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DSI Intakes, Ferri, and China's J-20

pegasus

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F-16 with DSI vs basic F-16:




Is the inlet larger or smaller when DSI is added?

J-10A vs J-10B:


If you compare the fixed intake with boundary layer diverter aka BLD with DSI, you will get this conclusion:
Pressure recovery tends to decrease at supersonic speed at all conditions. This is due to the fact that the shock waves at the inlet in supersonic condition causes additional pressure loss and hence it results in lower pressure recovery as compared (Goldsmith and Seddon 1993, Mattingly 2002). This phenomena is quite similar in fixed intakes (Ibrahim, Ng et al. 2011).
The results revealed that BLD intake configuration is more effective in subsonic regime as compared to DSI configuration, whereas at supersonic speeds DSI configurations gave superior performance.



Comparative Flow Field Analysis of Boundary Layer Diverter Intake and Diverterless Supersonic Intake Configuration I. Arif† , S. Salamat, M. Ahmed, F. Qureshi and S. Shah


However consider the fixed intake can not compete with a Variable geometry one



1571435774633.png

So while the DSI intake on F-16 is marginally better than the BLD type at supersonic speeds, both are inferior to the one on F-15 at supersonic speeds, Su-57 thus uses one with variable geometry and several shocks

1571436028054.png

So J-10B/C are also slower compared to the original J-10A and show inferior acceleration near their max speeds compared to J-10A.

Su-57 has the ideal intake for high speeds and it will have better pressure recovery than both J-10C, F-35 and J-20, just because it has variable geometry intakes

1571437033884.png

Even with WS-15, J-20 will not be able to compete with Su-35 nor Su-57 in terms of supersonic acceleration, so the Chinese are overhyping J-20 supersonic ability, very likely because with WS-10, or Al-31 and a fixed DSI intake has not really the best acceleration compared to Su-35 and Su-57, thus the purchase of Su-35 was justified as an aircraft with higher acceleration than J-20
 
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Inst

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IIRC, the study on the J-10 DSI showed that it could reach 87% TPR at Mach 2.

When someone says DSI, what you should hear is "conical inlet with more computing power", because that's what it is. It's a fixed shockwave generator that has more complex geometries than a traditional conical inlet because computing power's come a long way, baby, and computational fluid dynamics allow the simulation of conical inlets that are not strictly cones.

A DSI can be optimized for any Mach number, hell, there's been attempts at creating DHI (hypersonic).

So just because the J-20 has a DSI, you shouldn't assume it's highly compromised at high Mach.

The real question I have is more, what's the effective inlet area on the J-20? It's an important question because that allows us to know how it comes to the Su-27 effective inlet area; i.e, can the J-20 get the AL-31 to supercruise or near-supercruise performance by providing higher MFR at altitude?
 

pegasus

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IIRC, the study on the J-10 DSI showed that it could reach 87% TPR at Mach 2.

When someone says DSI, what you should hear is "conical inlet with more computing power", because that's what it is. It's a fixed shockwave generator that has more complex geometries than a traditional conical inlet because computing power's come a long way, baby, and computational fluid dynamics allow the simulation of conical inlets that are not strictly cones.

A DSI can be optimized for any Mach number, hell, there's been attempts at creating DHI (hypersonic).

So just because the J-20 has a DSI, you shouldn't assume it's highly compromised at high Mach.

The real question I have is more, what's the effective inlet area on the J-20? It's an important question because that allows us to know how it comes to the Su-27 effective inlet area; i.e, can the J-20 get the AL-31 to supercruise or near-supercruise performance by providing higher MFR at altitude?
at Mach 2 a pressure recovery of 87% is not 13% less thrust, it is not linear but it grows exponentially, so a TPR of 87% is not 13% less thrust it can be 25% or 30% less thrust, less thrust means less acceleration, the computing power does not change physics, higher speeds means the DSI will ingest more boundary layer thus it will need bleeding system that is already rendering DSI useless, it means more weight, the caret intake was considered less efficient because for F-35 it weighed more due to the bleeding system, you like it or not F-35 has the ideal intake and shows the limits of DSI fixed geometry intakes.


J-20 if overscan is right has 2 or at least one bleeding system on the cowl intake, this means it has design limits and making a bigger bump means more drag, thus they used a bleeding system, fixed geometry has limits, and DSI only has advantages because is fixed you might not like it but it is the truth, it has no mechanical parts no bleeding system, why it is better? simply it weighs less, disadvantages well it has its best performance at Mach 1.6, the Americans do not lie, F-35 is slower for such reason


In the graph you can see F-15 has TPR higher than 90% at mach 2, SR-71 has even higher TPR at Mach 2, this happens because they have variable geometry intakes

1571472037206.png
Supersonic DSI can not work well in all flight regimes, the Mach design limits means it is optimised for a given Mach number, the intake throat and capture area are not fixed that is the reason you have variable intakes that change capture area and intake throat area and auxiliary intake doors and bypass doors.

This scheme shows how a Mach 3 mixed compression intake works

1571473313663.png
 
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overscan

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J-10's DSI intake is designed for Mach 2.0, not 1.6. I expect J-20 to be the same.
 

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D50D175E-7D01-47F4-A6DE-89389FE3F945.jpeg

FF262742-EF94-4259-B5F5-BD6FF914E4CE.jpeg

现在即将装备的 J-10B,对腹部进气布局的 Bump 进气道的鼓包和进气唇口进行了修改(唇口截面改得更方了),2.05Ma 时出口平均总压恢复系数接近 0.9,是高空高速大马赫数下的推力增加约 4% 的主要方面
The J-10B being inducted has the DSI intake modified to sharpen the corners, leading to pressure recovery coefficent approaches 0.9 at M2.05, increasing the speed (as compared to J-10A) by 4%
 

pegasus

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J-10's DSI intake is designed for Mach 2.0, not 1.6. I expect J-20 to be the same.
they can still work well up to Mach 2, same is F-16 with its fixed Boundary Layer Diverter Intake , the efficiency is not as good as at Mach 1.6, can it reach Mach 2 yes it can but the efficiency is much lower, J-10B/C has very likely an intake optimised for Mach 1.7 too, why you can know that? well at low speeds capture area and throat area usually requiere a bigger mass flow, some intakes use auxiliary air intakes, since the intake is fixed, has no mechanical parts, no bypass nor bleed devices it does not work in the whole flight envelop well, thus Mach 1.7 will be the ideal pressure recovery, the engine might still work at Mach 2, but the efficiency is much lower than the efficiency of the intake of F-14, true it can achieve Mach 2, so J-20 very likely has lower swept wings and forebody to generate less drag and even the bleed system to ensure operation at Mach 2 more or less efficiently, but compared to a variable geometry intake it still is inferior, critics of Su-57 thought it was old fashioned, but in reality its intake was designed to operate up to Mach 3 efficiently, it might be slower, but the caret type has still advantages that DSI can not surpass except at speeds below Mach 2
 
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pegasus

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View attachment 620360

View attachment 620361

现在即将装备的 J-10B,对腹部进气布局的 Bump 进气道的鼓包和进气唇口进行了修改(唇口截面改得更方了),2.05Ma 时出口平均总压恢复系数接近 0.9,是高空高速大马赫数下的推力增加约 4% 的主要方面
The J-10B being inducted has the DSI intake modified to sharpen the corners, leading to pressure recovery coefficent approaches 0.9 at M2.05, increasing the speed (as compared to J-10A) by 4%
the DSI is still inferior to intakes of F-111, F-14 and F-15, when they say close to 90% it is around 89% or 87%, F-111 has 95% TPR at Mach 2 and F-14 is almost 95%, the
DSI is still inferior a difference of 4% can translate in 12% less thrust to put it simple if it uses Al-31, su-27 will have better TPR and thrust than J-20.

In fact using 117 engines the variable geometry intakes and carrying 4 AAM will allow higher thrust to the Su-35 thanks to variable geometry intakes, it translates well in better acceleration, against Su-57 well the Sukhoi will be much more efficient than J-20, better thrust also translates in better STR specially since the wing of J-20 has high induced drag at low AoA, delta configurations generally have a much higher induced drag penalty.
 
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Inst

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IIRC, the study on the J-10 DSI showed that it could reach 87% TPR at Mach 2.

When someone says DSI, what you should hear is "conical inlet with more computing power", because that's what it is. It's a fixed shockwave generator that has more complex geometries than a traditional conical inlet because computing power's come a long way, baby, and computational fluid dynamics allow the simulation of conical inlets that are not strictly cones.

A DSI can be optimized for any Mach number, hell, there's been attempts at creating DHI (hypersonic).

So just because the J-20 has a DSI, you shouldn't assume it's highly compromised at high Mach.

The real question I have is more, what's the effective inlet area on the J-20? It's an important question because that allows us to know how it comes to the Su-27 effective inlet area; i.e, can the J-20 get the AL-31 to supercruise or near-supercruise performance by providing higher MFR at altitude?
at Mach 2 a pressure recovery of 87% is not 13% less thrust, it is not linear but it grows exponentially, so a TPR of 87% is not 13% less thrust it can be 25% or 30% less thrust, less thrust means less acceleration, the computing power does not change physics, higher speeds means the DSI will ingest more boundary layer thus it will need bleeding system that is already rendering DSI useless, it means more weight, the caret intake was considered less efficient because for F-35 it weighed more due to the bleeding system, you like it or not F-35 has the ideal intake and shows the limits of DSI fixed geometry intakes.


J-20 if overscan is right has 2 or at least one bleeding system on the cowl intake, this means it has design limits and making a bigger bump means more drag, thus they used a bleeding system, fixed geometry has limits, and DSI only has advantages because is fixed you might not like it but it is the truth, it has no mechanical parts no bleeding system, why it is better? simply it weighs less, disadvantages well it has its best performance at Mach 1.6, the Americans do not lie, F-35 is slower for such reason


In the graph you can see F-15 has TPR higher than 90% at mach 2, SR-71 has even higher TPR at Mach 2, this happens because they have variable geometry intakes

View attachment 620346
Supersonic DSI can not work well in all flight regimes, the Mach design limits means it is optimised for a given Mach number, the intake throat and capture area are not fixed that is the reason you have variable intakes that change capture area and intake throat area and auxiliary intake doors and bypass doors.

This scheme shows how a Mach 3 mixed compression intake works

View attachment 620347
Correct, but your argument is more "fixed vs variable" inlets. In the F-22's case, the caret / F119 combo gets it to roughly Mach 2.45.

I think on SDF there was some research discussion of "hybrid" DSI that had variable components (cowl, mainly).

I mean, if you want me to bash the J-20's inlet type, I can go on about where the inlet fails:

-Too long. Long inlets increase weight, implying that the inlet length is necessary to achieve some effect (pressure recovery, tolerance for high-AOA, babying the notoriously bad Chinese engines).
-Basic inlet area seems too low. High-altitude / high-speed fighters tend to be optimized for low drag, but also huge inlets. See the inlet on the MiG-31, for instance. Without resorting to the cone-type inlet of the SR-71, the large MiG-31 inlet allows its turbofans decent thrust in thin air by allowing a high MFR. The drawback, of course, is poor low-altitude high speed performance.
 

pegasus

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Correct, but your argument is more "fixed vs variable" inlets. In the F-22's case, the caret / F119 combo gets it to roughly Mach 2.45.

I think on SDF there was some research discussion of "hybrid" DSI that had variable components (cowl, mainly).

I mean, if you want me to bash the J-20's inlet type, I can go on about where the inlet fails:

-Too long. Long inlets increase weight, implying that the inlet length is necessary to achieve some effect (pressure recovery, tolerance for high-AOA, babying the notoriously bad Chinese engines).
-Basic inlet area seems too low. High-altitude / high-speed fighters tend to be optimized for low drag, but also huge inlets. See the inlet on the MiG-31, for instance. Without resorting to the cone-type inlet of the SR-71, the large MiG-31 inlet allows its turbofans decent thrust in thin air by allowing a high MFR. The drawback, of course, is poor low-altitude high speed performance.
I am not bashing the intake nor belittling it, DSI simply has a mach designed number, From 0 km/h to Mach 1.7 the DSI of F-35, J-10C and JF-17 have a design number of Mach 1.6, from Mach 2 designers use variable geometry.

DSI only advantage was with respect the fixed intakes of F-16A or a fixed caret for JSF, was lower weight, this translated in lower maintenance and price, that weight difference was because it did not use bleeding system or any mechanical device.


Pretty much a DSI with moving part makes no sense because the main advantage is it is fixed and has no bleeding system and no moving or mechanical parts like bypass doors.

1571492530102.png

if you look F-35 has no bleeding system or mechanical devices, that is truly the whole concept behind DSI, it is the perfect DSI intake, J-20 is an aircraft that flies in that range too, Mach 1.7 or slightly more, to think adding variable geometry is pure non sense, it destroys the whole concept and advantage of DSI which is fixed and no mechanical parts or bleeding system

see what is written there, no bleeding system, no diverter cavity no mechanical variation that is what it makes it lighter than a fixed Boundary Layer Diverter Intake
1571493429937.png

System-level trade studies were performed to quantify the weight, cost, and benefits of the DSI, compared to more conventional inlets (e.g., F-22 and F/A-18E/F caret inlet systems). In these studies, a 30-percent inlet weight reduction was estimated for the DSI, relative to the reference caret inlet. The largest contributing factor was the elimination of the bleed and bypass systems. Studies performed by other ACIS contractors [25] indicated similar savings for diverter-less/bleed-less systems.

https://www.lockheedmartin.com/content/dam/lockheed-martin/eo/documents/webt/F-35_Air_Vehicle_Technology_Overview.pdf
 
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Inst

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Correct, but are you able to eyeball the J-20's Mach design number by its shape?



Obviously, the J-20's DSI was changed, first, from the 2001 / 2012 variant, to the 2017 variant / production variant.

Second, if the J-20's intended more for high-speed performance, what's the drawback of having a design Mach number that's abnormally high (Mach 1.8, Mach 2, etc)?

If it's pronounced spillage drag at low altitudes and high speeds, that's a major problem. But if TPR drops under these conditions, that's not necessarily a bad thing depending on the inlet design altitude; i.e, one theory I was floating on SDF was that the reason the J-20's shown such anemic low-altitude performance is precisely because the inlet isn't designed for low-altitude performance; to get supercruise / pseudo-supercruise / quasi-supercruise out of Al-31 class engines, you increase MFR at medium / high altitudes at the cost of low-altitude spillage drag.
 

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Latest J-20 DSI shape (from the Zhuhai airshow with the weapons load)

 

pegasus

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Correct, but are you able to eyeball the J-20's Mach design number by its shape?

Obviously, the J-20's DSI was changed, first, from the 2001 / 2012 variant, to the 2017 variant / production variant.

Second, if the J-20's intended more for high-speed performance, what's the drawback of having a design Mach number that's abnormally high (Mach 1.8, Mach 2, etc)?

If it's pronounced spillage drag at low altitudes and high speeds, that's a major problem. But if TPR drops under these conditions, that's not necessarily a bad thing depending on the inlet design altitude; i.e, one theory I was floating on SDF was that the reason the J-20's shown such anemic low-altitude performance is precisely because the inlet isn't designed for low-altitude performance; to get supercruise / pseudo-supercruise / quasi-supercruise out of Al-31 class engines, you increase MFR at medium / high altitudes at the cost of low-altitude spillage drag.
intakes have features that tell you the intake design mach number, there are two types one is fixed for speeds from 0 to Mach 1.8-1.9 and variable geometry intakes for 0 to Mach 3 or more.

First feature is the bump is fixed
Second feature the cowl is fixed
Third feature in order to have 3 external compression oblique and normal shocks it needs variable geometry, bleeding system and if it will have 4 or more oblique and normal shocks they need mixed compression with the same features.


At low speeds F-15 has a moveable cowl, MiG-29, MiG-23 and have auxiliary intake doors.

Sr-71 has by pass doors, these features tell you the speed it flies.

J-20 has a fixed intake, its design number is around Mach 1.7, you have to prove it is not fixed, but it is fixed all DSI are fixed in order to save weight,

The intake cowl on a DSI has to be placed away of the boundary layer spillage zone thus it is constrained by the bump fixed position and on F-35 and J-20 the bump is not centered but it is located slightly higher to create a shielded effect by the upper part of the intake cowl contrary to X-35 which has symmetric cowl lips with the cowl lip wedge coinciding with the bump center line, this gives better AoA handling to F-35 than X-35

1571497300688.png
Any way DSI intakes are for speeds of Mach 1.7

1571497075892.png
 
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overscan

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-Basic inlet area seems too low. High-altitude / high-speed fighters tend to be optimized for low drag, but also huge inlets. See the inlet on the MiG-31, for instance. Without resorting to the cone-type inlet of the SR-71, the large MiG-31 inlet allows its turbofans decent thrust in thin air by allowing a high MFR. The drawback, of course, is poor low-altitude high speed performance.
Your theory is incorrect no matter how many times you repeat it. MiG-31 maximum speed is Mach 1.23 at low altitude - there is no compromise in low altitude high speed performance involved here, that's about as fast as any aircraft ever managed.

MiG-31's intakes are designed to allow speeds up to Mach 2.83, and to supply the high mass flow requirements of the D30F-6 engine (150kg/s).
 

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I think Inst may be conflating altitude-related thrust lapse rate and Mach-related inlet momentum drag. Both these effects hit home in the top right corner of the flight envelope, but they are quite separate.

At a given Mach, thrust will decrease approximately in proportion with air density as you increase altitude, so the engine is starved of mass flow. Increase Mach at a given altitude however, and engine thrust will also decrease - despite intake compression ratio increasing dramatically (IIRC, in Concorde, it was ~7 at Mach 2!). There is definitely NO lack of mass flow in this case, but aircraft forward speed (to which inlet momentum drag is proportional) now approaches engine exhaust jet velocity, so net thrust eventually falls to zero.

If you don't consider exclusively the maximum speed at optimum altitude (where of course a handful of highly specialized aircraft like the SR-71 were faster), the MiG-31 makes a credible contender for the fastest aircraft - it's blazingly fast anywhere from sea level to the stratosphere!
 
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Maybe we should just ask the Israelis??
 

pegasus

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I think Inst may be conflating altitude-related thrust lapse rate and Mach-related inlet momentum drag. Both these effects hit home in the top right corner of the flight envelope, but they are quite separate.

At a given Mach, thrust will decrease approximately in proportion with air density as you increase altitude, so the engine is starved of mass flow. Increase Mach at a given altitude however, and engine thrust will also decrease - despite intake compression ratio increasing dramatically (IIRC, in Concorde, it was ~7 at Mach 2!). There is definitely NO lack of mass flow in this case, but aircraft forward speed (to which inlet momentum drag is proportional) now approaches engine exhaust jet velocity, so net thrust eventually falls to zero.

If you don't consider exclusively the maximum speed at optimum altitude (where of course a handful of highly specialized aircraft like the SR-71 were faster), the MiG-31 makes a credible contender for the fastest aircraft - it's blazingly fast anywhere from sea level to the stratosphere!
in Chinese forums, most people are ethnic chinese living in western countries most of them, say based upon certain reports J-20 is a very fast aircraft something like Mach 2.5 and perhaps supercruising speeds of Mach 1.9 thus when they try to reconcile the DSI mach number of F-35, JF-17 or J-10B/C they say J-20 has a new intake type with variable geometry, so they start with some theories, however some factors can not fit that explanation.
1571532582178.png

Aircraft like Mirage 2000 or Mirage 4000 have a moving cone, but have a traditional diverter, in the case of J-20 they say the intake cowl moves forward, so it adapts to relocate the oblique shock like a Mirage 2000 would do by moving its intake half cone, the question is the bump position and the cowl are set in a way the boundary layer is taken out of the intake so the cowl position and bump basically are set for 2 basic needs to position the oblique and normal shock in the cowl and divert the boundary layer.

It is the position of the bump relative to the intake that is the major difference and this shows how important the positioning is. It indicates that it is advantageous to place the maximum amplitude of the bump close to the cowl lips of the intake, so that they coincide with the shock from the bump surface.

A comparison between Intake & Mod 1 and Intake & Mod 2 show that high a amplitude of the bump is preferable to a low amplitude. This gives both higher pressure recovery as well as better boundary layer diversion



http://www.diva-portal.org/smash/get/diva2:221/FULLTEXT01.pdf

Abstract
Extensive experiments were conducted on a body-integrated diverterless supersonic inlet (DSI). Diverterless supersonic inlets are designed and developed in order to provide both supersonic flow compression and boundary-layer diversion by using a three-dimensional bump in combination with a suitable cowl lip. The present experiments were performed at three different freestream Mach numbers of M∞=0.75M∞=0.75, 1.65 (the design Mach number), and 1.85, as well as at 0 deg angles of attack and angles of sideslip. To model the performance accurately, the intake was integrated with a typical forebody including a nose with an elliptical cross section. Wind-tunnel tests were conducted at critical, subcritical, and supercritical operating conditions. The results showed that the present DSI has acceptable performance in these operating conditions and is able to provide the required mass flow and static pressure ratios. For all conditions examined in this study, as a significant result, the fixed geometry of the designed DSI showed acceptable performance in the ranges of supersonic Mach numbers tested: M∞=1.65–1.85M∞=1.65–1.85; furthermore, its operation in the subsonic condition of M∞=0.75M∞=0.75 was satisfactory. It should be mentioned that there were no movable parts or an auxiliary flow control system for this intake
https://arc.aiaa.org/doi/abs/10.2514/1.C035328


Therefore, at supersonic speed higher amplitude of bump is preferred over smaller amplitude. In case of Config 1 bump, shock on lip phenomenon is met since its maximum amplitude is kept near the cowl lip. Pressure above the intake duct is almost same in all the cases since intake duct is same for all the cases so.

https://www.eares.org/siteadmin/upload/8484EAP5171002.pdf





in my personal opinion is not posible to have a variable geometry DSI intake with a moving cowl due to positioning to take the boundary layer out of the intake

Taipei, Sept. 23 (CNA) Taiwan's defense minister said Monday that the U.S.-made F-16V fighter jet can outclass China's Chengdu J-20 in a dogfight.

Yen De-fa (嚴德發) made the comment at a hearing of legislative Foreign Affairs and National Defense Committee, which completed a preliminary review of a draft bill that would allow the government to create a special budget of up to NT$250 billion (US$8.07 billion) to buy 66 of the F-16V fighters from the U.S.


He was replying to legislators' questions about the ability of the F-16V compared to the Chengdu J-20, a fifth-generation stealth fighter developed by the Chengdu Aerospace Corp., in close aerial combat. The F-16V would have no problem beating the J-20, Yen said.
http://focustaiwan.tw/news/aipl/201909230017.aspx
 
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Inst

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I think Inst may be conflating altitude-related thrust lapse rate and Mach-related inlet momentum drag. Both these effects hit home in the top right corner of the flight envelope, but they are quite separate.

At a given Mach, thrust will decrease approximately in proportion with air density as you increase altitude, so the engine is starved of mass flow. Increase Mach at a given altitude however, and engine thrust will also decrease - despite intake compression ratio increasing dramatically (IIRC, in Concorde, it was ~7 at Mach 2!). There is definitely NO lack of mass flow in this case, but aircraft forward speed (to which inlet momentum drag is proportional) now approaches engine exhaust jet velocity, so net thrust eventually falls to zero.

If you don't consider exclusively the maximum speed at optimum altitude (where of course a handful of highly specialized aircraft like the SR-71 were faster), the MiG-31 makes a credible contender for the fastest aircraft - it's blazingly fast anywhere from sea level to the stratosphere!
As I've said before, the decrease in air density between sea level and 35000 feet is approximately 67%. The increase in speeds from Mach .9 (dogfighting) to Mach 2 is only 122%, which amounts to roughly a 27% decrease in MFR between 0 altitude Mach .9 to 35000 feet Mach 2 provided the inlet can minimize supersonic losses. Moreover, thrust = MFR * (exhaust velocity - airspeed), so the faster you go, the greater the exhaust speed needs to be to keep a constant thrust.

Variable inlets do have bypass ducts, so the inlet overflow can be limited to an extent on the MiG-31.

My interest in this is the "supercruise" requirement, recall that the Brits had supercruise capable aircraft in the 1950s. Getting high MFR at altitude ensures that you can break the Mach barrier with inferior engines.

The biggest problem, though, is that it appears that DSI by definition will not have bypass ducts, implying that if you go to high altitude supercruise MFR, you get hosed at low altitudes due to spillage drag.

Re: @pegasus:

As far as moving cowls go, we have no indication of such on the J-20, although there was apparently research studies on the concept.. My point is just that a DSI can be optimized for different Mach, and that the J-20 DSI could very well be optimized for high Mach instead of low Mach.

If we're talking SDF, I've already explained the situation in terms of Hegelian dialectic. Western commentators decided the J-20 was likely a striker or an interceptor based on its apparent large size. Chinese nationalists got pissed off because they wanted an air superiority fighter, then dragged out Song Wencong research papers about aiming for stealth, supercruise, supermaneuverability (which isn't the same as agility, the former being defined as post-stall maneuver ability), and short take-off. Then they began dragging out any evidence they could find that the J-20 is highly agile and more suited for anti-fighter roles (shallow bays, when the J-20 weapons bay is deeper than the F-22s and only slightly shallower than the F-22's). The "synthesis" move is when reports began coming out about the J-20's supersonic maneuverability (long arm canards) and speed records (for the PLAAF, the comparison points would be the J-10A, their MiG-21 knock-off, and possibly the Flanker knock-off); i.e, people are starting to believe that the J-20 is more akin to a fifth-gen version of a MiG-31 (which is already a contrast to a MiG-25, as the MiG-31 sacrifices max speed for maneuverability); i.e, it's competently agile subsonically, but focuses more on its supersonic performance.

The Chinese line is more "J-20 is a dogfighter, not an interceptor", not, as you've said, that the J-20 is "high speed".

And, if you look at how the J-20 DSI evolved since the prototype versions:

Therefore, at supersonic speed higher amplitude of bump is preferred over smaller amplitude.
The J-20's DSI has become progressively greater since its prototype versions, and the latest J-20 DSI bump seems even more pronounced than the J-20's prototype versions.

===

Lastly, if you want to talk Taiwanese views, one, it's a defense minister of a party that's pro-independence. Two, if the idea is that the F-16V can take the J-20 easily, by extension the F-16V can also take the F-35 or F-22 easily. HOBS means that dogfighting is dead, while stealth means that 4th gens die BVR long before they get into the merge. If the point is that the F-16V is HOBS capable, then sure, why not, J-20 vs F-16V at close ranges results in a mutual kill, just as, say, HOBS F-16V vs HOBS F-35 / F-22 results in a mutual kill provided the F-16V's IR sensors are good enough.
 
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pegasus

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Re: @pegasus:

As far as moving cowls go, we have no indication of such on the J-20, although there was apparently research studies on the concept.. My point is just that a DSI can be optimized for different Mach, and that the J-20 DSI could very well be optimized for high Mach instead of low Mach.



The Chinese line is more "J-20 is a dogfighter, not an interceptor", not, as you've said, that the J-20 is "high speed".

And, if you look at how the J-20 DSI evolved since the prototype versions:

Therefore, at supersonic speed higher amplitude of bump is preferred over smaller amplitude.
The J-20's DSI has become progressively greater since its prototype versions, and the latest J-20 DSI bump seems even more pronounced than the J-20's prototype versions.

===

Lastly, if you want to talk Taiwanese views, one, it's a defense minister of a party that's pro-independence. Two, if the idea is that the F-16V can take the J-20 easily, by extension the F-16V can also take the F-35 or F-22 easily. HOBS means that dogfighting is dead, while stealth means that 4th gens die BVR long before they get into the merge. If the point is that the F-16V is HOBS capable, then sure, why not, J-20 vs F-16V at close ranges results in a mutual kill, just as, say, HOBS F-16V vs HOBS F-35 / F-22 results in a mutual kill provided the F-16V's IR sensors are good enough.
I will tell you my opinion upon what i have read.
so J-20 is a large version of F-35, they needed to make it large because contrary to F-35 they lack engines in the class of F135 so they opted for 2 twin engined fighters, using the Al-31 and RD-93 as interim versions, in J-31 and in J-20.


Its DSI follow the same rules that F-35 follows and obeys, so it is a Mach 1.7 to Mach 2 fighter at the most and the DSI will not work as well at Mach 2.


Like JF-17 it has porous holes bleeding the boundary layer in the cowl, it might allow it to operate up to Mach 2 with relatively efficiency at the expense of weight and higher maintenance than F-35.

the bumps are huge so they also generate drag, and increase cross section thus to keep fineness ratio the fuselage is very long, part of it is because the canard takes a lot of space longitudinally and forces the wing to be further back.

It is heavy, at least 19-20 tonnes and 30 tonnes basic configuration ready for combat, so they made it with canards to generate the canard and wing vortex interaction and increment lift at G higher than 1 and AoA higher than 10 degrees.

It is very agile? i do not think so, however it is not an aircraft that can not dogfight but due to stealth its aerodynamics were limited so it needs either HMS and HOBS missiles, even with TVC nozzles agility is lift dependant.


So it is basically a larger version of F-35 with the expectation if might take some missions given to F-22, engine limitations make it more like a F-35 than F-22 though
 
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Inst

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I'd disagree on that; i.e, that the Chinese are pumping about 116 million a pop for a larger F-35. The choice of doing a heavyweight fighter, alongside increased pilot training, and the Chinese situation wherein, as a dual land/sea power, airpower bridges the gap between its ground capabilities, suggests that the Chinese are fully serious with the J-20 and aren't doing things like the Russians, where the Su-57 is extremely cheap (35-50mn a pop) and is somewhat compromised.

You have to remember, the WS-15 is going to be in the 160-200 kN range. If the WS-15 got the high-end of the target figures, the J-20 would end up having roughly a 1.64 T/W (implying they'll likely tone it down by adding 2D TVC to increase weight and decrease thrust).

As far as agility goes, the question is what is the comparison point? When I bring up the MiG-31, the point is that the MiG-31 was agile enough to go toe-to-toe with many 3rd generation fighters, although it couldn't really come to par vs 4th gen. What I assume, concerning the J-20, is that it's roughly a 4th generation (3rd in Chinese parlance) level of agility.

I'd say it's a good guess, because first, the J-20 has roughly 75 m^2 of wing area (slightly less than the Su-57 and F-22, but for size, much better than on the F-35), and the claim they're making is that body lift + lerx + canards + delta has roughly 20% lift increase over a basic (Gripen-class) canard delta (although most likely at high AOA). Wing loading is roughly in the 300-350/m^2 kg range at 60% fuel using a 25,000 kg loaded figure.

On the other hand, the choice of long-coupled canards makes it clear that subsonic agility isn't a priority (long-coupled is better for subsonic control), and the airframe is currently underengined with engines in the 130kN range instead of the 190kN range.

===

FYI, when it comes to agility, the J-20's demonstrated a 22.5-30 deg/sec instantaneous turn at between 2000-10000 meters. On the other hand, at present performances, it has shown mediocre sustained turn rates (15-18 deg/sec) to date at low altitudes.

===

I think the underlying concept of the J-20 is a stealthy fighter-interceptor. If you look at the F-35, the assumption American designers are making is that STR is obsolete, that HOBS makes any short-range engagement suicidal. But the J-20 is too expensive to spam, so the J-20 needs to seek superiority in another realm. Supersonic performance is one way to get around it and that seems to be the J-20's goal.

The question being made, however, is what is the Al-31's installed thrust curve vs altitude and speed? Is the J-20 inlet-engine combo optimized for low-altitude subsonic fighting (F-35, as an example)? Or did they decide to sacrifice low-altitude performance for better high-altitude performance? Remember, to the best of our knowledge the J-20 is an archetypal DSI fighter, with an absence of bypass ensuring spiillage drag.
 

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the DSI is still inferior to intakes of F-111, F-14 and F-15, when they say close to 90% it is around 89% or 87%, F-111 has 95% TPR at Mach 2 and F-14 is almost 95%, the
That probably correct, but if F-4 can reach Mach 2.4 with 87% pressure recovery at Mach 2, I think J-20 and J-31 can do something similar
 

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There’s a lot of likely incorrect and/ unsupported assumptions flying around these discussions.
Very different assumptions of the J-20’s design focus, bizarre assumptions of cost of J-20 versus Su-57 (and relevant how?), apparent need for some contributors to defend/ promote Russia’s variable-inlet approach (their chosen approach to meet their requirements which is fair enough but probably also influenced by their relative lack of experience or knowledge re: DSI’s).

And an extremely basic fact almost not mentioned above at all - adopting of the DSI approach is clearly greatly influenced by underlying “stealth” requirements which is one of the clear down sides of the variable inlet approach.
And to state again what some contributors have stated above - at this stage it appears it is just conjecture of what max speed the J-20 DSI inlet has been tailored for, just repeating it “must” be the approx. same as the F-35 doesn’t make it so.
 

pegasus

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That probably correct, but if F-4 can reach Mach 2.4 with 87% pressure recovery at Mach 2, I think J-20 and J-31 can do something similar
lower total pressure recovery means more fuel spent and less thrust, let us suppose J-20 achieves Mach 2.2 with Al-31, now you have Su-27 with better pressure recovery, it will spend less fuel and will have more thrust, if J-20 will supercruise needs excellent pressure recovery, having the same engine does not mean they will have the same thrust, to put you a simple example F-16 and F-15, at Mach 2, F-15 can get close to 100% of the potential thrust of F100 engine thanks to variable geometry intakes, but F-16 will get much much less of the max thrust of F100 than the F-15 because the air mass flow can not be slowed down as in the intakes of F-15, this translates in F-15 having better acceleration at Mach 2 than F-16 and longer range.

read

I. Introduction The inlet is a duct before the engine. Its basic function is to capture a certain amount of air from the freestream and supply it to the engine. Most gas turbine engines require the Mach number at the engine face at a moderate subsonic speed, to be about Mach 0.4. Therefore, for supersonic aircraft with a gas turbine engine, the inlet will reduce the supersonic freestream to subsonic speed, and provide a matched air mass flow rate to the engine. The gas turbine engine requires a supply of uniform high total pressure recovery air for good performance and operation, thus the quality of the airflow at the engine face will significantly affect the performance of the engine, especially the total pressure loss which affects the engine thrust and consequently the fuel consumption. For 1% total pressure loss, the engine will suffer at least 1% thrust loss. Therefore, it is important to maximize the total pressure recovery at the engine face. The total pressure recovery is the ratio of the total pressure of the airflow at the engine face to that of the freestream.

http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.559.484&rep=rep1&type=pdf

An aircraft inlet captures freestream air and reduces its velocity so the engine can process if in a stable and efficient manner. In order to minimize compressor work, inlet diffusion should be accomplished with a minimum of total pressure loss. The inlet should also deliver the working fluid with minimum distortion, all over a wide range of Mach number, angle-of-attack, angle-of-sideslip, and engine demand. The supersonic inlet for a tactical aircraft must also be sized to provide a maximum demand airflow which usually occurs at maneuver or acceleration -.- conditions. When the aircraft is at a subsonic cruise condition, however, the engine needs to process only a limited mass flow associated with 40-60 percent maximum dry thrust. The inlet, however, is still capable of processing larger mass flow closer to maximum demand.

The control of the shock wave position and prevention of shock induced flow separation in the inlet can be accomplished by bleeding boundary air from the inlet ramps, cowls, or sidewalls and dumping that flow overboard. This produces forces similar to the bypass flow which must be considered in supersonic inlet throttle dependent forces.
https://apps.dtic.mil/dtic/tr/fulltext/u2/a162939.pdf


Turbojet installed thrust 6 • Uninstalled thrust is obtained from engine manufacturer, preliminary cycle analysis or a fudge factor approach. • Every 10 years: 25% less SFC, 30% less weight, 30% less length, • Installed thrust = uninstalled thrust – installation effects – drag contribution assigned to the propulsive system


http://www.ae.metu.edu.tr/~ae452/lecture1_propulsion.pdf
 
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pegasus

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And an extremely basic fact almost not mentioned above at all - adopting of the DSI approach is clearly greatly influenced by underlying “stealth” requirements which is one of the clear down sides of the variable inlet approach.
And to state again what some contributors have stated above - at this stage it appears it is just conjecture of what max speed the J-20 DSI inlet has been tailored for, just repeating it “must” be the approx. same as the F-35 doesn’t make it so.
the caret intake is stealthy the fact F-22 uses it shows is very practical, you are misunderstanding DSI, the only reason Lockheed chose the DSI, was it was cheaper to build and cheaper to maintain, in fact i posted a Lockheed document where they say it, Caret intakes can be built with fixed geometries or Variable geometry, they are more expensive to build and maintain certainly, they are as stealthy as DSI but they are more expensive, trying to portrait DSI as the ultimate stealth intakes is false in fact the bump destroys the alignment the fore body chines and intake cowl have with the vertical tails, but from a frontal cross section is not a problem, but since they are spherical in nature the bump has a RCS that approaches a sphere, contrary to the caret intake that is aligned to the facets of F-22, the chines and cowl intake of F-22 are aligned with the vertical tails and wing leading edges, certainly the bump is not aligned as the caret intake is with the rest of the airplane
 
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pegasus

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Wing loading is roughly in the 300-350/m^2 kg range at 60% fuel using a 25,000 kg loaded figure.
I will give you a simple detail, F-35 which is much much smaller than J-20 has an empty weight of 13000kg, for J-20 to have such 60% it means it will be as light as F-35, since F-35 weighs around 30000kg fully loaded with 100% fuel.


F-22 is around 19000kg empty weight, J-20 weighs around 20000 kg empty, around 30000kg combat loaded and between 35000 kg fully loaded, why you can know that? if the J-20 is as light as Su-27, then Al-31 are enough to achieve 1.2:1 thrust to weight ratio and a take off weight of 24000 kg combat loaded, it simply means it does not need WS-15, if WS-15 are needed you can expect a weight of 30000 kg combat ready, so the supposedly wing loading of 300 kg/square meters you are quoting does not make sense.

Su-35 has excellent thrust to weigh ratio with 117, Su-57 does not have the ideal TWR with 117 thus they need T30.

In few words those figures you are quoting do not make sense, stealth aircraft like F-22 or J-20 have at least a max take off weight of 36000 kg and a combat ready of 30000 kg that is why they need high power engines
 
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Deino

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I had a thread on SDF but SDF gets pissy any time you try to compare the J-20 to an interceptor.
...

To admit, indeed now I - and only now - I get "pissy, But not since we don't want to "compare the J-20 to an interceptor" but since you once again twist the facts. :mad:

You are the one, who constantly wants to debate issues that were already so often discussed, You are the one who - in contrary to what is published in different academic papers - want to portray it as a pure interceptor; and nothing else but an interceptor. This was already discussed so often, ad nauseum and always we come to the conclusion, that we won't agree, something you don't seem to accept.

So please stick at least to the facts.
 
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Trident

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As I've said before, the decrease in air density between sea level and 35000 feet is approximately 67%.
And as I've said, at high Mach a multi-shock intake will compress the inlet mass flow by several hundred percent - low density, i.e. *lack* of mass flow, is patently NOT the problem in this condition! In fact, more mass flow than the engine can even accept generally becomes an issue, hence the need for either spill doors to get rid of the excess air at minimum drag penalty or intakes undersized for low speed where they are then supplemented with auxiliary doors.

Moreover, thrust = MFR * (exhaust velocity - airspeed), so the faster you go, the greater the exhaust speed needs to be to keep a constant thrust.
Exactly: net thrust = MFR * exhaust velocity - MFR * air speed = gross thrust - inlet momentum drag.

As air speed increases, the engine will eventually no longer deliver a sufficient margin in exhaust velocity - that's what drives the need for low BPR for efficient supersonic flight in dry thrust. Reheat increases jet velocity, but at a steep hike in fuel consumption, decreasing BPR improves non-afterburning specific thrust (i.e. thrust per mass flow, which according to the above equation means higher jet velocity).

My interest in this is the "supercruise" requirement, recall that the Brits had supercruise capable aircraft in the 1950s.
Yes - in part by designing an inlet which was egregiously undersized in low speed conditions, to avoid spillage drag at *high* Mach!!!

Getting high MFR at altitude ensures that you can break the Mach barrier with inferior engines.
We are not talking about high altitude exclusively however but high altitude and high speed in combination - inlet compression removes lack of mass flow rate as a consideration in this case. If anything, you are dealing with too much air for the engine - the real concern is not getting more mass flow (oversized intakes will just add drag on three counts: inferior fineness ratio, increased weight and higher spillage) but more thrust per mass flow, i.e. higher jet velocity.

The biggest problem, though, is that it appears that DSI by definition will not have bypass ducts, implying that if you go to high altitude supercruise MFR, you get hosed at low altitudes due to spillage drag.
Once more, with an external compression intake, spillage is NOT a low speed but a *high* speed problem! No reason why you could not provide a DSI with spill doors (again: for high speed!) either - arguably the Crusader III did exactly that, only that Lockheed had not come along yet and slapped a new buzz word on what was still known as a Ferri intake at the time.
 
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Inst

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I had a thread on SDF but SDF gets pissy any time you try to compare the J-20 to an interceptor.
...

To admit, indeed now I - and only now - I get "pissy, But not since we don't want to "compare the J-20 to an interceptor" but since you once again twist the facts. :mad:

You are the one, who constantly wants to debate issues that were already so often discussed, You are the one who - in contrary to what is published in different academic papers - want to portray it as a pure interceptor; and nothing else but an interceptor. This was already discussed so often, ad nauseum and always we come to the conclusion, that we won't agree, something you don't seem to accept.

So please stick at least to the facts.
I've told you repeatedly, I haven't been arguing that the J-20 is a pure interceptor.

You guys get pissy because you don't even know what the MiG-31 is or how it differs from the MiG-25.

The MiG-25 is a third-generation interceptor; it optimizes for max speed and max speed only. The MiG-31, in contrast, sacrifices much of its max speed, in part because hitting Mach 3.2 will destroy the MiG-25's engines, and instead the MiG-31 increases both its subsonic and supersonic maneuverability, in part by doubling the thrust-to-weight!

Look at the bloody MiG-31 planform. It's an interceptor with LERX, for Christ's sake! The airframe is definitely not 4th-gen competitive when it comes to maneuverability, but it's 3rd generation competitive when it comes to maneuverability.

In other words, the MiG-25 is a pure interceptor. The MiG-31 is not; it's a BVR-optimized platform that can acquit itself WVR vs older platforms, but if you think it's a pure interceptor, note that it's achieved about 15 deg/sec turn rates at low speeds and altitudes; not good by 4th generation standards, but far from terrible.

Back-compare to the J-20. The J-20 is a fifth-generation fighter-interceptor; it's stealthy, it's meant to get TVC, it has canards, albeit long-armed ones, as well as ventral strakes for high AoA performance. But its demonstrated low-altitude performance has been anemic compared to what we've seen of later 4th gen aircraft as well as 5th gen aircraft.

And, as we've discussed, the regime design for the J-20 isn't a problem since dogfighting in the era of HOBS is suicide or murder suicide.

====

@Trident:

What you're basically implying is violation of the law of conservation of mass.

In front of the J-20, you only have a certain amount of air which the J-20 can ram into its inlets. All the pressure recovery in the world won't do you any good if there's no pressure (density, rather) to recover; there's no such thing as a 1.5 total pressure recovery ratio and even in the best case scenario, the most you'll get is something close to 1, coming in from the .9 region.

Fact of the matter is, you design an inlet for a specific flight regime. The AL-31 on Su-27 is actually an example of designing for something above sea level altitude; thrust actually decreases at sea level the faster the Su-27 goes, which is a contrast to say, the F-14B/D, wherein thrust at sea level continues to increase as speed goes up.

I've been asking about the F-35's inlet area (was told it varied depending on flight regime), because the F-35 is a valuable comparison. Hell, we even have the MFR of the compressor (139.6 kg/ second), so that we get the MFR of the engine using the bypass ratio (219.172 kg/second).

Then we can push these numbers back into the F-35 inlet area (about .67 m^2 treating the DSI bumps as non-transparent). Density of air at sea level (at standard temperature) is roughly 1.225 kg / m^3. Put into the inlet area, you get 0.82075 kg / km/s. At, say, Mach .9 at sea level (1109.538 km/h), you get roughly 252 kg/second MFR.

Take it to Mach 1.6 (its design max speed) at 35,000 ft, you get about 295 m/s, with air density of 0.38 kg / m^3. Multiply by the inlet area, and you get a MFR of about 75 kg/second, which is far lower than the MFR at sea level at Mach .9 and not sufficient for the F135 to run at full power.

====

Of course, I'll mention the errata. It is hypothetically possible for forebody design to increase the airflow to the inlets by increasing the effective air capture area and routing it to the inlets. The same applies to the DSI bumps, the big question I've had and needed answered is, are the DSI bumps transparent (i.e, does not decrease effective inlet area)? And if so, at what speeds and altitudes, since it's a complex aerodynamic device?

====

Some errata on MiG-31: at 60% fuel, subtracting 40% fuel weight from gross weight, the MiG-31 has about a .9 T/W ratio, and about 560 kg / m^2 wing loading.

On the J-20, assuming 60% of 12kg fuel and a 18,000 kg empty weight, with 1000 kg of munitions, you get about 350 kg / m^2 wing loading and about 1.01 T/W.

On the F-35A, assuming 60% fuel (40% fuel weight removed from gross weight) with full munitions, you get about 440 kg / m^2 wing loading and about 1.02 T/W.

The Su-35BM, in contrast, under similar conditions gets about 400 kg / m^2 wing loading and about 1.15 T/W.

====

One last thing, @Trident ,

When you say pressure, I'm assuming you're referring to dynamic pressure, which does have an exponential increase (u^2) based on velocity. But dynamic pressure isn't the same as mass / density / stagnation pressure; if it were so, the implication is that by accelerating air, you can make more air come out of nothing.

And this dynamic pressure isn't that useful for the inlet; the job of an inlet is to slow air down to subsonic speeds where the turbofan can function. In such a condition, most of the dynamic pressure of the airflow is lost.
 
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Deino

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I had a thread on SDF but SDF gets pissy any time you try to compare the J-20 to an interceptor.
...

To admit, indeed now I - and only now - I get "pissy, But not since we don't want to "compare the J-20 to an interceptor" but since you once again twist the facts. :mad:

You are the one, who constantly wants to debate issues that were already so often discussed, You are the one who - in contrary to what is published in different academic papers - want to portray it as a pure interceptor; and nothing else but an interceptor. This was already discussed so often, ad nauseum and always we come to the conclusion, that we won't agree, something you don't seem to accept.

So please stick at least to the facts.
I've told you repeatedly, I haven't been arguing that the J-20 is a pure interceptor.

You guys get pissy because you don't even know what the MiG-31 is or how it differs from the MiG-25.

...
That's not the point, point is you constantly want to discuss - in fact you want to lecture - this is the J-20 thread and I already told you; I'm not interested, I'm not able to follow these discussions technically and I even less care about the difference between the MiG-25 and MiG-31 especially since this is so much off to the J-20.

So please leave it and in fact it's you, who is pi..ed of, only since no-one want to join the boat.
 

Inst

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I had a thread on SDF but SDF gets pissy any time you try to compare the J-20 to an interceptor.
...

To admit, indeed now I - and only now - I get "pissy, But not since we don't want to "compare the J-20 to an interceptor" but since you once again twist the facts. :mad:

You are the one, who constantly wants to debate issues that were already so often discussed, You are the one who - in contrary to what is published in different academic papers - want to portray it as a pure interceptor; and nothing else but an interceptor. This was already discussed so often, ad nauseum and always we come to the conclusion, that we won't agree, something you don't seem to accept.

So please stick at least to the facts.
I've told you repeatedly, I haven't been arguing that the J-20 is a pure interceptor.

You guys get pissy because you don't even know what the MiG-31 is or how it differs from the MiG-25.

...
That's not the point, point is you constantly want to discuss - in fact you want to lecture - this is the J-20 thread and I already told you; I'm not interested, I'm not able to follow these discussions technically and I even less care about the difference between the MiG-25 and MiG-31 especially since this is so much off to the J-20.

So please leave it and in fact it's you, who is pi..ed of, only since no-one want to join the boat.
FYI, no one wants to argue the topic definitively, and more so because Blitzo merged the thread into the inlet thread.

The inlet topic discussion basically ended up jumping here, and we have plenty of discussion (I am waiting Trident's response to calculated MFR at altitude).

I am annoyed at this point because you keep on trying to characterize my claims as claiming the J-20 is a pure interceptor, which is, as I've stated, the position held by Western analysts. I am basically trying to achieve synthesis between the general claims that the J-20 can't dogfight, the claims that the J-20 is a superb dogfighter, with the result being that the J-20 can dogfight, but it doesn't want to and that's not the purpose of the airframe.

You are trying to merge position 3 into position 1 because it opposes position 2.
 

Trident

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What you're basically implying is violation of the law of conservation of mass.

In front of the J-20, you only have a certain amount of air which the J-20 can ram into its inlets. All the pressure recovery in the world won't do you any good if there's no pressure (density, rather) to recover; there's no such thing as a 1.5 total pressure recovery ratio and even in the best case scenario, the most you'll get is something close to 1, coming in from the .9 region.
Density =/= *total* pressure! Density is a static value, total pressure (from which pressure recovery is calculated) includes the contribution of kinetic energy due to forward motion.

Take the following intake shock system (XB-70, so combined external & internal compression, but we'll look only at the external part - not least for brevity!):

SJh0h.png

The sketch notes ramp angles and Mach numbers, the oncoming air arrives from the left at M=3.0 and the first intake wedge turns it to an angle of 7°. Enter those numbers into the following calculator:


Lo and behold, you get the correct post-shock Mach number of 2.65. You'll also note the density ratio across this first oblique shock is 1.44, i.e. post-shock density is 44% higher than the pre-shock value! At the same time, total pressure ratio (=pressure recovery) is <1.0 as expected, with a total pressure loss of 1.4%.

If you string together the external compression shocks in the above manner (entering the post-shock Mach of the preceding wedge as the free-stream Mach of the next, as well as the amount of *additional* turning by the next ramp) you end up with a density ratio of 2.82, before we even start internal compression! Pressure recovery up to this point is an impressive 97.6%, thanks to the intake designers taking care to minimize total pressure losses by spreading compression over a large number of relatively weak shocks. Four shocks in we're still over Mach 2.0 while more typical intakes take the air from free stream to subsonic in less than this - but then a typical intake is not required to work at Mach 3 and be efficient enough to cruise for hours at that speed!

I've been asking about the F-35's inlet area (was told it varied depending on flight regime), because the F-35 is a valuable comparison. Hell, we even have the MFR of the compressor (139.6 kg/ second), so that we get the MFR of the engine using the bypass ratio (219.172 kg/second).
140kg/s is almost certainly the total intake air flow for the F135. Core compressor flow would then be 140/(0.57+1)=89kg/s (BPR = bypass flow / core flow).
 
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Trident

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Take it to Mach 1.6 (its design max speed) at 35,000 ft, you get about 295 m/s, with air density of 0.38 kg / m^3. Multiply by the inlet area, and you get a MFR of about 75 kg/second, which is far lower than the MFR at sea level at Mach .9 and not sufficient for the F135 to run at full power.
You are totally ignoring pre-compression by the inlet shock system which, as already pointed out, is what decelerates the air to Mach 1.0 at the inlet opening and dramatically increases pressure and density in doing so!
 

Trident

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Russia’s variable-inlet approach (their chosen approach to meet their requirements which is fair enough but probably also influenced by their relative lack of experience or knowledge re: DSI’s).
I doubt that's the reason. While I'll confess that I'm baffled by Sukhoi's choice, DSI isn't some kind of alien technology - it's a clever application of well-established supersonic cone flow math, feasible even with late-1950s design tools. What's more the idea (and its benefits to the more recent concept of stealth) was apparently no secret in Russia as early as the mid-1990s, probably years before what would become the J-20 adopted it:


So there is likely something about the spec that the Su-57 is required to meet which makes variable intakes a better solution - whatever that is (and whether it makes sense or not).
 

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What you're basically implying is violation of the law of conservation of mass.

In front of the J-20, you only have a certain amount of air which the J-20 can ram into its inlets. All the pressure recovery in the world won't do you any good if there's no pressure (density, rather) to recover; there's no such thing as a 1.5 total pressure recovery ratio and even in the best case scenario, the most you'll get is something close to 1, coming in from the .9 region.
Density =/= *total* pressure! Density is a static value, total pressure (from which pressure recovery is calculated) includes the contribution of kinetic energy due to forward motion.

Take the following intake shock system (XB-70, so combined external & internal compression, but we'll look only at the external part - not least for brevity!):

View attachment 620442

The sketch notes ramp angles and Mach numbers, the oncoming air arrives from the left at M=3.0 and the first intake wedge turns it to an angle of 7°. Enter those numbers into the following calculator:


Lo and behold, you get the correct post-shock Mach number of 2.65. You'll also note the density ratio across this first oblique shock is 1.44, i.e. post-shock density is 44% higher than the pre-shock value! At the same time, total pressure ratio (=pressure recovery) is <1.0 as expected, with a total pressure loss of 1.4%.

If you string together the external compression shocks in the above manner (entering the post-shock Mach of the preceding wedge as the free-stream Mach of the next, as well as the amount of *additional* turning by the next ramp) you end up with a density ratio of 2.82, before we even start internal compression! Pressure recovery up to this point is an impressive 97.6%, thanks to the intake designers taking care to minimize total pressure losses by spreading compression over a large number of relatively weak shocks. Four shocks in we're still over Mach 2.0 while more typical intakes take the air from free stream to subsonic in less than this - but then a typical intake is not required to work at Mach 3 and be efficient enough to cruise for hours at that speed!

I've been asking about the F-35's inlet area (was told it varied depending on flight regime), because the F-35 is a valuable comparison. Hell, we even have the MFR of the compressor (139.6 kg/ second), so that we get the MFR of the engine using the bypass ratio (219.172 kg/second).
140kg/s is almost certainly the total intake air flow for the F135. Core compressor flow would then be 140/(0.57+1)=89kg/s (BPR = bypass flow / core flow).
Thing is, mass rate flow is defined in terms of DENSITY, not pressure. If total pressure recovery is used, the point is that stagnation pressure is a reasonable surrogate for density.

As far as the F135's MFR goes, that seems to check out.

https://books.google.com/books?id=2Wy5rpdm3DMC&pg=PA214&lpg=PA214&dq=f100+pw-229+"mass+flow+rate"&source=bl&ots=5f4Gl_e3Sc&sig=ACfU3U3yQQZD0s4R8NcJFF8ViYFDAmWWKA&hl=en&sa=X&ved=2ahUKEwjK47qMhqzlAhXOqFkKHUpRAI8Q6AEwAnoECFEQAQ#v=onepage&q=f100 pw-229 "mass flow rate"&f=false

This source does not give, but gives sufficient inference for the F100-PW-229's MFR at about 81 kg/sec. Considering that the F135 is a more powerful engine, a 140 kg/s is roughly equal to 220 kN if we go linearly, and that's a benchtest result for the F135.

At a 35,000 ft cruising altitude for Mach 1.6, that's roughly 116 kg / sec MFR on an engine which needs 140 kg/s, or about 83% of peak MFR. If you treat the DSI bump as semi-transparent (effective inlet area is between .67 and 1.02 m^2), the engine should have enough air at that speed and altitude to operate, although how close TPR is to 1 is still a factor.
 

pegasus

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But how, in the context of being an integrated part of a low observatory (“stealthy”) airframe, does any of that remotely add up to a DSI being “unstealthy”?
Especially when DSI’s and other “fixed” inlets are probably easier and better from such a “stealth” design perspective than a variable geometry inlet approach?
Why would F-35s, J-20s and other designs have DSI if there were inherently flawed in this way?
Where are all the variable geometry inlet “stealth” aircraft?
I did not say it has no low observable treatment, they do, however the supposedly superiority over caret is not stealth, its price and maintenance, if you want to say they are superior, their only superiority is lower price, beyond that carets still enjoy some advantages, DSI are not perfect nor the ultimate stealth intake type, if you want an efficient fixed intake for low price yes they are good, beyond that carets have areas such as the ability to fit them variable geometry features that turn them better at higher speeds than Mach 2.

Just remember stealth is not radar invisibility, is lower signature treatment, and improvements in radar technology render stealth useless, diffraction and power density output of radars make stealth useless
 
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Take it to Mach 1.6 (its design max speed) at 35,000 ft, you get about 295 m/s, with air density of 0.38 kg / m^3. Multiply by the inlet area, and you get a MFR of about 75 kg/second, which is far lower than the MFR at sea level at Mach .9 and not sufficient for the F135 to run at full power.
You are totally ignoring pre-compression by the inlet shock system which, as already pointed out, is what decelerates the air to Mach 1.0 at the inlet opening and dramatically increases pressure and density in doing so!
You're still ignoring law of conservation of mass; i.e, while you can play with dynamic pressure all you want, you can't create air that never existed in the first place.

If you decelerate air to Mach 2 from Mach 1, you still have the same mass. Dynamic pressure definitely decreases substantially, and density should increase as well, but the quantity of mass and hence mass flow rate is still the same. You cannot increase density so that mass increases beyond the original captured mass; the mass there is the same.

), it's possible that the F135 has its full MFR requirement met at 35,000 ft and Mach 1.6.
 
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Trident

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How so? Compression is external, the air isn't truly captured before it has been decelerated to Mach 1.0, at the inlet opening. The mass flow passing through the opening/throat will have been occupying a very different area upstream of the intake shock system.
 
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Jemiba

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To all participants here: Cool down !
And return to an acceptable tone, please.
That's not the class room and no one has to grade others posts and make entries to the class register !
 

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Mass flow rate = air density x velocity x area.

Flying faster increases the velocity. Flying higher decreases density. Varying intake geometry alters area. There is a limit to how much mass flow rate can increased due to velocity as compressibility effects will come into play as the intake has to slow the velocity down to the engines required speed.
 

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How so? Compression is external, the air isn't truly captured before it has been decelerated to Mach 1.0, at the inlet opening. The mass flow passing through the opening/throat will have been occupying a very different area upstream of the intake shock system.

Think of a hypothetical pitot tube that flows through the air. You can make the pitot tube go as fast as you want, but the air capture rate won't exceed density of non-moving air * area of the tube inlet * velocity.

What you're suggesting is that there's aerodynamic effects extending the capture zone, but if you consider the Venturi effect, low pressure zones in a duct downstream can't "draw" air from high pressure zones upstream once flow approaches supersonic speed.

So if you think of the airflow, there's a fixed potential quantity created by the fuselage in front of the inlet, as well as the air in front of the inlet. You can't exceed this air mass with pressure manipulation tricks without violating the Law of Conservation of Mass, but you can approach its maximum.

In any case, some information on Mass Flow Rates for the J58 on the SR-71 is given here:


More importantly, there's also Mass Flow Rates for the J57 on the F-8 Crusader here:


This is a turbojet, requiring about 75 kg/s for 75 kN thrust with a 11:1 OPR.

There's also mass flow rates for the EJ200 with 75 kg/s for 90 kN thrust with a 26:1 OPR here:


So in general, the claim of 140 kg/s for the F135 is believable, and it's believable that at 35,000 ft at Mach 1.6, the F135 on the F-35 is getting its required mass flow rate based on the DSI inlet.

===

When you talk about compression effects on the external part of the inlet, I'm talking about "bump transparency", i.e, the air is being slowed down and the stagnation pressure (hence density) is being increased as it's being funneled into the inlet aperture. But I'm always thinking about this as relative to "free stream" flow, i.e, there's only so much air in front of the inlet, and supersonic effects mean that air closer to the inlet can't draw air further away from the inlet.

And when you talk about "transparency", the compression shocks are bleeding off part of the air approaching the inlet as seen with the DSI airflow diagrams, so at a given Mach, part of the free stream flow is being diverted.
 
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