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Why AJS-37 flying 1.32M above the ground in level flight?


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Overheating is making impossible to fly at this speed for any aircraft! Dear, Heatblur! Even pilots and test pilots claims that 1450 km/h indicated is maximum for even test machines! 1.1M was not a problem for usual aircraft!

1606512476_Viggen1.32Monthedeckinlevelflight.jpg.9a7822499cd936a5ea9f359323a57df4.jpg


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Max indicated speed is 1350 km/h, ca M1.2 at low level. Above that speed the skin temperature will be to high. During testflight IRL at 500 m I had to reduce thrust when reaching 1350. So, M 1.3 might be OK. I was not allowed to try!!

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It’s a recommended limit, the charts show that the afterburner zoneIII have enough power to accelerate further.

But the real limit seem to be the fuel pumps, it can’t keep up with the fuel needed for the engine.

“Flying at M1.1 will deplete the fuel within 10minutes, but the fuel feed won’t allow that, the engine will light out before the fuel is empty”

So perhaps the question would be for how long can the pumps keep up before a engine light out?

 

/F

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"Recommended limit" in the context of an aircraft means "it will probably do something highly unpleasant if you exceed this". While it has the excess thrust to reach that speed, I highly doubt it could maintain it safely - even a short burst is reasonable IMO but I've seen Viggens maintaining 1,600km/h IAS for entire sorties in MP. When the MiG-21 loses its engine if it overspeeds, the MiG-19 can't reach that speed except in a power dive and then loses stability and breaks apart, and even the MiG-29 can't keep up... something is wrong. I don't buy that the Viggen has some special attribute that allows it to blaze through its never-exceed speed without consequence, to a speed that no other aircraft of its era is capable of. Are we really sure Swedish materials science was that advanced compared to the two superpowers? Not that its engine would notice, being repurposed out of an airliner...

 

IMO the impression that the "Viggen is really fast at low level" is not an expression of speed, it is an expression of good pilot training and a good autopilot/stability control system allowing it to actually reach its never-exceed speed in reasonable safety at altitudes that other jets could not.

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No, it can't. It loses lateral stability and tears itself apart at 1590 kph. DCS Viggen goes faster. Stop lying.

 

That's an FCS bug and happens well before that speed, also not always. I've done 1.3 on the deck in a clean Hornet once too. Are you all of a sudden self-taught aeronautical internet engineers to make all these claims?

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Are you all of a sudden self-taught aeronautical internet engineers to make all these claims?

As a matter of fact he is. If I'm not mistaken he has Master Degree in Aeronautics. So he has some qualifications to make claims ;)

 

 

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As a matter of fact he is. If I'm not mistaken he has Master Degree in Aeronautics. So he has some qualifications to make claims ;)

 

 

Bachelor's. And I'm not making "claims".

 

 

That's an FCS bug and happens well before that speed, also not always. I've done 1.3 on the deck in a clean Hornet once too. Are you all of a sudden self-taught aeronautical internet engineers to make all these claims?

 

I didn't make any claims and I don't care why it happens since it's irrelevant to your post, I made an observation - in DCS the F-16 goes into oscillations and tears itself apart at those speeds. I didn't say anything about why it loses stability, I only acknowledged what I've seen happen in DCS. And what I've NOT seen happen in the Viggen in the same situation. Go shitposting on reddit, not here.

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Bachelor's. And I'm not making "claims".

 

 

 

I didn't make any claims and I don't care why it happens since it's irrelevant to your post, I made an observation - in DCS the F-16 goes into oscillations and tears itself apart at those speeds. I didn't say anything about why it loses stability, I only acknowledged what I've seen happen in DCS. And what I've NOT seen happen in the Viggen in the same situation. Go shitposting on reddit, not here.

 

I'm sorry but how am I shitposting? I too work in the field and am not foolish enough to take this broken game this seriously. Nor am I a test pilot. I merely explained to you why this oscillation happens (but rarely and under certain conditions) in the F16 (TLDR: it's a bug) and that you can reach and sustain Mach 1.3+ in the DCS Hornet and Viper when clean. So it's not just a Viggen issue. I do have the swedish AJS manuals on my computer and will take a look at the operational limitations section when I get the chance but from a pure aerodynamic standpoint I don't see why a semi-canarded delta wing aircraft with an afterburning JT8D low bypass turbofan shouldn't physically go that fast on the deck in a clean config or with a low drag index. It was literally designed for supersonic flight at low altitudes.

 

Are we really at the point of comparing a game to reality and bringing supersonic thermodynanics into play when we dont even have a decent atmospheric or weather depiction model in DCS? Among numerous other things. And why are we even comparing the F16 to the Viggen?

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It matters when other modules simulate structural/engine failure when oversped but the Viggen does not, for those of us who play multiplayer. If the charts all say 1350km/h, and a pilot has now said 1350km/h, then I'd expect that limitation to exist for a reason since the engineers aren't going to set it just for funsies.

 

As for why we're comparing the 16 to the Viggen (and no, I think you'll find most of us aren't), aren't you the one who brought up the F-16 in the first place?

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You're comparing apples to oranges though. Vne is set by different things for different aircraft, and as far as I know no DCS module simulates overspeeding issues related to heating. In the case of the Fishbed, the issue is less with heating and more with how the airflow goes to the engine, which is why at Mach ~2.1 you get a flameout in that jet.

 

In that sense, I think a comparison of the Viggen with the F16 is perfectly reasonable, since both jets have speed limits related to heating that are not simulated in DCS. On a practical note, you can only hit those speeds with a clean jet, so it doesn't impact combat operations that significantly.

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Maybe it is apples and oranges, maybe it is not. I would assume that like every other manual I've seen, Viggen documentation would have a separate section focusing on structural and flight limitations, not only indicating the speed limits but also reasons why they are there. For now all I've seen is just number - 1350 km/h at lower altitudes. But if someone can point to a specific page of manual where this is explained, it sure would be helpful.

 

And sidenote, TLTeo, you are wrong in regard to MiG-21bis. Or half wrong. MiG-21bis flight characteristics manual indicates that there are two speed limits - Mach 2.1 in general and Vne of 1300 - 1350 km/h indicated at lower altitudes.

Speed limit at low altitudes (up to around 10 km) based on instrument speed (indicated) is dictated by structural limitations of the engine. And it always was, be it for R-11F2-300, R-13-300 or R-25, all were unable to withstand airflow pressure (although at different speeds, for example R-11F2-300 was limited to 1100 - 1200 km/h). Especially the first stage of compressor, the blades and afterburner chamber which reaches thermal limit. The flameout we have in game is likely the best scenario pilot would expect. If engine was run for longer periods near the limit one could expect even a permanent damage.

 

The Mach 2.1 speed limit on the other hand has nothing to do with airflow but with directional stability. Or lack of it at this speeds. In early tests MiG-21 exhibited directional instability over Mach 2.05 and would become uncontrollable to a significant degree. After MiG-21PFM when tail fin was enlarged, aircraft gained in that department and limit was lifted to 2.1, although I dont think any real life tests were specifically conducted to test the limit as credible sources (i.e. a number of pilots) exceeded that limit and pushed aircraft past Mach 2.2

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And second question is the time limit for afterburner! We just need to find this limit for Viggen! It cant fly with 3d stage of afterburner for like 20 min i am sure!

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Maybe it is apples and oranges, maybe it is not. I would assume that like every other manual I've seen, Viggen documentation would have a separate section focusing on structural and flight limitations, not only indicating the speed limits but also reasons why they are there. For now all I've seen is just number - 1350 km/h at lower altitudes. But if someone can point to a specific page of manual where this is explained, it sure would be helpful.

 

And sidenote, TLTeo, you are wrong in regard to MiG-21bis. Or half wrong. MiG-21bis flight characteristics manual indicates that there are two speed limits - Mach 2.1 in general and Vne of 1300 - 1350 km/h indicated at lower altitudes.

Speed limit at low altitudes (up to around 10 km) based on instrument speed (indicated) is dictated by structural limitations of the engine. And it always was, be it for R-11F2-300, R-13-300 or R-25, all were unable to withstand airflow pressure (although at different speeds, for example R-11F2-300 was limited to 1100 - 1200 km/h). Especially the first stage of compressor, the blades and afterburner chamber which reaches thermal limit. The flameout we have in game is likely the best scenario pilot would expect. If engine was run for longer periods near the limit one could expect even a permanent damage.

 

The Mach 2.1 speed limit on the other hand has nothing to do with airflow but with directional stability. Or lack of it at this speeds. In early tests MiG-21 exhibited directional instability over Mach 2.05 and would become uncontrollable to a significant degree. After MiG-21PFM when tail fin was enlarged, aircraft gained in that department and limit was lifted to 2.1, although I dont think any real life tests were specifically conducted to test the limit as credible sources (i.e. a number of pilots) exceeded that limit and pushed aircraft past Mach 2.2

 

 

Makes sense, thanks for the clarification. My comment about the Viggen's limitations come simply from hilmerby's post above saying that it's related to the skin, rather than the engine. I agree it's not as good as an actual manual though, so more SMEs/devs chiming in would be nice.

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First and foremost to illustrate that not only the Viggen can sustain Mach 1.3+ (or comparable speeds) at sea level in max AB with no issues whatsoever, below are a few screenshots of various DCS modules doing exactly just that. Some aircraft like the Mig-21 and Tomcat obviously simulate various engine limitations and the effects of reaching Vne/Mmo, but even then in a rather simple and best case scanario manner. Something to also mention, there is a bug in DCS that when you have stores (fuel tanks, missiles) and jettison them in flight you will have less drag than if you spawn in the exact same clean config, with no prior stores. In the Viggen for example, if you spawn clean you can do 1.29 - 1.3 on the deck in stage 3, if you spawn with stores and jettison those you can usually do 1.35-36. Something worth mentioning I guess.

Screen_201030_144030.thumb.png.f681a640aede64e714b6b48b7f9eec2c.png

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Screen_201030_150844.thumb.png.7d51f3d4e9f1121bebf34523eb213b44.png

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Maybe it is apples and oranges, maybe it is not. I would assume that like every other manual I've seen, Viggen documentation would have a separate section focusing on structural and flight limitations, not only indicating the speed limits but also reasons why they are there. For now all I've seen is just number - 1350 km/h at lower altitudes. But if someone can point to a specific page of manual where this is explained, it sure would be helpful.

 

There is no such part of the manual to my knowledge. It just lists VNE as 1350km/h IAS on the AJ37, 1450km/h IAS on the JA37. The JA37 has a slightly different engine with an extra compressor stage that makes it more resistant to compressor stalls and it has a few percent more power, but is otherwise extremely similar to the AJ37 in most ways relevant to this issue. I believe that the aerodynamics compendium discusses some issues with available pitch authority/load factor at very high dynamic pressures because of limitations with the amount of force the hydraulics can exert on the elevons, but as far as I can recall it doesn't really go into much detail. JA37 does have a lot more pitch authority available throughout the entire flight envelope, as an aside.

 

Another concern pointed out in the aerodynamics compendium is that the aircraft decelerates extremely quickly from supersonic speeds at low altitude when engine power is reduced. When the dynamic pressure rapidly falls so do the elevon deflection angles required to achieve a particular load factor. While the Viggen isn't fly-by-wire it does have a system that attempts to keep "stick force per G" at least in the same neighborhood throughout most of the envelope, and in a situation where the dynamic pressure is quickly reduced this system has to react very quickly. The problem is though that it really can't do that - it's a pretty primitive system and the pitch gearing can take 15 seconds to move from supersonic to subsonic mode, while the aircraft is capable of decelerating faster than that.

 

And second question is the time limit for afterburner! We just need to find this limit for Viggen! It cant fly with 3d stage of afterburner for like 20 min i am sure!

 

There is no time limit for the afterburner listed in the declassified flight manuals for the real aircraft. If an operational restriction for time limits in zone 3 has ever existed I've never any evidence whatsoever for it. Presumably because you'll run out of fuel before you could realistically run into problems. Perhaps hilmerby knows more than the manual though.

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I don't buy that the Viggen has some special attribute that allows it to blaze through its never-exceed speed without consequence, to a speed that no other aircraft of its era is capable of. Are we really sure Swedish materials science was that advanced compared to the two superpowers? Not that its engine would notice, being repurposed out of an airliner...

 

The F-111 is from the same time period as the Viggen and is limited to Mach 1.2 at sea level for the same reason hilmerby mentions (skin temperature). The F-111 has equipment for warning about skin overheating, though, and the Viggen does not. There is nothing magical about neither of these aircraft, nor their engines.

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Triple posting, sorry about that, but it's relevant. In the book System 37 Viggen, Kenneth Nilsson (longtime Saab employee, head of the applied aerodynamics department 1992-2001) relates the following story regarding work he did at Saab in the early 1970's studying the feasibility of beating a few FAI records with the Viggen, in the hopes that it would help it on the export market. My translation.

 

Having been forced to abandon the attempt to beat the time-to-climb record, we licked our wounds and turned our attention to the top speed at minimum altitude record. "Minimum altitude" meant that the timed track, which is 3 kilometers long and must be completed twice in each direction, must be flown at less than 100 meters AGL (350 meters these days). [...] The record holder at the time was a F-4 Phantom since 1961, with 1452.8 km/h. According to the FAI rules the record must be beaten by at least one percent to be valid, which meant at least 1468 km/h.

 

In the first attempt to break this record with the F-4, the aircraft disintegrated in mid-air and the pilot was killed. The cause was powerful pilot-induced oscillations in pitch, as a result of the demand for extremely precise altitude control. This was in large part due to the control system being extremely sensitive at high speeds and low altitudes, which was not unusual for supersonic aircraft of the period, early versions of the J 35 Draken being one example.

 

[...]

 

Radar and other non-essential equipment was removed, resulting in an assumed weight reduction of 800 kg. This also had the nice side effect of moving the center of gravity aft, reducing the trim-induced drag at supersonic speeds. Of course all antennas, weapon stations and other pointy bits were removed and the aircraft was assumed to be polished to a mirror finish. The engine and fuel consumption were both uprated by 3%, which was something we believed Volvo Flygmotor could live with. One concern in this context is the great dynamic pressures exerted on the aircraft and particularly in the air intakes. Of course we would have to exceed the AJ 37's VNE - 1350 km/h IAS - significantly if we were to beat the record. It could be mentioned that the same limit for the JA 37 was 1450 km/h IAS.

 

The flight profile starts with an acceleration phase, and after the aircraft has passed the timed track it decelerates to subsonic speed before turning around to run the track again in the other direction. Turning while supersonic would consume too much fuel. After various calculations we managed to come up with a flight profile that involved acceleration and deceleration tracks 20 km long and turns at Mach 0.8 and 3.5G. After four timed laps and three turns the aircraft would be almost out of fuel. But the average speed was calculated to be 1535 km/h, 5.5% above the then-current record!

 

[...]

 

The effects of atmospheric conditions were never studied, but they were probably not of as vital importance as for the time-to-climb records. There were other issues, though. Flying at 1500 km/h at less than 100 meters above water (which was the only realistic option) requires good ground references. We considered putting out marker buoys along the entire flight path, around 50 km. Not the simplest task, though. And where would we attempt this? Mach 1.25 at minimum altitude makes a considerable bang, and the flight must be conducted near an airfield, that is near a town with inhabitants and greenhouses. We talked about doing it over the lake Vättern, taking off from Karlsborg, but...

 

In the end this planned record attempt was also abandoned, even though we thought we had a good shot. I've forgotten exactly why, maybe the marketing budget didn't allow for it perhaps they prioritized something else.

 

It took a few years until the record was beaten by a F-104A, achieving 1590.5 km/h. This heavily modified aircraft, built from parts from several different F-104 versions, flew for the first time in 1976. It was owned and operated by Darryl Greenmayer, former SR-71 test pilot, and was called "Red Baron". If what's written on the internet might be believed, the "Baron" was a bastard aircraft made from parts of dubious origin and definitely not approved by the USAF!

 

edit: to be clear, I think going much beyond Mach 1.2 at sea level in an AJ37 is probably unrealistic, but I have never seen anything that indicates that engine, intakes or airframe in general would give up the ghost if you exceed 1350 km/h IAS. The only sort of related thing I know of is that in 1978, during development of the JA37 and the RM8B, the first prototype aircraft (37-8) was lost due to an engine fire caused by overheating issues in the lubrication system at high speed and low altitude, and this did lead to temporary operational restrictions for all Viggens for a while. This problem was fixed though, at great expense, and all RM8A's were modified with this fix as well and the restrictions were lifted afterwards. The flight manual sections that discuss restrictions, stability and behavior in various flight regimes do not mention any particular risks related to overspeeding at low altitude at all. The closest thing is a mention that roll authority is reduced at M>0.98 at altitudes below 2000 meters because of elastic deformation of the wing. I'd bet that the restriction to 1350 km/h IAS at low altitude is as much for airframe and engine longevity as anything else. For one thing in the real aircraft any intentional load factor < 0 G is forbidden, not because it'll make the aircraft fall apart, but because it causes a lot of airframe stress and reduces lifetime.

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First and foremost to illustrate that not only the Viggen can sustain Mach 1.3+ (or comparable speeds) at sea level in max AB with no issues whatsoever, below are a few screenshots of various DCS modules doing exactly just that. Some aircraft like the Mig-21 and Tomcat obviously simulate various engine limitations and the effects of reaching Vne/Mmo, but even then in a rather simple and best case scanario manner. Something to also mention, there is a bug in DCS that when you have stores (fuel tanks, missiles) and jettison them in flight you will have less drag than if you spawn in the exact same clean config, with no prior stores. In the Viggen for example, if you spawn clean you can do 1.29 - 1.3 on the deck in stage 3, if you spawn with stores and jettison those you can usually do 1.35-36. Something worth mentioning I guess.

 

Fair enough.

 

I made some tests myself in a Viggen, default cold start so OAT 2 °C (where it is I don't know, it's DCS, but I assume it's gonna be SL temperature), clean config. Reached 1556 in ground effect at the moment of running out of fuel. Considering that I've seen documentary where a pilot claimed they could go 1470 kph I can believe that, though I don't know how accurate that claim was either. What I find hard to believe is that it would be as smooth as what I've experienced. I was flying 4m above the water surface at over 1550 kph and it felt almost on rails. Not to mention that I could hold this speed indefinitely, only limited by fuel usage. Here's the track file, take a look if you want.

 

https://1drv.ms/u/s!ArMDWm49dN3Cgdcu0mGRz66oRTnxUA?e=v9a8J0

 

I'm sorry but how am I shitposting? I too work in the field and am not foolish enough to take this broken game this seriously. Nor am I a test pilot. I merely explained to you why this oscillation happens (but rarely and under certain conditions) in the F16 (TLDR: it's a bug) and that you can reach and sustain Mach 1.3+ in the DCS Hornet and Viper when clean. So it's not just a Viggen issue. I do have the swedish AJS manuals on my computer and will take a look at the operational limitations section when I get the chance but from a pure aerodynamic standpoint I don't see why a semi-canarded delta wing aircraft with an afterburning JT8D low bypass turbofan shouldn't physically go that fast on the deck in a clean config or with a low drag index. It was literally designed for supersonic flight at low altitudes.

 

Almost every single sentence in your previous two posts was a sophism, that's what I call shitposting. And "it's a broken game" or "other modules are broken too" is not even an argument, it's another sophism. You don't need to take the game seriously, just take people you're talking to seriously.

 

Are we really at the point of comparing a game to reality and bringing supersonic thermodynanics into play when we dont even have a decent atmospheric or weather depiction model in DCS? Among numerous other things. And why are we even comparing the F16 to the Viggen?

 

Are you for real? Because you started it.

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Fair enough.

 

I made some tests myself in a Viggen, default cold start so OAT 2 °C (where it is I don't know, it's DCS, but I assume it's gonna be SL temperature), clean config. Reached 1556 in ground effect at the moment of running out of fuel. Considering that I've seen documentary where a pilot claimed they could go 1470 kph I can believe that, though I don't know how accurate that claim was either. What I find hard to believe is that it would be as smooth as what I've experienced. I was flying 4m above the water surface at over 1550 kph and it felt almost on rails. Not to mention that I could hold this speed indefinitely, only limited by fuel usage. Here's the track file, take a look if you want.

 

 

 

Almost every single sentence in your previous two posts was a sophism, that's what I call shitposting. And "it's a broken game" or "other modules are broken too" is not even an argument, it's another sophism. You don't need to take the game seriously, just take people you're talking to seriously.

 

 

 

Are you for real? Because you started it.

 

1) Yes. I did my tests at ISA conditions (or DCS ISA conditions rather) and had the above mentioned results. And as mentioned previously the smooth ride is due to the lack of any more complex atmospheric and environmental effect engine in DCS, as well as any interference of the shockwave our compressible boundary layer, dawnwash etc. with the ground. Not to say that ground effect in the takeff and landing config isn't modelled in most aircraft it's just that it doesnt seem to have a noticeable effect at compressible or supersonic flight.

 

2) Well, I stop taking people seriously whenever they compare DCS to real life in these kinds of places. It's a game, you either accept that or live in a constant state of dissatisfaction and pointing out various flaws. This is also the internet forum and I wasn't "talking" to you nor do I know you, I just see something that I disagree with and reply to it. There is absolutely no need to take this personally or feel attacked by someone on the internet you know literally nothing about. Again, I merely pointed out the flaws in your previous statements. We can agree to disagree here.

 

3) And yes I am for real. I brought up the F-16 because it is another DCS module that can easily achieve or even exceed these high Mach numbers at sea level. You then dismissed my statement saying "No it will shake itself apart", to which I replied pointing out that this is a bug and only happens in certain rare occasions. On the topic of the F-16, the published IAS/CAS limitations is 800kts CAS at sea level, it is however also mentioned that due to the great excess power of the F110/F100 this speed can easily be exceeded. Obviously there are hundreds of factors in the real world incolved with this flight regime and flight at these speeds and altitude which greatly exceed the scope of a desktop flight simulator / game. Again, all I tried to point out in my initial reply was that it is not a peculiarity of the Viggen but most other DCS jets.

 

Thus I think the developers can eventually look at the flight model and related drag values and check them against the real world data (however I doubt there even is flight test data for these speeds and alts).

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Here's the documentary I mentioned btw:

 

 

Note however that JA-37 uses the RM8B engine, with an additional fan stage. It's hard to tell how that will influence performance at max speed without data for both engines at hand.

 

Not by very much. The JA37 only has maybe 5 kN more thrust at M 1.1 at sea level. Thust-drag diagrams below. The main advantage of the RM8B is that it deals better with high alpha and is less susceptible to compressor stalls in general.

 

AJ 37:

 

LPDNBts.jpg

 

JA 37:

 

Iuifanq.jpg

 

See this thread for most of the original Swedish flight manuals, where these performance charts are taken from.

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It's obviously not super accurate, but probably in the ballpark. The two rightmost lines are based on the ISA+10 line and offset from that.

 

 

Screenshot_1.thumb.png.97ddafc6f8390b2727f46e218fd494db.png

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