J-20 Inlet Discussion


Inst

Senior Member
I'm starting up a new thread, because I'm evaluating the hypothesis that the J-20 is already capable of supercruise, using only the AL-31 / WS-10.

Of course, if the J-20 were capable of supercruise, why haven't we been told this yet? Why hasn't it been leaked that the J-20 is supercruise capable?

A few hypotheses:

1. The null hypothesis is simple, the J-20 can't supercruise under its present engines. For the J-20 to be able to supercruise, it needs an intake-engine combo that can bypass the Mach barrier's drag spike. The combination of the J-20's current intake and the WS-10 / Al-31 is insufficient to present sufficient thrust at altitude.

2. The J-20 already can supercruise, but it's being kept mum for strategic reasons.

3. The J-20 has pseudo-supercruise or quasi-supercruise, i.e, it can supercruise at Mach 1.2 after using afterburners to break the Mach barrier, or it can supercruise below Mach 1.5, which is how the US Air Force is defining supercruise these days.

Any of these hypotheses is true, but I think the most tenable ones are 1 and 3. Hypothesis 1 is "absence of evidence implies evidence of absence", in the non-logically rigorous sense (absence of evidence is not evidence of absence). 3, likewise, combines what we know about the J-20 (it's using the WS-10 simply as a stopgap engine and is a low-aspect ratio fighter with strong high-speed maneuverability).

To pursue hypothesis 3, however, we need to look at some interesting facts about the J-20 design that have heretofore been ignored. That is, inlet length. The J-20, for instance, has a particularly long inlet length relative to its body. This is a trait you don't see on the Su-27, and a trait you see moderately on the F-22 and moreso on the Eurocanards.

(To be continued)
 

Inst

Senior Member
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Discussing the inlet lengths, we get this:

The rough total inlet + engine length is about 13.6 meters using the length of the J-20 as 20.4 meters long.




The AL-31 is roughly 5 meters long. The WS-10 is known to be slightly shorter, although I haven't been able to find engine length information. The Snecma M88 that powers the rafale is believed to be about 3.5 meters long, while the EJ200 is about 4 meters long. The F119 behind the F-22 is slightly over 5 meters, while the F135 in the F-35 is about 5.6 meters.

This gives us about 8.6 meters for the length of the J-20's inlet.
 
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Inst

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The next step is to estimate the inlet+engine length of other fighters. Let's start with the F-22, a proven supercruising design.



Here' it's 172 / 318 pixels, with the F-22 known to be 18.9 meters or about 10.22 meters for the inlet+engine combo, or about 5 meters for the length of the F-22's inlet.
 

Inst

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Here's the F-35, an aircraft we know to be capable only of "pseudo-supercruise", i.e, it can break the Mach barrier with afterburners, then use dry thrust to sustain supersonic speed at about Mach 1.2 or Mach 1.3




This comes out to roughly 8.8 meters, which, when combined with the F135's known length, gives you about 3 meters of inlet length.
 

Inst

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Now let's evaluate the Eurofighter, which is known to be capable of quasi-supercruise at Mach 1.4 with an air-to-air loadout:




The Eurofighter has roughly 9.7 meters of engines + inlet, or roughly 5.7 meters of inlet length.
 

Inst

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And for the Rafale, which is also claimed to be Mach 1.2-1.4 cruise capable with an air-to-air loadout:




We get about 8.6 meters, with an inlet length of about 5.1 meters once the Snecma M88s are subtracted.
 

Inst

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For comparison purposes, we should also include the Su-27 / J-11. This aircraft uses about the same engines as the J-20, but we know the Su-27 is not supercruise capable.




This gives us an engine + inlet length of 8.58 meters, or an inlet length of about 3.6 meters.
 

Deino

Lieutenant General
Staff member
Super Moderator
Discussing the inlet lengths, we get this:

The rough total inlet + engine length is about 13.6 meters using the length of the J-20 as 20.4 meters long.




The AL-31 is roughly 5 meters long. The WS-10 is known to be slightly shorter, although I haven't been able to find engine length information. The Snecma M88 that powers the rafale is believed to be about 3.5 meters long, while the EJ200 is about 4 meters long. The F119 behind the F-22 is slightly over 5 meters, while the F135 in the F-35 is about 5.6 meters.

This gives us about 8.6 meters for the length of the J-20's inlet.
Sorry ... but you really want to start an earnest highly technical discussion based on that drawing?? :eek::confused:o_O
 

Inst

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Sorry ... but you really want to start an earnest highly technical discussion based on that drawing?? :eek::confused:o_O
I need some measurements to get this started. The objective fact is, from any given set of pictures of the J-20 the J-20 has much longer inlets than any other 5th gen fighter in existence.

Some of this inlet length is used for stealth; i.e, the S inlet that blocks engine reflections. However, the longer the inlet is, the more distance you have to stuff in devices to bleed off boundary layer air, to compress air to increase relative pressure, and to decompress air (these are contradictory, I know), to slow the air down so prevent engine stalling due to supersonic shock.

The obvious reverse effect is poorer subsonic performance; i.e, the J-20 engines have to pull harder to get the first few gulps of air so that when turbine suck isn't being dominated by airspeed blow, the J-20 engines are going to be oxygen-starved. Which helps to explain why some fighters prefer very short air intakes; supersonic performance is less crucial than subsonic maneuverability.
 
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Deino

Lieutenant General
Staff member
Super Moderator
Honestly ... You "need some measurements to get this started" but if you want to make it correctly by comparing length of and ratio between esp in order to compare and the deduct whatever you want, I would never start with such a crappy drawing, which is all wrong!??
 

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