J-20 Inlet Discussion


Inst

Senior Member
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  • #11
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!??
I don't see the point..I drew this out because it had the LERX shown in 2011 and later models, whereas other diagrams tend to have round LERX as on the 2001.

But even if you go by photographs, you still end up with an inlet only length in the 7-8 meter range.

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I think the overall point of this thread is to explore what the inlet means and how it applies to the J-20's operating characteristics.

Check out this NASA page:

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Here, the difference between a supersonic and a subsonic inlet is defined. In a supersonic inlet, you want more length so you have more space to bleed out the boundary layer and dissipate shockwaves. For subsonics, on the other hand, you want as short an inlet as is practical for the reasons given above.

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The long J-20 inlet is suggesting one or both of two things: first, the J-20 can supercruise if its inlet can keep the AL-31 / WS-10 working at high efficiency. You only need about 50 kN thrust per engine to break the Mach barrier,at altitude, and the Al-31 / WS-10 should have around 75kN base thrust. But that's a sea-level rating, not a rating at altitude,

Second, the long inlet design is an optimization for high-speed fighting, not low-speed fighting. The longer your intake is, the more your engines are going to have to suck, and until your suck has been overwhelmed by speed-dependent blow, you're at a disadvantage.

You know what other aircraft has long intakes?

.

The point being, regarding the MiG-31, is that its inlets are designed to maintain usable airflow to its engines at high speed, i.e, it draws in a lot of air.to work as altitude, and is long enough to absorb multiple supersonic shock waves. The J-20's inlets are somewhat smaller, but it has a similar length to that of the MiG-31, with the latter having around 14 meters of length
 

Deino

Lieutenant General
Staff member
Super Moderator
...
I think the overall point of this thread is to explore what the inlet means and how it applies to the J-20's operating characteristics.
...
Now I understand ... you are only back on track to prove, the J-20 is an interceptor!
 

secretprojects

New Member
Registered Member
This is some kind of 'cargo cult' method of aerodynamics. Correlation does not equal causation. Very odd.
Intake length varies as a tradeoff between many factors including weight and complexity, engine stability requirements, and stealth. Longer intakes help conceal engine compressor faces with s-ducts.

Supercruise is a function of available thrust versus drag.
 

Inst

Senior Member
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  • #15
This is some kind of 'cargo cult' method of aerodynamics. Correlation does not equal causation. Very odd.
I'll respond to Deino at the same time I respond to you.

My original interest was in seeing whether the J-20 could still supercruise, even though the Al-31 can't supercruise on the Su-27. So I'm focusing on the differences; the Su-27 has a short inlet, while the J-20 has a long inlet.

As for the interceptor claims; I'm sure most people are aware that I don't like characterizing the J-20 as an "air superiority fighter" and would prefer fighter-interceptor due to its leaked characteristics of "good subsonic maneuverability, excellent supersonic maneuverability". This has gotten very histrionic as posters on this board get very insulted when people claim that the J-20 isn't an air superiority fighter in the same mode as the F-22 or Su-57.

People here know what I think, as well as what others think, so to diffuse the situation, I'll also point out that the Rafale and Eurofighter have relatively long intakes for their size.

Simply having long intakes does not make for an interceptor, as the former two show, but they do seem to correlate to good high-speed performance, as the Eurocanards are supercruise capable, the F-22 is supercruise capable, the MiG-31 is known for its high cruising speeds (but not supercruise, as it requires afterburners).
 

Inst

Senior Member
  • Thread Starter Thread Starter
  • #16
Intake length varies as a tradeoff between many factors including weight and complexity, engine stability requirements, and stealth. Longer intakes help conceal engine compressor faces with s-ducts.

Supercruise is a function of available thrust versus drag.
But available thrust is also a function of inlet design; i.e, we can go back to the F-14, which was reengined mid-life to much higher thrust, yet its max speed didn't change as the inlets were never modified. The main changes were in maneuverability and acceleration due to the increase in thrust.
 

ougoah

Senior Member
Registered Member
I reckon we give the supercruise speculations a well deserved rest. Unless one member can prove they really know what they're talking about, it's pointless to say an intake length of x distance allows for/ does not allow supercruise. I'm certain it's not quite as simple as that even if patterns can be observed. This stuff is barely in conjecture territory. If someone comes up with meaningful observations, please share but so far there's an absolute absence of quality speculation when it comes to J-20 supercruise.
 

latenlazy

Colonel
I reckon we give the supercruise speculations a well deserved rest. Unless one member can prove they really know what they're talking about, it's pointless to say an intake length of x distance allows for/ does not allow supercruise. I'm certain it's not quite as simple as that even if patterns can be observed. This stuff is barely in conjecture territory. If someone comes up with meaningful observations, please share but so far there's an absolute absence of quality speculation when it comes to J-20 supercruise.
Pardon my French, but bullshit can’t substitute for physics.
 

Inst

Senior Member
  • Thread Starter Thread Starter
  • #19
I reckon we give the supercruise speculations a well deserved rest. Unless one member can prove they really know what they're talking about, it's pointless to say an intake length of x distance allows for/ does not allow supercruise. I'm certain it's not quite as simple as that even if patterns can be observed. This stuff is barely in conjecture territory. If someone comes up with meaningful observations, please share but so far there's an absolute absence of quality speculation when it comes to J-20 supercruise.
We know that the J-20 was designed to be able to supercruise with inferior engines. Since available thrust at a given altitude is a function of both the engines and the inlets, and the J-20 is currently trapped with the WS-10 / Al-31, it stands to reason that if Chengdu wanted to give the J-20 supercruise, it would resort to screwing with the inlet duct to increase available power.

From the drag calculations we've seen, the J-20, if its Al-31 or WS-10 were to operate at sea-level dry thrust at altitude, it could easily hit Mach 1.6. But it won't, because the Al-31 / WS-10 end up being oxygen-starved and there's not even enough dry thrust to break the Mach barrier.

Since Chengdu can't fix the engines (that's SAC's job), it can still try to give the engines as much support as possible via the inlet.

You can't possibly deny that the J-20 has one of the longest inlets of any fighter in existence, matched or surpassed mainly by the MiG-31. That's an interesting feature that people haven't touched upon recently, although it'd probably come up in discussions of the J-20's stealth (S-duct).
 

secretprojects

New Member
Registered Member
Actual real relevant factors for supercruise can be found in
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The article by Song Wencong on J-20 configuration includes several relevant factors.

Due to the future fighter's requirement for supercruise, supersonic drag characteristic is a critical design point and designers must avoid using aerodynamic measures that may potentially increase supersonic drag. As a result, the wing shape and wing twist coefficient can't be selected based on trans-sonic lift to drag characteristics alone. It is necessary to employ wing-bending mechanisms but its aerodynamic efficiency has already been exhausted.

Further decreasing the aircraft's longitudinal relaxed stability is an excellent solution to this problem. Diagram 1 shows how the variation tendency of trim-drag coefficients against longitudinal instability of a conventional fighter aircraft in a tight, sustained turn. Modern fighters fix their longitudinal instability at 3% the average aerodynamic chord length. The future fighter could enjoy a significant improvement in lift-to-drag if the longitudinal instability could be increased to a magnitude of around 10%.

...

Supersonic drag characteristics

The key to lowering supersonic drag is to minimize the max cross sectional area of the aircraft. Accomplishing this requires excellent high level design skills. Placement of the engines, engine intakes, landing gears, cartridge receiver, weapons bay, and main structural support all influence the max cross sectional area of the aircraft. Attention to details and careful considerations are necessary to design decision making.

Wing shape has profound effects on supersonic drag characteristics. Small aspect ratio wings with large backsweep have low supersonic drag but are detrimental to low speed lift and trans-sonic lift to drag characteristics. If we select the liftbody LERX canard configuration we can expect to retain relatively good lift to drag characteristics while using medium backsweep wings. Under high AOA conditions, liftbody LERX canard configuration aircraft concentrate lift on the body and inner portions of the wings so moderately lowering the aspect ratio will not only not lower the max lift coefficient but raise it (see figure 10). Because of this, employing small aspect ratio wings on a lift-body LERX canard configuration aircraft will settle the conflicts among supersonic drag characteristics, low speed lift characteristics, and trans-sonic drag characteristics.
Supersonic drag is largely wave drag. The key to minimizing wave drag is minimal frontal area (cross section), excellent area ruling and a high fineness ratio. J-20 has a higher fineness ratio than any of its rivals.

Delta wings are a good choice for low supersonic drag.

Inlet length is not a good indicator of supercruise. Just ask Boeing... 2707 SST....
2707-200-085-1600x915.jpg
 

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