Intake design and general stealth discussions

[Ronny]

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

From frontal, DSI is stealthier than caret inlet because the smooth curve will have fewer surface scattering point than multiple sharp edges, surface discontinuity and moving parts of variable inlet. That is an advantage for VLO aircraft. Second generation of stealth aircraft use blended curve instead of flat facet like F-117 for the same reason.

 

[Inst]

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.

I've told you repeatedly, I haven't been arguing that the J-20 is a pure interceptor.

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

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

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

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

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

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

====

@Trident:

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

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

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

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

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

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

====

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

====

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

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

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

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

====

One last thing, @Trident ,

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

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

 

[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.

I've told you repeatedly, I haven't been arguing that the J-20 is a pure interceptor.

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

...

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

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

 

[Inst]

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.

I've told you repeatedly, I haven't been arguing that the J-20 is a pure interceptor.

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

...

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

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

FYI, no one wants to argue the topic definitively, and more so because Blitzo merged the thread into the inlet thread.

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

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

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

 

[pegasus]

From frontal, DSI is stealthier than caret inlet because the smooth curve will have fewer surface scattering point than multiple sharp edges, surface discontinuity and moving parts of variable inlet. That is an advantage for VLO aircraft. Second generation of stealth aircraft use blended curve instead of flat facet like F-117 for the same reason.

Ronny stealth is better understood if you consider light.

Light behaves like particle and wave, waves have diffraction, waves go around corners, see this point, this is very important.

I will give you what is really stealth, imagine a person in the middle of a street running from north to south, his eyes are are looking to the south and his back is facing the north.

there is a perpendicular street, running from east to west ahead of him, where a tank that is going to the west is in front of him, the tank has its cannon aiming towards the west and two lamps at the front of the tank, pointing at the west too, so the tank crew can see what is ahead of the tank.

It is a dark night, when the tank fires a shell, it is like a particle, the shell goes in a direction from east to west, then it will never harm the person standing on the street running from north to south.

Now the tank turns on the lights, and the same person standing in the middle of the street that goes from north to south can see the tank, why can that person see the lights? why? simple light does not travel only from east to west but also from north to south and south to north.

If you understand that you can see and understand stealth, light and electromagnetic waves bend around corners, they do not behave only like particles, does not matter if it is DSI or caret intakes, some part of the electromagnetic wave always goes back to the radar, always, and the creeping wave exists because light and electromagnetic waves bend around corners, so you always detect DSI or caret intakes; for a powerful radar both are as visible as the whole aircraft, in few words stealth only exist if instead of looking at the tank that person is trying to see a firefly.


If you are walking at night in a very dark night and a person ahead of you is holding a lamp facing ahead of both of you, you will still be able to see your friends back, that is the creeping wave, the shadow is not total still there is some light because light travels in all directions.


Nothing is invisible, the caret or DSI intakes for powerful radars are visible, in the same way, B-2 is not invisible either, the main reason is diffraction, not reflection, reflection concentrates the electromagnetic emission in one direction, but diffraction goes everywhere so it does not matter the round surfaces, in fact in that respect is better the facets because they tend to concentrate the electromagnetic emission into a single direction, but diffraction and power density makes even a faceted object visible either to light or other frequencies, so remember light has also constructive interference in few words two lamps will concentrate light in one direction, so several radars will unmask stealth aircraft too, nothing is truly stealthy, people fantasize about stealth, the only thing that makes something stealthy is a very weak radar only that

 

[Trident]

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

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

Density =/= *total* pressure! Density is a static value, total pressure (from which pressure recovery is calculated) includes the contribution of kinetic energy due to forward motion.

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



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

Oblique Flow Relations - engineering.com

Computes ratios to free stream values across an oblique shock wave, turn angle and associated Mach numbers.

www.engineering.com

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

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

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

140kg/s is almost certainly the total intake air flow for the F135. Core compressor flow would then be 140/(0.57+1)=89kg/s (BPR = bypass flow / core flow).

 

[Trident]

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

You are totally ignoring pre-compression by the inlet shock system which, as already pointed out, is what decelerates the air to Mach 1.0 at the inlet opening and dramatically increases pressure and density in doing so!

 

[Trident]

Russia’s variable-inlet approach (their chosen approach to meet their requirements which is fair enough but probably also influenced by their relative lack of experience or knowledge re: DSI’s).

I doubt that's the reason. While I'll confess that I'm baffled by Sukhoi's choice, DSI isn't some kind of alien technology - it's a clever application of well-established supersonic cone flow math, feasible even with late-1950s design tools. What's more the idea (and its benefits to the more recent concept of stealth) was apparently no secret in Russia as early as the mid-1990s, probably years before what would become the J-20 adopted it:

https://www.secretprojects.co.uk/threads/tupolev-tu-160-projects.26/post-299593

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

 

[Inst]

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

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

Density =/= *total* pressure! Density is a static value, total pressure (from which pressure recovery is calculated) includes the contribution of kinetic energy due to forward motion.

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

View attachment 620442

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

Oblique Flow Relations - engineering.com

Computes ratios to free stream values across an oblique shock wave, turn angle and associated Mach numbers.

www.engineering.com

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

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

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

140kg/s is almost certainly the total intake air flow for the F135. Core compressor flow would then be 140/(0.57+1)=89kg/s (BPR = bypass flow / core flow).

Thing is, mass rate flow is defined in terms of DENSITY, not pressure. If total pressure recovery is used, the point is that stagnation pressure is a reasonable surrogate for density.

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

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

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

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

 

[pegasus]

But how, in the context of being an integrated part of a low observatory (“stealthy”) airframe, does any of that remotely add up to a DSI being “unstealthy”?
Especially when DSI’s and other “fixed” inlets are probably easier and better from such a “stealth” design perspective than a variable geometry inlet approach?
Why would F-35s, J-20s and other designs have DSI if there were inherently flawed in this way?
Where are all the variable geometry inlet “stealth” aircraft?

I did not say it has no low observable treatment, they do, however the supposedly superiority over caret is not stealth, its price and maintenance, if you want to say they are superior, their only superiority is lower price, beyond that carets still enjoy some advantages, DSI are not perfect nor the ultimate stealth intake type, if you want an efficient fixed intake for low price yes they are good, beyond that carets have areas such as the ability to fit them variable geometry features that turn them better at higher speeds than Mach 2.

Just remember stealth is not radar invisibility, is lower signature treatment, and improvements in radar technology render stealth useless, diffraction and power density output of radars make stealth useless

 

[Inst]

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

You are totally ignoring pre-compression by the inlet shock system which, as already pointed out, is what decelerates the air to Mach 1.0 at the inlet opening and dramatically increases pressure and density in doing so!

You're still ignoring law of conservation of mass; i.e, while you can play with dynamic pressure all you want, you can't create air that never existed in the first place.

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

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

 

[Trident]

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

 

[Jemiba]

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

 

[overscan (PaulMM)]

Mass flow rate = air density x velocity x area.

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

 

[Ronny]

Ronny stealth is better understood if you consider light.

Light behaves like particle and wave, waves have diffraction, waves go around corners, see this point, this is very important.

I will give you what is really stealth, imagine a person in the middle of a street running from north to south, his eyes are are looking to the south and his back is facing the north.

there is a perpendicular street, running from east to west ahead of him, where a tank that is going to the west is in front of him, the tank has its cannon aiming towards the west and two lamps at the front of the tank, pointing at the west too, so the tank crew can see what is ahead of the tank.

It is a dark night, when the tank fires a shell, it is like a particle, the shell goes in a direction from east to west, then it will never harm the person standing on the street running from north to south.

Now the tank turns on the lights, and the same person standing in the middle of the street that goes from north to south can see the tank, why can that person see the lights? why? simple light does not travel only from east to west but also from north to south and south to north.

If you understand that you can see and understand stealth, light and electromagnetic waves bend around corners, they do not behave only like particles, does not matter if it is DSI or caret intakes, some part of the electromagnetic wave always goes back to the radar, always, and the creeping wave exists because light and electromagnetic waves bend around corners, so you always detect DSI or caret intakes; for a powerful radar both are as visible as the whole aircraft, in few words stealth only exist if instead of looking at the tank that person is trying to see a firefly.


If you are walking at night in a very dark night and a person ahead of you is holding a lamp facing ahead of both of you, you will still be able to see your friends back, that is the creeping wave, the shadow is not total still there is some light because light travels in all directions.

Nothing is invisible, the caret or DSI intakes for powerful radars are visible, in the same way, B-2 is not invisible either, the main reason is diffraction, not reflection, reflection concentrates the electromagnetic emission in one direction, but diffraction goes everywhere so it does not matter the round surfaces, in fact in that respect is better the facets because they tend to concentrate the electromagnetic emission into a single direction, but diffraction and power density makes even a faceted object visible either to light or other frequencies,

That isn't exactly correct.
A complex object will reflect radar wave in many ways

But in general they can be grouped into:
Specular return: this is the most significant form of reflection, surface acts like a mirror for the incident radar pulse. Most of the incident radar energy is reflected according to the law of specular reflection ( the angle of reflection is equal to the angle of incidence).
Traveling/Surface wave return: an incident radar wave strike on the aircraft body can generate a traveling current on surface that propagates along a path to surface boundaries such as leading edge, surface discontinuous …etc, such surface boundaries can either cause a backward traveling wave or make the wave scattered in many directions
Diffraction: wave striking a very sharp surface or edge are scattered instead of following law of specular reflection.
Creeping wave return: this is a form of traveling wave that doesn’t face surface discontinuous and not reflected by obstacle when traveling along object surface , thus it is able to travel around the object and come back at the radar. Unlike normal traveling wave, creeping wave traveled along surface shadowed from incidence wave (because it has to go around the object). As a result, the amplitude of creeping wave will reduce the further it has to travel because it can’t feed energy from the incident wave in the shadow region. Creeping wave mostly traveled around a curved or circular object.


When the object size is at least 10 times the wavelength, we say it is in optical region, in this regime, specular mechanisms dominate. In this regime, “surface wave” mechanisms are small contributors to RCS, but are still present. If the wavelength is small relative to the surface, these waves are weak and their overlap will generate maximum backscatter when the radar signal is at grazing angles. When these currents encounter discontinuities, such as the end of a surface or change in material or sharp edge, they abruptly change and emit edge waves or edge diffraction. That why DSI is stealthier than a variable inlet, because DSI can be smooth while variable inlet must have gaps and discontinuities





To reduce the effect of discontinuities scattering at panel gaps or trailing edge, one common solution is serration



When the wavelength approach the size of the object (for simple object such as a sphere that mean 1 < 2πa/λ < 10 ) we say the object is in Mie region, in this region, a creeping wave travels around the object and back towards the receiver where it either interferes constructively or destructively with the specular return. So it can either increase or reduce the total RCS value. With that said, the magnitude of the creeping wave return is much smaller than specular return even in this case.


Some objects are easier to curve around than the other, stealth aircraft are designed in a way that would minimize the creeping wave return coming back to the source, such as not using pure cylinder shape

so remember light has also constructive interference in few words two lamps will concentrate light in one direction, so several radars will unmask stealth aircraft too, nothing is truly stealthy, people fantasize about stealth, the only thing that makes something stealthy is a very weak radar only that

Stealth isn't invisible, stealth is making your vehicles harder to detect to the enemy so you can attack them first. Everything is stealth if you stay far enough and nothing is stealth if you are close enough. The goal of stealth design is to make this distance shorter.

 

[Ronny]

improvements in radar technology render stealth useless

It doesn't as long as there are improvements in jamming technology because jamming depends on S/N ratio so stealthier aircraft cut down burn through distance

diffraction and power density output of radars make stealth useless

It doesn't, there are various ways to reduce/eliminate edge diffraction such putting RAM on trailing edge and leading edges

 

[Inst]

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

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

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

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

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

Dylan Stapp

Thrust and Design Analysis of the J58 Engine Dylan T. Stapp The University of Alabama, Department of Aerospace Engineering & Mechanics,...

dtstapp.blogspot.com

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

Pratt & Whitney J57 - Wikipedia

en.wikipedia.org

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

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

https://www.mtu.de/fileadmin/EN/2_Engines/2_Military_Aircraft_Engines/1_Fighter_Aircraft/EJ200/ProductLeaflet_EJ200.pdf

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

===

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

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

 

[Inst]

Lastly, working with new calculations, the MFR of AL-31 at Mach .9 and 35000 ft is about 65 kg/sec, when, if we assume the AL-31 is roughly an analogue to the F100-PW-229 engine, it should be about 81 kg / sec requirements. This implies a thrust loss of about 20%, ignoring factors like the fuselage diverting increased air into the inlets. The thrust loss in the chart, however, is roughly 50% from the chart you posted, so simply blaming MFR for thrust loss is inaccurate.

 

[pegasus]

That isn't exactly correct.
A complex object will reflect radar wave in many ways
View attachment 620459
But in general they can be grouped into:
Specular return: this is the most significant form of reflection, surface acts like a mirror for the incident radar pulse. Most of the incident radar energy is reflected according to the law of specular reflection ( the angle of reflection is equal to the angle of incidence).
Traveling/Surface wave return: an incident radar wave strike on the aircraft body can generate a traveling current on surface that propagates along a path to surface boundaries such as leading edge, surface discontinuous …etc, such surface boundaries can either cause a backward traveling wave or make the wave scattered in many directions
Diffraction: wave striking a very sharp surface or edge are scattered instead of following law of specular reflection.
Creeping wave return: this is a form of traveling wave that doesn’t face surface discontinuous and not reflected by obstacle when traveling along object surface , thus it is able to travel around the object and come back at the radar. Unlike normal traveling wave, creeping wave traveled along surface shadowed from incidence wave (because it has to go around the object). As a result, the amplitude of creeping wave will reduce the further it has to travel because it can’t feed energy from the incident wave in the shadow region. Creeping wave mostly traveled around a curved or circular object.


When the object size is at least 10 times the wavelength, we say it is in optical region, in this regime, specular mechanisms dominate. In this regime, “surface wave” mechanisms are small contributors to RCS, but are still present. If the wavelength is small relative to the surface, these waves are weak and their overlap will generate maximum backscatter when the radar signal is at grazing angles. When these currents encounter discontinuities, such as the end of a surface or change in material or sharp edge, they abruptly change and emit edge waves or edge diffraction. That why DSI is stealthier than a variable inlet, because DSI can be smooth while variable inlet must have gaps and discontinuities





To reduce the effect of discontinuities scattering at panel gaps or trailing edge, one common solution is serration
View attachment 620476


When the wavelength approach the size of the object (for simple object such as a sphere that mean 1 < 2πa/λ < 10 ) we say the object is in Mie region, in this region, a creeping wave travels around the object and back towards the receiver where it either interferes constructively or destructively with the specular return. So it can either increase or reduce the total RCS value. With that said, the magnitude of the creeping wave return is much smaller than specular return even in this case.


Some objects are easier to curve around than the other, stealth aircraft are designed in a way that would minimize the creeping wave return coming back to the source, such as not using pure cylinder Stealth isn't invisible, stealth is making your vehicles harder to detect to the enemy so you can attack them first. Everything is stealth if you stay far enough.

you are correct, the point is light is another electromagnetic wave, these diagram most of the time present stealth as if reflection is the only aspect of radars because diffraction while we experienced it every day is far more complex than the drawing you posted; on your every day life you see what a radar does, your eyes are radar receivers, they only happen to use a limited range of frequencies, light in fact presents what radars do, shadows, search lights, mirrors, they basically do what you are describing in those pictures, however we have a big advantage over aircraft, the sun has too much power density, aircraft radar have very limited power thus stealth seems far too complex, but is not, aircraft are more like us at night with lamps and search lights, thus RAM, faceting and creeping waves seem more complex, but they are not, radars by increasing the power density will see stealth aircraft well.

When you facet an aircraft or you add chines and flat sides, a weak radar is like a person looking at the distance trying to see details his eyes can not let him see due to distance.

Then stealth becomes "useful" most radars will see well F-22 at close ranges, it does not matter stealth treatment at short distances; at long distances then the ability gets reduced and chines, planform alignment then they become useful, why? simple because there are so little radar signals that faceting will weaken even further the signal and it will seem the aircraft has disappeared from radar.

The reality is if you have enough power density, in few words if the radar is powerful enough, it will become like daylight stealth will not work, why because on your daily life the sun let you see flat surfaces, round surfaces only some gases will appear invisible or some materials transparent like glass.


So when people think DSI or caret are really stealthy the reality is radars are so weak, that a few tricks here and there will reduce their visibility from some distances, add powerful radars and these are very visible. Is not that the creeping wave disappears by flattening the sides of F-22 forebody, if the radar signal is weak, it will be weakened enough for some types of radar to disappear from their view, but believe me your daily life at night and obscurity puts you in the same situation aircraft are, but add power density and you will discover nothing is stealth, it is only that aircraft are blind or have weak lamps aka weak radars

 

[Inst]

For Ronny and Pegasus's convo, I'll leave this here:

Quantum radar - Wikipedia

en.wikipedia.org

I.e, compromised, but otherwise jamming-immune radar.

The other thing is, Ronny has a point about how potent the jam / stealth combo is; if we go to a visible light analogy, imagine the object you're looking for is already very dark, a few lumens above black. Then someone flashes a searchlight in your eyes.

===

Regarding MFR, I'm going back to the NASA page and they're still implying that it's loss of static pressure at altitude that results in loss of thrust at altitude. Which is rather weird, because for a sufficiently sized inlet moving at a sufficient speed, you should have all the static pressure / MFR you need.

Here's a different article discussing why Turbojets (not Turbofans!) lose performance at altitude, albeit an old one:

https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930087116.pdf

 

[pegasus]

For Ronny and Pegasus's convo, I'll leave this here:



The other thing is, Ronny has a point about how potent the jam / stealth combo is; if we go to a visible light analogy, imagine the object you're looking for is already very dark, a few lumens above black. Then someone flashes a searchlight in your eyes.

https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930087116.pdf

you want to pretend to see i will be jammed, the reality is you will know some one is in front of you in the middle of the night, a jammer is a good way to know where an enemy aircraft is, basically anti-radiation missiles do that, basically that is the whole point when a fighter illuminates you, you can simply let his own radar to guide you where is him, thus passive system are always a way to attack, radar silence or jumping from one frequency to another is what modern stealth aircraft do.

But at the end the best stealth is passive detection, if someone is jumping from red to green or yellow lights, in example traffic light, you still will be able to see the traffic light, well radars are the best way to know where you are that is the reason they use IRST, in fact if we are 2 or 3 people they might find me and blind me but the other 2 will see the enemy position very clear so snipper will beat the enemy.

Will a good radar now detect DS I intakes? yes it will and same caret intakes, it is only when it is very dark that stealth works, F-117 was effective for radars on MiG-21 or MiG-23, modern aircraft like Su-35, F-22 or J-20 supposedly need to find F-117 at relatively longer ranges than MiG-21 that is why F-117 was obsolete and phased out.

The major problem for radars is energy is absorbed by the atmosphere so the signal weakens , is like a lamp at night there is a limit where you lights will let you see, cars at night driving have the same problem driving on highways, their lights only let them see not very far but sometimes they can be seen by some one else at longer distance, so radar sensitivity plays a very important part

 

[sferrin]

If you decelerate air to Mach 2 from Mach 1, you still have the same mass.

How are you going to do that without compressing it? And if it's compressed the mass per capture area goes up.

 

[Trident]

That's not the class room and no one has to grade others posts and make entries to the class register !

Fair enough, it was rather a rather arrogant thing to say. Sorry.

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

Actually it will, that's why pitot air speed measurements need a compressiblity correction for high speed flight: https://en.wikipedia.org/wiki/Equivalent_airspeed

The crux is that the air IS moving with respect to the pitot tube and constant density along its path into the pitot tube is merely an approximation that holds for low subsonic (as in <0.3) Mach numbers. Once supersonic, with the mass flow also crossing shock waves between free stream and inlet opening, you have to apply a new set of equations. They too conserve mass, but work rather differently in some regards, for example a pipe diffusor to decelerate air is now convergent rather than divergent.

 

[kaiserd]

It is a shame to see some contributors posting what appears to be little better than warmed up 90’s Russian-fan-boy “stealth is rubbish” narratives mixed in with some legitimate and interesting technical detail on this topic.
The problem is trying to unpick one from the other while the former acts to undoubtedly undermining the credibility of the latter.
Literally no one is saying that DSI are ”stealth perfection” or purporting the permanent magical powers and unquestionable immortal invulnerability of low-observatory aircraft.
But some of the generalised arguments presented above by some contributors are almost equally absurd in the opposite direction.

 

[pegasus]

It is a shame to see some contributors posting what appears to be little better than warmed up 90’s Russian-fan-boy “stealth is rubbish” narratives mixed in with some legitimate and interesting technical detail on this topic.
The problem is trying to unpick one from the other while the former acts to undoubtedly undermining the credibility of the latter.
Literally no one is saying that DSI are ”stealth perfection” or purporting the permanent magical powers and unquestionable immortal invulnerability of low-observatory aircraft.
But some of the generalised arguments presented above by some contributors are almost equally absurd in the opposite direction.

Radar cross-section σ is as defined as:

σ =

4·π·r2·Sr/St

mit

σ: measure of the target's ability to reflect radar signals in direction of the radar receiver, in [m²] St: power density that is intercepted by the target, in [W/m²] Sr: scattered power density in the range r, in [W/m²]

https://www.radartutorial.eu/01.basics/Radar Cross Section.en.html






see how it is basically the formula treats energy as reflected energy, basically as particles, best example a mirror or a metal plate, however light is not only reflected but also diffracted, F-22 will be detected, yes at short ranges for some powers densities, at longer ranges it will be not detected, then if you change power density, the RCS fluctuates upon the power density reflected, it is not fixed, yes σ is based upon the geometry of the aircraft but since F-22 is close to the radar power density is enough to allow a weak radar to detect it at closer ranges, but at long ranges it will not, you change the power density you change the range you will detect it at longer ranges, is F-22 invisible to radar? no it is not, it simply says low power density will make it harder to detect.

As i told before, the simplest example is the human eye, if you look at something close you can see details, further it goes from you details disappear. and if it is very far you will not see the object at all.


Thus is not that the object is invisible because is not, it means the power density is low, in this case of light, the amount of lumens is low.

DSI intakes are very visible at short ranges, radars detect them well, it is at longer ranges when shape or RAM will aid to reduce visibility. only that, the formula gives you that


The target radar cross-sectional area depends of:

• the airplane’s physical geometry and exterior features,
• the direction of the illuminating radar,
• the radar transmitters frequency,
• the used material types.

number two tells you that a plate at different angles from the radar will reduce or increase reflection RCS thus F-22 will have different RCS to different radars positioned at different directions but at the same distance, see that because detectability is also dependant upon the radar network, so what stealth does is makes it more expensive radar networks, but F-22 is not invisible nor DSI the best intake, it is detectable

 

[kaiserd]

It is a shame to see some contributors posting what appears to be little better than warmed up 90’s Russian-fan-boy “stealth is rubbish” narratives mixed in with some legitimate and interesting technical detail on this topic.
The problem is trying to unpick one from the other while the former acts to undoubtedly undermining the credibility of the latter.
Literally no one is saying that DSI are ”stealth perfection” or purporting the permanent magical powers and unquestionable immortal invulnerability of low-observatory aircraft.
But some of the generalised arguments presented above by some contributors are almost equally absurd in the opposite direction.

is not about Russian or not is about physics,

Radar cross-section σ is as defined as:

σ =

4·π·r2·Sr/St

mit

σ: measure of the target's ability to reflect radar signals in direction of the radar receiver, in [m²] St: power density that is intercepted by the target, in [W/m²] Sr: scattered power density in the range r, in [W/m²]

https://www.radartutorial.eu/01.basics/Radar Cross Section.en.html



see how it is basically the formula treats energy as reflected energy, basically as particles, best example a mirror or a metal plate, however light is not only reflected but also diffracted, F-22 will be detected, yes at short ranges for some powers densities, at longer ranges it will be not detected, they if you change power density, the RCS fluctuates upon the power density reflected is not fixed, yes σ is based upon the geometry of the aircraft but since F-22 is close to the radar power density is enough to allow a weak radar to detect it at closer ranges, but at long ranges it will not, you change the power density you change the range you will detect it at longer ranges, is F-22 invisible to radar? no it is not, it simply says low power density will make it harder to detect.

As i told before, the simplest example is the human eye, if you look at something close you can see details, further it goes from you details disappear. and if it is very far you will not see the object at all.


Thus is not that the object is invisible because is not, it means the power density is low, in this case of light, the amount of lumens is low.

DSI are very visible at short ranges, radars detecte them well, it is at longer ranges when shape or RAM will aid to reduce visibility. only that, the formula gives you that


The target radar cross-sectional area depends of:

• the airplane’s physical geometry and exterior features,
• the direction of the illuminating radar,
• the radar transmitters frequency,
• the used material types.

number two tells you that a plate at different angles from the radar will reduce or increase reflection RCS thus F-22 will have different RCS to different radars positioned at different directions but at the same distance, see that because detectability is also dependant upon the radar network, so what stealth does is makes it more expensive radar networks, but F-22 is not invisible nor DSI the best intake, it is detectable

I hope it is clear my comments are not in anger but in bewilderment.

Yes I assume literally everyone reading this understands the basics of most of what you are saying above, we understood it well before you said more or less the same thing 3-4 previously above.

It’s the conclusions that you choose to draw that are where you start loosing me completely.

The echo of Russiab-fan-boy-nonsense is various extremely self-serving (to your argument - no offence meant) assumptions around the availability, close proximity, and power of various powerful radars (plus associated command and control issues) and the required lack of counter measures (passive and active) by the low observatory aircraft in question. That remains an unlikely scenario and the worlds combat aircraft producers continue to produce new aircraft that are to lessor or greater extents low observatory in nature.

And going back to the actual topic here; what on earth does your argument actually have to do with DSI inlets?
DSI inlets are understood to be be superior to other more traditional “fixed” inlet designs, including but not limited to from a “stealth” perspective, as part of the overall integrated design of the aircraft in question.
And while you are correct a fully variable inlet with moving parts etc. should still have some advantages over a DSI in some parts of an aircraft’s performance envelope it also has other disadvantages such as weight, complexity and a greater difficulty in managing signature management.
As such statements like “DSI are very visible a short range” make little real sense.
More visible than an equivalent variable geometry inlet? More visible than an equivalent more traditional fixed inlet?
As I previously mentioned

 

[pegasus]

I




And going back to the actual topic here; what on earth does your argument actually have to do with DSI inlets?
DSI inlets are understood to be be superior to other more traditional “fixed” inlet designs, including but not limited to from a “stealth” perspective, as part of the overall integrated design of the aircraft in question.
And while you are correct a fully variable inlet with moving parts etc. should still have some advantages over a DSI in some parts of an aircraft’s performance envelope it also has other disadvantages such as weight, complexity and a greater difficulty in managing signature management.
As such statements like “DSI are very visible a short range” make little real sense.
More visible than an equivalent variable geometry inlet? More visible than an equivalent more traditional fixed inlet?
As I previously mentioned

I never said they are not effective i know when we write is hard to express ideas, DSI are good intakes, economic with good performance either in aerodynamics or stealth, F-35 has the ideal DSI intake, simple and effective, are they limited? yes to 1.6 Mach that is the ideal speed and pressure recovery, So basically it is cheap and easy to maintain, i only said that any technology has advantages as well as disadvantages, why JF-17 has porous holes for its bleeding system? i find it as a disadvantage, simpler is better on DSI intakes, they are fixed and inferior in subsonic speeds to traditional intakes in performance.

So if F-16 basically flies most of the time bellow Mach 1 then why Lockheed will do what Chengdu did with J-10, start building F-16s with DSI intakes, it makes no sense, J-10B/C unless it flies most of the time at supersonic speeds does not have any advantage in performance over F-16 with its traditional intake with boundary layer diverter.

On F-35 makes sense you need an aircraft with stealth, on J-10B/C is only price since it will not fly most of the time at supersonic speeds.

On J-20 if it has 2 porous holes nets for its bleeding system, it added weight and complexity, so tell me what is better for a supercruising aircraft? variable geometry or Fixed? if i want an aircraft to fly well between Mach 1.6 to Mach 2.4 i prefer a Caret with variable geometry, yes it is more expensive, but it is more effective; if the aircraft will fly below Mach 2 and supercruise at speeds in the region of Mach 1,6, then i will take DSI intakes like J-20

 

[Ronny]

you are correct, the point is light is another electromagnetic wave, these diagram most of the time present stealth as if reflection is the only aspect of radars because diffraction while we experienced it every day is far more complex than the drawing you posted; on your every day life you see what a radar does, your eyes are radar receivers, they only happen to use a limited range of frequencies, light in fact presents what radars do, shadows, search lights, mirrors, they basically do what you are describing in those pictures, however we have a big advantage over aircraft, the sun has too much power density, aircraft radar have very limited power thus stealth seems far too complex, but is not, aircraft are more like us at night with lamps and search lights, thus RAM, faceting and creeping waves seem more complex, but they are not, radars by increasing the power density will see stealth aircraft well.

When you facet an aircraft or you add chines and flat sides, a weak radar is like a person looking at the distance trying to see details his eyes can not let him see due to distance.

Then stealth becomes "useful" most radars will see well F-22 at close ranges, it does not matter stealth treatment at short distances; at long distances then the ability gets reduced and chines, planform alignment then they become useful, why? simple because there are so little radar signals that faceting will weaken even further the signal and it will seem the aircraft has disappeared from radar.

The reality is if you have enough power density, in few words if the radar is powerful enough, it will become like daylight stealth will not work, why because on your daily life the sun let you see flat surfaces, round surfaces only some gases will appear invisible or some materials transparent like glass.


So when people think DSI or caret are really stealthy the reality is radars are so weak, that a few tricks here and there will reduce their visibility from some distances, add powerful radars and these are very visible. Is not that the creeping wave disappears by flattening the sides of F-22 forebody, if the radar signal is weak, it will be weakened enough for some types of radar to disappear from their view, but believe me your daily life at night and obscurity puts you in the same situation aircraft are, but add power density and you will discover nothing is stealth, it is only that aircraft are blind or have weak lamps aka weak radars

The wavelength of visible light is too small compared to human size object for the creeping wave return phenomenon to happen and human eye doesn't suffer from side lobes clutter like a radar so I don't think they are really that similar.



personally, I think of stealth aircraft as camouflage sniper. They both trying their best to reduce signature, only a very small part of the total signature comming back to the observer and that signature is harder to distinguish from clutter. You see, even with a powerful emitter like the sun, it is still very hard to detect a camouflaged sniper at long range. That isn't because the sun isn't bright enough, but because the surrounding scenery will also reflect signals to you. Similarly, a powerful radar will not only see stealth aircraft, but its wave will also be reflected from birds, insects, ground, sea surface. ..etc. Doppler shift help mitigate this issue somewhat, but not totally because it also depends on aspect angle with the threat. An aircraft moving 700 km/h perpendicularly to your radar can easily has less Doppler shift than a bird flying straight at your radar. An insect 50 meters from your radar can have bigger reflection strength than an F-16 from 300 km away. So stealth work not only with very weak radar. The aim isn't 100% invisible, the aim is cut down detection range short enough so you can attack first. A stealth aircraft isn't invisible but it doesn't have to, like a sniper, if your enemy can detect you from dozen meters away that is already enough

you want to pretend to see i will be jammed, the reality is you will know some one is in front of you in the middle of the night, a jammer is a good way to know where an enemy aircraft is

Who say the jammer and the aircraft have to be at the same location? There are many methods to separate the physically distance between the jammer and the thing it supposed to protect.
From stand off jammer like EA-18G, EA-6B, B-52 CCJ to stand in jammer such as MALD-N, SPEAR-EW



They can also operate jamming between 2 or more jammers

basically anti-radiation missiles do that, basically that is the whole point when a fighter illuminates you, you can simply let his own radar to guide you where is him

While that sounds like a great method in theory, in practice, it is very terrible. Because aircraft can fly very fast, A2A missile have to aim at a predicted location of target instead of its current location. This predicted location is calculated base on the range to enemy aircraft as well as their direction of travel, speed and altitude. All these vital information is normally measured by your radar or LRF. But when your radar is jammed, using RWR you only get the direction, you do not know how far is your enemy or how fast they travel. Anti radar missiles must use pure pursuit guidance so, they have almost zero chance of hitting a maneuvering aircraft and they will miss if your enemy happens to turn off their radar in terminal phase.

 

[Ronny]

Will a good radar now detect DS I intakes? yes it will and same caret intakes

Can you see an ant with your eye? yes
Can you see an elephant with your eye? yes
But they are not equally visible at the same range.

F-117 was effective for radars on MiG-21 or MiG-23, modern aircraft like Su-35, F-22 or J-20 supposedly need to find F-117 at relatively longer ranges than MiG-21 that is why F-117 was obsolete and phased out

The major reasons for F-117 phase out were:
_ Limited weapon load and weapons of choice: only 2 internal LGB or JDAM
_ Limited sensors: only FLIR no Radar, no RWR, no IRST, no MWS
_ Limited defense: no supersonic speed, no agility, no chaff, no flares, no ECM system
_ High maintenance cost.
But F-117 stealth was very effective

 

[TomcatViP]

The end meaning of this conversation has long switched to Emcon 1.

 

[Ronny]

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.

Would you mind telling me where that value come from? I only have the chart for Mig-25 but not for Mig-31

 

[pegasus]

The wavelength of visible light is too small compared to human size object for the creeping wave return phenomenon to happen and human eye doesn't suffer from side lobes clutter like a radar so I don't think they are really that similar.

our eyes while use small wavelength high frequencies, radar also use small frequencies , in fact an F-16 is pretty huge for radar frequencies, to understand waves you have to see them as waves, they are not particles, they are not rays or arrows, in fact the fact you can jam a radar means they are having diffraction interference, basically a shadow explains you why light is not a particle but a wave, objects generate shadows, but if you are behind the shadow you still can see, electromagnetic waves try to fill everything, even a laser is visible some of its light is not coherent some of its light comes to our eyes, the net effect is creeping waves exist in light.

Creeping waves exist because electromagnetic waves interact with objects and diffraction exist, in particles to bend its trajectory you need a magnetic or electric field, in big objects gravity, but objects are not waves.

Particles by nature a bullet for example can not generate an interference pattern, so basically can not be jammed so if light was a particles you will have perfect darkness in a shadow and once you enter a shadow zone you will not be able to see even in daylight, but light bends around corners, so you can see

As the wavelength gets bigger waves behave more like waves and not like particles with respect aircraft, so the current effect stealth loses its meaning.

basically the facets or flat walls of the F-22 sides are mirrors for radar frequency so what they do is if the creeping wave is weak, they focus it in the direction of the flat sides of the chines or sidewalls, but it does not get eliminated 100% because radar is like light and not particles, some will go back to the radar, chines offer aerodynamic benefits too so not everything is stealth,

 

[Ronny]

our eyes while use small wavelength high frequencies, radar also use small frequencies , in fact an F-16 is pretty huge for radar frequencies, to understand waves you have to see them as waves, they are not particles, they are not rays or arrows, in fact the fact you can jam a radar means they are having diffraction interference, basically a shadow explains you why light is not a particle but a wave, objects generate shadows, but if you are behind the shadow you still can see, electromagnetic waves try to fill everything, even a laser is visible some of its light is not coherent some of its light comes to our eyes, the net effect is creeping waves exist in light.

Creeping waves exist because electromagnetic waves interact with objects and diffraction exist, in particles to bend its trajectory you need a magnetic or electric field, in big objects gravity, but objects are not waves.

Particles by nature a bullet for example can not generate an interference pattern, so basically can not be jammed so if light was a particles you will have perfect darkness in a shadow and once you enter a shadow zone you will not be able to see even in daylight, but light bends around corners, so you can see

At this point, I think everyone in this discussion understands the particles-wave duality of radio wave so I don't think it is necessary for you to re-explain the same thing.
Visible light doesn't curve a full circle around human size object (creeping return) because the wavelength isn't big enough.

 

[pegasus]

At this point, I think everyone in this discussion understands the particles-wave duality of radio wave so I don't think it is necessary for you to re-explain the same thing.
Visible light doesn't curve a full circle around human size object (creeping return) because the wavelength isn't big enough.

CREEPING-WAVE DIFFRACTION CONCEPT When a wave is incident upon an opaque object which is large compared to the wavelength, a shadow is formed. However, some radiation penetrates into the shadow region due to diffracted rays as shown in figure 5. (See refs. 12 to 16.) These rays are produced by incident rays which are tangent to the surface of the body. Each tangent ray splits at the point of tangency with one part continuing along the path of the incident ray and the other traveling along a geodesic on the surface of the body. At each following point, it splits again with one part traveling along the geodesic and the other reradiating along a tangent to the geodesic. rays are produced, one of which is reradiated at each point of the geodesic. These waves traveling around the opaque body have been designated as creeping waves introduced first by Franz and Deppermann (ref. 12) for the interpretation of scalar diffraction by circular cylinders and spheres.

https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19700009872.pdf
On your dally life you see shadows, in fact as i said to you, if light was particles, once you enter in the shadow region you will not be able to see, the radiation penetrates the shadow because light bends around corners,



Going back to DSI intake , the DSI intake since it has round bumps, they help the creeping wave




see the two DSI intake are pretty round, they will aid the creeping wave


The concept is a flat surface has only one tangent since it is flat, however DSI have huge circular bumps with turning sets of tangents, like any circle, so you see the flat Caret plates go better with stealth, just remember with enough light you can see pretty well in the shadow side, in fact every time you have a lamp in your hand, and it is facing ahead of you can see thanks to creeping waves

 

[Ronny]

CREEPING-WAVE DIFFRACTION CONCEPT When a wave is incident upon an opaque object which is large compared to the wavelength, a shadow is formed. However, some radiation penetrates into the shadow region due to diffracted rays as shown in figure 5. (See refs. 12 to 16.) These rays are produced by incident rays which are tangent to the surface of the body. Each tangent ray splits at the point of tangency with one part continuing along the path of the incident ray and the other traveling along a geodesic on the surface of the body. At each following point, it splits again with one part traveling along the geodesic and the other reradiating along a tangent to the geodesic. rays are produced, one of which is reradiated at each point of the geodesic. These waves traveling around the opaque body have been designated as creeping waves introduced first by Franz and Deppermann (ref. 12) for the interpretation of scalar diffraction by circular cylinders and spheres.

https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19700009872.pdf
On your dally life you see shadows, in fact as i said to you, if light was particles, once you enter in the shadow region you will not be able to see, the radiation penetrates the shadow because light bends around corners,

View attachment 620522

Ok, I see where your confusion come from.
I didn't say there are no radiation penetrated into the shadow region. I said very specifically: "Visible light doesn't curve a full circle around human size object (creeping return) because the wavelength isn't big enough". That is to help you distinguish between Traveling wave scattering return and Creeping wave return.
The former is generated when the wave travels along the surface of the aircraft hitting some discontinuities and they scatter in many directions

while the Creeping wave return happens when the traveling wave doesn't strike any discontinuities and not reflected by obstacle when traveling along object surface so it is able to travel a full circle around the object and either interfered instructive or destructive with the specular return. The two phenomena are very closely related but they are not exactly the same thing. The creeping wave return only happen when the wavelength of the radar closer to the size of the object

Going back to DSI intake , the DSI intake since it has round bumps, they help the creeping wave
see the two DSI intake are pretty round, they will aid the creeping wave
The concept is a flat surface has only one tangent since it is flat, however DSI have huge circular bumps with turning sets of tangents, like any circle, so you see the flat Caret plates go better with stealth, just remember with enough light you can see pretty well in the shadow side, in fact every time you have a lamp in your hand, and it is facing ahead of you can see thanks to creeping waves

DSI inlet has a smooth bump with no gaps, so it reduces surface scattering from surface discontinuities and edges compared to a variable inlet. The same reasons more modern stealth aircraft all have blended circular shape unlike the F-117 with flat facets.



Creeping wave return isn't an issue because radar wave curve a full circle into and around the inlet duct won't go back to the illuminate radar.

 

[pegasus]

Ok, I see where your confusion come from.
I didn't say there are no radiation penetrated into the shadow region. I said very specifically: "Visible light doesn't curve a full circle around human size object (creeping return) because the wavelength isn't big enough". That is to help you distinguish between Traveling wave scattering return and Creeping wave return.
The former is generated when the wave travels along the surface of the aircraft hitting some discontinuities and they scatter in many directions

while the Creeping wave return happens when the traveling wave doesn't strike any discontinuities and not reflected by obstacle when traveling along object surface so it is able to travel a full circle around the object and either interfered instructive or destructive with the specular return. The two phenomena are very closely related but they are not exactly the same thing. The creeping wave return only happen when the wavelength of the radar closer to the size of the object
View attachment 620531



DSI inlet has a smooth bump with no gaps, so it reduces surface scattering from surface discontinuities and edges compared to a variable inlet. The same reasons more modern stealth aircraft all have blended circular shape unlike the F-117 with flat facets.



Creeping wave return isn't an issue because radar wave curve a full circle into and around the inlet duct won't go back to the illuminate radar.

Ronny the illustrations you give do not really show you what is diffraction since basically the illustration can be confused for reflection of electromagnetic waves


this illustration shows you what is diffraction, basically it means now the light rays moving in new trajectories

my first comment about the tank can be explained like this


the creeping wave exists due to diffraction in fact the nasa document says it all:

CREEPING-WAVE DIFFRACTION CONCEPT When a wave is incident upon an opaque object which is large compared to the wavelength, a shadow is formed. However, some radiation penetrates into the shadow region due to diffracted rays as shown



the last illustration tells you why you can detect targets with radars, since the waves do not follow a narrow strait path, but an angular one, thus the document says


These rays are produced by incident rays which are tangent to the surface of the body. Each tangent ray splits at the point of tangency with one part continuing along the path of the incident ray and the other traveling along a geodesic on the surface of the body. At each following point, it splits again with one part traveling along the geodesic and the other reradiating along a tangent to the geodesic. rays are produced, one of which is reradiated at each point of the geodesic.



So basically radars do the same




You are thinking DSI ntakes can eliminate creeping waves, no they can not in fact the sharp pointy lip cowls are helping diffraction, stealth is simply a way of reducing RCS using distance, low power density and wave control, no object is invisible none, DSI intakes are not perfect, they are visible, and their bumps are not flat.

But do not get me wrong with the right shaping they make harder to detect by radars at longer ranges, so stealth is no visibility at long ranges aided by shaping and materials, can they reduce their RCS? yes they can, are they visible all the time? yes they are, can they confuse radars? yes they can, can they be undetected if proper shaping materials and tactics are used? yes they can

 

[Ronny]

Ronny the illustrations you give do not really show you what is diffraction since basically the illustration can be confused for reflection of electromagnetic waves
this illustration shows you what is diffraction, basically it means now the light rays moving in new trajectories
my first comment about the tank can be explained like this
the creeping wave exists due to diffraction in fact the nasa document says it all:
CREEPING-WAVE DIFFRACTION CONCEPT When a wave is incident upon an opaque object which is large compared to the wavelength, a shadow is formed. However, some radiation penetrates into the shadow region due to diffracted rays as shown
the last illustration tells you why you can detect targets with radars, since the waves do not follow a narrow strait path, but an angular one, thus the document says
These rays are produced by incident rays which are tangent to the surface of the body. Each tangent ray splits at the point of tangency with one part continuing along the path of the incident ray and the other traveling along a geodesic on the surface of the body. At each following point, it splits again with one part traveling along the geodesic and the other reradiating along a tangent to the geodesic. rays are produced, one of which is reradiated at each point of the geodesic.
So basically radars do the same

Pegasus, yes radar wave can bend around the corner ( diffraction) but like I said how much it can bend around the corner depend on the relative size of wavelength versus the object size. If the wavelength is small compared to the object, such as visible light versus human, there is very little diffraction and there is no creeping wave return. Because the diffraction isn't enough for the wave to go a full circle around the object. Only when the wavelength is very close to the object in size that we have the creeping return effect.

You are thinking DSI ntakes can eliminate creeping waves, no they can not in fact the sharp pointy lip cowls are helping diffraction, stealth is simply a way of reducing RCS using distance, low power density and wave control, no object is invisible none, DSI intakes are not perfect, they are visible, and their bumps are not flat.

I don't think DSI eliminate creeping wave, DSI reduce surface discontinuities diffraction compared to variable inlet because it doesn't have as many gaps and discontinuities. Like I explained, that the same reason B-2, F-35, F-22 doesn't have as many sharp edges as F-117.
Beside, creeping wave effect is often reduced by RAM.

 

[overscan (PaulMM)]

I'm losing the will to live with the argument on how radar works. I may split it off the topic or just delete it.

Shadows are not truly black on earth due to atmospheric scatter effects, its got nothing to do with wave interference. Its not the same on the moon, for example.

 

[pegasus]

Pegasus, yes radar wave can bend around the corner ( diffraction) but like I said how much it can bend around the corner depend on the relative size of wavelength versus the object size. If the wavelength is small compared to the object, such as visible light versus human, there is very little diffraction and there is no creeping wave return. Because the diffraction isn't enough for the wave to go a full circle around the object. Only when the wavelength is very close to the object in size that we have the creeping return effect.
View attachment 620544





I don't think DSI eliminate creeping wave, DSI reduce surface discontinuities diffraction compared to variable inlet because it doesn't have as many gaps and discontinuities. Like I explained, that the same reason B-2, F-35, F-22 doesn't have as many sharp edges as F-117.
Beside, creeping wave effect is often reduced by RAM.

There are important aspects you forget, if you are in a shadow still there is light, the concept of creeping wave is also related to power density, you are comparing an antena to the sun, the sun can fill the whole horizon, basically, no mountains will stop it can not compare, you are giving me an antena with very limited radiation power, remember light is also absorbed by the atmosphere, radar too, it weakens plus the human body is not homogeneous, is not perfectly smooth, there is plenty of diffraction around it, there are creeping waves in human body, creeping waves exist simply because the Huygens principle.




in Fig 4 you have a description of concentric circles for long waves and a narrow ray for microwave, however both have the same nature,


DSI intake can deal with creeping waves to an extent like i said to you because at 100 km the atmosphere has absorbed enough power density that it is very weak,.
The caret intake you still do not want to admit, is for higher speeds, so it can keep pressure recovery of 95% at Mach 2, DSI can not do that does not matter how much you want to argue about its smoothness, in aerodynamic terms it will not help you compared to caret variable geometry intakes.


The bump and sharp edges are diffraction sources, you still think the bump by being smooth is invisible to radar, it is not. in fact you have a contradiction F-22 has chines good for eliminating creeping wave, F-15 has round cross section no good for creeping wave, but you say the bump is good


To understand stealth you have to see with distance the radar sensibility and power density go down, that is why stealth works.


If you look a Boeing B-787 aircraft from 5 meters you see lots of details on it, at 100 meters you can not see so many details, at 8km you can not see the eye color of a person looking through one its windows, radar is the same farther the target is, less detail you can see, stealth only is weakening the same signal.

Basically if you consider when an object is very far. an aircraft at 200 km from you, you can not see it, however it is not invisible for a person at 30 meters from it, he or she see it well, but you at 200 km you can not see it, radar is the same.

 

[Inst]

That's not the class room and no one has to grade others posts and make entries to the class register !

Fair enough, it was rather a rather arrogant thing to say. Sorry.

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

Actually it will, that's why pitot air speed measurements need a compressiblity correction for high speed flight: https://en.wikipedia.org/wiki/Equivalent_airspeed

The crux is that the air IS moving with respect to the pitot tube and constant density along its path into the pitot tube is merely an approximation that holds for low subsonic (as in <0.3) Mach numbers. Once supersonic, with the mass flow also crossing shock waves between free stream and inlet opening, you have to apply a new set of equations. They too conserve mass, but work rather differently in some regards, for example a pipe diffusor to decelerate air is now convergent rather than divergent.

When I say conservation of mass, what I mean is that you have free stream air with a MFR of roughly density * area * velocity moving into the inlet. The fixed variable for a given altitude is density. We can also treat velocity as fixed for the purposes of our thought experiment.

The compressibility is extremely important for decelerating flow, but because you're limited by the MFR of the free stream air, it's hard to see how compressibility can get you something out of nothing. You can decelerate the air as it comes into the inlet by compressing it, but the change in density does not generate mass and thus does not increase MFR.

The only way I can see MFR increasing as a result of supersonic velocity would be treating area as a dependent variable; i.e, as you go supersonic, the area of free stream flow capture increases because of supersonic effects.

But we have a few problems here. First, if the compression is being done by a normal shockwave, I don't think there's any possible capture here. If it's an oblique shockwave, if you visualize the vector diagram, the only increased flow could come from upstream, not downstream. And upstream relative to the inlet won't come from outside the inlet, leading to no increased flow.

The area increase we COULD obtain incidentally has nothing to do with the inlet itself. It has to do, instead, with the air captured by the fuselage and the resulting boundary layer. But how much of that boundary layer needs to be diverted off, and how much of that boundary layer is recoverable?