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


Shmal

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I thought negative drag was an issue on specific aircraft, not the entire game, but I don't know if that fully explains it. I've seen people doing 1580-1610 with some air to air missiles still onboard. I was kind of hoping renhanxue would turn up at some point with this kind of stuff, because it does add more detail to what the limitations were and from that, pushing through for a sprint seems reasonable. On the other hand, being able to maintain an average speed of over 1600 (which I've now seen several times) across most of a sortie is still exceeding what Saab expected to be able to do with a stripped-down, hotted-up aircraft - so maybe the overall drag modelling needs some adjusting.

 

It's worth note that I know a few of the faster Viggen rocket riders I've seen have a habit of flipping some of the switches along the right wall of the cockpit, at least one of which regulates fuel flow IIRC. I'm not sure how much that factors into it as well, but I'd imagine it does.

 

Ultimately the problem is that (as Skysurfer says) it's a game, and people in games will minmax things. You don't need to worry about whether the jet is going to be flyable in 5 years, you don't even need to care if it's flyable in 5 hours, but it makes it pretty easy to just capitalise on that and make life miserable for other people in MP whose jets might not have limitations that can be at least temporarily overlooked.

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I thought negative drag was an issue on specific aircraft, not the entire game, but I don't know if that fully explains it. I've seen people doing 1580-1610 with some air to air missiles still onboard. I was kind of hoping renhanxue would turn up at some point with this kind of stuff, because it does add more detail to what the limitations were and from that, pushing through for a sprint seems reasonable. On the other hand, being able to maintain an average speed of over 1600 (which I've now seen several times) across most of a sortie is still exceeding what Saab expected to be able to do with a stripped-down, hotted-up aircraft - so maybe the overall drag modelling needs some adjusting.

 

Yeah, doing 1600km/h on the deck with external payload seems extremely unrealistic to me, there's no way I can see it having enough thrust to do that. Clean maybe, it's at least conceivable, but with payload? Can't see it happening.

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Yeah, doing 1600km/h on the deck with external payload seems extremely unrealistic to me, there's no way I can see it having enough thrust to do that. Clean maybe, it's at least conceivable, but with payload? Can't see it happening.

 

Well, there is no flight-test data for it and no one has attempted it. Like people mentioned in reality you'd worry about thermal heatup and shockwaves forming on undesirable parts of the aicraft and engine components. The Mig-25 had problems or limitations with all of that even at altitude, even though the engine would probably be fine, it's the airframe structure and aerodynamic stability that would cause problems first.

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Yea it's been mentioned repeatedly in this thread that for many aircraft, thrust available is not the only limiting factor to top speed. It absolutely is unrealistic that no plane in DCS overheats or runs into other nasty effects, but it's an issue that isn't limited to the Viggen.

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Well, there is no flight-test data for it and no one has attempted it. Like people mentioned in reality you'd worry about thermal heatup and shockwaves forming on undesirable parts of the aicraft and engine components. The Mig-25 had problems or limitations with all of that even at altitude, even though the engine would probably be fine, it's the airframe structure and aerodynamic stability that would cause problems first.

 

What? I just posted a thrust-drag diagram that clearly shows even the JA37 not having enough thrust to reach its VNE at sea level with the lightest loadouts listed. Just follow the line marked 1 in the diagram and you'll reach T=D well below M 1.2. Or just extrapolate where the line marked 0 (clean aircraft) is going and it's pretty obvious that you'll run out of thrust just above M 1.2/1450 km/h IAS even then. I can easily see 1500 km/h being possible with a clean aircraft and you can probably do a bit better than that on a cold day even. With any payload at all though you're gonna get limited by drag below that.

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What? I just posted a thrust-drag diagram that clearly shows even the JA37 not having enough thrust to reach its VNE at sea level with the lightest loadouts listed. Just follow the line marked 1 in the diagram and you'll reach T=D well below M 1.2. Or just extrapolate where the line marked 0 (clean aircraft) is going and it's pretty obvious that you'll run out of thrust just above M 1.2/1450 km/h IAS even then. I can easily see 1500 km/h being possible with a clean aircraft and you can probably do a bit better than that on a cold day even. With any payload at all though you're gonna get limited by drag below that.

 

Yes, I can read diagrams. The question is, is the Vne indicated of 1350 an artificially placed limitation, thus cutting the graph off artificially? Because you can see the extrapolated red lines go past said limits.

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Yes, I can read diagrams. The question is, is the Vne indicated of 1350 an artificially placed limitation, thus cutting the graph off artificially? Because you can see the extrapolated red lines go past said limits.

 

I don't understand what you're trying to say. For a clean aircraft, yes, the diagram ends at VNE before thrust=drag is reached, so we don't know exactly what the thrust-limited top speed actually is, or even if there is another limit other than available thrust (I don't think there is, but as you say, we don't know that). For an aircraft with payload, this is not the case - there is a known top speed (where drag > thrust) and it's below VNE, and we can see this from the diagrams. Hence when Rossmum said that he's seen aircraft doing 1600km/h IAS at sea level with external payload, I responded that I can't see how that could be possible in reality, judging by the data available.

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I don't understand what you're trying to say. For a clean aircraft, yes, the diagram ends at VNE before thrust=drag is reached, so we don't know exactly what the thrust-limited top speed actually is, or even if there is another limit other than available thrust (I don't think there is, but as you say, we don't know that). For an aircraft with payload, this is not the case - there is a known top speed (where drag > thrust) and it's below VNE, and we can see this from the diagrams. Hence when Rossmum said that he's seen aircraft doing 1600km/h IAS at sea level with external payload, I responded that I can't see how that could be possible in reality, judging by the data available.

 

And I agree with that with regards to payload. But from what I'm seeing in the first two graphs and from my very limited understanding of Swedish is that the "zone 3" AB thrust curve might be artificially cut off to respect Vne by the looks of it, based on what we see from the last graph. Just a healthy asumption here and even if that's the case the deviations and excess thrust and speed should probably not be as great as what we see in DCS. And again, I mostly agree with all of you on that one but there simply wont be any actual data "busting" operational limits to know for a fact.

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And I agree with that with regards to payload. But from what I'm seeing in the first two graphs and from my very limited understanding of Swedish is that the "zone 3" AB thrust curve might be artificially cut off to respect Vne by the looks of it, based on what we see from the last graph. Just a healthy asumption here and even if that's the case the deviations and excess thrust and speed should probably not be as great as what we see in DCS. And again, I mostly agree with all of you on that one but there simply wont be any actual data "busting" operational limits to know for a fact.

 

No there won't be, which is why anything outside of the envelope requires numerical analysis and careful modelling of known physical phenomena ocuring in this regime. Either that, or just break stuff apart once it goes out of envelope, like some modules do. The problem with the former approach is that no you won't find much vanguard for this kind of stuff because no one does this these days for any professional applications of flight simulation software, you cross the envelope - the simulation stops, the instructor walks in and smacks you in the head. And if you're doing failure simulations there's no point in reaching this level of abstraction in the first place. Stuff like this would have been done back in the day though, in fact it must have been done by Saab when they came up with those limitations, so it doesn't mean no attempt should be made because DCS is the one environment where it does matter, otherwise people will just exploit it. DCS is not an arcade shooter, if we're to stand by "lul it'z a shitty game who cares about accuracy" then we should have stuck to Flaming Cliffs, but that's not what they're doing, the mission statement of DCS is to get things as close to realism as they possibly can and enforcing physical limitations should be the priority here, higher priority than all the "cockpit clicking simulator" stuff. If you look into sim racing market you will see that all resources there are poured into driving model and accurate replication of tracks while system modelling is the lowest priority. Because that is more important in a PvP environment where people compete against each other. In DCS, the same should apply to performance limitations and weapon limitations, this is why HB did CFD on their missiles for the F-14 instead of relying on official data - which is inaccurate by design - as well as why there's so much flaming over targetting and countermeasures modelling, globally, which is laughable at the level of fidelity they are aiming for. And as it stands, cold war servers that feature Viggen while using 3rd gen jets as primary A2A platforms devolve into Benny Hill simulators, where Viggens fly perpetually at Mach 1.3+ immune to all acts of aggression. Or better, go UP, where the FM is definitely inaccurate and where HB admitted in the past it's inaccurate, and go at completely stupid speeds. I remember reddit posts boasting about Mach 2.75+ at high altitude (while simultaneously the ceiling is lower than it should be). Perhaps that's what we should check next.

IMO this warrants attention and should be researched more by HB, that's what I'm saying. I know that they're busy with the Tomcat but Viggen really needs some love, it seems to have been sidelined over the last year. FM inaccuracies are not its only issue these days.

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Yea it's been mentioned repeatedly in this thread that for many aircraft, thrust available is not the only limiting factor to top speed. It absolutely is unrealistic that no plane in DCS overheats or runs into other nasty effects, but it's an issue that isn't limited to the Viggen.

 

The problem is that the Viggen is one of those aircraft that can reach speeds where this would become an issue with no effort. In fact it takes effort to keep it within safe operational limits. No one would complain about inaccurate supersonic modelling for an A-10 or Su-25.

 

I thought negative drag was an issue on specific aircraft, not the entire game, but I don't know if that fully explains it. I've seen people doing 1580-1610 with some air to air missiles still onboard. I was kind of hoping renhanxue would turn up at some point with this kind of stuff, because it does add more detail to what the limitations were and from that, pushing through for a sprint seems reasonable. On the other hand, being able to maintain an average speed of over 1600 (which I've now seen several times) across most of a sortie is still exceeding what Saab expected to be able to do with a stripped-down, hotted-up aircraft - so maybe the overall drag modelling needs some adjusting.

 

It's worth note that I know a few of the faster Viggen rocket riders I've seen have a habit of flipping some of the switches along the right wall of the cockpit, at least one of which regulates fuel flow IIRC. I'm not sure how much that factors into it as well, but I'd imagine it does.

 

Ultimately the problem is that (as Skysurfer says) it's a game, and people in games will minmax things. You don't need to worry about whether the jet is going to be flyable in 5 years, you don't even need to care if it's flyable in 5 hours, but it makes it pretty easy to just capitalise on that and make life miserable for other people in MP whose jets might not have limitations that can be at least temporarily overlooked.

 

If the negative drag bug is what's happening here, then it would explain a lot. And definitely needs attention. It would be one thing if it was A-10. It's another when it's an aircraft designed for speed and the sim puts a black hole into its nosecone.

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Agreed. Sadly there hasn't been much attention or incentive to push the Viggen out of EA with its remaining issues. The FM and engine performance were reviewed and tuned a while back but I haven't really flown it too much to really see a noticeable difference. Sadly we probably won't see any attention to the Viggen this year anymore. The Tomcat on the other hand has an excellent FM, since they get a lot of SME feedback and inputs and doesn't exhibit any out of the ordinary flight performance features - apart from those few "exploits" some people use that obviously break it.

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OK, I have a winner. Rossmum was right, there are gamebreaking exploits that occur naturally just by following normal procedures and let you go stupidly fast. I haven't checked the fuel pump exploit yet but just by starting with all pylons occupied with Rb-24Js and center tank, then jettisoning everything and launching the remaining two missiles (so in the end, I was not fully clean, I should have had additional drag from the sidewinder rail) I got the aircraft to go over 1650 kph at sea level... this is just stupid.

 

Also note that that chart shows Mach 2.0 max at ISA-15, not +10. So lower temp. This here was done in ISA on the other hand. Mach 1.34 on indicator. I didn't even bother expending all fuel, and ground effect barely changed anything, like 1kph difference in and out, I could have flown a proper mission profile like this, using the warp speed to give myself an unfair advantage on egress.

 

Screen_201031_223714.thumb.png.1e0acdb11ee72862aa91dcf201fe446c.png

 

Track file

 

https://1drv.ms/u/s!ArMDWm49dN3CgdcvTtDYg-XimM-BGQ?e=Sdo2uJ

 

I think this explains the ludicrous speeds we've seen online. I'm gonna see if I can replicate the fuel exploit, I can smell 1700+. This is bad and people who use it in PvP should feel bad.

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  • 4 weeks later...

Each missile fired increases the speed further. Flew ISA with 6 Rb 24J and belly tank, fired the former in pairs and the latter was jettisoned. No messing with anything else, this was purely to check "negative drag" effect from firing weapons. The trackfile is attached.

 

On an earlier test run (using default DCS weather, so a bit warmer), and 11m above the Black Sea, here are the speeds the aircraft stopped accelerating at:

 

1466km/h IAS with 6 Rb 24J and tank (~M 1.19)

1555km/h IAS with 6 Rb 24J (tank jettisoned) (~M 1.26)

1633km/h IAS with 4 Rb 24J (2 fired, tank jettisoned) (~M 1.32)

1719km/h IAS with 2 Rb 24J (4 fired, tank jettisoned) (~M 1.40)

1804km/h IAS with all Rb 24J fired and tank jettisoned (~M 1.47)

 

In the attached trackfile the speeds reached are slightly slower, but follow the same pattern and with roughly equivalent increments between each 'stage'.

 

All of these are obviously above the documented and attested-to never-exceed speed of 1350km/h IAS for the AJ/AJS 37 and even above the documented and attested-to never-exceed speed of 1450km/h IAS for the JA 37. The only consequences for doing this are obviously the fuel use, and the fact that you only go from very high speed to ludicrous speed as you expend missiles - which is fine, and arguably even ideal, in multiplayer. It means you can make slashing attacks with virtually no chance of reprisal, as not even the 4th gens can catch you and even most SAMs won't be able to reach you unless they're very lucky.

 

This is a game-breaking bug in multiplayer and I don't think any amount of SAAB marketing can explain how an aircraft which was limited to 1350km/h specifically due to skin temperature is able to achieve and sustain nearly M 1.5 at sea level while the F-111 needed one eye kept on a skin temperature gauge for crews to be able to approach the same speeds. As I understand it, "negative drag" is not an issue limited to the Viggen, but in any case there should be some kind of consequence for overspeeding that severely. Short sprints up to 1450-1500 are one thing as it is, but sustaining 1600+?

viggen overspeed.trk

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3 hours ago, rossmum said:

Each missile fired increases the speed further. Flew ISA with 6 Rb 24J and belly tank, fired the former in pairs and the latter was jettisoned. No messing with anything else, this was purely to check "negative drag" effect from firing weapons.

 

Just want to make sure I understand this correctly: Those speeds can only be achieved by expending weapons? So if you equip a jet with an empty loadout it will not go as fast as a jet that had missiles and expended them?

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26 minutes ago, QuiGon said:

 

Just want to make sure I understand this correctly: Those speeds can only be achieved by expending weapons? So if you equip a jet with an empty loadout it will not go as fast as a jet that had missiles and expended them?

Yup.

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6 hours ago, rossmum said:

This is a game-breaking bug in multiplayer and I don't think any amount of SAAB marketing can explain how an aircraft which was limited to 1350km/h specifically due to skin temperature is able to achieve and sustain nearly M 1.5 at sea level while the F-111 needed one eye kept on a skin temperature gauge for crews to be able to approach the same speeds. As I understand it, "negative drag" is not an issue limited to the Viggen, but in any case there should be some kind of consequence for overspeeding that severely. Short sprints up to 1450-1500 are one thing as it is, but sustaining 1600+?

viggen overspeed.trk 1.09 MB · 2 downloads

We already established above that anything above 1500-ish is unrealistic even clean, as both Saab employees and the flight manual agree on this point, so there's no need to be snippy. Saab marketing certainly hasn't ever made any claims about the speeds you're seeing, and nobody else has tried to argue otherwise either since last time this was brought up.

 

It's good to have numbers though, and I agree it's a significant bug, so thank you for testing it. I'm curious though how closely it matches the flight manual numbers clean.


Edited by renhanxue
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So using the cold start mission and doing a high speed test with an unarmed Viggen, i.e. no weapons launched or tanks dropped, the plane appears to top out at M1.3 1550kph @ 50% fuel (also only rises to 1552kph at 24% fuel). Seems a little more "reasonable" (i.e. still slightly too fast) and suggests the 1780kph speed demonstrated in Rossmum's track is a weird released stores weapon drag bug.

 

Edit: This is with DCS 2.7.1.6430 Open Beta and my point being there appears to be two bugs here. 1) The Viggen is slightly too fast (if a stripped down special version was predicted to do 1542kph runs then perhaps 1450-1500kph should be the limit for military version, gut feeling here based on a physics degree) and should have consequences for sustained high speed runs to limit them (airframe overheat or engine fatigue/damage, but bearing in mind this was more modern tech than the MiG21 and a purpose built low level interdictor), probably HB remit to fix and 2) Weapon release excessive drag reduction bug which appears to affect several aircraft and is probably an ED remit to fix. Given we are missing the sensory feedback to detect a cockpit or airframe getting warm it would be interesting to discuss how this could be indicated in the sim.

 

image.png


Edited by Sideburns
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16 hours ago, Skysurfer said:

Yup.

 

I see, that makes it even worse. Definitely needs to be fixed!

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On 5/20/2021 at 11:37 PM, Sideburns said:

So using the cold start mission and doing a high speed test with an unarmed Viggen, i.e. no weapons launched or tanks dropped, the plane appears to top out at M1.3 1550kph @ 50% fuel (also only rises to 1552kph at 24% fuel). Seems a little more "reasonable" (i.e. still slightly too fast) and suggests the 1780kph speed demonstrated in Rossmum's track is a weird released stores weapon drag bug.

 

Edit: This is with DCS 2.7.1.6430 Open Beta and my point being there appears to be two bugs here. 1) The Viggen is slightly too fast (if a stripped down special version was predicted to do 1542kph runs then perhaps 1450-1500kph should be the limit for military version, gut feeling here based on a physics degree) and should have consequences for sustained high speed runs to limit them (airframe overheat or engine fatigue/damage, but bearing in mind this was more modern tech than the MiG21 and a purpose built low level interdictor), probably HB remit to fix and 2) Weapon release excessive drag reduction bug which appears to affect several aircraft and is probably an ED remit to fix. Given we are missing the sensory feedback to detect a cockpit or airframe getting warm it would be interesting to discuss how this could be indicated in the sim.

 

image.png

 

 

Yes, it most definitely is a bug, the sidewinder launch rails have negative drag coefficients. And it happens 100% of the time, you get ~50kph speed boost per empty launch rail. With 6 empty sidewinder rails it goes up to ~1800kph or Mach 1.46 (wow), with just 2 it's around 1650 (Ma=1.34). Most hilariously, jettisoning the 4 launch rails almost smears you against the front glass, it goes down from 1800 to 1650 as if you opened the airbrake. This is a massively game breaking bug. So far haven't found it happened with anything other than sidewinders - which is still bad, cause on most cold war servers Viggen is primarily used as CAP platform - I assume this is part of the reason why. I have suspicions towards ECM/CMS pods as well, but need to test more. And it doesn't only extend to max speed, with 6 empty launchers you can almost fly level on flight idle, the autothrottle will hold like half of the throttle setting it would in clean config and it's legitimately hard to slow down on approach.

 

This needs to be fixed. Negative drag rails is an old bug in DCS but other modules that had it seem to have fixed it by now, Viggen is the outlier.

 

As for heat - I'm pretty sure running the entire flight at 1800 kph at sea level would do more than just slightly heat up the skin. It would cause damage that would be apparent after landing if not catastrophic in the air. We're not talking about going a bit over max speed estimates, we're talking about going over 200 kph past it. We're talking over 100kts higher than actual low level speed records. It breaks the suspension of disbelief at this point.


Edited by m4ti140
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10 hours ago, m4ti140 said:

 

As for heat - I'm pretty sure running the entire flight at 1800 kph at sea level would do more than just slightly heat up the skin. It would cause damage that would be apparent after landing if not catastrophic in the air. We're not talking about going a bit over max speed estimates, we're talking about going over 200 kph past it. We're talking over 100kts higher than actual low level speed records. It breaks the suspension of disbelief at this point.

 

Again, I don't think anyone has actually argued that the real aircraft is even remotely capable of reaching those speeds at sea level, let alone sustaining them, so this is sort of academic. The speed alone is well past suspension of disbelief on its own.

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At this point it's just becoming more hyperbole about it being a "massively game breaking" bug.

 

My previous post was intended as a suggestion in the interests of sticking to facts, getting the bug fixed asap and guesses on which teams should resolve each issue given that there appears to be two issues here. Other aircraft are also affected (core DCS aero bug? weapons being ED's responsibility now?) but the Viggen does seem to be the most affected by this negative drag bug, as well as missing overspeed damage (also not unique in that respect) and being a little too fast even without the negative drag bug. I did search on the forums and asked on the ED Discord yesterday if there was an existing bug for negative drag from weapons release but no answer so far.

 

W.r.t the excessive speed effects there should probably be effects, such as but not limited to skin, airframe, equipment and engine damage, starting at Vne to stop continual over speeding at or above that speed. Hence my comment it would be useful to have some kind of surrogate environmental feedback given the comment earlier that the cockpit and airframe would be noticeably warm and served as a warning in real life, it would be nice if this feedback was universal across all fast aircraft. My original comment was on the assumption the negative drag bug allowing the Viggen to do 1780kph (the maximal speed with evidence so far) will be resolved. As per @renhanxue's response I don't think there is anyone who has claimed the Viggen should be capable of sustaining or surviving unscathed a speed like 1800kph down low, of course at such speeds you would expect a more severe effect.

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@Sideburns

Quote

At this point it's just becoming more hyperbole about it being a "massively game breaking" bug.

People using Warp Drive to outrun missiles in a2a combat is game breaking. We have tons of acmi files of this at this point. It's just depressing, the moment a Viggen arrives in the area people just ignore it or bug out, hoping they won't be targetted, those who do not just waste missiles, because any attempt at retaliation is futile unless the Viggen player doesn't know what they're doing. If they are aware of this exploit they use it and good luck chasing them.

 

Quote

My previous post was intended as a suggestion in the interests of sticking to facts

We are sticking to facts.

 

Quote

Other aircraft are also affected (core DCS aero bug? weapons being ED's responsibility now?) but the Viggen does seem to be the most affected by this negative drag bug

I'm not aware of any module this has not been fixed in up to now, point to a specific one with numbers to prove me wrong.

 

Quote

negative drag bug allowing the Viggen to do 1780kph (the maximal speed with evidence so far) will be resolved

SSL - 1789:

Screen_210517_192818.png?width=901&heigh

 

20°C (default DCS settings), credit to Ross - 1804:

unknown.png?width=901&height=507

 

A better description would be Ma~=1.46. Remember the speed of sound increases with temperature.


Edited by m4ti140
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