Blackstone
Brigadier
What about copying the 117 engine? China can't produce its own, so its options are limited. Su-35 buy allows it to reverse engineer the Russian engine.
J-20 5th Gen Fighter Thread V
- Thread starter Deino
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Blackstone
Brigadier
Is there any creditable evidence the J-20 was designed with an airframe that supports supercruise?
they will get the WS-15 right eventually, it takes everybody time and a very specific expertise, China manufactures Many, Many, Many, commercial jet engines.
The F-119, F-135 are on a different planet structurally and performance wise, they run V ERY HOT! and eat run of the mill parts, they are very precise. So give the Chinese time, the WS-10 is coming right along, and I really don't think they want the hassle and waste of OVT??? we decided to omit that on the F-35, and I think they prolly realize we did that for a very good reason.....
As to supercruise, that's also a big deal, but when the J-20 gets the WS-15, it will "supercruise", so its basically a game of tweaking things until you get them just right!
Then we will know how they did on reducing specific drag at supersonic speeds??
Ohh come on ! And they lay these newly acquired engines right down onto an 3D-printer and voila, they have enough engines ???
- The Chinese have engines that are working ... at least for the Flankers, so please STOP this sort of argument.
- How long will it take to "copy" these engines ... and in parallel the latest news are quite positive in regard to the WS-15.
- By the way - and not only You, but also B787 - these is NO way China will get the 117 ! This engines is solely restricted for the T50. The engine in question is the 117S and this is a different beast. Simply get it.
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In return: Just look at the fuselage .. is there anything that supports Your theory the J-20 is NOT designed for supercruise ??
To admit this endless forth and back on the same points is already boring !
So I am out.
Deino
Blackstone
Brigadier
Agreed China's merged large SOEs into a giant SOE will eventually get the WS-15 functional, the question isn't if but how many decades and how high the costs.
I feel like this thread had gone back in time three or four years.
I can't believe people are still talking about the engine that J-20 uses, its length, or whether it's meant to have supercruise.
Jeez.
All of this should already have been settled; I'm not sure why these subject matters are suddenly a topic of discussion again just because a few lousy articles about J-20 popped up, written by people who probably don't know the difference between 052C and 052D or KJ-200 from KJ-500.
I can't believe people are still talking about the engine that J-20 uses, its length, or whether it's meant to have supercruise.
Jeez.
All of this should already have been settled; I'm not sure why these subject matters are suddenly a topic of discussion again just because a few lousy articles about J-20 popped up, written by people who probably don't know the difference between 052C and 052D or KJ-200 from KJ-500.
shen
Senior Member
That's the problem. The threads are so long, and for people like me who don't follow the discussion on a regular basis, it is difficult to know what has been discussed. I wish there is an authoritative site regularly updated that would summarize all the know facts and analysis. Or maybe a sticky thread for a particular topic where especially good informative posts are archived for easy reading.
Let's do this exercise in earnest, just to lay out the exact process by which we are arriving at our conclusions.
The F119's max dry thrust is about 116 kN (from all the sources I could find). It's unlikely to be higher than that.
The F-22's empty weight per wikipedia is about 43,000 lbs, and its internal fuel capacity is 18,000 lbs. That means at max fuel load the F-22's takeoff weight is about 61,000 lbs. 116 kN is about 26,000 lbs of thrust. With two engines that's about 52,000 lbs of dry thrust. *IF* we assume the F-22 can reach its max supercruise speed at max fuel load, then that means it would need a T:W ratio of 0.85 to do so (I do not think this is actually the case of course).
Assuming that the J-20 has roughly the same cross sectional area distribution and volume as the F-22, at around 20.5 meters the J-20 would be about 8.5% heavier than the F-22 (these are big ifs, as I will highlight later on). For the sake of simplicity, let's be generous about the J-20's take off weight at max fuel load and say it carries the same amount of internal fuel as the F-22. Under those assumptions, that would mean the J-20's extra weight would all be empty weight. For the sake of this exercise, that would mean the J-20's empty weight should be 46,655 lbs. Again, for the sake of simplicity, let's round that up to 47,000 lbs. With 18,000 lbs of fuel, the J-20's take off weight at max fuel load would be 65,000 lbs. Let's *assume* that Deino's reporting on this is right and the J-20 uses AL-31F M2s, and that they have similar dry thrust figures to the 117S at 86 kN, or roughly 19,500 lbs of dry thrust. With two engines that would put the J-20's T:W ratio at max fuel load at 0.60.
Now, let's assume that the J-20 and F-22 have similar drag characteristics, so that they are roughly comparable but for T:W ratio (not a good assumption to make, as I will also elaborate on later). For the J-20 achieve a T:W ratio of .85 on 39,000 lbs of dry thrust, it would have be about 45,000 lbs. In other words, to get even close to that, it would have to be running empty. Under the assumptions of this exercise, that would mean the J-20 could in fact supercruise at 1.8 M (or at least supercruise at the same speed as the F-22 with a full tank), but not practically so. Assuming that drag scales linearly with Mach number (another bad assumption, we know it does not!), that would mean, assuming the F-22 can supercruise at 1.8 M at a T:W of 0.85, the J-20 should be able to do about 1.3 M at a T:W of 0.60, given that 0.60 is 70% of 0.85.
Now, let's look at some of the assumptions we just made.
For one, we do not know if the J-20 is actually heavier than the F-22, and if so whether the difference is significant. The J-20 is certainly a longer plane, but dimensionally speaking mass is a function of volume and density, not length. I keep asking B787 if he has cross sectional measurements of the J-20 because without those "eyeballing" doesn't do squat to get us closer to understanding roughly how much the J-20 should weigh. For example, let's say for the sake of the thought exercise that the J-20 is deceptively on average a half meter smaller in its cross sectional area than the F-22, maybe because it's wings are thinner (I think they are, actually), or maybe because it has much smaller control surfaces (which I don't think anyone can dispute). That half meter in cross sectional area multiplied over the length of the plane could add up. Or, let's say that the story about the J-20 having a 40% lighter titanium structure than the F-22 is true because of 3D printed bulkheads. Perhaps, because the J-20 uses older engines with no 2D TVC, weight from the engines are also much lighter? Perhaps the J-20 has lighter subsystems than the F-22 because its subsystems are newer? Do I believe any of these points to be factually true? No. But I do not take it as a given the J-20 must be heavier, because eyeballing is a terribly fuzzy way to determine weight. We know the J-20 is dimensionally longer by about 8-9%, and we know that the J-20 looks like it should have comparable cross sectional area to the F-22 (they do share roughly the same wingspan after all). Beyond that, we don't know a thing about its volume or density. There are far too many unknown parameters to make confident claims.
Second, should we actually assume that the J-20 and the F-22 have similar drag characteristics? I think there are several reasons for assuming not. If the J-20 is longer but has roughly the same cross sectional area as the F-22, that would mean it should conform to a tighter area rule. The J-20 is also a canard delta, which means generally speaking it should have better supersonic drag characteristics (lower trim drag). Furthermore, it would appear that the J-20 has slightly narrower wing sweep than the F-22 (From what I've heard, Lockmart relaxed the wing sweep of the F-22 from the YF-22 to optimize a bit more for subsonic flight after determining that the YF-22's supersonic flight characteristics were more than satisfactory). We know that the J-20's original design study was concerned about hitting supercruise requirements with inferior engines, and talked extensively about how the design they pursued could be optimized for better supersonic drag characteristics. These considerations suggest to me that for the same T:W ratio, the J-20 may be a better supercruiser than the F-22.
Third, within the specific parameters of the thought exercise we just engaged in, we ascertained that the J-20 should be able to attain 70% of the F-22's Mach number at max fuel given that it has about 70% of the F-22's T:W ratio under those conditions. However, we know that drag with each increasing mach increment does not scale linearly, but as a polynomial function. That means for every extra increment of mach number, you need proportionally greater thrust than the last. Ergo, the J-20 should be able to do at least a bit better than 70% of the F-22's supercruise number with 70% of its T:W ratio.
So to pull this all back to the original question, can the J-20 supercruise with its current engines? I'd say probably. Can it supercruise as well as the F-22? Most likely not, *but* perhaps it does not compare nearly as poorly as some here might suggest.
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Huitong's site fits the bill quite well.
Yes. It's called Song Wencong's original design study, *as well as* the stated requirements for the J-XX program, which we've known about now for more than a decade.
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