As luck would have it, I did a J-20 OEW and fuel capacity estimate recently - I had not meant to post it anywhere as the method is a bit crude and of my own invention, but the result is pretty decent IMHO and some of the claims here are just outrageous. Apart from the well-known overhead drone photo, I used the following documents:
-----------------------F-22---------J-20
Length:--------------18.92m
1-----20.9m
2
Span:----------------13.56m
1-----12.9m
2
Wing Area:----------78m^2-------75m^2
3
LE Sweep:-----------42°-----------50°
3
Fuselage x-section:-5.3m^2------5.9m^2
4
Fuselage length:----16.8m--------19.7m
5
Bay length:----------4.3m---------4.4m
6
Bay width:-----------2.1m---------2.3m
7
Wing box area:-----23m^2-------25m^2
8
Internal fuel:--------8.2t----------10.5t
9
H-stab area:--------12.8m^2----7.2m^2
10
Thrust:--------------32+t---------25+t
11
OEW:----------------19.7t--------21.2t
12
OEW would increase slightly to approximately 21.7t with a final engine similar to the F119 in thrust and TWR - a sufficiently large thrust bump means engine TWR improvement is outpaced in terms of airframe weight impact (aircraft TWR still improves markedly of course). I don't really expect the real figure to be below this value on an apples to apples basis unless actual fuel capacity is markedly lower than my estimate (BTW, if F-22 fuel capacity is actually 9.3t, J-20 internal fuel goes up to >11.5t).
As it is, I'm crediting the J-20 with a slightly bigger weapons bay and an almost 30% increase in fuel capacity for a <10% OEW penalty - that's pushing it already (the F-35 gains 20% OEW over the Typhoon for a similar fuel capacity increase, but of course that also comprises the weapons bays)!
siegecrossbow;483343 said:
1) Smaller (proportionally speaking) net wing area and thinner wings.
Accounted for in my estimate. I doubt the J-20 has thinner wings BTW, they have conical camber only in the leading edge area as opposed to along their entire chord on the F-22 (think Concorde). Combined with the twist that does reduce projected frontal area, but doesn't affect their actual thickness.
siegecrossbow;483343 said:
2) Much smaller vertical stabilizers (even accounting for the ventral strakes).
Accounted for in my estimate, perhaps even too generously (I don't think J-20 fin area with the ventral strakes is reduced as dramatically as h-stab/canard size over the F-22, yet I assumed that it is).
siegecrossbow;483343 said:
3) DSI inlets (reportedly saved several hundred kilos on the F-35).
Ok, let's say getting rid of the BL diverter gap and associated RAM yields 100kg, per side, 200kg total (J-20, unlike F-35, retains BL bleeds and ducting). Makes my estimate 21.0t (about 21.5t with definitive engines).
siegecrossbow;483343 said:
4) Fiber optic wiring instead of copper wiring.
Used on the F-22 as well, in fact pretty much a necessity for handling the data through put for a integrated processor avionics architecture with sensor fusion. No benefit applicable for the J-20 over the F-22.
siegecrossbow;483343 said:
5) Composite material (unsure how much is used).
Swings and roundabouts. Minor variations in material fractions will not give major differences in OEW, consider Typhoon and Rafale for example (~1t difference which is almost entirely explained by the slight overall size difference).
siegecrossbow;483343 said:
6) Lack of gun + associated mechanism.
Good point, M61A2 (F-22) weight ~90kg, empty M61A1 feed system on the F/A-18 (capacity 100 rounds more than F-22) ~120kg, let's say another 200kg saved. We're now at 20.8t (some 21.3t with definitve engines).
siegecrossbow;483343 said:
7) Use of simpler mechanism within the side bays compared with the F-22.
Simpler? Different, yes, but I don't see where a weight difference (one way or the other) should come from. No benefit applicable for the J-20 IMHO.
manqiangrexue;483342 said:
Of course there are other measures that shave weight off J-20 (lack of TVC for sure, no gun?, material upgrades, other things that its designers know that we don't) but I think that other than the TVC, it'd be hard for us to quantify those.
I'm also accounting for the lack of TVC via reasonably accurate engine TWRs, but we've not added ~100kg for an EOTS - so my best and final estimate actually ends up at 20.9/21.4t.
It's an untested method, but accounts for practically everything we know with reasonable certainty and delivers what I consider a remarkably plausible result (anything up to a 4 ton difference would have been within the limits of credibility IMHO -> F-15C vs. Su-27S). Of course there are significant error margins in both the assumptions and the measurements, but I'm happy to believe J-20 OEW hovers around the 21t mark until compelling evidence to the contrary emerges. 15t would put it within less than 2t of the lightest F-35 variant, for crying out loud!
1 F-22 blueprint checked against known span/length ratio - span proportionately too large in many published drawings!
[noparse]
2 measured from near-perfect planform shot using J-16 reference
3 measured from near-perfect planform shot
4 measured from near-perfect frontal shot of LRIP airframe, foreshortening checked with canard/wing span ratio, matches planfrom shot
5 nose tip to nozzle exit, measured from near-perfect planform shot
6 measured from decent LRIP in-flight belly shot, bay length to overall length (same pitch angle), may be a bit too large for both
7 measured from decent LRIP in-flight belly shot, bay width to wing span (same roll angle), may be a bit too large for both
8 without fuselage, control surfaces & LE flaps - proxy for wing tank size for fuel capacity estimate
9 scaled using fuselage volume (x-section*length) & wing box area based on actual F-22 fraction of fuel in wings
10 measured from near-perfect planform shot, used as proxy for total empennage area reduction (fins somewhat reduced on J-20 as well)
11 if we assume a more advanced AL-31F with higher thrust (even with better TWR), weight benefit vs. the F-22 may not be as large
12 scaled using fuselage volume (weighted 35%), wing area (35%) & empenage area (10%), assuming ~1.6t for J-20 engine 1.8t for F-22
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