J-20 Inlet Discussion

The other key to supercruise is suitable engines. You need a high dry thrust, and typically this is achieved within a certain physical size by using a lower bypass ratio so more of the air mass flow is going through the core. Proportionally, the core is larger compared to the overall engine size.

Olympus 593 none (turbojet)
F119 - 0.3:1
M88 - 0.3:1
EJ200 - 0.4:1
F125 - 0.57
AL-31F - 0.57
F110-GE-129: 0.76

The key factor to look for in WS-10 and WS-15 is not thrust per se (Concorde can cruise at Mach 2 with a much lower thrust to weight) but bypass ratio. If the engine is designed for optimal supercruise it will be in the area of 0.3:1.

secretprojects said:

The other key to supercruise is suitable engines. You need a high dry thrust, and typically this is achieved within a certain physical size by using a lower bypass ratio so more of the air mass flow is going through the core. Proportionally, the core is larger compared to the overall engine size.

Olympus 593 none (turbojet)
F119 - 0.3:1
M88 - 0.3:1
EJ200 - 0.4:1
F125 - 0.57
AL-31F - 0.57
F110-GE-129: 0.76

The key factor to look for in WS-10 and WS-15 is not thrust per se (Concorde can cruise at Mach 2 with a much lower thrust to weight) but bypass ratio. If the engine is designed for optimal supercruise it will be in the area of 0.3:1.

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You're sort of ignoring the existing conversations, and that's irritating. We've gone over several times on how the J-20 is designed as a low-drag fighter with less compromises in maneuverability; the LERX-Canard-LERX-Delta formula was found to enable strong lift at high AoA while keeping a short fineness ratio.

The question being presented right now is about how the inlet / engine combination can deliver more dry thrust at altitude.

Your position is basically that it's all in the bypass ratio, but the bypass ratio isn't everything. If, for instance, an aircraft engine was forced to "suck" through a literal straw, the engine wouldn't have enough airflow to operate. On the other hand, if the engines were given considerable excess airflow, even a high bypass engine could supercruise.

The challenge, however, is getting the pressure down through the inlet so that the engine doesn't meet excessive pressure at the airflow it's getting. This arguably results in complex inlet geometry to compensate for this loss.

I also want to mention that the J-20 is designed both for the 130-140kn WS-10 and Al-31, as well as the 170-180kn WS-15. There's also a significant difference in both aircraft's dry thrust, being around 100+kn in the WS-15's case vs 75kn in the WS-10 and Al-31's case. This implies that the WS-15 needs more airflow than the WS-10 does, and that if the J-20's inlets are designed to fully feed the WS-15, it will have excess airflow for the WS-10 which could potentially be used to force supercruising.

I thought some basic facts were useful in this new topic you created. You don't seem to be a fan of facts however. There's a bit of wiggle room in terms of mass flow rate for a given intake design but an oversized intake would typically reduce performance not increase it. Your intake must be matched to deliver the correct amount of air that the engine can handle.

Its likely that the WS-15 doesn't need a higher mass flow rate (that's the technical term for airflow) because it is designed to fit in the same diameter as the AL-31F , but has a much lower bypass ratio. This means more of the airflow goes through the high speed compressor section and less just goes around it, effectively giving a larger engine inside the same overall diameter. Now, due to having less bypass air to feed into the afterburner, the percentage increase in thrust in afterburning will be lower, and fuel consumption in economic cruise will likely be higher. That's the tradeoff for a low bypass ratio.

If your figure of 170-180kN is correct, and I am correct that it has a bypass ratio in the region of 0.3:1, then you'd expect military thrust to be in the region of 113kN -120kN.

I don't have a figure for AL-31FM2 mil thrust, but lets guess its 76.2 (original AL-31F )+ 15-20% - that would mean about 87-3 - 91.44 kN. This means mil thrust of WS-15 could be 23% - 37% greater than with AL-31FM2. That level of thrust increase is going to be tranformational for supercruise.

@secretprojects :

The issue is thrust at altitude. Drag increases the lower you go, and many aircraft can't break Mach 1 at sea level, even with afterburners. Via Trident:

https://www.sinodefenceforum.com/attachments/al-31f-p037-png.48266/

The apparent issue is that the Al-31F can't get enough air at 10,000 meters, where the Vtech study suggests a drag of 20,000 lbf (89 kN) to hit Mach 1 (the Vtech study isn't perfectly accurate and works with about 15,500 kg for the J-20's weight, when I tend toward 17,750 kg). Practical thrust using the Su-27's inlet gets you to about 30kn per engine, or a total of 60 kN force, which isn't enough to supercruise.

So what I'm suggesting is that the J-20's inlets could be designed to have a higher mass flow rate so that when altitude drops air density, you still have enough airflow to keep the Al-31 / WS-10 at near sea-level thrust. This means you sacrifice low-altitude performance or otherwise have to bleed out a substantial amount of intake air.

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About the WS-15, it's designed to fit the J-20 and have approximately the same dimensions as the Al-31 / WS-10, but it's not going to be precisely the same. I think I read somewhere on this forum that they've had to put in spacers to get the Al-31 to fit, so the WS-15 will likely have a greater diameter / length than the Al-31.

Via this resource, we know that at 35,000 feet (what the Vtech study used for drag), you have about a 68% drop in air density, and the other chart shows about a 58% drop in thrust. The entire point of going after the inlets is to see to what degree larger inlets + diverters / bleeders can give you performance equivalent to lower-level performance at altitude.

The most favorable calculation I have of the J-20's inlets is about 30% greater than that of the Su-27. That gives you about the equivalent of the Su-27 AL-31F's performance at 27,500 ft, or about 8382 meters. This gets you out to 37.5 kn, or about 75 kn of actual thrust off the Al-31Fs, which is still below the thrust needed to break Mach 1.

I'm trying to ask around for precise calculations of the J-20's intakes alongside that of the Su-27, so maybe once we get that we might get more results.

That's... not how jet engines work.

secretprojects said:

That's... not how jet engines work.

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In what regard? When you brought up the dry thrust figures of high-bypass vs low-bypass, my eyes glazed over because the issue isn't the level of dry thrust, but the level of dry thrust at altitude. Yes, low-bypass fares better at high altitudes compared to high-bypass, but the point remains that you can't just compare dry thrust at altitude in a low bypass ratio bird to a dry thrust at altitude in a high bypass ratio bird and assume the performance increases are linear.

secretprojects said:

I thought some basic facts were useful in this new topic you created. You don't seem to be a fan of facts however. There's a bit of wiggle room in terms of mass flow rate for a given intake design but an oversized intake would typically reduce performance not increase it. Your intake must be matched to deliver the correct amount of air that the engine can handle.

Its likely that the WS-15 doesn't need a higher mass flow rate (that's the technical term for airflow) because it is designed to fit in the same diameter as the AL-31F , but has a much lower bypass ratio. This means more of the airflow goes through the high speed compressor section and less just goes around it, effectively giving a larger engine inside the same overall diameter. Now, due to having less bypass air to feed into the afterburner, the percentage increase in thrust in afterburning will be lower, and fuel consumption in economic cruise will likely be higher. That's the tradeoff for a low bypass ratio.

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The J-20 has been designed around the larger core diameter of the WS-15, note that after 001, 002/004, the intakes were reshaped/reduced in diameter to reduce air flow volume and optimize the J-20 for the AL-31FN in the same overall redesign that clipped the tips and other minor design changes to optimize J-20 RCS/Performance overall.

Once the WS-15 engines are incorporated, the inlets will be enlarged and reshaped to optimize flow and WS-15 performance, giving the J-20 the ability to supercruise, J-20's performance will increase across the speed range with the WS-15 decreasing take off roll, increasing climb rate, and sustained turn rate..

While I myself am not convinced the WS-15 will incorporate thrust vectoring, many here seem confidant the WS-15 will incorporate OVT.

Air Force Brat said:

The J-20 has been designed around the larger core diameter of the WS-15, note that after 001, 002/004, the intakes were reshaped/reduced in diameter to reduce air flow volume and optimize the J-20 for the AL-31FN in the same overall redesign that clipped the tips and other minor design changes to optimize J-20 RCS/Performance overall.

Once the WS-15 engines are incorporated, the inlets will be enlarged and reshaped to optimize flow and WS-15 performance, giving the J-20 the ability to supercruise, J-20's performance will increase across the speed range with the WS-15 decreasing take off roll, increasing climb rate, and sustained turn rate..

While I myself am not convinced the WS-15 will incorporate thrust vectoring, many here seem confidant the WS-15 will incorporate OVT.

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Problem is, reshaping the inlets means that the stealth work has to be done all over again as the inlets are rather large and three-dimensional.

As far as OVT goes, the J-20 is already testing OVT on a WS-10 platform.


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On the general subject, let's mention the SR-71's inlets and how these contributed to thrust:



I'll acknowledge the SR-71 was a short inlet, but treating inlets as simply ways to get stable airflow into an engine is lazy and naive.