J-20 Inlet Discussion

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

  1. ougoah
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    ougoah Senior Member
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    @Inst could you explain the inlet engine matching topic you are presenting? I fail to see what you are trying to say but I'm interested. You have quoted that "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] " but how does that show "how important inlet-engine matching is"??
     
  2. Inst
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    Inst Senior Member

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    @secretprojects

    Strongly suggest that you don't post your e-mail on the public internets, or if you do, have very good spam protection. Thank you for the quotes, though.

    The French source also suggests that the long subsonic diffuser reduces distortion (i.e, air becomes more even and easier for the compressor to process without stalling or seeing other engine malfunctions).

    ===

    I'd also disagree with your reading on the section of "the amount of air ingested by an engine does not depend directly on intake size or forward speed".

    https://www.quora.com/What-is-the-ramming-effect-in-jet-engines

    One way to think of it is that the engine is trying to suck in a constant amount of air, presumably slowed down by the diffuser of the inlet into a processable form. At high altitudes and low speed, the inlet is choking the engine because the amount of ram air entering the inlet is insufficient for the engine to get sufficient pressure and oxygen for function.

    At low altitudes and high speeds, on the other hand, the inlet is drowning the engine because the ram air is excessive. The ram air is beyond what the engine can process, and as mentioned in your quote, the excess airflow is being either expelled (creating drag) or stalling the engine.

    The subject of discussion concerning the J-20's inlet is whether there's a deliberate attempt to create excess air flow at low altitudes and high speeds (i.e, worse low-speed performance due to induced drag) so that at high/medium altitude and high speeds the engine isn't choking, or at least is choking less.

    Put in other words, the engine has a maximum air input which, if exceeded, damages the engine (MiG-25) or stalls it. There's only so much air the engine can process.

    On the other hand, the "engine-inlet system" (I emphasize the difference) does not have the same maximum air input. In the F-22, for instance, there's overpressure plates that bleed off excess air because the inlet is fixed, keeping the available air to the engine at stable quantities where the F119 can maximize its performance.

    I assume you continue to disagree with this interpretation, but the interesting point is, how and why?

    ====

    @ougoah

    https://www.quora.com/Why-does-the-...-designated-as-F110-100-is-mounted-on-an-F-16

    There's a few Quora posts discussing the Navy's refusal to upgrade the F-14's inlets. There are some claims online that the TF-40 provided a higher maximum speed on the F-14 than the F110, despite the increased thrust.

    You can Google for how the F-14 with F110 wasted much of the F110's potential due to a refusal to redesign the inlets (which is reasonable, given that the F-14 was out of production, and that the Iranians went rogue and presented a parts control issue).
     
    #52 Inst, Oct 16, 2019
    Last edited: Oct 16, 2019
  3. secretprojects
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    secretprojects New Member
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    The problem I have is that it makes no logical sense.


    1) This is purely an assumption on your part. When evidence suggests this condition actually occurs in a real fighter and is the cause of lower thrust at high altitude?
    2) What relevance does high altitude and LOW speed engine performance have for supercruise optimisation? At altitude and Mach 0.9+ engines typically need smaller intakes than they do at low speed - typical fighter inlets are sized for optimal operation at precisely high subsonic speeds, which is smaller than they would ideally be at low altitude and slow speeds. That's why they sometimes have blow-in auxillary intakes, though as Whitford suggests an undersized intake can ingest larger volumes anyway if the engine is turning and sucking the air in. At very high speeds, lots of the air entering the intake is typically dumped straight out of bypass doors because it's not needed by the engine. Note that the Lightning with its seriously undersized intake was designed that way to optimise it for low drag at supersonic flight. I don't see how an argument can be constructed that a extra large intake is some kind of supercruise adaptation.
     
  4. Inst
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    Inst Senior Member

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    1.
    https://aviation.stackexchange.com/questions/16727/how-does-turbojet-thrust-change-with-altitude

    Via Stack Exchange.

    So you have a few things going on here.

    -First, exhaust velocity is relatively fixed, although increased pressure increases the velocity further.
    -Second, theoretical max thrust decreases as the aircraft goes faster.
    -Third, mass flow to the engine decreases dramatically with altitude, holding speed constant.

    To put into perspective how air density (and thus mass flow) changes with altitude, check out this graph:

    [​IMG]

    2. I'm just using it as an example of how an engine-inlet system can choke.

    https://www.grc.nasa.gov/www/k-12/airplane/thrsteq.html

    mass flow rate = density * velocity * area.

    Note that mass flow rate is linear with respect to velocity, and not a square.

    An engine has a fixed mass flow rate requirement, but the actual mass flow rate is going to vary depending on speed and altitude.

    The practical issue with inlet design for supercruise is that you have three fields of control; the speed of the aircraft, the variability of the inlet, and the fixed design of the inlet (with the variability being a component).

    With air density varying by 400% (~25% atm at ~10,000 meters), designing inlets for supercruise with bad engines means that you want near-optimal mass flow rate at a given altitude and speed.

    On the other hand, the mass flow rate creates a separate limitation. If your mass flow rate through the inlet is too high, you'll have to bleed off the air, creating drag instead of thrust. This could, for instance, make it impossible to take off if you're optimized for too different a flight regime than ground conditions. And even if you do take off, your lift to drag could make it impossible to ascend to sufficient altitudes, although the situation should improve as you get higher.

    ====

    Just to put things in perspective again, this is the Su-27 AL-31 dry thrust graph at various altitudes:

    [​IMG]

    What kind of speed and altitude does the Su-27 AL-31+inlet combo seem optimized for? The AL-31 and its inlets seem very much optimized for low-altitude cruise when dry. The situation likely changes dramatically when afterburners are turned on, however.

    More likely, an insufficient mass flow rate and low dry thrust at altitude is actually a designed-for feature, since lower thrust implies lower fuel consumption and thus more efficient cruise.
     
    #54 Inst, Oct 16, 2019
    Last edited: Oct 16, 2019
  5. Inst
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    Inst Senior Member

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    One last possibility is that you're assuming oxygen content is varying with altitude and that's affecting engine performance.

    Nope, I checked. First thing I thought of as well.

    https://www.wildsafe.org/resources/...itude-safety-101/high-altitude-oxygen-levels/

    ====

    Amusingly, in complaining about the thread merger, I mentioned that "one possibility for the J-20's anemic displayed aerodynamic performance is that the aircraft's inlet induced drag at low altitude is what's actually preventing us from seeing the J-20 layout's true potential". Most of the airshow stuff is done at low altitude, i.e, when, if we assume the J-20's inlet is designed for the WS-15, and is otherwise optimized for supercruise at high altitudes with the WS-10 / AL-31, is going to involve a lot of induced drag from the inlet having to bleed off too much air.
     
  6. Inst
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    Inst Senior Member

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    One last thing, if we assume the J-20 inlets deliver about 50% more mass flow than the Su-27's, then cross check it with atmospheric pressure graphs (this one should be slightly more accurate: https://www.engineeringtoolbox.com/standard-atmosphere-d_604.html ), what we see is that the J-20 with Al-31 effectively functions as though it jumped 1 line in the Al-31 graph, i.e, mass flow rate at 11 km is now equivalent to 8 km, and so on.

    When we go back to the J-20 chart:

    [​IMG]

    We know for a fact that the J-20 can't quasi-supercruise, because it can't breach the Mach barrier in level flight. However, it can definitely turn on afterburners to supercruise, or alternatively, it can dive to break the Mach barrier. I.e, the J-20, if we assume mass flow rate is the determinant of high-altitude engine performance, and that the increased inlet size allows the J-20 to jump a level in thrust, the J-20 can pseudo-supercruise like the F-35 and achieve a max speed of around Mach 1.4 at 11,000 meters altitude.

    Using the Al-31 chart and the pressure chart, what we can also assume is that the thrust of the J-20 at about 5000 meters (about 16000 feet) is about the same as it is at 2000 meters, using the pressure chart. Below 5000 meters, however, the J-20 begins to drown as its inlets are going to bleed out a lot of air.

    For an airshow altitude comparison, the USN Blue Angels fly at a minimum height of 50 feet, or about 15 meters. The highest altitude the Blue Angels do, on the other hand, is about 3000 meters.

    In other words, if the J-20 inlet-engine situation is as I speculate, the J-20 literally can't perform at airshows because its inlet-engine combo bleeds too much at low altitudes and is forced to resort to ultra-low speeds to avoid engine bleed.

    The J-20 aerodynamic formula might have sacrificed low speed agility (Mach .3) etc, so we don't see anything interesting there. For comparison purposes, the F-16A has peak ITR at around Mach .6 at 5000 ft / (2000 meters).
     
  7. Inst
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    Inst Senior Member

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    There's some point in studying the DSI. Arguably, the Chinese have the most experience with DSI to date given that they've put out two production fighters (JF-17, J-10B) that utilize a DSI and a third DSI fighter is between development and mass production (J-20). The Americans, in contrast, only have F-16 experimental DSI and the F-35 fighter.

    One theory I'm investigating is whether the J-20 might use a dual DSI system. A conventional single DSI, according to the French source, is a Pitot-type inlet. These inlets need "supersonic compression", i.e, shockwaves need to be generated to reduce air pressure. The DSI does this once, removing the need for an splitter plate or an intake cone (both shockwave generators) as well as performing the functions of a splitter plate (boundary layer remover). However, while DSIs have good performance subsonically and adequate performance up to Mach 2, if you need an aircraft that can perform beyond Mach 2, a single DSI is not sufficient to slow the air past Mach 2, if the DSI doesn't fail completely (i.e, fails to divert boundary layers or fails to generate shockwaves).

    Given that information on the J-20's inlet system is so sparse, we have to resort to studying that on the F-35, which is known to have performance issues beyond Mach 1.5 (which is what the EADS VP was warning about concerning Pitot-type inlets).

    Pictures of F-35 inlet ducts:

    https://news.northropgrumman.com/ne...f-35-air-inlet-duct-from-key-turkish-supplier

    [​IMG]

    Notice the flat shape of the inlet duct. In the J-20, a flat inlet duct is achievable because of the shape of the weapons bay:

    [​IMG]
    [​IMG]
     
  8. Inst
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    Inst Senior Member

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    Reading up on DSI, the problem with DSI is that mass flow rate as a function of velocity flatlines at Mach 1.

    https://www.grc.nasa.gov/WWW/K-12/rocket/mflchk.html

    The net effect is that as velocity continues to increase, the only effect is that density decreases as in m dot = r * V * A.

    A variable inlet, in contrast, can modify its geometry to increase total area. Things like feeders along the air inlet also become more important (even if they destroy stealth) because that's the only way you can further increase mass flow rate.

    Basically, one interesting thing is that I've been assuming the DSI bumps are ignored (i.e, create an area of low pressure to reaccelerate boundary layer air). But that actually states that in practice, the DSI, if it creates supersonic shockwaves, is actually blocking the inlet. (in other words, we go to .70-.80 m^2 area for the J-20 inlet from around .95 m^2. Repeated measurements of the F-22 inlets imply about 1.05 m^2 on a fixed inlet that lacks DSI bumps for engines operating in 110-160 kN).

    A further fact is that the mass air flow is bottlenecked effectively by any region in the inlet duct that goes to Mach 1.
     
  9. Air Force Brat
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    Thank you, there are work arounds to minimize the fixed inlets minor performance issues... and those Chinese engineers have a lock on this!
     
  10. Inst
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    Inst Senior Member

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    The way I'm parsing this is as follows:

    Compare the F-35 and X-35 inlets:

    [​IMG]
    [​IMG]

    Also consider the SR-71 inlet:

    [​IMG]

    Inlet area DOES matter, but mainly for subsonic speeds where the air velocity is less than Mach 1.

    https://www.grc.nasa.gov/WWW/K-12/rocket/mflchk.html

    Mass Flow Rate = Density * Velocity * Area.

    For an efficient inlet, the velocity can be assumed to be capped at Mach 1, i.e, the inlet entrance or its foresector is a flow choke. However, density is a modifiable variable beyond normal air speed functions depending on the compressor / supersonic diffuser devices employed.

    I.e, in the case of the SR-71 inlet, the external compressor, depending on airframe speed, allows it to screw with the density value partially independent of velocity.

    Expressed differently, there's actually two Mass Flows Rate at work, because of two separate density formulas.

    Mass Flow Rate Outside Duct = Independent Density * Velocity * Area

    Mass Flow Rate Inside Duct = Dependent Density * Velocity * Area

    Outside the duct, density is not a function of the velocity of mass flow rate because until the air gets into the duct or into the object's airstream, the velocity of the air is 0 or near 0. Once it gets inside the duct / airstream, however, the air becomes accelerated and its density changes as a result of the stream.

    The role of the external compressor, then, is to slow down the air as quickly as possible while obeying the first equation before it begins to obey the second equation. But that still implies a mass flow rate outside the duct limiter on the inlet duct itself, #1, #2, the effectiveness and efficiency of the compressor at a given mass flow rate is going to affect the mass flow rate inside the duct itself.

    ===

    This also means my assumption regarding DSI as an inlet limiter is incorrect. At supersonic speeds, the DSI bump works as an external compressor acting on the air OUTSIDE the inlet area, i.e, the roughly .95 m^2 bumpless inlet area of the J-20 can be used at supersonic speeds. At subsonic speeds, the DSI bump may continue to compress air and increase the effective inlet area and the total mass flow rate.
     
    #60 Inst, Oct 17, 2019
    Last edited: Oct 17, 2019
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