World's 5th gen fighters - relative strengths and weaknesses


Inst

Senior Member
Do you think, at this angle, placing the canards behind the main wings will reduce rcs?
Canards are inferior to tailed aircraft in stealth terms, think of it this way:

Tailed aircraft:

Reflection from main wing (large), reflection from the tail not blocked by the main wing (very small), reflection from the tail reemitted from the main wing (very very small).

Canard aircraft:

Reflection from canards (small), reflection from main wing not blocked by the canards (slightly less large), reflection from main wing reemitted from canards (very small), reflection from canards from radar that was reemitted by canards and reemitted further by the main wing (very very small).

But generally the penalty from a canard layout isn't sufficient that a canard aircraft can't be VLO; it's like the YF-23 vs the YF-22; both were stealthy, the YF-23 was stealthier than the YF-22, but the degree of stealth penalty wasn't sufficient that the YF-23 was chosen over the YF-22.
 

Pmichael

Junior Member
Actually the stealth penalty of a canard design is significantly. Not just we have a completely visible control surface frontally the gap between airframe and canard is a trap for radar waves.

Something things can be compensated with the right flight control system and efficient RAM coating. But nothing will fully compensate for a visible control surface frontally. Even ignoring that the J-20 is quite the bastard design which also deploys all moving fins and deep central strakes.
 

Inst

Senior Member
Actually the stealth penalty of a canard design is significantly. Not just we have a completely visible control surface frontally the gap between airframe and canard is a trap for radar waves.

Something things can be compensated with the right flight control system and efficient RAM coating. But nothing will fully compensate for a visible control surface frontally. Even ignoring that the J-20 is quite the bastard design which also deploys all moving fins and deep central strakes.
Welcome to SDF, 60% China fanboys / Chinats, 30% China bashers, 10% moderates.

Remember that the NATF YF-23 was supposed to be a canard aircraft, that the JSF had concepts that were canard aircrafts, and that the US has 6th generation concepts that are canard aircrafts.

Canards impose a penalty, but nowhere as great as you imply. Moreover, most American stealth aircraft have truly massive tailfins as a result of the tailed planeform; check out the F-35 as an paragon. The J-20, in comparison, has small tailfins relative to the F-22 and the Su-57.

As I've stated repeatedly, the dream of canard stealth aircraft is to ditch the tailfins altogether with TVC, hence the American 6th generation designs, without having undue risk due to using the TVC as a key control surface. This would make canard stealth a decisive advantage over tailed tailfin stealth, but the closest we've gotten to realizing it is the reduced tailfin size on the J-20.
 

Inst

Senior Member
I mean, what do you want to talk about? We have the Song Weicong documents explaining why the J-20 layout was chosen. We have my discussion on how the J-20's weapons bay is crap. We have the discussion on the future of air combat and how agility will become less important due to ultra-maneuverable AAMs.

The fact of the matter is, we can compare the J-20 to an F-22 and an F-35. The J-20 isn't an LO aircraft as China-bashers want to claim, but neither is it likely to have a stealth advantage over American warplanes unless China metamaterials design matures and the J-20 is refitted with metamaterials. Then American aircraft can also be refitted with metamaterials, and the advantage disappears.

We can also compare it in terms of maneuverability. It has a 75 m^2 wing area for an empty weight range in 18000-22000 kg, I prefer the lower figures given known Chinese advances in 3D materials. This roughly puts it at a wing loading comparable to the F-22, but it doesn't give it advantages over the Su-57. We are aware that the lerx long-coupled canard lerx body lift delta planeform is superior to canard deltas (in the J-10-style configuration) by 20% in terms of lift, but this doesn't necessarily give it an advantage over lerx long-coupled canard lerx planeforms like the Rafale and Eurofighter, which have far superior wing loading compared to the J-20.

In terms of thrust-to-weight, it's competitive with 4th gens at 60% fuel and with missiles, but without the WS-15 it doesn't have a decisive advantage.

In terms of weapons bay capacity and strike capability, it's better than the F-22, about the same as a Su-57, but significantly worse than the F-35 Joint Strike Fighter given that its loadout has no decisive advantage over the F-35 despite greater size.

In terms of supercruise, while I've tried to argue that the air intake on the J-20 permits supercruise, it's apparent from the volume of the intake that the J-20 intakes are comparable to Su-27 intakes and no further mass can be obtained. The Vtech drag diagrams say that total drag build-up for the J-20 at Mach 1 is 9 kt (metric), at Mach 1.1 this drops to 6.8 kt, assuming 35kft or 10.68 km. The Al-31 engine outputs about 2.65 kt of force at Mach 1.0 at 10km and about 2.85 kt at Mach 1.1. The dry thrust of the Al-31 is 7.6 kN at sea level, implying a 65% thrust reduction at altitude. At Mach 1.1, you'd need about 3.4 kt per engine and to break Mach 1 you'd need 4.5 kt per engine, or roughly you'd need a 20% increase in dry thrust to achieve pseudo-supercruise and a 70% increase in dry thrust to achieve true supercruise. Even the Al-41F-1S on the Su-57 can't achieve pseudo-supercruise on a J-20 platform without tweaking to high altitude dry thrust.

In terms of max speed, the DSI and stealth coatings on the J-20 prohibit it from having a strong max speed due to the let-off in engine thrust at high speeds and the need to preserve the engines.

In terms of cost, the F-22 is out of production, the F-35 has seen massive cost reduction since its LRIP versions (around 80 million a pop these days), and the Russians are claiming they've gotten the Su-57 down to ridiculously low prices that can have you losing 2 Su-57s per F-35 and not bat an eye. The rumored J-20 price is 100 million (especially with exchange rate changes over the past few years).

In terms of sensor subsystems, the J-20 is superior to every other stealth aircraft in that it has full EODAS as with the F-35 and has a full-sized AESA around the 1m diameter.

In terms of other subsystems, the J-20 has advantages in the operation PL-15 system, but the PLAAF lacks micromissile development as even the Russians are attempting such to fit their tiny Su-57 side weapons bay. The PL-21 can't fit on the J-20, the J-20 seems to lack an interceptor missile for very long-range interception against AEW&C and tankers, the J-20 seems to lack an anti-radiation strike missile for the SEAD mission. The Su-57 has a DIRCM that the J-20, to date, lacks, although as I've pointed out, the J-20 is very pod ready given its side weapons bay.

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As I've said repeatedly, the one huge advantage the J-20 has is its development potential. The least disputed element is the WS-15, which should add TVC, but moreover, will increase thrust by 25-44%.

Slightly more speculatively, the J-20 is likely to be the only airframe with a pod advantage over its competitors. The F-35 was slated to receive a laser pod, but this seems to be cancelled. However, the layout of the J-20's side weapons bays means that pods can be deployed to the side of the J-20, allowing the J-20 to incorporate new capabilities, including laser point defense and dazzlers.

Even more speculatively, once the WS-15 is ready, the J-20 is no longer limited by engine power meaning that aggressive modifications to the airframe
 
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Inst

Senior Member
In short, one possibility that can be brought up is that the J-20 IS going to be the Chinese 6th generation fighter. When you look at the J-20's development and design, it's clearly a J-10 derivative (and a key feature is that it inherits the anhedral-dihedral canard wing layout as the J-10 has a modified inverted gull wing from the inner section of its wing being dihedral). The ability to modify the basic J-20 design for other purposes and enhanced capabilities suggests that modding the J-20 can be how the PLAAF gets a sixth generation aircraft up by 2035 or even 2030. It could, in all honesty, be a 5.5th generation aircraft, but it's close enough to 6th generation to form a stop-gap. That also allows the PLAAF to develop more aggressive 6th generation designs that would be riskier than the PLAAF could support without a stop-gap fighter like J-20 derivatives.
 

Inst

Senior Member
Basically, fanboy position -> canards are better than tailed stealth aircraft. Fanboy-Moderate position: canards impose some penalty from an all-aspect perspective, but not to direct. Moderate position: canards impose penalties, but not prohibitively so. Basher position: canards suck and reduce the J-20 to LO!

A paper was posted here a while back detailing canard design and treatment for a speculative canard delta stealth fighter. It stated that with exhaustive ram treatment on the interface between the canards and the main body, RCS contribution can be reduced to around -40 dBsm.

This is not wholly different from the F-22, which has inlet gaps that create huge RCS contributions, but are aggressively treated with RAM.
 

Pmichael

Junior Member
I think it's not a good showcase of discussing in good faith if people accuse other people of being fanboys or bashers or anything. It's mostly tiresome.

Also talking about stealth. It doesn't take a lot to realize the more visible movable control surfaces are from the frontal arc (including undesirable elements like the gap between airframe and canards) the potential worse is the RCS.

Delta-Canards designs (there are more than enough such planes in service) have very interesting and impressive aerodynamic performance characteristics. But the stealth implications basically killed it for any American, Russian and European future concepts.
 

Inst

Senior Member
I think it's not a good showcase of discussing in good faith if people accuse other people of being fanboys or bashers or anything. It's mostly tiresome.

Also talking about stealth. It doesn't take a lot to realize the more visible movable control surfaces are from the frontal arc (including undesirable elements like the gap between airframe and canards) the potential worse is the RCS.

Delta-Canards designs (there are more than enough such planes in service) have very interesting and impressive aerodynamic performance characteristics. But the stealth implications basically killed it for any American, Russian and European future concepts.


Boeing F/A-XX concept.

Canards are, of course, not an ideal set up for a stealth aircraft, but no stealth fighter is an ideal set-up for a stealth aircraft and as the ideal stealth shape looks something like this:



Every deviation from this ideal shape is a compromise of some sort, and canards aren't the deadly compromise you imagine it to be; they're roughly as bad as the inlet design on the F-22 where you have a massive reflector covered with RAM between the inlets and the main body. This is likely why the J-20 resorted to DSI; if it had gone caret intakes as in the original design proposal, it'd basically have the F-22's intakes RCS contribution doubled.

The advantage of a canard for the J-20 design is that it allows smaller tailfins, which by themselves add to RCS, and allows the J-20 to use narrower wings, which also reduce the RCS.

Does it impose an RCS penalty by itself? Yes, it does. But if RCS were the paramount thing the Chinese would have been looking for something at the F-35 scale, which by virtue of its size can be stealthier than a larger equivalent, in the same way a F-15A lights up on radar in a way a F-16A doesn't.
 

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