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Stall + High AoA behaviour...


Robin885

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So it's been since april 2020 that the flight dynamics at high angles have been broken in two ways:

1: The critical AoA seems to be gone, you can just pull on the stick as far as you wish the plane will keep flying. This issue appeared to have been fixed at some point during summer but is now bugged again

2: After pulling the stick too far back, the plane will violently rock on the yaw axis in a totally unrealistic manner. This has never been fixed since april.

 

So... can we hope to see a fix for a bug that, according to me at least, must be a top priority or is the Fishbed doomed to stay in the hangar forever?

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1. 21 is a delta wing, it doesn't stall the same way straight or swept wings do, what limits your AoA is stability and engine thrust first, as L/D goes to drain past a certain point even before lift starts falling. The previous behaviour where at a certain AoA the aircraft turned into a brick was wrong, not even a rectangular wing stalls as violently as the 21 did in game back then, a delta loses lift more gradually and at angles of attack significantly higher than a conventional wing.

2. And the wing rock is precisely that stability limit, it's a real phenomenon with this aircraft and is what limits the operational AoA, this is what the yellow and red regions on AoA gauge warn you about.

As it is in game right now there seems to be a second region of stability if you survive the wing rock and increase the AoA even further, not sure if this part is correct, nor if the lift should be sufficient for level flight at that point but even then, once you enter this region you don't have enough thrust to overcome the drag while generating enough lift for level flight, unless you're very low on fuel AND have emergency afterburner on. And you have to go back through unstable region to recover from that, so it's generally not a good idea to go there.

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I was expecting this kind of answer... But no, none of these two points are realistic, and it is a bug. It's not about the way the plane stalls, it's that it simply doesn't stall. The manual states that after 33° AoA, the plane reaches it's critical AoA and won't go further. This limit does not exist in game today.

And about the wing rock, I'm not talking about stability. Just watch it by yourself:

It just looks like the plane reaches an area where the FM is not even "coded" anymore
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Except the wing rocking is exactly because the aircraft loses stability. It is a well documented behaviour in Soviet manuals and in US exploitation program material. It is a behaviour exhibited in many aircraft of this era and if you incorrectly try to counter it with aileron, you'll end up in a spin.

 

What makes this seem so "unrealistic" to you? The plane very clearly stalls. Stalling does not mean the plane falls out of the sky immediately, it means airflow has separated from the wing and you are losing lift rather than producing it. The onset of this and what effect it has on the aircraft depends on a large number of variables, it's why different aircraft have different and unique stall characteristics.

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Wing rocking isn't a departure in yaw, it's a departure in roll. I think a more accurate FM would have a lot less yaw stability than is demonstrated in DCS.

 

"The manual states that after 33° AoA, the plane reaches it's critical AoA and won't go further." That's not what stalling is, stalling is simply the maximum value in the CL curve. It says nothing about controllability or feasibility of flight at angles greater. The wing isn't a control surface although reduction in its lift can affect control. The UUA-1 values which correspond to stall only apply to a semi-narrow Mach range anyway. At low speed 33 is not CLmax (or at least not on the real airplane).

 

My impression is that MiG-21bis turning capability is limited by stability and controllability in the post-stall regime primarily. Lift post-stall is quite high and provided stability and control authority hold up the airplane flies quite well there. Naturally this is a very high drag regime so nothing is going to remain high speed for very long. Russian manual restrictions are quite conservative and other nations have exploited the airplane much beyond them.

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Except the wing rocking is exactly because the aircraft loses stability. It is a well documented behaviour in Soviet manuals and in US exploitation program material. It is a behaviour exhibited in many aircraft of this era and if you incorrectly try to counter it with aileron, you'll end up in a spin.

 

What makes this seem so "unrealistic" to you? The plane very clearly stalls. Stalling does not mean the plane falls out of the sky immediately, it means airflow has separated from the wing and you are losing lift rather than producing it. The onset of this and what effect it has on the aircraft depends on a large number of variables, it's why different aircraft have different and unique stall characteristics.

 

And again, as I stated before, I am not saying that the plane doesn't suffer from any kind of roll/yaw stability issue. In fact my post isn't even about stability issues. I am just talking about the violent yaw imput in itself. Again, posting that here

. It's totally unreal, just looks like a bomb exploded on one side of the vertical stabilizer. I get and I agree that there should be a phenomenon that makes the plane be unstable on both axis, but definitly not on the way it departs. No buffeting, no slip, nohing. Just looks like a sript that makes the plane go apeshit as soon as the needle reaches 33° AoA above a certain airspeed.
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So let me ask you this:

What do you think should be happening and what do you base that on?

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Per Soviet documentation the aircraft does not exhibit stall buffet at very low speeds. It's a quirk of the design. Again, aircraft have their unique characteristics for a variety of design or construction reasons. If you think comparing a tailed delta MiG-21 to a Mirage, or a Tomcat, or an F-86, or a Cessna 152 will give you anything meaningful, I don't know what to tell you.

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So let me ask you this:

What do you think should be happening and what do you base that on?

 

You guys surely know better than me what should happen. But the thing is I'm not trying to say how the plane should react, but why I believe it doesn't react realistically. The plane goes from perfectly controlled flight to wing rocking in nothing, no amount of time. There is no transition between the two phases of flight, and it's not a way to say that it is too violent, there is literally no phase between the two regimes. It's just like driving your car and stoping without the braking phase, 80-0 without deccelaration. Even disregarding any buffeting, or anything that could potentially warn the pilot of what's about to happen, I am fairly certain that this is just not possible. I'm not doubting the wing rocking tendency of the plane, nor the yaw oscillations that ensues (although I believe they are too strong but I could very well be wrong, whatever). But the transition between the two regimes... no way.


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Per Soviet documentation the aircraft does not exhibit stall buffet at very low speeds. It's a quirk of the design. Again, aircraft have their unique characteristics for a variety of design or construction reasons. If you think comparing a tailed delta MiG-21 to a Mirage, or a Tomcat, or an F-86, or a Cessna 152 will give you anything meaningful, I don't know what to tell you.

 

I'm not gonna try to be annoying and stuff but you tell me where I made such a comparison... And I'm fairly certain the Mig21 did experience buffeting. Maybe not at lower speeds as you say, but that's only a portion of the flight enveloppe.

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@Robin885, please don't take it the wrong way, but "I believe" and "I'm fairly certain" doesn't constitute a valid complaint. They are just weasel words to justify your belief, but with no actual proof behind it. As someone who says that the MiG-21 is not behaving correctly at high angles of attack your job now is to find the necessary documentation to prove your assertions and test it out until you get verifiable and repeatable results. Only once you complete this step should you consider filing a bug report to the devs. Unless you find some legitimate flight test documents and/or pilot reports to base your assertions on and then compare them to the in-game flight characteristics of the MiG-21 then we have little to discuss. To paraphrase a certain English writer, "what may be asserted without evidence, may be dismissed without evidence".

 

Please don't take it as an ad personam attack, I'm merely trying to remind you that the burden of proof lies with you, not us or the devs.

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@Robin885, please don't take it the wrong way, but "I believe" and "I'm fairly certain" doesn't constitute a valid complaint. They are just weasel words to justify your belief, but with no actual proof behind it. As someone who says that the MiG-21 is not behaving correctly at high angles of attack your job now is to find the necessary documentation to prove your assertions and test it out until you get verifiable and repeatable results. Only once you complete this step should you consider filing a bug report to the devs. Unless you find some legitimate flight test documents and/or pilot reports to base your assertions on and then compare them to the in-game flight characteristics of the MiG-21 then we have little to discuss. To paraphrase a certain English writer, "what may be asserted without evidence, may be dismissed without evidence".

 

Please don't take it as an ad personam attack, I'm merely trying to remind you that the burden of proof lies with you, not us or the devs.

 

No offense taken, and you're right. But the thing is I don't know how to prove it appart from showing it in video, as I did above^^ Regarding that whole wing rock thing, I'm not trying to correct something and explain how it should act, but rather point something that looks very suspicious and why.

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I was expecting this kind of answer... But no, none of these two points are realistic, and it is a bug. It's not about the way the plane stalls, it's that it simply doesn't stall. The manual states that after 33° AoA, the plane reaches it's critical AoA and won't go further. This limit does not exist in game today.

And about the wing rock, I'm not talking about stability. Just watch it by yourself:

It just looks like the plane reaches an area where the FM is not even "coded" anymore

 

None of what you just said is true. The performance manual (real life one) for the 21 describes what happens at critical AoA and why you shouldn't go there. The wing rock happens before lift starts to drop off. And yes, delta wing doesn't stall in the same sense conventional wing does. It transitions to vortex lift, and then the lift starts slowly dropping with increasing AoA (more or less rapidly depending on whether the wing has leading edge extensions), while drag keeps increasing until L/D goes to drain. What you get in that video is correct, the vertical stabiliser ends up in the aerodynamic shadow of the wing, and the aircraft starts rapidly yawing to whichever side it had more sideslip on, until the tail encounters free flow again, which throws the aircraft to opposite side, rinse and repeat. Look at your sideslip indicator when the wing rock happens.

Keep in mind that the guy who coded the flight model is an actual MiG-21 pilot. So you need good sources to base your claims upon when you say something's incorrect. Remember that this is a delta wing with conventional elevator layout and no FBW or even SAS, it won't behave like most conventional aircraft do.

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Wing rocking isn't a departure in yaw, it's a departure in roll. I think a more accurate FM would have a lot less yaw stability than is demonstrated in DCS.

 

Because it's not the same kind of wing rock you encounter with conventional wing, it's a departure in yaw, wing rock is a secondary effect that is the first thing you'll notice, because of how violent it is.

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No offense taken, and you're right. But the thing is I don't know how to prove it appart from showing it in video, as I did above^^ Regarding that whole wing rock thing, I'm not trying to correct something and explain how it should act, but rather point something that looks very suspicious and why.

 

Apologies for misunderstanding your intentions, I judged your claim based on the fact you posted this in the bug reports sub-forum. I'm glad you didn't take my post the wrong way.

 

As for verifying your assertions, I'd start with a look through the official MiG-21bis flight manual (they're out there if you know where to look, I don't want to post links here for the fear of breaking forum rules) as well as test pilot reports from US evaluations of the type. Note that most of the currently available declassified US sources cover the MiG-21F and its later Chinese derivatives. Although similar in overall layout and design, the bis has a different enough airframe and engine that its handling shows a slightly different picture compared to the F-13. Unfortunately for your claim most (if not all) of the other users who posted above appear to have these bases well and truly covered. That doesn't mean you can't verify what they're trying to explain. My suggestion would be to get into a Quick Start mission or your own customs scenario with all the relevant flight parameters written down and then simply test them repeatably until you get a clearer picture. Note your results and show them here so others can verify them. If there are any significant abnormalities then you'll be able to pick them up easily.

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"And yes, delta wing doesn't stall in the same sense conventional wing does."

Never said otherwise.

 

"What you get in that video is correct, the vertical stabiliser ends up in the aerodynamic shadow of the wing, and the aircraft starts rapidly yawing to whichever side it had more sideslip on, until the tail encounters free flow again, which throws the aircraft to opposite side, rinse and repeat. Look at your sideslip indicator when the wing rock happens."

Well, you see, that's the thing. It's not RAPID, it's INSTANT. That shadowing you're talking about is progressive, with the problem amplifying while the angle of attack rises. It could very well be rapid, almost sudden, but definitly not from 0 to 100 without any in-between like it does in game.

 

"Keep in mind that the guy who coded the flight model is an actual MiG-21 pilot"

I'm not doubting that having such a person coding the module is an excellent thing, and that he certainly is one of the best people you'd want making the flight model. But that absolutely doesn't make it immune to problems, defaults and possible bugs. Otherwise there wouldn't even be a "Flight dynamics" sub-forum to express the players' opinions to what they see in-game.

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"And yes, delta wing doesn't stall in the same sense conventional wing does."

Never said otherwise.

 

"What you get in that video is correct, the vertical stabiliser ends up in the aerodynamic shadow of the wing, and the aircraft starts rapidly yawing to whichever side it had more sideslip on, until the tail encounters free flow again, which throws the aircraft to opposite side, rinse and repeat. Look at your sideslip indicator when the wing rock happens."

Well, you see, that's the thing. It's not RAPID, it's INSTANT. That shadowing you're talking about is progressive, with the problem amplifying while the angle of attack rises. It could very well be rapid, almost sudden, but definitly not from 0 to 100 without any in-between like it does in game.

 

"Keep in mind that the guy who coded the flight model is an actual MiG-21 pilot"

I'm not doubting that having such a person coding the module is an excellent thing, and that he certainly is one of the best people you'd want making the flight model. But that absolutely doesn't make it immune to problems, defaults and possible bugs. Otherwise there wouldn't even be a "Flight dynamics" sub-forum to express the players' opinions to what they see in-game.

 

 

 

You see, m4ti140 explained the whole process how he understand it. You just again go and say "but definitly not from 0 to 100 without any in-between like it does in game". Definitely? How you can say definitely if you just said you don;t know how? You contradicting yourself through this whole thread. Explain process how you see it and why it should be like it, or it will be another circle of self contradiction I guess...

AKA LazzySeal

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You see, m4ti140 explained the whole process how he understand it. You just again go and say "but definitly not from 0 to 100 without any in-between like it does in game". Definitely? How you can say definitely if you just said you don;t know how? You contradicting yourself through this whole thread. Explain process how you see it and why it should be like it, or it will be another circle of self contradiction I guess...

 

I'm opposing his explanation to my in-game observations, and trying to say that it just doesn't match the way it acts in-game.

In DCS the plane will go from controlled flight to massive yaw oscillations passing 33° angle of attack with absolutely no delay, warning or preemptive sign. It's either not there, or fully there. This is factual and shown twice in the above posts.

And the thing is that it does not match m4ti140 explanation (no offense to him, I appreciate that people take time to explain stuff). Wing rock does not appear before the loss of lift, but at the same time and brutally. The plane does not start to yaw rapidly on one side or the other, it just directly transitions to violent oscillations without amplification. And I'm not making that up over what I think is right or not, it's by observing the plane behaviour in-game.

 

Now I understand that I came up with that pretty barehanded, without any proof of what should be happening, and both you and Firefly are very right in reminding this to me. Aswell the first two posts may have been a little cocky on my side. However, I do make clear in the followings that I'm not trying to explain how it should act, but that I strongly believe the current behaviour of the aircraft is inaccurate and why.

I believe I'm not the first one to have brought this up in the forums aswell, and so to believe that it is incorrect.

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And the thing is that it does not match m4ti140 explanation (no offense to him, I appreciate that people take time to explain stuff). Wing rock does not appear before the loss of lift, but at the same time and brutally. The plane does not start to yaw rapidly on one side or the other, it just directly transitions to violent oscillations without amplification. And I'm not making that up over what I think is right or not, it's by observing the plane behaviour in-game.

 

It may be not matching because of array of reasons. Maybe you are not in parameters for it to be developing slow, therefore it slams instantly (means process stays same but it happens faster because of too violent pull).

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Worth note that this behaviour wasn't so much introduced as fixed - it used to be present but some issue or other caused it to disappear for quite a while. If you started flying it at a similar time I did you wouldn't have experienced this behaviour the first time around, but in older videos of the module it's present.

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Critical AoA from a dev perspective is extremely difficult to simulate. This module has changed a lot over the years, but I think the flight model is in a pretty good place now.

There was a time a few years back when the flight model became messy because it was thinkered with back and forth every week... I don't think anyone wants to go back into that carousel again

unless there is something seriously wrong.

 

Reading OP's complaints, my understanding is that the major issue is not really the stall behaviour, but the suddenness. It is the lack of feedback right before the behaviour sets in. Feedback such as sound or buffeting.

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  • 1 year later...

OK so I want to believe that high AoA situations lead to wing rock and unstable roll/yaw axis due to stalls.

But the current implementation just feels really silly. I do not own a Mig-21 IRL, so I can not check, but does Magnitude really want to tell me that it departs in the exactly same way, every single time? In a left hand turn, at exactly 16° AoA (DCS, not indicator), the plane rolls right, yaws left and then in the opposite direction. Right hand turn exact opposite.

It doesn't even lose any lift during this "wing rock", because the g-load stays pretty constant. If it's a form of stall, should it not decrease somehow?

To me it looks less like an aerodynamic phenomenon and more like Dolphin jumped into my cockpit and started to wiggle my controls to mess with me for fun 😉

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It's a 2014 module coming at first from a LOMAC mod, devs probably did the best they could back then. That doesn't mean it shouldn't start wing rocking at 16º AoA if that's the critical AoA for the matter, and we don't know internally what is modelled or not, if it loses lift because of that, or not. Subjective impressions are usually off.

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