J-20 Inlet Discussion

Discussion in 'Air Force' started by Inst, Oct 12, 2019.

  1. Inst
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    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.
     
  2. Jura
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    Jura General

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    TLDR
    isn't this thread about semantics?
     
  3. Bltizo
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    Bltizo Moderator
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    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.
     
  4. secretprojects
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    secretprojects New Member
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    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 Ray Whitford's "Design for Air Combat" (there are scanned copies floating around on the internet) which has a detailed chapter on exactly this.
     
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  5. Inst
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    Inst Senior Member

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    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.
     
  6. secretprojects
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    secretprojects New Member
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    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.
     
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  7. Inst
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    Inst Senior Member

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    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.[3] " 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.
     
    #47 Inst, Oct 16, 2019
    Last edited: Oct 16, 2019
  8. Inst
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    Inst Senior Member

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    @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.

    http://www.icas.org/ICAS_ARCHIVE/ICAS2002/PAPERS/643.PDF

    From EADS Research VP.
     
  9. secretprojects
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    secretprojects New Member
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  10. secretprojects
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    secretprojects New Member
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    A few quotes from Ray Whitford's book for consideration. You can contact me via email - [email protected] - or my forum.
     
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