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Chengdu J-20 news and analysis Part III

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TomcatViP

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The picture discussed in above link:


Notice also the different shape of the sensor on the inlet lip (below the twin bypass outlets)
 
FighterJock

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TomcatViP said:
The picture discussed in above link:


Notice also the different shape of the sensor on the inlet lip (below the twin bypass outlets)
Click to expand...

I wonder what the range of the twin seat variant of the J-20 will be over the single seat variant? Considering that there will obviously be less space for fuel tanks.
 
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Josh_TN said:
Is there any public range figure for the single seat version?
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Nothing official, estimates range from 1250 km (slightly higher than F-35C) to 2000 km combat radius. There's a claim somewhere of a tanked range of 5500 km. Compare over 3000 km F-22 ferry range, 4500 km Su-57 ferry range with external tanks.

For all the J-20's short-comings, it has very good range.

===

As for your comment about interceptor / fighter bomber, the J-20 has too poor a weapons bay to be either. Its weapons bay volume is roughly comparable to that of a F-35, except that an F-35 is a much smaller jet.

Interceptor -> can't dogfight, but can strike / intercept
Striker -> can dogfight, can strike, but can't intercept
Air superiority fighter -> can't strike, but can intercept or dogfight.

The poor weapons bay is a major short-coming, but air superiority fighters can and have been developed into strike fighters (Su-30, F-15E).

===

The bigger question, of course, is whether it's a good air superiority fighter (mediocre kinematics despite having essentially the Eurofighter AMK's layout). But that's more a function of the engine (surprised that they haven't tried stuffing TVC onto some operational fighters) and engine output (WS-10C is apparently in the same range as the Su-57's engines).
 
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GARGEAN said:
Inst said:
WS-10C is apparently in the same range as the Su-57's engines
Click to expand...
Eeeeeegh... That's optimistic)
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Erm, should have specified AL-41 / 117S, not Izdeliye 30. Should be in the 137-142kN range. Note that the intended WS-15 engine is supposed to be in the 17-18 ton range, and that the Chinese really screwed up by developing AL-31 / F110-class engines, instead of F404-type engines, where the smaller engine would have presented lesser technical challenges and allowed the Chinese to compensate for technological inferiority by simply using more, but smaller engines.
 
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Here's an important and relevant question:

Chinats on Chinese websites seem to attach a combat radius of 2000 km to the J-20. How viable is this on internal fuel?

For instance, a F-106 Delta Dart, on about a 30% internal fuel fraction, can make 930 km combat radius with a turbojet. The J-20 is certainly a draggier design, and has a higher wing loading to boot, but it uses a canard-lerx-delta-body lift design, which purports, for its 75 m^2 of wing-body lifting area, to achieve 80% higher lift (vs delta only and 20% vs Gripen/J-10-type canard-delta, and has a likely 40% fuel fraction.

In such a set-up, can the J-20 reach 2000 km combat radius on internal fuel alone?

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The point is basically the F-35's point. The F-35 is a high fuel fraction fighter whose T/W and wing loading (although the wing loading should be decreased by about 23% due to tail lift in unstable parts of its regime) is terrible at maximum range, but excellent when engaging at "normal" ranges. In other words, it can be treated as having CFTs built in. If the J-20 has an even higher combat radius on internal fuel, first, the extreme combat radius allows it to afterburner in on defensive missions and later supercruise in. Second, since the J-20 needs much less of its fuel to get home, it can fight at very low fuel fractions and improve its wing loading / thrust to weight to Eurofighter levels even on a WS-10C.

===

One additional point is that the F-35, on an air to air mission, can reach about 1410 km. Since it's not designed for high speeds like the J-20, it's further possible that the J-20 can reach or near 2000 km on internal fuel on an air to air mission.

By the way, some calculations: the F-35 can reach around 274 kg / m^2 wing loading using tail lift in unstable (about -23% to wing loading) flight regimes to a combat radius of 600 km. The J-20 can likewise reach 271-282 (depending on whether canards are adding lift or neutral in the flight regime) kg / m^2 wing loading. The respective thrust / weights would be 1.22 and 1.32 T/W (using 137 kN for the J-20, mind you).
 
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Inst said:
Erm, should have specified AL-41 / 117S,
Click to expand...
The Su-57 has a 117M engine with 15 tons of thrust.

How many tons of thrust the WS-10 has, as far as I remember, was not officially reported. It is unlikely that it is fundamentally different from the latest versions of the AL-31.
 
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Just use Breguet range equation to do some calculations for range estimates. Feed in some feasible L/D, SFC or fuel fraction (e.g. compare with other aircraft published data) then see what you get out.
 
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red admiral said:
Just use Breguet range equation to do some calculations for range estimates. Feed in some feasible L/D, SFC or fuel fraction (e.g. compare with other aircraft published data) then see what you get out.
Click to expand...

Well it's actually bit more complex than that.. Used as it is. The breguet range would give "ferry range" which is... the "1 way" range. More iterations are needed. Like fuel utilizations, calculations of SFC during combat and returning to base, the aircraft's L/D's.

But anyway did that in excel with following assumptions :

Empty weight : 17000 Kg
Fuel weight of : 11000 Kg
L/Dmax of : 11 (looks reasonable, as far as i see typical fighters are about 10-13)
Cruise L/D is assumed to be 93.4% of the L/D max
Payload of 4000 Kg (weapons, pilot, etc)
Mission altitude of 12000-15000 m

Also Fuel utilizations need to be defined too. The radius or combat range is then effectively the cruise fuel.

The plane will "supercruise" throughout the mission at Mach 1.2. and doing afterburner during the combat as it would need some maneuvers. Then RTB, the fraction for RTB can be less than cruise phase one as the plane would drop payload. so it's lighter.

So what i got was that the combat range for my hypothetical J-20 there is 1384 Km. With enough fuel for 10 minutes of combat or dogfight with afterburner, and enough fuel for RTB with 6 minutes allowance for loiter awaiting landing.
 

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red admiral

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Also need to consider the SFC values you're using. SFC at SLS uninstalled is very different from what you'll actually get at cruise conditions. The value you're currently using is more representative of a modern civil high bypass ratio turbofan
 
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red admiral said:
Also need to consider the SFC values you're using. SFC at SLS uninstalled is very different from what you'll actually get at cruise conditions. The value you're currently using is more representative of a modern civil high bypass ratio turbofan
Click to expand...

My sheet also take account of that. This is the "calculated parameters" section of the sheet.

Notice that i do Specify Altitude. That can be used to predict the SFC of the Jet engine.

The Atmosphere model i use is based on ICAO. Which should be good until 22000m.

The SFC or Specific Fuel Consumption of the engine roughly follows the following simple relationship RElationship-Engine.png

Where Ct is the SFC at SLS condition. a is the speed of sound in the operating altitude while the asl is the speed of sound in sea level condition.

Thus for given parameters (SLS SFC : 0.75) and cruising altitude of 15000 m. the estimated SFC of the engine at that altitude would be :

a(15000 m) : 924.9 ft/s
a(SLS) : 1115 ft/s

SFC(15000m) = 0.75 *(924/1115)
SFC(15000m) = 0.621 lb/lbhr.
 

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overscan (PaulMM)

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GARGEAN said:
overscan (PaulMM) said:
WS-10C is analogous to Izdeliye 117, WS-15 to Izdeliye 30.
Click to expand...
May be. But from what I see we don't know enough about either WS-15 or Izd. 30 to make such claims.
Click to expand...
WS-10C is a developed WS-10. WS-15 is a new design.

Izd. 117 is a developed AL-31F. Izdeliye 30 is a new design.

Analogous doesn't mean comparable.
 
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stealthflanker said:
red admiral said:
Just use Breguet range equation to do some calculations for range estimates. Feed in some feasible L/D, SFC or fuel fraction (e.g. compare with other aircraft published data) then see what you get out.
Click to expand...

Well it's actually bit more complex than that.. Used as it is. The breguet range would give "ferry range" which is... the "1 way" range. More iterations are needed. Like fuel utilizations, calculations of SFC during combat and returning to base, the aircraft's L/D's.

But anyway did that in excel with following assumptions :

Empty weight : 17000 Kg
Fuel weight of : 11000 Kg
L/Dmax of : 11 (looks reasonable, as far as i see typical fighters are about 10-13)
Cruise L/D is assumed to be 93.4% of the L/D max
Payload of 4000 Kg (weapons, pilot, etc)
Mission altitude of 12000-15000 m

Also Fuel utilizations need to be defined too. The radius or combat range is then effectively the cruise fuel.

The plane will "supercruise" throughout the mission at Mach 1.2. and doing afterburner during the combat as it would need some maneuvers. Then RTB, the fraction for RTB can be less than cruise phase one as the plane would drop payload. so it's lighter.

So what i got was that the combat range for my hypothetical J-20 there is 1384 Km. With enough fuel for 10 minutes of combat or dogfight with afterburner, and enough fuel for RTB with 6 minutes allowance for loiter awaiting landing.
Click to expand...

Thanks for doing the work, although I'd dispute the empty weight (Trident elsewhere thinks it's around 21 tons, Chinats came out with a rumor of 15 tons and 17.5 tons, I work with 18.5-19.5 tons based on rough dimensions and 3D-printed titanium weight reduction) and payload (it's an air superiority / air defense fighter, it won't be carrying heavy strike missiles because it literally can't).

That said, I'm more interested in non-supercruising combat radius than supercruising combat radius. I think it's interesting that most Western analysts are thinking about the J-20 as an offensive and aggressive interceptor aimed at shooting down AEW&C and tankers. But on a defensive mode, speed and range are actually quite interesting.

For instance, if you have 2000 km non-supercruising range, you can now put rudimentary bases on the periphery of your territory, but base the aircraft with their weapons perhaps 1000-2000 km inland. The intent is that the aircraft take off from inland bases, cruise or supercruise to the border, dogfight, then return not to their original bases but the rudimentary frontline bases, allowing you to slash combat T/W while at the same time reducing your vulnerability to enemy attacks on your airfield.

Likewise, the speed and range allows you to attempt defeat in detail tactics; because you're faster than your opponent, you can concentrate the entire PLAAF onto a small portion of the enemy force. If you're in a range of 0.8 vs 1.2 equivalency to enemy fighters you can easily achieve 2:1 numerical superiority in a small area so that even if you don't have a qualitative or big qualitative advantage, quantity has a quality all its own. The quantitative advantage can help you minimize losses, then move on to the next segment of the enemy force.

===

Moreover, a 2000 km combat radius allows the J-20 to reach, from Chengdu basing (where, incidentally, the J-20 is produced), both Lhasa and Shanghai. This allows the Chinese to put a reserve force in Lhasa and Shanghai and allow them to handle both the Indian and Taiwan theaters simultaneously, perhaps even supercruising onto a combat mission using reduced fuel reserves with the aim of landing at a front airbase instead of the original airbase.
 
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