Weird Inverted Stall

[LtMacGowan]

I've noticed in the DCS World module Su-27 mission "gauntlet" that when im making violent maneuvers i keep going into this weird stall were i invert upside down and just can't not either turn right side up, or get the nose to point down, every time i try and push the stick up or use rudder, its like the plane automatically rights itself, and i have to start rocking it, go as far as i can to get the nose down, the plane tries to right itself upside down, i pull back, and after 4-5 times i get enough momentum to get the nose down and pull out of the stall..

 

What the hell is causing this? I've never had a plane fight me before like this yet alone try and put itself back in an upside down stall over and over again.

 

Is there some kind of autopilot assistance im missing? or trimmer?

[Ironhand]

SOme links you might find interesting:

 

Su-27 Flight Model Discussion

 

Is there a way to recover

[ShuRugal]

what is basically happening here is that the SU-27 FCS allows you to put the plane into AoAs that guarantee extreme departure. The aircraft is particularly sensitive to negative G pushovers.

 

Unfortunately, the aircraft is very stable once it achieves the inverted position, thanks to how tail-heavy it is.

 

What those threads really boil down to is that the new flight model requires the Flanker to be flown with a gentle hand, and positive Gs.

[ineth]

i flew the plane just today and i faced the same problem. At around 12.000 feet it was apparent i had a deep stall issue and the only way out of it was to throw on it the Falcon 4.0 bible. I found where the fly by wire manual override is and i rocked the little babe out of it. (textbook approach out of the bible)

 

If the plane had the vector thrust engines this would be allot of fun to deal with.

 

What i find weird though is that it was impossible to get over 10000 meters (30.000 feet). It should be easy to get near 50.000 feet. I allmost made it to 11.000 meters using a ballistic path.

 

Also while i had to sweat every bit of experience with flight sims so i could land the Su25s and A10, landing the Su27 was effortlessly as eating Nutella out of the bucket with a spoon. (liking the interior is a tad harder though)

 

I come to wonder if the flight model of the Su27 is over-oversimplified in any way.

offcourse it could just mean that the guys at Sukhoi really did a good job with the Su27 too.

 

Fly safe all.

[Ironhand]

...

 

What i find weird though is that it was impossible to get over 10000 meters (30.000 feet). It should be easy to get near 50.000 feet. I allmost made it to 11.000 meters using a ballistic path...

 

You should be able to get above 10K meters without much trouble at all. Take off and climb. Set your throttle at 95% altering your climbing angle to maintain around 700-800 km/hr TAS all the way up. You should get above 10K fairly quickly.

Edited January 2, 2016 by Ironhand

[Stuge]

It does land beautifully, doesn't it :) The landing dynamics are just really nice, nothing simplified there...

 

About your climb problems.. if the thing doesn't want to climb then most likely you are flying too slow! When going for high altitude fly minimum Mach 0.9 all the way! If you're too slow, you have to dive for more speed and then retry the climb. Being too slow means high angle of attack which means high drag and also slow speed means less power from the engines. This is why you can get "stuck" if you try to climb at too low airspeed.

 

Also make sure that gear, flaps and airbrake are all retracted ;)

[DarkFire]

Another tip: don't use the autopilot above 10,000m. For some reason it becomes unstable below ~560 Km/h indicated air speed (not true air speed) and at any altitude above 10,000m unless you're in afterburner your IAS will be below the critical value. I still think this is a bug, though I could be wrong.

 

Otherwise, for a climb to high altitude as others have commented you need high TAS for the duration of the climb. I generally don't go above 25 m/s vertical velocity at anything above 5,000m altitude and it's also helpful to level off periodically to gain TAS before resuming your climb.

 

Zoom climbs are another matter entirely and you should easily be able to achieve 110,000 feet. At that altitude, and in particular on the way back down, you really start to notice how low in terms of TAS the sound barrier is, and the weird handling that can happen when mach 1 actually occurs at a rather slow TAS :)

[Aluminum Donkey]

I've noticed in the DCS World module Su-27 mission "gauntlet" that when im making violent maneuvers i keep going into this weird stall were i invert upside down and just can't not either turn right side up, or get the nose to point down, every time i try and push the stick up or use rudder, its like the plane automatically rights itself, and i have to start rocking it, go as far as i can to get the nose down, the plane tries to right itself upside down, i pull back, and after 4-5 times i get enough momentum to get the nose down and pull out of the stall..

 

What the hell is causing this? I've never had a plane fight me before like this yet alone try and put itself back in an upside down stall over and over again.

 

The real-life Flanker is indeed prone to loss of control at excessive negative angles of attack, especially when flying at low airspeeds. I heard about it as a kid in the early or middle 80s although I don't remember where, but it left a lasting impression on me. Why would the Russians deliberately allow the use of a flight control system that makes it possible to easily lose control of the aircraft?

 

The answer is that military pilots are very highly trained and are well aware of the limitation, much as all pilots have to be aware of positive AoA limits, minimum speeds, Vne etc. The airplane doesn't have a more complex control system that prevents the pilot from exceeding negative AoA--the aircraft designers simply assumed that he knows better and wouldn't do it anyway because he's trained not to.

 

Same goes for the autopilot oscillations below certain speeds--the real-life Flanker does that, too. The Russian fighter has simplified computer-assisted controls unlike today's newest generation fighters with 'carefree' handling characteristics.

 

It's still a very capable aircraft, though, and the PFM makes it great to fly! It finally feels like flying a large, heavy, but powerful and agile airplane instead of just playing a video game :)

 

Peace and happy warfare

Ian

[ineth]

You should be able to get above 10K meters without much trouble at all. Take off and climb. Set your throttle at 95% altering your climbing angle to maintain around 700-800 km/hr TAS all the way up. You should get above 10K fairly quickly.

 

Yes true, but honestly now. 30.000 feet are the normal economic flight altitude for long range missions. An F16 cruises up there with Mach .8 or so with ease. With Air to Air weaponery it shoudl be able to get at 40.000 easily too.

 

The Su27 has two monster engines and is notorious for flying long distances. I strongly believe there is smt wrong here.

 

As i said, it was the first day with the bird so i will look into this a bit more.

 

Otherwise it glides like a seagull. And i love seagulls

 

Fly safe.

[Sweep]

^Good luck with the Flanker...it might not be too fast at going fast, it loves inverted deep stalling everywhere, and sometimes it will pop your brains out all over the 'pit...but that thing can TURN!

 

I don't fly it much for BVR combat because of the avionics, but its a very fun aircraft to fly in dogfighting...

[DarkFire]

I did a bit of investigation around cruise speed at various altitudes which produced some interesting results and also produced some recommended cruise throttle settings for various altitudes. It's a bit in-depth but for interested parties read this thread:

 

http://forums.eagle.ru/showthread.php?t=151731

 

Leading on from that, for a fairly in-depth analysis of the Flanker range performance have a read of this thread:

 

http://forums.eagle.ru/showthread.php?t=151731

 

The work involved in this last thread also revealed an... anomaly... of the autopilot system in that it has to be engaged at an indicated air speed of over 560 Km/h to produce stable flight. For some reason the true air speed is irrelevant. Engaging the autopilot an anything under ~550 Km/h produces all sorts of weird yaw oscillations.

 

As for flying the Flanker in general, it takes much longer to get used to it than something like the Eagle, and your thumb will certainly get sore from constant trimming, but when everything clicks in to place it's a genuine pleasure to fly and is horrifyingly capable in WVR / close air combat / dogfighting.

 

Perseverance and practice is definitely the key to learning the Flanker :)

[Ironhand]

...The work involved in this last thread also revealed an... anomaly... of the autopilot system in that it has to be engaged at an indicated air speed of over 560 Km/h to produce stable flight. For some reason the true air speed is irrelevant. Engaging the autopilot an anything under ~550 Km/h produces all sorts of weird yaw oscillations...

I think you know this but this is for some of the newer guys. The aircraft handles, more or less, in accordance with IAS (Indicated Air Speed--the number you see on the HUD) rather than TAS (True Air Speed--the number you see on the HDD). In a sense, the IAS is the force of the air flowing over your control surfaces (it flies as if you are going ??? km/hr). TAS is simply how fast you are actually going. But the airplane doesn't care about that.

 

 

Perseverance and practice is definitely the key to learning the Flanker :)

And, once you have it, it's a hell of a lot of fun.

[Angel101]

SOme links you might find interesting:

 

Su-27 Flight Model Discussion

 

Is there a way to recover

 

regarding to this:

 

How is it that so many people are entering into unrecoverable inverted spins? Just for chuckles, here's my approach.

 

After entering an inverted spin you should:

• Smoothly throttle the engines back to idle
  
• Move the control stick to neutral on both the pitch and roll axes
  
• Stop any rotation with opposite rudder, then the rudder pedals go to a neutral position
  
• As the nose falls below the horizon, roll upright and slowly increase airspeed
  
• Once Nadia stop complaining, level off slowly being careful not to exceed the critical angle of attack
  

 

I think there are many people that fails to recognize the flame out engine that appears sometimes in that situation. If the engines stops, then it´s impossible to get out of the inverted spin.

 

I advise to add to the "checklist" -Check engine´s and restart if needed.

[DarkFire]

I think you know this but this is for some of the newer guys. The aircraft handles, more or less, in accordance with IAS (Indicated Air Speed--the number you see on the HUD) rather than TAS (True Air Speed--the number you see on the HDD). In a sense, the IAS is the force of the air flowing over your control surfaces (it flies as if you are going ??? km/hr). TAS is simply how fast you are actually going. But the airplane doesn't care about that.

 

And, once you have it, it's a hell of a lot of fun.

 

Got it :) I've been thinking about this autopilot anomaly a lot and maybe, if we take IAS as being an indication of the actual mass flow over the control surfaces, maybe it does make sense. I'd love to know what the actual Su-27S flight manual says about autopilot operation at different speeds...

 

And hell yes, the Su-27 is easily my favourite aircraft in DCS at the moment, though I'm looking forwards to spending some time learning the Mirage :joystick:

 

I'd pay nearly any amount of money for a DCS: Su-27 module...

[Ironhand]

...I'd love to know what the actual Su-27S flight manual says about autopilot operation at different speeds...

There are minimum altitude requirements, particular pitch and roll requirements depending on the ACS mode selected, etc but there is no mention whatsoever of a minimum airspeed requirement. Not that I've found, anyway. That leads me to believe, especially after looking the section on landing at the programed airfield under ACS control that there is no minimum requirement other than the aircraft's minimum based on weight, etc.

 

Rich

Edited January 4, 2016 by Ironhand

[DarkFire]

There are minimum altitude requirements, particular pitch and roll requirements depending on the ACS mode selected, etc but there is no mention whatsoever of a minimum airspeed requirement. Not that I've found, anyway. That leads me to believe, especially after looking the section on landing at the programed airfield under ACS control that there is no minimum requirement other than the aircraft's minimum based on weight, etc.

 

Rich

 

Aha! I'd completely forgotten about that! yes of course, the Su-27 ACS is supposed to be able to control the aircraft during final approach. That definitely suggests that there's either a bug in the ACS system or more likely a response curve error which manifests itself at IAS values under about 550 Km/h.

 

Since the autopilot actually works more or less perfectly at higher IAS values the problem probably isn't an actual programming bug in the code but must be an issue somewhere in the way that the control system feed-back loop is modelled. I'll try to get a track of this later and I think I'll submit an official bug report.

[Nedum]

The real-life Flanker is indeed prone to loss of control at excessive negative angles of attack, especially when flying at low airspeeds. I heard about it as a kid in the early or middle 80s although I don't remember where, but it left a lasting impression on me. Why would the Russians deliberately allow the use of a flight control system that makes it possible to easily lose control of the aircraft?

 

The answer is that military pilots are very highly trained and are well aware of the limitation, much as all pilots have to be aware of positive AoA limits, minimum speeds, Vne etc. The airplane doesn't have a more complex control system that prevents the pilot from exceeding negative AoA--the aircraft designers simply assumed that he knows better and wouldn't do it anyway because he's trained not to.

Same goes for the autopilot oscillations below certain speeds--the real-life Flanker does that, too. The Russian fighter has simplified computer-assisted controls unlike today's newest generation fighters with 'carefree' handling characteristics.

 

It's still a very capable aircraft, though, and the PFM makes it great to fly! It finally feels like flying a large, heavy, but powerful and agile airplane instead of just playing a video game :)

 

Peace and happy warfare

Ian

 

Here I have to ask what an advantage will a Russian well trained pilot get if he can pull so much negative Gs on a plane that the plane would go in such a condition for 100% inverted?

Or the other way around!

What is the reason to not protect the super expensive/skilled Russian pilots to NOT make mistakes in an extreme situation, which can be easily done whilst doing extreme maneuvers to survive a missile attack, if I know the weak point of this plane are nnegative Gs and so an inverted stall?

I myself would do ALL I can to protect my high skilled and expensive pilot of all costs and if the cost is only a simple line in a program, what devil will ride me to not write this line of code down?

Because they are well trained?

Even so you can say all not Russian pilots are not well trained and so they need this extra protection which would give them NO disadvantage. Sounds for me not good at all too.

 

Because of this ugly behavior a test pilot dies. And his dead were the reason the Russians protects their pilots from doing this.

The real Su27 can only behave like in DCS if the Pilot would override the protection (in game with “S”).

I had often inverted stalls even with full afterburner nose 15° down and POSITIVE Gs.

From time to time I get in a stall, I then let the nose drop till I gain speed up to 300+ kph, now I pitch up, then suddenely I get a new stall warning with a speed around 350 kph. The nose becomes suddenly sluggish and rises even if I hold the stick full down. My speed is now around 250 kph. Now the nose drops slowly and the speed stays at 250 kph and raises, but the nose doesn’t stop the movement. The nose flips around even if I pull the stick. No reaction anymore. The "best thing" is, the engine is working the whole time and it doesn’t matter if I change the thrust. The speed rises only because of the gravity effect. The thrust of the engines has NO effect!

The DCS Su27 gained NO speed from the engines in these situations.

But why?!

Full afterburner MUST produce a force, like every rocket engine would do.

Till today I think this behavior is wrong, because physic will say NO to this behavior!

If you have thrust you will move but not the DCS Su27 if inverted. Why?

Nose down, full AB and no movement in nose direction! Any explanation for that behavior?

Every force produces a counter force but what is the counter force to counter two fully working engines with full afterburners? Air drag? For sure not! So what is this special force?

And from time to time I have another strange behavior.

I start the Su27 with full afterburner and I have the whole flight positive Gs up to 8000 meters and suddenly the engines blow out? These 8000 meters seems to be a magic line for the Su27. At this height she becomes extreme sluggish and inverted stalls are easy to go.

I will try to do this next vacation and show some tacview files!

Yes you can fly the DCS Su27 without these problems IF you are always fly this plane like a raw egg, but why should any engineer built such a plane, so a pilot must always look not to smash this raw egg? Why not give him any help to focus the fight and not simple pitching?

Why is there a special override button if the plane even without any override can kill any pilot very quick if he is only 1/1000 second not looking for his “raw egg”?

Say what you want, I think that FM should not so easy affected by inverted stalls!

 

But that are only my thoughts! ;)

[ineth]

Nedum......

 

You are right as far as your argument go but the facts prove you wrong. i totaly agree that with the engine running it should produce the force needed to keep flying to a certain degree.

This would make sense with an aerodynamicaly stable plane. But in the modern fighter case this is not the deal and i pressume more factors come into play.

 

While i am very fresh to DCS every simulator i know of that is worth its salt has brought me with many type of planes to some kind of a deep stall.

 

FYI the F16 does the same thing on positive g's. If you wish to get it in a deep stall it is as simple as getting some altitude lower the speed at around 170 knots, drop the nose a bit and then pull hard. VOILA.

 

THe plane will not react on any command no matter the power setting. You have to kick in the FBW override and start rocking the plane into a slow downward slope to gain speed.

 

The second i realised on the Su27 i had a deep stall i did the same thing and it worked like a charm. Mind you thoygh, with vector thrust this would be a totaly different story.

 

Fly safe.

[DarkFire]

Same goes for the autopilot oscillations below certain speeds--the real-life Flanker does that, too. The Russian fighter has simplified computer-assisted controls unlike today's newest generation fighters with 'carefree' handling characteristics.

Ian

 

While I agree that the FBW system on the Su-27S is very much "1st generation" it certainly shouldn't exhibit the low speed yaw oscillations. The actual Su-27 flight manual clearly states that the ACS autopilot is able to fly the Flanker along a final approach to landing, as long as the airfield has a Russian-mode ILS system installed. At that point the IAS will be between 300 and say 260, which is VASTLY lower than the ~550 Km/h minimum stable speed that it has now.

[rrohde]

I believe I remember Yo-Yo saying that the oscillating ACS is still something they're working on. That's a rather frustrating experience as it is (esp. along final), and waiting for some improvement from ED in that regard as well.

[DarkFire]

I believe I remember Yo-Yo saying that the oscillating ACS is still something they're working on. That's a rather frustrating experience as it is (esp. along final), and waiting for some improvement from ED in that regard as well.

 

If they're aware of the issue then all well and good :) personally I always fly a landing manually. The biggest problem the autopilot oscillation causes for me is that it makes using the autopilot impossible above 10,000m because above that altitude your IAS is always below 560 Km/h on dry thrust.

[Ironhand]

...

I had often inverted stalls even with full afterburner nose 15° down and POSITIVE Gs....The "best thing" is, the engine is working the whole time and it doesn’t matter if I change the thrust. The speed rises only because of the gravity effect. The thrust of the engines has NO effect! The DCS Su27 gained NO speed from the engines in these situations.

 

So, I have to ask, would you feel better if the sim depicted you crabbing along but remaining in a stall and dropping rather than what it does now? :) Because that's actually your complaint. If the airflow over your surfaces is from a direction that doesn't produce sufficient lift, you are going to continue to fall regardless of thrust. Or am I misunderstanding your objection?

 

 

...From time to time I get in a stall, I then let the nose drop till I gain speed up to 300+ kph, now I pitch up, then suddenely I get a new stall warning with a speed around 350 kph. The nose becomes suddenly sluggish and rises even if I hold the stick full down. ...

If I understand the situation correctly, you drop the nose until you barely have enough speed to fly and, then, pull back on the stick and stall the aircraft a 2nd time? Or is it your trim setting pulling the nose up?

 

What you describe from this point sounds suspiciously like the tip stall you can get in a swept wing aircraft under certain conditions. Not sure that's what's going on but that's what it sounds like. The root is still flying but the outer wings have stalled. This has the effect of moving the lift component further forward compared to the center of gravity. This causes the nose to pitch up. Then once the swing momentum stops, the nose either drops forward or flips the aircraft on its back depending on the attitude at the time.

[Nedum]

So, I have to ask, would you feel better if the sim depicted you crabbing along but remaining in a stall and dropping rather than what it does now? :) Because that's actually your complaint. If the airflow over your surfaces is from a direction that doesn't produce sufficient lift, you are going to continue to fall regardless of thrust. Or am I misunderstanding your objection?

 

Crabbing?

What the .....! What's wrong with you? :huh:

I ask for a physically correct behavior, that's all!

What you say is very strange!

Your announcement would say that thrust will only be produced as long as there is lift. How that?

 

If you were any near correct there would be no space flight/controlled object movement in space till today! There is no lift in space! So why can a space ship move without any lift? You know that the ISS is changing/correctures their position with only using some gyroscopes?

Too start a rocket and too change the direction of a rocket there is NO need of any lift! A bullet produces no lift and nevertheless she is moving.

Please read this.

 

http://www.britannica.com/science/law-of-force

The engines, if working, will produce a force in the opposite direction even if there is no lift. So if the nose of the plain shows 25° down and the engines produce 75.22 kN each, then the plane MUST move in the direction in which the nose is pointing. There is NO way that the 2x75.22 kN (full afterburner) have no influence on the flight path because there is no lift. Again, if you were any near right there would be no space flight possible!

Even without lift EVERY object must move if a force affects the object as long there is no counterforce with the same power!

As long as the afterburner is working the Su27 MUST react on this input and lift has nothing to do with this! Counterforces would be gravity (pseudo Force) and the sum of forces called "air drag".

If a Su27 can riding on her engines straight into the sky or reach a SL mach +1.0, I can't see any real force which could counter this engines force.

Action = Reaction!

If there is Action but no Reaction then there is something wrong!

 

The stall behavior of the DCS Su27 is physically not possible as long the engines produces enough power to force a direction change.

 

 

But you try to kidding me, right? :smilewink:

 

 

 

If I understand the situation correctly, you drop the nose until you barely have enough speed to fly and, then, pull back on the stick and stall the aircraft a 2nd time? Or is it your trim setting pulling the nose up?

What you describe from this point sounds suspiciously like the tip stall you can get in a swept wing aircraft under certain conditions. Not sure that's what's going on but that's what it sounds like. The root is still flying but the outer wings have stalled. This has the effect of moving the lift component further forward compared to the center of gravity. This causes the nose to pitch up. Then once the swing momentum stops, the nose either drops forward or flips the aircraft on its back depending on the attitude at the time.

Yes and no!

I am testing very often all aspects of the flight model!

This was a test I've done very often.

I stall the plane only the first time with full intent. The second stall is a product of the first one.

Even at a speed a way over 300 kph the stall begins with NOSE DOWN at 300+ kph and me pulling the stick gently back. In this situation suddenly the stick/movement of the plane becomes sluggish and the stall begins with NOSE down again. The nose itself starts to go up and the stall sound appears; now the nose goes down on its own again and the plane flips inverted.

 

Edit:

1. stall with full intend start with Nose up. (engines working, no reaction on thrust change)

2. nose down, speed gain, gently pull up of the nose, speed over 300+ kph (engines working, no reaction on thrust change)

3. new stall warning even the speed is 250+ kph, the nose goes up by itself, controlls extreme sluggish (engines working, no reaction on thrust change)

4. speed lost down to 100 kph with nose up (engines working, no reaction on thrust change)

5. nose goes down, no input reaction, speed gains up to 200 (engines working, no reaction on thrust change)

6. nose flips inverted (engines working, no reaction on thrust change)

7. nose commutes from +5° - -20 ° (engines working, no reaction on thrust change)

8. nose down engines now blown out (engines stoped, no reaction on thrust change)

 

Again I would really like to hear an explanation why the "laws of forces" are not working for the DCS Su27 whilst stalling or a little time before the stall appears!?

If the engines working with military or AB they must affect the movement/direction path of the object, especially it the nose of the plane shows to the ground and the gravity works in the same direction the engines try to move the plane (gravity and engines force working together)!

 

EDIT 2:

I've repeated my test!

You must test it with full weapons!

Without any weapons the reaction on thrust changes is a way different anD the Su27 feels really good, like I would think they should feel with weapons. As soon as I put weapons on her, she becomes very sluggish.

Without weapons she is a dream; a little bitchy but I like it! ;)

With weapons the drag seems a way to high (has this to do with the drag problem of the missiles?) and so the reaction on thrust is too sluggish.

I would like it more inbetween those both with weapons suited.

Right now the Su27 becomes another plane with missiles on it.

 

So yes, I am wrong if the Su27 is without weapons. The the reaction on thrust changes is there but a way different as with weapons.

Edited January 5, 2016 by Nedum

[Ironhand]

Nedum,

 

I ask this in all seriousness. Would you tell me what a stall is and its relationship to airspeed? As I read what you've written, I get the feeling that you believe that, if you have airspeed, you should not be in a stall and that increased thrust should always lead to more speed.

[GGTharos]

EDIT 2:

I've repeated my test!

You must test it with full weapons!

Without any weapons the reaction on thrust changes is a way different anD the Su27 feels really good, like I would think they should feel with weapons.

 

Do you know what it should be like? Because what you think it should feel like is not actually important, and will not cause anyone to change anything. :)

 

As soon as I put weapons on her, she becomes very sluggish.

You mean as soon as you put on several metric tonnes of weight and drag generating devices? Sounds like what every loaded aircraft will experience :)

 

With weapons the drag seems a way to high (has this to do with the drag problem of the missiles?) and so the reaction on thrust is too sluggish.

I would like it more inbetween those both with weapons suited.

Right now the Su27 becomes another plane with missiles on it.

It's only important how it acts in RL, and Yo-Yo has some pretty good sources for that.

All aircraft suffer from drag induced by their external payloads; but the flanker in particular likes to carry a LOT of very heavy weapons.

 

Perhaps you should start arming up with only 6 missiles, and not take any of the E versions of the R-27, like the real RuAF? ;)