SDF Aerospace and Aerodynamics Corner

Engineer said:

Actually, I understand how intakes work better than you do, and in fact I have corrected you on many instances. Throwing out random terminologies does not indicate you know what you are talking about, especially when you need to be corrected on concepts as simple as area vs. volume.




DSI isn't being fixed isn't comparable to pitot intake. In fact, DSI creates both oblique and normal shocks.




Wrong! F-35's DSI is one type of intake whereas fixed-inlet is another type, therefore DSI being fixed does not make it an isentropic intake.



Yet they cannot reach Mach 3. You claimed by employing variable-geometry inlets alone, aircraft fly efficiently and go fast. Yet, the fact these aircraft cannot fly at Mach 3 while other aircraft can indicates while inlet efficiency is important, it does not determine the speed of an aircraft.



The more shocks you create, the more complex the inlet becomes. More complexity leads to increase weight, which cancels out advantages in increased pressure recovery ratio. SR-71 and XB-70 can only fly fast and do nothing else, and both of these aircraft turn like airliners. This illustrates my point quite clearly.

Fighter aircraft do not fly at Mach 3 because the excess weight of being able to fly at Mach 3 means no maneuverability. Within the operating speed of a fighter aircraft (from 0 up to and slightly exceeding Mach 2), DSI offers better pressure recovery ratio than fixed inlets like those found on F-4D. That's all the matters. Bringing in Mach 3 aircraft doesn't challenge this point in anyway.

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to start you have not corrected me, all your concepts here are wrong.

To start, variable geometry intakes are used to increase the flight envelop for an aircraft that fixed can not increase, a fixed intake has limits and variable geometry intake surpass those limits.


F-15 has higher pressure recovey than F-4D and DSI intakes and they do it by varying the intake size capture area and increasing the number of shocks, fixed intakes can not do that


Second one of the reasons SR-71 can not turn has nothing to do with weight, which shows how wrong you are and you lack of knowledge of physics


At mach 3 any aircraft flies almost a kilometer a second thus it can not have a small turn, second all jets when they fly faster drag increases thus lift reduces, so it is not weight but speed and drag, a jet with DHI or DSI will not fly and turn at Mach 3 better than one with variable geometry intakes

In fact lower pressure recovery means more fuel burnt, thus a fixed DSI means the need to carry more fuel than a variable geometry intake.

Engineer said:

Translating the intake cone more is not going to make a MiG-21 reach Mach 3. If it is that simple, the designers would have done it and they would have a Mach 3 MiG-21. Yet, they didn't, because they couldn't. Inlet does not power the aircraft and so does not make an aircraft fly fast. What makes an aircraft fly fast are airframe geometry, thrust and exhaust velocity of the engine.



.

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this is one of the things that show you do not understand intakes


The inlets of the Mach 3+ SR-71 aircraft are specially designed to allow cruising flight at high speed. The inlets of the SR-71 actually produce thrust during flight


The variable-geometry inlets for the engines were quite complex and intricate. The most prominent feature was a hydraulically-actuated conical spike which was automatically moved forward or aft by the Air Inlet Computer as required to keep the supersonic shockwave properly positioned in relation to the inlet throat. Working in conjunction with a series of bypass ducts and doors, the spike prevented supersonic air from entering the inlet and maintained a steady flow of subsonic air for the engine. At Mach 3.2 cruise the inlet system itself actually provided 80 percent of the thrust and the engine only 20 percent, making the J58 in reality a turbo-ramjet engine

MiG-29 said:

to start you have not corrected me, all your concepts here are wrong.

Click to expand...

I have corrected you many times. To name a few, you equate capture area with intake volume; optimal speed with top-speed; DSI with fixed inlet. You think you know what you are talking about, but you do not. Trying to win an argument by randomly throwing out terminologies without understanding what they actually mean and how they relate to one another is why you got many things wrong.

MiG-29 said:

To start, variable geometry intakes are used to increase the flight envelop for an aircraft that fixed can not increase, a fixed intake has limits and variable geometry intake surpass those limits.

F-15 has higher pressure recovey than F-4D and DSI intakes and they do it by varying the intake size capture area and increasing the number of shocks, fixed intakes can not do that

Click to expand...

Once again, you are making the mistake of equating DSI with fixed-inlet. Yes, DSI is fixed, but this is different from the fixed-inlet like that found on F-16. Your attempt to lump DSI with fixed inlets does not alter the fact that DSI performs better than some variable-geometry inlets.

The role of ramps on variable-geometry inlets is to generate multiple shockwaves, with the trailing part of the ramp allowing air to diverge and further slow down from transonic speed. Fixed intake like that on F-16 does not do this, but DSI does and does so without complex mechanisms. It increases pressure recovery ratio without needing to create higher number of shocks.

For comparison, the 2D variable-geometry inlet on F-4D creates three shocks. The J-10B's DSI creates one oblique and normal shocks yet achieve better pressure recovery ratio.

MiG-29 said:

Second one of the reasons SR-71 can not turn has nothing to do with weight, which shows how wrong you are and you lack of knowledge of physics

At mach 3 any aircraft flies almost a kilometer a second thus it can not have a small turn, second all jets when they fly faster drag increases thus lift reduces, so it is not weight but speed and drag, a jet with DHI or DSI will not fly and turn at Mach 3 than one with variable geometry intakes

Click to expand...

SR-71 can barely turn even at subsonic speed! The lack of ability for this aircraft to turn is because its design aims to go fast and forgoes any maneuverability. To go fast, more shocks are needed to slow down air, thereby increasing complexity of the inlet. This complexity leads to more weight, cancels out advantages gained in improvement of the inlet system. This is actual physics, not the pseudo physics that you make up on the spot in your desperate attempt to portray your opinion as facts.

MiG-29 said:

In fact lower pressure recovery means more fuel burnt, thus a fixed DSI means the need to carry more fuel than a variable geometry intake.

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Wrong! The amount of fuel needed is dependent on the efficiency of engines and the airframe. Efficiency of inlet is a factor, but employing variable-geometry inlet does not magically makes an aircraft better than one that employs fixed inlets.

We have made this comparison before: MiG-31 needs to carry more fuel for comparable performance of a F-22, as MiG-31 needs afterburner in supersonic flight while F-22 doesn't. F-14, F-15 cannot supercruise without afterburner. Using afterburner means more fuel burnt, despite the use of variable-geometry inlets. Facts contradict your claim.

MiG-29 said:

this is one of the things that show you do not understand intakes


The inlets of the Mach 3+ SR-71 aircraft are specially designed to allow cruising flight at high speed. The inlets of the SR-71 actually produce thrust during flight

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The variable-geometry inlets for the engines were quite complex and intricate. The most prominent feature was a hydraulically-actuated conical spike which was automatically moved forward or aft by the Air Inlet Computer as required to keep the supersonic shockwave properly positioned in relation to the inlet throat. Working in conjunction with a series of bypass ducts and doors, the spike prevented supersonic air from entering the inlet and maintained a steady flow of subsonic air for the engine. At Mach 3.2 cruise the inlet system itself actually provided 80 percent of the thrust and the engine only 20 percent, making the J58 in reality a turbo-ramjet engine

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Click to expand...

You did not address the fundamental point: MiG-21 cannot fly at Mach 3, despite using variable-geometry inlet. You attributed this to the fact that MiG-21 inlet cone does not have as much movement as SR-71's, but you cannot explain why the designers at MiG did not increase the range of movement if this is all they need to do to get the aircraft to reach Mach 3.

The reality is that employing a particular inlet system does not magically makes an aircraft go fast, hence MiG-21, F-14, F-15, etc. cannot reach Mach 3. SR-71's ability to go fast goes far beyond its variable-geometry inlets. It is the combination of airframe design, and the unique ability of SR-71 to convert its engines into ramjets that allows this aircraft to fly at Mach 3.

Finally, I want to add that using bold letters and large font size does absolutely nothing to enhance your arguments. It is no different than shouting instead of arguing logically.

Engineer said:

I have corrected you many times. To name a few, you equate capture area with intake volume; optimal speed with top-speed; DSI with fixed inlet. You think you know what you are talking about, but you do not. Trying to win an argument by randomly throwing out terminologies without understanding what they actually mean and how they relate to one another is why you got many things wrong.



Once again, you are making the mistake of equating DSI with fixed-inlet. Yes, DSI is fixed, but this is different from the fixed-inlet like that found on F-16. Your attempt to lump DSI with fixed inlets does not alter the fact that DSI performs better than some variable-geometry inlets.

The role of ramps on variable-geometry inlets is to generate multiple shockwaves, with the trailing part of the ramp allowing air to diverge and further slow down from transonic speed. Fixed intake like that on F-16 does not do this, but DSI does and does so without complex mechanisms. It increases pressure recovery ratio without needing to create higher number of shocks.

For comparison, the 2D variable-geometry inlet on F-4D creates three shocks. The J-10B's DSI creates one oblique and normal shocks yet achieve better pressure recovery ratio.



SR-71 can barely turn even at subsonic speed! The lack of ability for this aircraft to turn is because its design aims to go fast and forgoes any maneuverability. To go fast, more shocks are needed to slow down air, thereby increasing complexity of the inlet. This complexity leads to more weight, cancels out advantages gained in improvement of the inlet system. This is actual physics, not the pseudo physics that you make up on the spot in your desperate attempt to portray your opinion as facts.



Wrong! The amount of fuel needed is dependent on the efficiency of engines and the airframe. Efficiency of inlet is a factor, but employing variable-geometry inlet does not magically makes an aircraft better than one that employs fixed inlets.

We have made this comparison before: MiG-31 needs to carry more fuel for comparable performance of a F-22, as MiG-31 needs afterburner in supersonic flight while F-22 doesn't. F-14, F-15 cannot supercruise without afterburner. Using afterburner means more fuel burnt, despite the use of variable-geometry inlets. Facts contradict your claim.

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As the SR-71 increases its speed, the inlet varies its exterior and interior geometry to keep the cone-shaped shock wave and the normal shock wave optimally positioned. Inlet geometry is altered when the spike retracts toward the engine, approximately 1.6 inches per 0.1 Mach. At Mach 3.2, with the spike fully aft, the air-stream-capture area has increased by 112 percent and the throat area has shrunk by 54 percent.



here is one of your great fallacies, you never corrected me in fact you made two very false statement, one is intake geometry does not change capture area and SR-71`s intake does generate thrust.

Engineer said:

Secondly, you got many points wrong:
Capture area refers to the cross sectional area of the intake, not the volume of air that intake takes. See, one is "area" which is in m^2 and the other one is "volume" which is m^3.
The movement of the cone has the same purpose as adjustment of ramps, and that purpose is to position the oblique shockwaves close to the intake lip to improve pressure recovery.
Mass flow to the engine is determined by size of the inlet and the intake ducts. These are fixed.

.

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MiG-21 is slower because it is not a turboramjet, simple like that. but MiG-21 and SR-71 have the same type of intake an axisymmetric one

In fact E-166 a MiG-21 derivative achieved Mach 2.82


Max. speed at altitude 15,4 km, km/h (M=) 3030 (2,82)

MiG-29 said:

As the SR-71 increases its speed, the inlet varies its exterior and interior geometry to keep the cone-shaped shock wave and the normal shock wave optimally positioned. Inlet geometry is altered when the spike retracts toward the engine, approximately 1.6 inches per 0.1 Mach. At Mach 3.2, with the spike fully aft, the air-stream-capture area has increased by 112 percent and the throat area has shrunk by 54 percent.

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Variable-geometry inlets vary their geometry to keep the oblique and normal shockwaves optimally positioned with respect to the intake lip. What SR-71 does as far as shockwaves are concerned is no different. As I have pointed out, MiG-21 also has an adjustable intake cone, yet it cannot reach Mach 3.

What makes SR-71 special is that it can convert its engines into ramjets, and has an airframe specially designed to fly fast. This makes the aircraft far more complex than even that of F-14 and F-15, at the cost of weight. As I have mentioned before, this increase in weight cancels out advantages of gain in performance. All design effort goes into making the aircraft fly fast, and the results is that SR-71 lacks maneuverability and turns like an airliner.

DSI on the other hand save weight, translate indirectly to less demand for the engines. With just an oblique and normal shock it nevertheless has better performance over 2D variable-geometry inlets with three shocks. A comparison which you are still afraid and refuse to acknowledge is as follow:

Pressure recovery ratio of J-10B's DSI intake: 0.87 at Mach 2.0, 0.91 at Mach 1.8
Pressure recovery ratio of F-4D variable-geomtry inlet: 0.87 at Mach 2.0, 0.89 at Mach 1.8



You kept talking about pressure recovery ratio, but when figures are presented and disagree with your own opinions, you choose to ignore them. You even go as far as coming up with pseudo physics to explain away your errors. Unfortunately for you, that doesn't make your claim less wrong.

Observe what your own has to say regarding DSI:

How Things Work said:

The F-35 inlet, however, is positioned flush against the fuselage, and just in front of the inlet opening is a raised surface, or bump, that pushes much of the boundary layer air off to the sides and away from the inlet. The bump serves another purpose: During supersonic flight, it compresses and slows the air passing over it into an oblique shock wave. The air is still moving supersonically, however, and it is slowed down to subsonic speeds after passing through a normal shock wave that forms at the mouth of the inlet. The simplicity of the JSF design makes for an inlet that requires less maintenance, reduces aircraft weight by 300 pounds, and costs $500,000 less than a traditional fighter inlet.

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I have corrected you almost in every single post. I now have to do it again.

MiG-29 said:

here is one of your great fallacies, you never corrected me in fact you made two very false statement, one is intake geometry does not change capture area and SR-71`s intake does generate thrust.

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Referring to truth as fallacies show you do not understand what constitute as fallacies, and that you are in denial. You are unable to accept the reality that your claims are simply wrong.

Let me tell you what fallacy is. Fallacy is improper argument and reasoning. Examples include:

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  - you argue variable-geometry inlets appear on fast flying aircraft, then conclude that aircraft speed is a function of inlet design. In reality, it is the other way around with inlet design being the function of speed.
  
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  - you assume DSI has poor performance to argue DSI has poor performance.
  
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  - you use one example of a fast flying aircraft to argue variable-geometry inlet makes an aircraft superior, while ignoring aircraft such as J-10B and F-22 that has better performance than many aircraft that employ variable-geometry inlets.
  
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  - you claim by using variable-geometry inlets alone makes any aircraft fly more efficiently and faster. Yet, when you are challenged with exceptions of aircraft using variable-geometry inlets that cannot fly as fast, you choose to disregard them. You also disregard the exception that hypersonic vehicles do not employ variable-geometry inlets.
  
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  - you incorrectly classified DSI as a fixed inlet even though DSI and fixed-inlet are distinct classifications.
  
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  - you incorrectly assumed inlet design determines performance of an aircraft, when it is the combination of airframe design, thrust and exhaust velocity of an engine that determines flight performance. Another example is your attribution of performance to speed alone, when performance is determined by many factors including maneuverability.
  
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  - you got challenged on your claims and is shown that DSI is not inferior with examples. Instead of addressing the point of contention you go on and on about things that are irrelevant. SR-71 and XB-70 are not fighter aircraft and has nothing to do with your original claim that J-10B and J-20 using DSI have inferior performance.
  

Aside from the above, you also incorrectly treat capture area as volume of air that an inlet can take; you incorrectly equate optimal speed (or design speed) as absolute top-speed; you incorrectly claimed F-22 fixed-inlet to be traditional fighter inlet; you incorrectly claimed DSI to be the same as F-104 inlet; etc.

You are wrong in claiming inlets produce thrust, as it is the engine and afterburner which produces thrust.

You are also wrong in claiming variable-geometry inlet can change the capture area. In your quote, the air-stream-capture-area is so named because it is an imaginary capture area. It does not refer to the physical area of the inlet, or the so called cowl-capture-area.

I have corrected you, and continue have to correct you on all of the above wrong statements.

MiG-29 said:

MiG-21 is slower because it is not a turboramjet, simple like that. but MiG-21 and SR-71 have the same type of intake an axisymmetric one

Click to expand...

In other words, it is not variable-geometry inlet that determines the performance of the aircraft. This is what I have been pointing out all along.

MiG-29 said:

In fact E-166 a MiG-21 derivative achieved Mach 2.82


Max. speed at altitude 15,4 km, km/h (M=) 3030 (2,82)

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Click to expand...

It is as you said a derivative, with redesigned airframe and a more powerful engine. The increase in performance is a result of these changes, not by the mere adoption of variable-geometry inlet.

My point still stands: if variable-geometry inlet alone determines the performance of an aircraft, then MiG-21 should be able to fly at Mach 3. There would not need to be a derivative. The fact it cannot means there are other factors involved; by your own admission, MiG-21 cannot convert its engine into ramjet.

Engineer said:

Variable-geometry inlets vary their geometry to keep the oblique and normal shockwaves optimally positioned with respect to the intake lip. What SR-71 does as far as shockwaves are concerned is no different. As I have pointed out, MiG-21 also has an adjustable intake cone, yet it cannot reach Mach 3.

What makes SR-71 special is that it can convert its engines into ramjets, and has an airframe specially designed to fly fast. This makes the aircraft far more complex than even that of F-14 and F-15, at the cost of weight. As I have mentioned before, this increase in weight cancels out advantages of gain in performance. All design effort goes into making the aircraft fly fast, and the results is that SR-71 lacks maneuverability and turns like an airliner.

DSI on the other hand save weight, translate indirectly to less demand for the engines. With just an oblique and normal shock it nevertheless has better performance over 2D variable-geometry inlets with three shocks. A comparison which you are still afraid and refuse to acknowledge is as follow:

Pressure recovery ratio of J-10B's DSI intake: 0.87 at Mach 2.0, 0.91 at Mach 1.8
Pressure recovery ratio of F-4D variable-geomtry inlet: 0.87 at Mach 2.0, 0.89 at Mach 1.8

You kept talking about pressure recovery ratio, but when figures are presented and disagree with your own opinions, you choose to ignore them. You even go as far as coming up with pseudo physics to explain away your errors. Unfortunately for you, that doesn't make your claim less wrong.

Observe what your own

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has to say regarding DSI:

Click to expand...

Hahaha
the MiG Ye-166 can fly at　Mach 2.82 and has a MiG-21 style intake.

The F-15 flies faster and has better pressure recovery than DSI.

DSI are fixed can not change the geometry of the intake, simple like that.

Because DSI is fixed it has a intake capture area fixed or a fixed intake cowl mach number.

DSI is inferior in pressue recovery to the mixed or external intakes of F-14, F-111, F-15, SR-71, XB-70 and Ye-166 simply because variable geometry intakes adapt the intake cowl to different mach numbers.

Su-35S is as agile as F-22 in poststall.

Su-35S has a mixed compression intake and flies faster than J-10B

Apart from the two dimensional configuration, axisymmetric mixed
compression intakes are widely used in fighter aircrafts. While 2D intakes of this type
can be found on modern aircrafts like F18 and Su30, axisymmetric intakes of the
same type are present on Mig 21 and SR71 Blackbird.

Engineer said:

I have corrected you almost in every single post. I now have to do it again.



Referring to truth as fallacies show you do not understand what constitute as fallacies, and that you are in denial. You are unable to accept the reality that your claims are simply wrong.

Let me tell you what fallacy is. Fallacy is improper argument and reasoning. Examples include:

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  - you argue variable-geometry inlets appear on fast flying aircraft, then conclude that aircraft speed is a function of inlet design. In reality, it is the other way around with inlet design being the function of speed.
  
• Please, Log in or Register to view URLs content!
  - you assume DSI has poor performance to argue DSI has poor performance.
  
• Please, Log in or Register to view URLs content!
  - you use one example of a fast flying aircraft to argue variable-geometry inlet makes an aircraft superior, while ignoring aircraft such as J-10B and F-22 that has better performance than many aircraft that employ variable-geometry inlets.
  
• Please, Log in or Register to view URLs content!
  - you claim by using variable-geometry inlets alone makes any aircraft fly more efficiently and faster. Yet, when you are challenged with exceptions of aircraft using variable-geometry inlets that cannot fly as fast, you choose to disregard them. You also disregard the exception that hypersonic vehicles do not employ variable-geometry inlets.
  
• Please, Log in or Register to view URLs content!
  - you incorrectly classified DSI as a fixed inlet even though DSI and fixed-inlet are distinct classifications.
  
• Please, Log in or Register to view URLs content!
  - you incorrectly assumed inlet design determines performance of an aircraft, when it is the combination of airframe design, thrust and exhaust velocity of an engine that determines flight performance. Another example is your attribution of performance to speed alone, when performance is determined by many factors including maneuverability.
  
• Please, Log in or Register to view URLs content!
  - you got challenged on your claims and is shown that DSI is not inferior with examples. Instead of addressing the point of contention you go on and on about things that are irrelevant. SR-71 and XB-70 are not fighter aircraft and has nothing to do with your original claim that J-10B and J-20 using DSI have inferior performance.
  

Aside from the above, you also incorrectly treat capture area as volume of air that an inlet can take; you incorrectly equate optimal speed (or design speed) as absolute top-speed; you incorrectly claimed F-22 fixed-inlet to be traditional fighter inlet; you incorrectly claimed DSI to be the same as F-104 inlet; etc.

You are wrong in claiming inlets produce thrust, as it is the engine and afterburner which produces thrust.

You are also wrong in claiming variable-geometry inlet can change the capture area. In your quote, the air-stream-capture-area is so named because it is an imaginary capture area. It does not refer to the physical area of the inlet, or the so called cowl-capture-area.

I have corrected you, and continue have to correct you on all of the above wrong statements.




In other words, it is not variable-geometry inlet that determines the performance of the aircraft. This is what I have been pointing out all along.



It is as you said a derivative, with redesigned airframe and a more powerful engine. The increase in performance is a result of these changes, not by the mere adoption of variable-geometry inlet.

My point still stands: if variable-geometry inlet alone determines the performance of an aircraft, then MiG-21 should be able to fly at Mach 3. There would not need to be a derivative. The fact it cannot means there are other factors involved; by your own admission, MiG-21 cannot convert its engine into ramjet.

Click to expand...

hahaha

Ye-166, Sr-71 and MiG-21 have the same type of variable geometry intake just optimised to different cowl intake mach number

hahaha you can not admit your knowledge is wrong haha

MiG-29 said:

Hahaha
the MiG Ye-166 can fly at　Mach 2.82 and has a MiG-21 style intake.

Click to expand...

Yet, MiG-21 cannot fly as fast even with a similar intake system. This shows it is not the inlet that determines the performance of the aircraft. What determine the performance are the airframe and engine, both are modified on the Ye-1666.

MiG-29 said:

The F-15 flies faster and has better pressure recovery than DSI.

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This does not dispute the fact that with only two shocks, J-10B's DSI in 2005 has better performance than the inlets of F-4D, the latter being variable-geometry generating three shocks.

MiG-29 said:

DSI are fixed can not change the geometry of the intake, simple like that.

Click to expand...

DSI does not have to change its geometry. It is an advantage, not a disadvantage. In fact, by not having to change geometry, it reduces weight significantly as well as drag. The result is increase in performance over variable-geometry inlets like that on F-4D.

MiG-29 said:

Because DSI is fixed it has a intake capture area fixed or a fixed intake cowl mach number.

Click to expand...

Cowl capture area is the physical size of the mouth of the inlet, and is fixed also in the case of variable-geometry inlets. What variable-geometry inlets do is adjust the position of the oblique shockwaves to position them as close to the intake lip as possible. It does not make the mouth of the inlet wider or narrower.

MiG-29 said:

DSI is inferior to mixed or external intakes of F-14, F-111, F-15, SR-71, XB-70 and Ye-166 simply because variable geometry intakes adapt the intake cowl to different mach numbers.

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And variable-geometry inlet like that of F-4D is inferior to DSI, even though the variable-geometry inlet on F-4D is supposed to adapt the intake cowl to different mach number.

Also, in your list of aircraft, F-111 have poor maneuverability compared to F-14 and F-15. SR-71 and XB-70 have worst maneuverability of all. Complex inlet system leads to increase in weight and cancels out advantages gained in any increase of pressure recovery ratio.

MiG-29 said:

Su-35S is as agile as F-22 in poststall.

Click to expand...

Su-35S cannot supercruise at the same speed as F-22, despite employing what you claimed to be more efficient inlet system. This shows performance of an aircraft is based on other factors, and not solely on inlet design.

MiG-29 said:

Su-35S has a mixed compression intake and flies faster than J-10B

Click to expand...

This is only your opinion, based on your assumption that employing variable-geometry inlets automatically means an aircraft has better performance. We do not know what the top-speed of J-10B other than that it is perfectly capable of reaching and exceeding Mach 2.

MiG-29 said:

Apart from the two dimensional configuration, axisymmetric mixed
compression intakes are widely used in fighter aircrafts. While 2D intakes of this type
can be found on modern aircrafts like F18 and Su30, axisymmetric intakes of the
same type are present on Mig 21 and SR71 Blackbird.

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Click to expand...

And many of these aircraft employing variable-geometry intakes cannot fly at Mach 3. What does this say? This says other factors such as airframe geometry and engine choice are involved. Use of variable-geometry intakes does not automatically means an aircraft has better performance.