Intake design and general stealth discussions

[marauder2048]

IIUC, the crucial distinction between the Lockheed DSI and prior art is that the compression surface in the Lockheed version produces a non-axisymmetric flowfield.

The "lost" Hamstra et al. paper, "Development, Verification, & Transition of an Advanced Engine Inlet Concept for Combat Aircraft Application" (attached)
goes into some more detail relative to the patent.

I'd also point out that the forthcoming X-59a has a DSI.

https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20180006432.pdf

 

[Trident]

Very informative paper! Not sure the non-axisymmetric nature of the flow field is that novel though, it stems mainly from setting the virtual cone at an AoA to "bias" the resulting flow field for more favourable aircraft integration. By the looks of it this may well have been the case on the Crusader III intake too, for example.

 

[Inst]

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

What I'm doing here is to try to get a discussion up on the J-20's inlets, in comparison to the F-35, F-22, and other aircraft.

The J-20's inlet length is rather unique; it's estimated at around 7-8.5 meters when the F-22 at max reaches around 5.5 meters and the F-35 reaches around 3 meters.

The initial theory I was floating was that the long inlet improves pressure recovery compared to shorter inlets, resulting in better mass flow rate into the engines at high speeds and altitudes, but the venturi effect implies that MFR maxes at Mach 1, i.e, once you get into the subsonic section of the inlet you can't use compression in a subsonic diffuser to increase MFR.

The other tentative theory is that the long inlet is designed to exploit the J-20's nose geometry, i.e, the chines and the nose airflow of the J-20 are intended to create shockwaves and pre-compress air entering the inlet, resulting in a higher pressure and thus higher MFR.

===

There's another question about the J-20 and F-35 DSI geometry. In leaked reports, the J-20 is a high-speed aircraft with very good top-speed. On the other hand, the F-35 is only rated to Mach 1.6, and begins to suffer performance issues related to the inlet near the top of its Mach range.

Both the J-20 and F-35 use DSI, with the J-20 having seen an increase in the size of the DSI bump between the prototype and the final version. A DSI bump, to an extent, is basically a variation of a conical inlet, so depending on its shape, it shouldn't create issues with supersonic performance.

How are the J-20 and F-35 DSI bumps different? How would the J-20 inlets deliver better high-speed performance than on the F-35?

 

[overscan (PaulMM)]

The JF-17 DSI is designed for Mach 1.6-1.8. The J-10B/C DSI is designed for Mach 2.0, like the F-16 DSI was. The geometry is different, and the J-10B DSI doesn't need bleed holes on the bump like the JF-17 DSI does. The J-20 is the third iteration of DSI intake by Chengdu.

The J-20 DSI will at least be good for Mach 2.0, and the grills added to the intakes from 2011 onward may indicate further optimization for high speeds.

 

[overscan (PaulMM)]

Merged with existing topic on DSI intakes and J-20.

 

[Inst]

The JF-17 DSI is designed for Mach 1.6-1.8. The J-10B/C DSI is designed for Mach 2.0, like the F-16 DSI was. The geometry is different, and the J-10B DSI doesn't need bleed holes on the bump like the JF-17 DSI does. The J-20 is the third iteration of DSI intake by Chengdu.

The J-20 DSI will at least be good for Mach 2.0, and the grills added to the intakes from 2011 onward may indicate further optimization for high speeds.

I came here because one of your colleagues on SDF suggested this forum, and I thought, F-16.net isn't interested, SDF doesn't like discussion of the J-20's high speed capability, might as well try Secret Projects.

With one of your colleagues, one of the hang-ups is why thrust decreases at altitude.

Searching one of the factors online, one comment is that Thrust = Mass Flow Rate (Exhaust Velocity - Airspeed), i.e, implying that MFR decreases at altitudes due to air pressure decreasing roughly hyperbolically

To list key assumption being made:

-MFR determines high-altitude performance, due to the thrust equation. Inlet design is crucial for high-altitude / high-speed performance, but there are also effectively trade-offs at lower speeds (high altitude MFR implies low-altitude excess MFR with diversion of excess air creating drag).

Some further elements that need clarification is:

Does the Venturi Effect apply to supersonic nozzles? What I've read on it suggests that it prevents low pressure zones from drawing air once the flow goes supersonically, but this might not apply to ram air being forced into a supersonic inlet.

Where's the maximum MFR achieved on an inlet? For instance, on paper, the F-22 has about 1.05 m^2 inlet area, but in practice, the caret-type inlet results in lower inlet area once the fixed devices for slowing supersonic flow are included.

How do inlet cones affect MFR both supersonically and subsonically? Most conical-type inlets tend to have low areas compared to pitot-type or modified pitot (diverter) inlets. For instance, if we look at the inlets the Electric Lightning, MiG-21, and SR-71, all these inlets seem to have low area, unless the cone is treated as transparent.

What type of DSI bump shaping is best for high-speed performance?

 

[overscan (PaulMM)]

I'm registered as Secretprojects on Sinodefenceforum.com - that was me

 

[overscan (PaulMM)]

Some recent Chinese DSI articles by Chengdu-associated researchers I can't download -

Reverse design of compression bulge outside Bump inlet based on close principle

基于密切原理的Bump进气道外压缩鼓包逆向设计--《航空动力学报》2018年09期

鼓包 密切 曲率中心 横向压力分布 位移厚度

www.cnki.com.cn

Double cone Bump compression surface design and aerodynamic characteristics

双锥Bump压缩面设计及气动特性--《航空动力学报》2013年01期

Bump进气道 运动间断边界算法 反设计方法 运动间断拟合 计算流体力学

www.cnki.com.cn

 

[overscan (PaulMM)]

And here's an article by researchers in the same university reverse engineering the F-35 DSI inlet from 2005.

Volume 26 Issue 3 Aviation Journal Vo l. 26 No. 3
May 2005 ACTA AERONAUTICA ET ASTRONAUTICA of May 2005
Received date: 2004-03-19; revised date: 2004-07-05
Fund Project: “Five-Year” Air Force Pre-Research Funding Project
Article ID: 1000-6893 (2005) 03-0286-04

Reverse Design of Diverterless Inlet and Mechanism of Diversion of Boundary Layer

Liang Dewang, Li Bo (School of Energy and Power, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China)

Reverse Design of Diverterless Inlet and Mechanism of Diversion of Boundary Layer

Abstract: The diverterless inlet and forebody of a fighter are constructed by using the Three Dimensional Reversion Technique of Photos and the external and internal flowfields of the inlet and forebody are calculated.

The Mach contours inside and outside the inlet and the pressure distribution on the bump surface are presented. Mechanism of diversion of boundary layer on diverterless supersonic inlet (DSI) is also analyzed. It can be Seen from the results that there is an initial compression angle at the front point of the bump and an isentropic Compression surface after the shock. The investigation shows that the diverterless supersonic inlet or bump inlet Creates a high pressure area o n the bump compression surface which pushes the air in boundary layer away
From the inlet.
Key words: diverterless supersonic inlet (DSI);bump inlet ;integrated design ;computational fluid dynamics
(CFD)

The design of the air intake is one of the keys to fighter design. in When designing, not only must the intake port be considered within the flight envelope The engine provides sufficient air flow and also considers the overall layout Constraints and requirements for integrated design, but also must meet the battle The overall stealth requirements of the bucket machine.

The United States first adopted the "without the surface layer" on the X-35 aircraft. Channel Supersonic Inlet (DSI) design technology, in the intake The port is not provided with a conventional fixed boundary layer compartment, but Counting a three-dimensional curved protrusion (or bulge), this bulge Compress the airflow. This novel design cancels the traditional Complex mechanisms associated with the control of the inlet layer of the inlet, such as the boundary layer The isolation panel, deflation system, and bypass system reduce the weight of the aircraft. This also reduces production and use costs [1 ~ 4].

The air intake design of the machine reflects the idea of stealth design. First of all, the inlet pipe is designed to be short S-bend, which can effectively The main radar that shields the engine's fan or compressor blades Wave scattering source. Secondly, the lip of the inlet is designed to be forward-swept. Avoid the formation of angular reflections of radar waves in the lips, and also improve The angle of attack of the inlet. In addition, the inlet layer of the inlet channel Is a large radar wave scattering source, due to the use of no channel Design, greatly improved stealth performance.

However, what kind of configuration is the inlet bulge? How to eliminate it? For the purpose of this, this article uses Nanjing Airlines according to the photo of the aircraft. Aerospace University's "Photo 3D Restoration Technology" restored the aircraft's front The shape of the body and the air intake (including the bulge) was analyzed and used. NAPA software carried out the flow field inside and outside the fuselage/inlet Numerical simulation, the flow characteristics are given, and the drum package exclusion is analyzed. The mechanism of the surface layer.

1 Restoration of a certain aircraft and air intake

The research object is the front fuselage and air intake of the aircraft, not including The wing, smoothed and simplified the belly. Figure 1 shows The restored air inlet lip angle and fuselage surface mesh Figure, inlet inlet cover is 3 lip structure, inlet installation The angle is - 5°. According to the symmetry, the left half of the aircraft is selected.

Generate a grid, the entire calculation grid has a total of 60 blocks, a total of grid nodes Counting 1,031,556. Due to the lack of in-port profile data, plus No. 3 Liang Dewang et al.: Anti-design of the inlet without the passage and analysis of the removal mechanism of the boundary layer

Fig. 1 Cowl angles and surface mesh on fuselage

The focus of this paper is on the shape of the bulge and the masking machine. Therefore, it is roughly designed based on the shape of the inlet and the surface of the drum. a small section of the shape of the inlet, the length and the length of the cover are the same, At the same time, considering the back pressure of the inlet of the inlet, the pipe is relatively straight. Not designed to be a short S-bend.

2 bulge shape and flow analysis

The bulge profile is the key to a non-channel intake. according to The result of the photo restoration is found (Fig. 2), and the drum kit can be shaped according to the shape. Divided into two sections, the first half is similar to a flat conical shape, the second half The section is the transition section and forms part of the duct profile in the inlet.

Fig. 2 Three-dimensional shape of the bump shape

Since the first half of the drum pack acts as a transfer surface layer and forms waves The role of the system, so the focus of this study is on the first half of the drum kit. The profile analysis. Translating and rotating the front half of the drum Then lay it flat in the XOZ plane, from the vertical symmetry plane XOY surface Start, make a cut surface every 5° in the circumferential direction around the X axis. X axis), the intersection of the cut surface and the surface of the bulge is the drum wrapped in different weeks. To the position of the profile, as shown in Figure 3, define the section at Z =0

The position is the basic line position. Then cut the guilloche The wires rotate around the X-axis XOY plane at their respective circumferential angles. The shaped line and the basic line are on the same plane. Next Other types of lines other than this type line circulate around the bulge in the XOY plane (ie, the origin of the coordinates) rotates clockwise, making all the profiles line to the baseline Close together, the results obtained by dimensionless are shown in Figure 4. Compressed in the first half of the flow direction along the apex of the drum kit The face analysis of the face fits the mathematical expression of the bulge profile.

Fig. 3 Slice positions on bump

Fig. 4 Dimensionless slice lines of bump

Its basic line passes through the symmetry plane of the apex of the bulge, at the starting point and water The flat direction is a small angle, approximate to a straight line, the starting angle
Degree θ ≈ 11 °, followed by a concave curve with a small change in angle An upwardly convex curve is leveled at the end profile. In order to analyze the mechanism of the bulge to remove the boundary layer, according to the reverse The calculated bulge line recreates the bulge compression surface, while One spiral body (general cone) is constructed according to the reference line to divide Analysis of comparison. Figure 5 and Figure 6 show the bulge and the spine, respectively.

Fig. 5 Surface pressure distribution on bump

Fig. 6 Surface pressure distribution on cone

Surface pressure distribution map. As can be seen from Figure 5, in the head of the drum There is 1 high pressure zone, other areas have lower pressure, and the flow does not have Axisymmetric, so there is a difference in height from the surface of the bulge Such a pressure gradient along the direction of the extension, this pressure distribution can The air flow is pushed to the sides. For the body, there is also a pressure on the head. Higher area, but lower pressure, and this pressure distribution is the axis Symmetrical, the flow is also axisymmetric, flowing along the surface of the spiral body
The circumferential direction is evenly distributed, so there is no effect of migrating airflow. Correct The cone flow (positive cone) we are familiar with, the parameters of the conical surface The distribution is uniform, there is no pressure gradient, so it can't play The role of the transfer surface layer. Analysis believes that it is due to the pressure in the drum The middle height on the shrink surface and the low pressure distribution on both sides make The pressure gradient acts to move the boundary layer to the sides, It acts to eliminate the flow of the boundary layer. Of course, the inlet lip Sweeping is another guarantee that the incoming flow layer does not enter the intake duct.

Key [ 1 , 4] .

3 front fuselage / inlet integrated flow analysis
(1) Calculation conditions Since the boundary layer barrier is removed, the intake air The mouth and the front fuselage of the aircraft become an integrated design, the front of the air intake The velvet bag surface is merged with the front fuselage. Produce a pressure distribution that pushes the boundary layer away from the air inlet, and at the same time To the compression of the airflow. In the mechanism of the DSI inlet In the process of analysis, it is also necessary to adopt an integrated idea to Considering the front fuselage as a whole, analyzing the drum package surface and progress The interaction between the angle and shape of the lip mask.

The specific tactical technical performance requirements for the aircraft are not yet available. Announced, from the information already available, its design flight Mach The number is around 1. 5 or so. Therefore, select 1.5 as the calculated flow horse. Hertz.

(2) Wave system structure Figure 7 and Figure 8 show no attack Angle, no side slip angle, vertical symmetry plane of the aircraft and bulge of the drum The Mach number of the flow field in the horizontal section. As can be seen from Figure 7, The supersonic flow forms a oblique shock wave at the head of the aircraft, and the airflow is in the wave. After decelerating and boosting, and then due to the shape of the fuselage, the airflow Born and expanded, a second oblique shock wave was formed in front of the upper cockpit. The airflow in the rear of the cockpit expands and accelerates. Can be seen from Figure 8. Out, the supersonic flow flows through the oblique shock wave of the aircraft's head, and the airflow is reduced. Speed boost, and then due to the shape of the fuselage, airflow expansion Speed, the Mach number of the airflow before the inlet of the inlet is about 1.44, in the drum The apex of the packet forms a oblique shock wave, the oblique shock wave does not have the upper lip Intersect, but at a certain distance in front of the upper lip, the airflow passes through the first After the oblique shock wave, the deceleration is supercharged in the front of the drum and formed before the inlet.

The second shock wave, the shock angle is larger, and it is close to the surface of the bump Straight, the wavefront Mach number is about 1.2, the wave is the subsonic flow, the whole

Fig. 7 Iso-Mach number contours (symmetric plane, α = 0°)

Fig. 8 Iso-pressure contours on horizontal plane(α= 0°)

The airflow in the air intake is subsonic.

(3) Exclusion of the surface layer due to the inlet bulge with - 5° installation angle, for this purpose also calculates the aircraft's 5° angle of attack The flow field during flight, so that the direction of the flow before the bulge can be basically It is facing the axial direction of the drum kit. Figure 9 shows the flow of the fuselage surface Line and pressure distribution, there is a higher pressure area in the head of the drum bag, Most of the forward flow was pushed to the sides of the drum. Due to the drum kit Close to the import lip on both sides.

 

[Inst]

I'm registered as Secretprojects on Sinodefenceforum.com - that was me

Thanks for the help, then, and I appreciate the message. What are the odds that we have someone competent on inlet design here?

Would be great to be just directly shot down on the "MFR at altitude doesn't determine high-altitude thrust", or for someone to confirm it, then explain the various optimizations to get MFR working supersonically.

For people who haven't seen the SDF thread:

The J-20 inlet area is between .67 and .95 m^2, depending on whether the DSI bump is treated transparently, and whether we're talking prototype 2012 (early inlets) or 2017+ (increased DSI bump size). For comparison purposes, the Su-27 is roughly .67 m^2, the F-22 is about 1.05 m^2 (but in practice, less, due to sloping inside the inlet), the MiG-31 is 1.1 m^2, the F-35 (treating bumps as transparent) is about 1.05 m^2 for two engines.

The F-35 inlet length is about 3 meters, compared to about 5.3 for the F-22, 8-8.5 on the J-20, 5.7 meters on the Eurofighter, 3.6 on the Su-27, 5.1 meters on the Rafale.

The Al-31 inlet diameter is about 900 cm, which gives us an area of .64 m^2 and a ratio of 4:1 in length to engine inlet diameter for the Su-27. There's 109 cm on the F135, which gives us a ratio of about 22:10 for the F-35, the F-22 gets a ratio of around 5:1 (F119 inlet diameter doesn't seem to be public), while the Rafale has about 73:10 ratio and the Eurofighter has about a 77:10 ratio.

This, of course, is ignoring the actual geometry of inlets (S-ducts), which will increase inlet length further.

As far as inlet length goes,

http://www.icas.org/ICAS_ARCHIVE/ICAS2002/PAPERS/643.PDF

This source (EADS VP) indicates that inlet length has a major drawback of weight; i.e, the shorter inlet can cut on aircraft weight. On the other hand, it's indicating that longer inlets reduce distortion (evenness of air pressure across the engine fan).

 

[Inst]

F-16 with DSI vs basic F-16:

Is the inlet larger or smaller when DSI is added?

J-10A vs J-10B:

 

[pegasus]

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

What I'm doing here is to try to get a discussion up on the J-20's inlets, in comparison to the F-35, F-22, and other aircraft.

The J-20's inlet length is rather unique; it's estimated at around 7-8.5 meters when the F-22 at max reaches around 5.5 meters and the F-35 reaches around 3 meters.

The initial theory I was floating was that the long inlet improves pressure recovery compared to shorter inlets, resulting in better mass flow rate into the engines at high speeds and altitudes, but the venturi effect implies that MFR maxes at Mach 1, i.e, once you get into the subsonic section of the inlet you can't use compression in a subsonic diffuser to increase MFR.

The other tentative theory is that the long inlet is designed to exploit the J-20's nose geometry, i.e, the chines and the nose airflow of the J-20 are intended to create shockwaves and pre-compress air entering the inlet, resulting in a higher pressure and thus higher MFR.

===

There's another question about the J-20 and F-35 DSI geometry. In leaked reports, the J-20 is a high-speed aircraft with very good top-speed. On the other hand, the F-35 is only rated to Mach 1.6, and begins to suffer performance issues related to the inlet near the top of its Mach range.

Both the J-20 and F-35 use DSI, with the J-20 having seen an increase in the size of the DSI bump between the prototype and the final version. A DSI bump, to an extent, is basically a variation of a conical inlet, so depending on its shape, it shouldn't create issues with supersonic performance.

How are the J-20 and F-35 DSI bumps different? How would the J-20 inlets deliver better high-speed performance than on the F-35?

J-20 has huge bumps, this increases drag, in fact F-35 has an ideal intake no bleed system, and it is optimised to the ideal pressure recovery of the intake, after Mach 1.6 pressure recovery falls drastically, by Mach 2 the engine will suffer drastic thrust loses, adding bleed systems only shows the intake is becoming inefficient, internal and external compression becomes only efficient after Mach 2.5, and they use mechanical variable geometry intakes such as those seen in SR-71 that move the intake central forebody cone, in fact adding bleed systems shows how inefficient the DSI intake has become, the main advantage of the DSI intake over the caret type is because it has no mechanical parts, no bleed system, if they reduce the bump size also it become less efficient, J-20 is a Mach 1.8 fighter, specially when its intakes are reducing pressure recovery the thrust is less efficient, the boundary layer also has to coincide with the intake cowl so by going farther than is design Mach limit it is ingesting more boundary layer and adding mechanical parts simply destroys the whole purpose of the design, it was no mechanical parts to reduce weight.

The Chinese like propaganda, J-20 in order to have a truly efficient intake needs variable geometry intakes after Mach 2, adding any mechanical system makes DSI less attractive, in such a case it is better to opt for a caret type like F-22 and a caret type with variable geometry intake like Su-57



The pressure losses and distortions caused by the ingested boundary layer increase with Mach number and, beyond approximately M0.6, some form of boundary layer treatment becomes necessary. This can be done by diverting it or by bleeding it out of the captured airstream, a process that inevitably incurs a drag penalty.¶

https://eprints.soton.ac.uk/46202/1/AIAA-26830-529.pdf

 

[overscan (PaulMM)]

Intakes should ideally be as short as possible while fulfilling their requirements to slow the air down via shocks to Mach 0.5/0.6 or so (required speed for engine) and a nice even pressure distribution across the engine intake. Stealth adds line of site blocking which tends to increase length while inlet location may be set for aerodynamic considerations e.g. a pitot intake in the nose to eliminate boundary layer.

Longer intakes don’t mean extra thrust or higher speed.

 

[pegasus]

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





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



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



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

 

[Inst]

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]

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


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

 

[overscan (PaulMM)]

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

 

[Ronny]

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

http://www.afwing.com/intro/j20_dsi/j20_dsi-3.htm

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]

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

 

[pegasus]

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

http://www.afwing.com/intro/j20_dsi/j20_dsi-3.htm

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.

 

[aonestudio]

J-10C/J-10CE brochure from 2018 ZHUHAI AIRSHOW. Max. Mach number 1.8

 

[Inst]

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]

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.



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


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/cont...webt/F-35_Air_Vehicle_Technology_Overview.pdf

 

[Inst]

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.

 

[Inst]

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

 

[pegasus]

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


Any way DSI intakes are for speeds of Mach 1.7

 

[overscan (PaulMM)]

-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).

 

[Trident]

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!

 

[Fluff]

Maybe we should just ask the Israelis??

 

[pegasus]

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.


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

 

[Inst]

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.

 

[pegasus]

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

 

[Inst]

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.

 

[Ronny]

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

 

[kaiserd]

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]

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

 

[pegasus]

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

 

[pegasus]

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

 

[Deino]

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.

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.

 

[Trident]

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.