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38Deg AoA Wobble at 600+kph IAS. Thoughts ?


LupinYonder
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As far as I'm concerned the behavior the OP posted about is what you would call a tip stall, or put another way, one wing stalling slightly earlier than another. I seem to recall the previous FM would break cleanly and straight ahead IRC. The new stalling behavior seems believable to me, but I have no idea if the real Mig21 stalls straight ahead or not.

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I don't know why you expect anything other than an accelerated stall when hitting 38 degrees AOA.

 

Who, me? The original poster described this as a “wobble” and “snap to horizontal” that the OP said felt wrong and scripted. Based on the words used, I am not sure if the OP understood this to simply be a stall. When I flew according to the OPs speeds, I got the same behavior and interpreted it to be an accelerated stall, as opposed to a problem with the FM.

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Same backtrim as I used before, stick back 3/4, rotates around 280-300, ease off stick as necessary to keep the nose from rising too much. It's more sensitive than it used to be and it took me a little adjusting to (mainly because of the nose's tendency to bounce) but honestly, if you hold the nosewheel off the tarmac long enough to take the stiffer suspension out of play, the takeoff is pretty much no different.

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Further reading on Tip and Accelerated Stall

 

Thanks to everyone for an interesting read. The replies to my OP post gave me a lot to think about.

 

It seems that the behaviour I described in the opening post is referred to as an accelerated stall. I'll be honest, I haven't heard this term before so I decided to try and educate myself a bit more before I draw further, possibly inaccurate conclusions on the Mig 21 flight model.

 

Accelerated Stalls. They are due to a critical AoA and can happen at any speed ( depending on aircraft ).

 

In other words... A higher load factor (measured in G's) leads to higher stalling speed. Load factor increases with

1. Increase in weight

2. Steep turns

3. Abrupt changes in aircraft attitude.

 

Here's a great quote I found from http://www.flightliteracy.com ( you may all know more about flying than me but I'm learning more every day and that's why I love sim flying )

 

"Any aircraft, within the limits of its structure, may be stalled at any airspeed. When a sufficiently high AOA is imposed, the smooth flow of air over an airfoil breaks up and separates, producing an abrupt change of flight characteristics and a sudden loss of lift, which results in a stall." - https://www.flightliteracy.com/load-factors-part-two/

 

Before the Accelerated stall at critical AoA, buffeting or other undesirable behaviours of the aircraft should be obvious to the pilot. I'm not sure I can spot these behaviours in the current FM and only the AoA indicator tells me how close I am to stall.

 

I believe the yawing behaviour after our accelerated stall can be called incipient spin? While looking at flying instructional videos, incipient spins seem to be characterised by a sudden nose down in the direction of turn but I feel our Mig's reaction being quite different. This may be because my turn is not properly coordinated with the rudder. Just a thought

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It happens at basically the exact same point it used to, most likely you weren't noticing it before because the aircraft would stop moving forwards and move down instead. The number of times I watched people stall all the way into the ground because they had no idea what was going on and kept trying to 'fix' their sudden descent by pulling the stick was painful.

 

The only real difference is that now the departures are a bit more spirited, so it's more noticeable and takes a second or two to recover from compared to previously.

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It happens at basically the exact same point it used to, most likely you weren't noticing it before because the aircraft would stop moving forwards and move down instead. The number of times I watched people stall all the way into the ground because they had no idea what was going on and kept trying to 'fix' their sudden descent by pulling the stick was painful.

 

The only real difference is that now the departures are a bit more spirited, so it's more noticeable and takes a second or two to recover from compared to previously.

 

Could be, remember being able to power my way in at high AoA and not stalling. Maybe the speeds have changed or like you said, it's more spirited

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I don't want to pick sides on this discution because I think that current FM need a lot of love but

here is a video af a real Mig21 puling a sustained turn at 30 AOA at 2:20 and a high resolution photo of that plane cockpit so you can check numbers on instrument.

And this was on RIAT 2019 so there's safety margin on that.

Lets face it. The vast majority of us are just entusiast without real and suported knowledge of this.

 

 

Cockpit of Romanian MIG 21

38272835_e7e22157cc_b.jpg


Edited by alexbap

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Yes, that's relatively easy to do in DCS as well (more so now because of the FM issues this patch). I'm a habitual turnfighter so most of my combat flying in the DCS MiG-21 is done in the red zone on the UUA. You just have to try and keep awareness of where the needle is while also watching your target, and it's when you lose awareness and get too greedy with the stick that you stall. The current FM does not let you stall at all even with the UUA pegged against the stop and F2 camera telemetry (showing true AoA) indicating 25+ degrees.

 

Remember though, the UUA-1 doesn't read true AoA, because the airflow is already disturbed as it passes over the sensor vane on the left side of the nose - 30ish on there corresponds to around 20-22 degrees actual AoA.

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a real Mig21 puling a sustained turn at 30 AOA

30 Units is child's play!

 

 

I can currently pull G's with the AOA instrument pegged in the red, off the scale... all day long. Yes I know they're investigating it.

This is 10km MSL, MAX Power and pull until AOA needle pegs. The manual says it should stall at 33 Units. That doesn't happen.

 

 

It'll pull 4G's at 35+ units all day long until you run out of gas!:lol::lol:

NCIqkjB.png

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I went from thinking the previous patch stall was extream. Then I understood better what accelerated stall and that it could happen at any speed.

Now it feels like we have reverted to a much older FM with no stalling at all. It was mentioned to me that the delta shape wing stalls differently to other wings so that will be my next area of study.

 

Good reference with that YouTube video, but.others are correct in that it's always been possible as long as you didn't go all the way to 38 on the indicator.

 

It feels like the Devs thought " we tried to update the FM and everyone hates it, let's just give them an old one with no stall characteristics to stop them whining"

 

Devs, we all love that the FM is getting some attention, don't be discouraged! We love odd quirky character in our FM when it's believable. Please be more transparent with FM changes so we can discuss it properly without guessing what's happening under the bonnet ( hood, for Americans ).


Edited by LupinYonder
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I might be alone, but I completely missed what was "wrong" in previous patch' take-off and landing... It didn't seem strange, alien, difficult, too simple or whatever to me. What was the issue?

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The nose felt a bit lighter, combined with the bouncy suspension - if you didn't rotate early enough, you could get a bad bounce and end up with a fairly unpleasant, but entirely controllable, takeoff. I never had a tailstrike from it though I did have the stall warning lights flash once or twice.

 

I think there was a bit of a mountain being made out of a molehill about it to be honest - like, yeah, 3/4 back stick is one thing, but if the nose starts coming up abruptly you don't just keep holding it there. Countering it by easing off backpressure was easy enough. It wasn't ideal but it didn't really make it near as hard as you'd think from reading the forums.


Edited by rossmum
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If you have a look here https://forums.eagle.ru/showthread.php?t=266878

they describe some of the strange behaviour from the previous patch.

 

This is all separate from the issue/feature I was describing in my original post on this thread. I was describing odd stall behaviour at high AoA which I believe now was simply a rather scripted simulation of an accelerated stall.

 

In the current model, the accelerated stall, in fact it seems all stalls have been totally removed. As if we have reverted to an older FM until the issues can be sorted but that is just an opinion. Also the FFB has stopped working for those who use it in the current release.

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