J-20 Inlet Discussion


Inst

Senior Member
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The advantage of the term I prefer, fighter-interceptor, is that it best describes the J-20 to a layman at a glance. It implies an aircraft that has long range, very good high-speed characteristics, while being able to acquit itself in an air superiority role.

Moreover, given the importance of interception (targets of opportunity) in modern air combat, it also adequately describes the J-20's combat role, as well as how it will engage enemy fighters.

The J-20 is likely to have a speed, supersonic maneuverability, and acceleration advantage over enemy fighters. This means the J-20 can concentrate and achieve a local numerical superiority, as well as fire missiles at long range, then run away.

====

The other term, which is what I think other posters might gravitate to, is "high-speed air superiority fighter".

The main problem with the air superiority claim is that it calls to mind 4th generation air combat, where dogfighting was paramount and BVR was more crucial for draining enemy fighters of energy (evasive maneuvers) before hitting the merge. This isn't how 5th generation aircraft fight, and if the J-20 only has 4th-gen level subsonic agility, it makes the J-20 look bad.

Moreover, it calls the J-20 into direct comparison to the F-22 and Su-57, when all these aircrafts are substantially different in performance.

The F-22, for instance, emphasizes stealth, both IR and radar. It has strong supersonic maneuverability because of TVC providing high control authority at supersonic speeds.

The Su-57, on the other hand, emphasizes cost and maneuverability, using the LEVCON innovation alongside off-axis TVC to provide superlative agility. It's an aircraft that wants to get in close and get into the knife fight, even if it doesn't guarantee exceptional kill-loss ratios.

The J-20, on the other hand, wants to use its sensors to spot the enemy first, fire off its BVR or long-range WVR missiles, then close in for the kill or run.

When you put the three heavyweight 5th generations in comparison, the J-20 is going to end up being worse off. In the 4th gen air superiority fighter race involving 5th gen platforms, the J-20 is the loser.

And finally, there's the novelty of it. To the layman, what the hell is a high-speed air superiority fighter? It's a term no one's ever heard of before, and you immediately have to explain how it differs from "normal" stealth air superiority fighters.

In contrast, a "fighter-interceptor" is a concept they can easily wrap their heads around, and is something the J-20 excels at doing. If the counter-argument is that the J-20 can be defeated by aircraft labeled "air superiority", all you have to do is mention that in 5th gen land, you're never going to get close enough to do this, #1, and #2, even if you do, you're fighting attritional battles with HOBS missiles, so you're only going to be able to trade off the J-20 1:1 at best.

====

That concludes my statement on this subject. There's definitely aspects I've missed, like how WS-15 can transform the J-20's performance, but as people on this forum are beginning to catch on, no one wants to dogfight anymore!

5th generation air combat is incredibly simple, once the complexity of the target tracking war has been smoothed out. See the target first, shoot the target first, and run away fast if the target tries to shoot back.
 

Bltizo

Lieutenant General
Staff member
Super Moderator
You've already made a new thread to write what you want about J-20, we don't need another one that covers a similar topic especially one that's been discussed to so much depth already in past threads either here or on CDF to about the same conclusion every time.

I'm merging the posts with the other thread.
 

secretprojects

New Member
Registered Member
As others have said, I don't think anyone here is an expert or even an educated layman on engine inlets here. This seems to be even more of a black art than engines, since people will talk about OPRs, compressor / turbine stages, bypass ratios, etc. This is why this stuff is worth discussion and investigation.
Well, if you want to actually learn how air intakes on fighter aircraft work, I'd suggest a good book for the non-specialist is
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(there are scanned copies floating around on the internet) which has a detailed chapter on exactly this.
 

Inst

Senior Member
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  • #45
TLDR
isn't this thread about semantics?
If you're talking about J-20 as air superiority fighter, there's a Hegelian triple here.

First, when the J-20 came out, Western commentators immediately called it a striker or an interceptor based on its apparent size (F-111) or dimensions (high aspect ratio for a conventional fighter). This is the Thesis.

Second, pro-Chinese commentators immediately lashed out, as they had been following rumors of a PLAAF 5th generation for years, and dragged out stuff like the J-20's actual size (trucks were used first to scale the aircraft, then satellite photos) and the Song Wencong research papers, to argue it's an air superiority aircraft. I sat in this camp for quite some time, until I started noticing that the J-20 failed to show exceptional maneuverability in videos. This is the Antithesis.

Third, leaks began to come in talking about the J-20's exceptional high-speed performance, as well as its relatively average or "good" subsonic performance. Moreover, actual measurements of the J-20's bays didn't fall in with Blitzo's claims that the J-20 had bays too shallow for strike missions, although the weapon bay was definitely too short for existing Chinese strike missiles.

Hegel's theory is that history moves in three steps, first with a Thesis, an Antithesis to combat it, and then a Synthesis that combines Thesis and Antithesis.

The Synthesis I am pushing, and am getting push back from advocates of the Antithesis, is that the J-20 is intended to combat other fighters, which is traditionally considered "air superiority", but it does so in ways much more akin to an 3rd generation fighter-interceptor as it does not want to go WVR (guaranteed telefrag due to HOBS missiles) and is more suited for high speed, long range action that boosts the effective range of its IR missiles.

What advocates of the Antithesis can't stand is the compromise with the Thesis, which accepts that the J-20 bears many similarities to an interceptor or striker. Thesis advocates, on the other hand, generally don't push back anymore because, #1, the F-35 is not an exceptional dogfighter either, #2, don't really care about the J-20 beyond bashing it.

Anyways, this discussion is off-topic, and if my thread de-merger (deletion or moving into a hidden forum) is approved, this post should also be removed.

This thread is about the physics of the J-20's inlet. If you have comments to be made on the J-20's inlet, please argue here. I'm not discussing the interceptor vs air superiority issue on this thread further.
 

secretprojects

New Member
Registered Member
The topic of intakes on the J-20 is potentially an interesting one, but conceding you aren't even an 'educated layman' means your argument is constructed on foundations of sand. I'd suggest reading the book I suggested to at least appreciate the complexity of the topic.
 

Inst

Senior Member
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  • #47
Well, if you want to actually learn how air intakes on fighter aircraft work, I'd suggest a good book for the non-specialist is
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(there are scanned copies floating around on the internet) which has a detailed chapter on exactly this.
Thank you for the suggestion, and I'll try to locate a scanned copy. I still insist that the present J-20 is the antithesis of the F-14; the F-14's original engines were unreliable and underpowered, but the F-14 was designed for it and when its engines were upgraded, the inlets weren't up to the task. When you have lines like:

"The F110-GE-400 engine produced 23,400 lbf (104 kN) of thrust with afterburner at sea level, which rose to 30,200 lbf (134 kN) at Mach 0.9.
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" it shows how important inlet-engine matching is.

In the F-22's case, it's fixed inlets with overpressure panels; i.e, at certain flight regimes the diffuser fails or there's just too much pressure in the engine. Why can't the J-20 have something similar to gain better performance out of AL-31 / WS-10 / WS-10IPE/G?.I've shown from visual measurements that the J-20's individual engine inlet is approximately equal in area to the F-35's dual inlets, but the J-20 has a far weaker engine indicating a substantially lower air flow requirement. This continues to imply that the J-20, in many flight regimes, has excess air flow which is bled off, and in regimes where the Su-27's AL-31 begins to choke, the J-20's engines can keep going due to this excess air flow.

===

I also want to bring up a few things.

First, I'm shocked no one rebutted the inlet length arguments by arguing that the length-diameter ratio is more important. Professional discussions of inlet length usually involve such, i.e, the diffuser works proportionally to the diameter of the inlet.

Second, going over Al-31 performance charts and assuming 50% more mass flow due to larger inlets, you're still not getting enough thrust to supercruise, except perhaps barely, at most speeds. You need a re-engining to an engine with better high-altitude performance (lower bypass ratio or higher dry thrust). This might not even require an Al-41, just a bit more dry thrust from improved Al-31 or WS-10.
 
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Inst

Senior Member
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  • #48
@secretprojects

Unfortunately, I can't find the scans, nor can I directly contact you about the subject (blocked or PMs disabled).

One further difficulty is that inlets is the preferred term in the United States, while intakes are preferred in the UK.

Some interesting surrogates do show up, however.

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From EADS Research VP.
 

secretprojects

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Registered Member
The amount of air ingested by a jet engine does not depend directly on intake size or forward speed. The engine doesn't have air forced into it, but takes in what it needs.At low forward speed and high engine RPM the capture stream tube (see Fig 112) is larger than the intake and must converge and thereby accelerate into the intake, giving a sucking condition. At high forward speeds with the engine throttled back (i.e.reduced RPM) the capture stream tube is smaller than the intake, provoking some of the air to spill around the intake lips.

The size of an intake depends principally on the intended use of the aircraft,though it should be recalled that the engine takes in what it needs rather than having air forced into it. On aircraft designed to cruise at high altitude and high Mach number (e.g.the XB-70 and SR-71) the intake is sized for this condition. Excess airflow is then inevitably experienced at transonic speed, even when account is taken of the airflow required for boundary-layer control, environmental control systems and engine cooling. On the more common run of supersonic aircraft,which have no specific design point, the intake is usually sized for high subsonic speed,the emphasis being on maximum acceleration in the transonic phase. Any excess airflow again has to be diverted back to the free stream as efficiently as possible. A case in point, the Lightning's intake was greatly undersized in order to minimise the spillage drag's effect on the slender thrust margins available from the early Rolls-Royce Avon turbojets. The extent of the undersizing meant that at high RPM the intake was increasingly full of strong shockwaves down to zero forward speed. These were intensified when the engine mass flow was increased after the intake design was frozen for manufacture. The later F-4 Phantom underwent a series of intake size increments to allow for increased mass flow as more powerful engines were introduced. The original F-4A had J79s requiring 75kg/sec airflow, whereas the Spey engines of the F-4K needed l00kg/sec. More recently still, the YF-16's intake was deliberately sized to allow for a 10% airflow increase in the course of engine development.

Cowl lip shape

Thick, rounded lips are needed to avoid lip flow separation at low speeds. At higher subsonic speeds,or at moderate speeds with the engine throttled, the intake is likely to spill flow around the outside of the cowl, though careful design can avoid a drag penalty.As aircraft speed increases into the high-subsonic regime, the flow, in spilling, will accelerate locally to supersonic speed. A shock wave will form on the blunt lip, terminating the supersonic flow and causing turbulence over so much of the flow that the drag increases severely.

Further spillage strengthens the shockwave and eventually the boundary layer separates, giving even more drag. At supersonic flight speed the shock-wave drag resulting from a very blunt lip would be quite prohibitive. It is clear that some compromise is necessary in the form of sharper lips,variable lip shape,auxiliary intakes, or some combination of these.
 

secretprojects

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Registered Member
Intake duct length and shape

After the air enters the intake and passes through the shock-wave pattern,further diffusion is needed down to the Mach number of about 0-4-0.5 which is acceptable to current engines. This takes place in the section of duct called the subsonic diffuser. The velocity seen at the engine face is maintained into the combustor, where there is a limit to the speed at
which combustion of the air/fuel mixture can take place.

To secure the minimum possible supersonic drag,the cross-section area of the aircraft in the centre-fuselage section must be closely controlled, and the intake duct turned into the centre fuselage as rapidly as is compatible with good duct lines. The Tornado is a good illustration of the elongated \"S\" shape commonly employed. Inlet/engine compatibility can be affected quite significantly by the design of the subsonic diffuser, which may either reduce or amplify flow distortion.The critical parameter is usually duct length, though using extra length to achieve compatibility is expensive in weight. Nevertheless, when the intake and duct shape require large geometry changes (e.g.the F-lll which also lacked adequate boundary-layer handling), then increasing duct length may be the only acceptable way of reducing flow distortion if vortex generators are inadequate.

The F-16 has a relatively long diffuser, the duct length from inlet throat to engine facebeing 5.4 times the engine face diameter It has large-radius turns and an area variation to produce a near linear Mach number chang ealong the duct, resulting in reduced diffuser losses The aircraft was originally proposed with the much longer duct shown in Fig 121 but shortening it not only saved duct and fuselage weight 18kg/m) but also offered a saving in vertical tail weight, a result of the reduced destabilising effect of the smaller forward side area. The weight savings more than offset the added drag of the boundary-layer diverter, the mission radius increased by 40km and the area distribution improved.
A few quotes from Ray Whitford's book for consideration. You can contact me via email - [email protected] - or my
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.
 

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