60 without stealth coatings.

Aviation Leak tends to have pretty good info, but I've never seen any actual hard figure for any piece of Chinese hardware, particularly any of the newest toys. I would tend to assume this is their own estimate unless they specifically indicated otherwise; if someone had a hard figure for this aircraft I think it would be screamed from the hills and fan boys from every side would immediately present their arguments for how this meant it was inferior/superior to F-35/F-22/Typhoon/PAKFA/sliced bread.

I don't know nearly enough about airframes to have an opinion worth listening to, but in the case of the F-35 I believe the fuel fraction is over 50% which is rather unique for a fighter a/c; I *think* a more typical figure for jet fighters historically is more like 30%. So this particular design may be a bad comparison to other designs, particularly one with two engines. I wouldn't read too much into it either way.

[Inst]

According to Aviationweek, these were indicated figures from AVIC, which is the Chinese state body in charge of military -aviation. The roughness of the figures suggests estimates and guidelines for the program; of course, so they may represent unattainable performance targets.

To Skywatcher: the issue with Chinese 3D titanium printing is that it seems revolutionary; if you note the hullabaloo with the MiG-25, the Americans expected heavy titanium use in the MiG-25, which would have enabled ridiculous thrust to weight and wing loading numbers. The Soviets, to their misfortune, were forced to use a nickel alloy, however, because of the sheer cost of machining titanium. As a result, the MiG-25 could only perform as an interceptor due to its relatively heavy weight.

With Chinese 3D printing, you get something like 80% cost reductions in the cost of titanium parts. This means that you can make much more extensive use of titanium, as well as more finely shape your titanium.

According to Chinese reports, if 3D titanium were used on the F-22, it would result in a 40% reduction in weight, although whether in reference to the parts themselves or the total aircraft weight I'm not sure.

This is what I'm really trying to find out; whether or not the 3D titanium technology can be used to enable massive weight reductions in aircraft. A F-35, with 40% of its weight reduced by titanium, would fly at the intended 17,500 take-off weight of the J-31. The J-20, with 40% of its weight reduced by titanium, would have both robust wing-loadings and thrust to weight ratios with mature engines.

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Of course, just to allay any sense of being a fanboy, Boeing has similar additive manufacturing technology, but it seems as though the current generation of American aircraft are not designed with 3D titanium in mind; note the heavy use of nanocomposites on the F-35, for instance.

[GGTharos]

An F-15C is sitting 48000lbs at nominal TO weight with missiles and a single bag. It's 30000lbs empty (allegedly so is the 35A), and an F-35C is around 34000lbs empty, IIRC.

The notion that the J-31 has a 38500lbs TO weight with fuel and can technologically compete with an F-35 is just funny. IMHO that J-31 has no avionics, no weapons, and I don't know what else would be missing to make that sort of weight. 3D titanium printing is great, but it isn't a magical weight-loss wand.

Another thing is, what they do have flying right now is a prototype. It will probably grow in weight.

And yet another thing, perhaps it isn't a competitor to the F-35 at all, but more of a stealthy F-18/MiG-29 analogue.

[Blademaster]

An F-15C is sitting 48000lbs at nominal TO weight with missiles and a single bag. It's 30000lbs empty, and an F-35 is around 34000lbs empty, IIRC.

That is the C version, the carrier version. The A version is 29,300 lbs empty.

Its amazing that the F-35 weighs the same as the twin-engined F-15 but that's probably due to the internal weapons bay which accounts for greater weight. However the trade off is that F-35 is more aerodynamically efficient than F-15 when carrying weapons and F-15 pays a fuel expenditure cost for carrying weapons externally.

[GGTharos]

That is the C version, the carrier version. The A version is 29,300 lbs empty.

Yep.

Its amazing that the F-35 weighs the same as the twin-engined F-15 but that's probably due to the internal weapons bay which accounts for greater weight. However the trade off is that F-35 is more aerodynamically efficient than F-15 when carrying weapons and F-15 pays a fuel expenditure cost for carrying weapons externally.

Also, you could probably stick both F100's inside the F135 ...

[Inst]

GGTharos: If you go with 17.5 ton take-off weight, then you'd have to have an airframe at 10.5 tons empty, which is about that of a J-10, with a fuel fraction of 40%.

One big advantage of the J-31 is that it's not going to be sold with an EODAS system at least in its initial offering, so it can probably skip about a ton or so of weight.

[xinhui]

After this thread, I have to share this pic. (now with 50% extra)

[GGTharos]

TO weight (not max, but nominal) for the F-35A with full fuel and standard weapons load of two 1000lbs (or is it 2000lbs?) weapons is 23 tons, almost 24 with 2000lbs weapons, and about 21.5 if you're all about AMRAAMs in the internal bay.
That's 4 tons of difference, which is about 9000lbs of fuel. My point is that the J-31 airframe is way lighter than that of the F-35, probably quite a bit more so than you can achieve by machining certain parts differently, so there's something different about the overall concept and design. I don't think it's designed to compete with the F-35 exactly, I think it's more of a MiG-29/F-18 than an F-35.

@xinhui: Very funny :)

GGTharos: If you go with 17.5 ton take-off weight, then you'd have to have an airframe at 10.5 tons empty, which is about that of a J-10, with a fuel fraction of 40%.

One big advantage of the J-31 is that it's not going to be sold with an EODAS system at least in its initial offering, so it can probably skip about a ton or so of weight.

[Inst]

I'd say it would be more something between a Mig-29 and a F-35; with the weight reductions on the scale of 3 tons, I definitely doubt it'd be as stealthy as the F-35; something has to give.

Hypothesis A:

AVIC is full of shit, or just has overly ambitious planning on their J-31 project.

Hypothesis B:

This is for the F-60 export version, which will ditch heavy stealth coatings to save on weight.

Hypothesis C:

The F-35 project is screwed up and has gone over-weight; the J-31 project doesn't include EODAS on its initial variant, so it should be lighter to start with.

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I guess A is most likely; that something is wrong with the 17.5 tons figure, perhaps this is for AA only loadout, which in PLA parlance means full fuel and two IR-guided dogfight missiles, or they're just using 50% weight. But until more information is available, we just have to wait and see.

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In the same way, the J-20 has pretty suspect weight estimates. A fairly good estimate made from measurements estimates that it should weigh around 34 tons loaded, which is extremely heavy and ruins its wing-loading and thrust to weight.

[GGTharos]

I believe it's all about the aircraft's intended purpose.

I don't know that stealth coatings are really that heavy. F-16's use some RCS reduction coatings for example, and the F-117 did as well.

The J-20 fully loaded is all about delivering air to surface ordnance AFAIK, more of a deep strike fighter like an F-15E, and possibly closer to the F-35 than the J-31 ... or perhaps it's even more like a stealthy F-111/Su-24/34.

I don't see any official figures being talked about though.

PAK-FA seems similarly low-weight ... it's all pretty suspicious IMHO.

[Inst]

About the J-20, it's not air-to-surface or a dedicated interceptor; its weapons-bays are too short to load up decent AShMs so it's unlikely for it to function that way. In the same way, as a dedicated interceptor, the complex canard-lerx-delta arrangement adds RCS and drag that a dedicated interceptor would not need.

It is actually an air superiority aircraft, but one with heavy emphasis placed on interceptor qualities; range was a big priority on the J-20, so much effort was made to reduce drag at the cost of maneuverability and perhaps sustained turn rate.