manqiangrexue said:
So the first thing I noticed is that he gave the engine thrust figures as 3.2 tons for F-22 and 2.5 tons for J-20. F119 are 156x2=312kN, which is roughly 31.3 tons (not accounting for rectangular nozzle reductions which I've heard to be from 3%-17%). Although the thrust for J-20's engines are not known, the best known AL-31X that was sold to China is AL-31FN3 with 137kN for J-10B. 2 of these is 274kN, which is 27.5 tons, not 25 tons. He's off by 10% on something that could be calculated as simply as that and that's ignoring the possibility that these are further upgraded AL-31X for J-20; for reference, the best AL-31 variant of the RuAF is FM3 with 145kN, but since there is no evidence that China got these, I used 137kN as the lowest figure possible, and he somehow went a full 10% lower. And for the Raptor, he rounded 31.3 to 32 instead of 31 or keeping the extra sig fig so based on that, I reserve my doubts for the rest of his numbers based on known inaccuracy and perceived bias towards the Raptor.
F119 thrust is officially stated as 35000 pounds *class* - 35000lbf in SI units works out to 156kN, but in making the conversion people are inclined to drop the "class" and thus forget that the exact figure is not public. Long story short, there's no point in trying to be exact to a Kilonewton.
As for the AL-31F, we have no confirmation on which variant is used on the J-20 and since on derivative engines especially thrust growth generally outpaces TWR improvement for weight impact, taking the lowest-thrust variant actually biases the J-20 OEW estimate *down*. In other words, an improved-thrust AL-31F is most likely going to be at best the same weight as the vanilla variant.
manqiangrexue said:
For some reason, he states the F-22 fuel capacity at 9.3 tons when it is 8.2 tonnes? Did he convert metric tons to American tons for this analysis??? Why? Is his analysis in tons or tonnes?
With all due respect, you might want to read my post again. I am baselining 8.2t (firmly in the metric camp, BTW) but the aircraft hazards document leaves open the possibility that F-22 fuel capacity (including the F1 compartment) is in fact 9.3t, so I provided the impact on the fuel capacity estimate of that eventuality.
manqiangrexue said:
He took off 200kg for the DSI. Why? No explanation of why that number was used. Someone reported prior that the F-35 saved several hundred kg due to DSI.
Sure I explained it: I gave siegecrossbow the benefit of the doubt with his "several hundred kilos" and noted that the J-20 (unlike the F-35) retains some conventional intake features in the shape of boundary layer bleeds and associated ducts. So I assumed a figure at the lower end of the range implied by "several hundred kilos" - how about asking for the source for the original claim, because I could not find a source with even such a vague quantification of the weight saving (only statements that there IS a benefit)?
It's not a bad question actually, but you should not be aiming it at me...
manqiangrexue said:
He dismissed airframe composition as having any effect!
Again, please read what I actually wrote and refrain from putting words in my mouth - I clearly said *minor* variations! Reading this thread, I got the impression that the best available information suggests composite content in the J-20 is not dramatically different from the F-22 (as between Rafale & Typhoon).
manqiangrexue said:
He gave 200kg off for the gun. I'd like to see him cite his source.
Google is your friend. Honestly, it was a big job to compile all the info that went into this estimate, so I'm not going to spoonfeed people on questions of this nature. Ask about the method all you want, that's not something you'll get an answer about any other way, but I think it's reasonable to expect a modicum of independent research effort from critics of figures like this.
manqiangrexue said:
He does not see any weight reductions from the weapons bay. He could be correct as I also don't see any obvious weight reductions could come from but simpler indicates reduction of parts and thus lighter to me.
Again, how is the J-20 solution simpler? It's clearly different, even elegant in its own right, but I see no grounds to assume a weight advantage.
manqiangrexue said:
He adds 100kg for EOTS on J-20. Where is that figure coming from?
EOTS combines IRST and LDP functions. IRSTs are anywhere between 50kg and 100kg in weight (Russian systems, AN/AAS-42, PIRATE), laser designator pods usually way 200 to 250kg. Because it is internal and thus doesn't need its own cooling system etc, EOTS will be lighter than the latter, but probably heavier than the former. It's possible that it's a bit less than 100kg, but do we really want to haggle about <50kg one way or the other when discussing a figure of ~21t?
manqiangrexue said:
He does not account for the know differences in engine weight between F-119 and AL-31X. Even the older AL-31F weighs 230kg less per engine than F-119. That's 460kg and that's not accounting for further weight reduction measures implemented in the modernization of the older 122.6kN AL-31F.
He does not account for the weight of the F-22's TVC. He mentions it but does not mathematically account for their weight. Someone else gave a figure of 800kg. I don't know if it's accurate but they look pretty big and bulky to me.
Dry weight figures for the AL-31F may be not directly comparable to figures quoted for Western engines, and in any case the only public weight information for the F119 is also estimated (and would include the nozzles - 16t thrust at 1.8t weight works out to a rather conservative TWR of 8.9). Once more, there is no point in pretending to be more accurate than the quality of your data allows in the first place.
True, it's not 100% perfect, but good enough that it likely won't get much better until they release an official three-view drawing.
