Su-27 trim issue is in need of explanation

[Svend_Dellepude]

Hummingbird, why do you ask questions if you don't at least try to understand the answers?

Spend some time digging the forum as there are other threads like this.

[Hummingbird]

Hummingbird, why do you ask questions if you don't at least try to understand the answers?

Spend some time digging the forum as there are other threads like this.

 

The issue is not understanding the answers, thats easy, the issue is wether the answers are correct.

 

So far I've seen no evidence to suggest that the real aircraft suffers such a heavy change in pitch, esp. not with the stability system on.

 

Also AFAIK no real Su27 has ever verified the FM.

[Hummingbird]

Well that is your opinion. I have not expressed one. You might not like the system, but that's the way the system works, I have even tried to explain why and yet you don't 'believe' me. Well you don't have to, but that doesn't change the facts.

 

Edited: Just to clarify, since you seem to have misunderstood part of my last post. Transitioning from another positively stable aircraft, to the SU27.

 

You call it fact, but has any real Su27 pilot said so?

 

Thats the problem with the Su27 model atm, we don't have anyone with any authority on the matter to judge wether its correct or not.

[FSKRipper]

You call it fact, but has any real Su27 pilot said so?

 

Thats the problem with the Su27 model atm, we don't have anyone with any authority on the matter to judge wether its correct or not.

 

ED is in contact with several actual and former pilots even of the MiG 31. Only because they don't come here and introduce themselves doesn't mean that they don't exist. I'm also a bit puzzled... If you ask and get an answer that you don't like you are free to do believe what you want but why asking further? Do you hope that some day you will hear something more matching your opinion?

[NineLine]

You call it fact, but has any real Su27 pilot said so?

 

Thats the problem with the Su27 model atm, we don't have anyone with any authority on the matter to judge wether its correct or not.

 

As has just been said, ED has many contacts, most of which for various reason cant or wont be shared publicly. If you dont buy that, you are welcome to track down and find your own Su-27 specialist. Su-27 is still in beta as well, not everything is final, something to keep in mind.

[DrDetroit]

The pilot in the article below, who was able to test the SU-27 back in 1990, never really mentions any weird trim characteristics, and he gets a bit in-depth with the article. He does mention a few times about the "positive" flight characteristics, but that's about it.

 

Check it out - posted elsewhere a few days ago - looks relevant to this discussion:

 

http://aviationweek.com/site-files/aviationweek.com/files/uploads/2015/01/1990-%20Su-27%20Pilot%20Report%20%282%20of%202%29.pdf

 

Good day,

 

DrDetroit

[Hummingbird]

It's unfortunate that healthy scepticism in the face of lacking real life data is so frowned upon by so many in here, makes it hard to have a serious and learnful debate. It's a shame.

[Hummingbird]

The pilot in the article below, who was able to test the SU-27 back in 1990, never really mentions any weird trim characteristics, and he gets a bit in-depth with the article. He does mention a few times about the "positive" flight characteristics, but that's about it.

 

Check it out - posted elsewhere a few days ago - looks relevant to this discussion:

 

http://aviationweek.com/site-files/aviationweek.com/files/uploads/2015/01/1990-%20Su-27%20Pilot%20Report%20%282%20of%202%29.pdf

 

Good day,

 

DrDetroit

 

Thank you detroit!

[Hummingbird]

Well having read the article I must say that if a seasoned western fighter pilot as experienced as Sir Harding in his very detailed description of the Su-27's handling characteristics fails to mention that the aircraft supposedly requires constant and significant trimming with changes in airspeed, well then I am inclined to believe that this is because he experienced no such tendency from the aircraft.

 

Specifically the ingame Su27 starts pitching up straight after take off and requires constant trimming after that as the speed builds: It would be very strange if Harding didn't comment on that had the real thing behaved this way.

 

Infact my impression reading the article is that the aircraft was remarkably easy to adjust to for a pilot used to the handling of western jet fighters.

Edited October 22, 2015 by Hummingbird

[NineLine]

I'm having a serious case of deja vu....

[Yo-Yo]

Well having read the article I must say that if a seasoned western fighter pilot as experienced as Sir Harding in his very detailed description of the Su-27's handling characteristics fails to mention that the aircraft supposedly requires constant and significant trimming with changes in airspeed, well then I am inclined to believe that this is because he experienced no such tendency from the aircraft.

 

Specifically the ingame Su27 starts pitching up straight after take off and requires constant trimming after that as the speed builds: It would be very strange if Harding didn't comment on that had the real thing behaved this way.

 

Infact my impression reading the article is that the aircraft was remarkably easy to adjust to for a pilot used to the handling of western jet fighters.

 

The most wearing place on the right flight glove of Su-27 is the thumb... it's a fact. What do you think about the reason?

[Hummingbird]

The most wearing place on the right flight glove of Su-27 is the thumb... it's a fact. What do you think about the reason?

 

Again I'm not saying the Su27 doesn't require trimming, I'm saying I am sceptic about the amount of trimming required in the ingame plane with even modest speed changes. This needs to be made clear.

[FSKRipper]

My Impression from this article that it is a nice but superficial description of a short flight from which you can't create FM graphs.

 

Don't get me wrong, I'm sure that Mr. Harding has the competence to judge on specific issues. The problem I see:

 

1. The report is quite short and made for the public. His flight was also rather short. The report is full of compliments for the aircraft (positive pitch control during slow speeds and other numerous events). That's fine for me but I think you have to watch this trough his eys in this special era. It was recovery from the cold war and if he had said something bad about this bird in the public it could have been the last time a western pilot were allowed to drive such a beauty :music_whistling:

 

2. According to his report, he never exceeded 700km/h. In fact most maneuvers were flown between 220-300 kt.

 

3. He never controlled the plane during extreme maneuvers like his backseater (understandable) and never used the AoA limiter override himself.

 

So my conclusion would be that the absence of trim comments doesn't mean that there is an issue between sim and the real one. But maybe I'm totally wrong. We will see it in the future. Remember it's a beta.

 

One last thing, as these discussions come up with every module, you should understand that the dev Team will ignore critics without proof. If you can bring a pilot (Su-27 for sure) willing to talk on this topic or authentic FM-data I'm sure they will answer you personally and you have their full Attention. Critics based on opinions, feelings and chit-chat will be ignored (for some good reason in my opinion)

 

EDIT: After reading trough his career in the RAF, I would doubt that Sir Peter Harding gained more than a couple of hours on modern fighter planes. He is for sure a very experienced bomber pilot. I'm not sure of british regulations, but the german airforce takes pilots out of active duty after reaching an age of 41 (for combat aircraft). There are always exceptions like personal pilots for government personnel or non regular flights at airshows like described in this article. But i would suppose that his experience in the F-16 and F-18 like mentioned in the article is very limited, especially if you take into account that this was the 100th different airframe (including helicopters) he "piloted".

Edited October 23, 2015 by FSKRipper

[kolga]

It's unfortunate that healthy scepticism in the face of lacking real life data is so frowned upon by so many in here, makes it hard to have a serious and learnful debate. It's a shame.

 

One more thing to keep in mind: none of us have a joystick straight out of an SU-27, which is a big shame.

[Svend_Dellepude]

It's unfortunate that healthy scepticism in the face of lacking real life data is so frowned upon by so many in here, makes it hard to have a serious and learnful debate. It's a shame.

 

Seems to me that it's difficult to have a meaningfull debate about something that none of us have any reallife knowledge about ( at least i don't, don't know about you).

Besides, as mentioned by others, the FM is still in beta so things may, or may not change. I don't understand the amount of trim required anymore than you, but for now that's just the way it is. :prop:

[OnlyforDCS]

Again I'm not saying the Su27 doesn't require trimming, I'm saying I am sceptic about the amount of trimming required in the ingame plane with even modest speed changes. This needs to be made clear.

First you demanded an explanation from a real life Su27 pilot, didn't believe that the system was designed to work that way and didn't believe that ED actually had pilot input. Now you think maybe it does but it's bugged somehow and requires too much trimming.

 

You asked for a healthy discussion and yet did not bring one shred of evidence that something is actually wrong. You even disregarded yoyo's comment in this thread and went all defensive when people asked for evidence instead of a hunch or a "feeling" that something is actually wrong.

 

At first I wanted to help you understand as Ive been there, but now I realize that I probably shouldnt have even engaged you on this matter. There is a word for what you are doing in this thread, it's called "trolling".

 

Sent from my HTC One M8s using Tapatalk

[LJQCN101]

Well, I have managed to implement the same airspeed trimming law to the F-16Demo MOD and achieved the same result as in Su-27. No big deal, just a simple control law.

[Nedum]

This is what you are asking right??

 

Well what Im trying to explain is that zero trim with FBW "on" and zero trim with FBW "off" is not the same trim setting. The horizontal stabilizer doesn't move because the plane isn't trimmed by the horizontal stab, it is trimmed by it's trim tabs.

 

I need a closer explanation for this!

Please can you show me those "trim tabs" at the Su27 please.

I never heard about those things at the Su27.

And if they are there, what is the logical reason to make the "S"hutt down of the ACS to a "Pilot Kill Switch" because the plane will behave completely different after a change of the "Flight Mode"?

I only talk from me as if I would design such a mode... all I would do, would be to make it as secure as it could be for a SUPER expensive Pilot if he really need this "Kill Switch".

And why is the trim so different if they could make it equal with no problem (the real Su27 engineers).

Why do they give a pilot an extreme complicated system if it would cost no money to build it in such a way so it could act the same way for any Flight Mode?

And who is this Su27 Pilot who has said that the real DCS SU27 behaves in this situation like the real one? Never read this interview/posting!?

I cannot believe that the Russians have built in "Pilot Kill Button" in their planes. And if a plane would really react like the DCS one does, I would take this button away of all Su27 if I were a Russian engineer.

I need much more information for this, and especially of those "trim tabs"!

How do they work and why do I need another/worse trim if the other one works much better/smoother?

Only to say “a real Su27 Pilot said so” seems not to be enough, does it?

Edited October 23, 2015 by Nedum

[Nedum]

The most wearing place on the right flight glove of Su-27 is the thumb... it's a fact. What do you think about the reason?

 

This may be right, but that's not really the point as I can see if I take a look at the start posting.

It's more about the different behaviors for the trim and why is one of them extreme good and smooth and the other one bad/not so smooth.

If one is much better, why not use this one for all "Flight Modes"?

I myself want to understand the reason for this behavior.

And in which way those "trim tabs" working and if they allow a smoother/better trim, why we can't use them the whole flight.

Till today there was no good explanation about those differences... or I can't find them. I would really like to take a deeper look at this point! :thumbup:

[OnlyforDCS]

Ok, let's get some facts on the board. When I say 'trim tab' what I mean is elevator position, the plane trims in pitch by moving the entire elevator up and down. The horizontal stabilizers aren't involved.

 

Hello Nedum, been a while. I see you are still quoting out of context, and haunting these boards with your views, and opinions without ever producing one single piece of evidence to support them.

 

All the answers to your questions have been provided in this thread, as well as the one I linked for Hummingbird.

Edited October 23, 2015 by OnlyforDCS

[OnlyforDCS]

One other thing to consider: The Su27 FBW system also simulates the 'feel' of the airflow as over traditional air deflection surfaces at least I believe it does, as I get considerable warning through my stick if Im approaching critical AoA. I haven't experienced this in the F15C for example, and I believe it wouldn't be possible if the FBW were configured similarily in the Su27. Unfortunately you will have to take my word on this, unless you own a force feedback joystick yourself. This is a huuuge help when maneuvering and pulling heavy G's.

Edited October 23, 2015 by OnlyforDCS

[Nedum]

Hello Nedum, been a while. I see you are still quoting out of context, and haunting these boards with your views, and opinions without ever producing one single piece of evidence to support them.

 

All the answers to your questions have been provided in this thread, as well as the one I linked for Hummingbird.

 

Is there a way to ask something and you are not insult the one who ask something that you obviously don't like?

If you think I am an idiot please ignore me, but please don't insult people only because you think their knowledge is not good enough to discuss with you!

 

But anyway!

Thanks for this single answer!

 

So If I understand it the right way, then you say there are two trim methods for each FM?

The normal flight mode gives us a trim with “something” and if we hit the “S” -button we get a more sensitive trim control with “only” the elevator?

 

 

During each trim, during normal flight mode, the elevator start moving and after the level position is reached the elevator returns to the center position. Isn’t this a little bit strange/not good if the elevator “helps” or “overreacts” to reach a stable position and after the trim is made he “swings” back which must give the plane another short pitch change?

How does this technic work?

The elevator moves back to the center position after the trim. How can the plane hold the right position/the level flight?

 

 

As I can see if I hit the S key only one thing happens; the sensitivity of the stick inputs will increase overall and because of this you must set the trim setting to the neutral position before you hit the S-key, because with less stick move I'll get much more elevator reaction. How the trimming overall works is the same way as before! And this is something that I would never do if I would give a Pilot the ability to override the agility of his plane! I would give him only more agility (after pressing the “S” –key) if he would push or pull the stick over a special point of degrees. This would be easy to make even only with a hardware system and this would save the pilots lives!

The pilot could hit the “S”-key in any Situation without kill himself and because this would be easy to make with hydraulic hardware. Why the Russians engineers have built such a “killer system” I will never understand!

 

 

Another thing I have noticed during the flight tests is that the elevator is moving by himself like a pendulum during moving on ground. Every bump gives the elevator an up and down, even if I pull the elevator full up or push him full down.

I’ve watched some RL vids and couldn’t see any of those behaviors at those RL Su27s.

Could this be a reason for those “big steps/swings” for the trimming behavior of the DCS Su27?

Or is this only at ground?

What is the reason for this behavior at ground until the plane is in the air?

As I can see if I use the trim on ground the elevator change the AoA “extreme” and in the air the elevator returns again to the center position.

Shouldn’t the elevator be stable if the Pilot pushes or pulls the stick even during the start?

Another thing is, if I pull the stick and hold this position the elevator kicks short in and then the elevator returns to the center position with a short “swing”.

Is this the RL behavior?

Shouldn’t the elevator hold the position until the stick will be pushed forward?

 

As I have said I would really like to have the whole picture, but right now most of the answers are only pieces of other discussions/a big puzzle and its really hard to put all this pieces together without a helping hand.

Could make more discussion much nicer if people only have to read one single explanation without to puzzle it together alone. And because of this many discussion could be really good without me idiot asking stupid (?) questions!

Perhaps we could make a section like “Questions for runaways” in which we “idiots” can ask something without to insult the pros?

Would be nice to ask something without getting a kick in the nuts all the time!

Edited October 23, 2015 by Nedum

[Hummingbird]

First you demanded an explanation from a real life Su27 pilot, didn't believe that the system was designed to work that way and didn't believe that ED actually had pilot input. Now you think maybe it does but it's bugged somehow and requires too much trimming.

 

You asked for a healthy discussion and yet did not bring one shred of evidence that something is actually wrong. You even disregarded yoyo's comment in this thread and went all defensive when people asked for evidence instead of a hunch or a "feeling" that something is actually wrong.

 

At first I wanted to help you understand as Ive been there, but now I realize that I probably shouldnt have even engaged you on this matter. There is a word for what you are doing in this thread, it's called "trolling".

 

Sent from my HTC One M8s using Tapatalk

 

*sigh*

 

Again you really need to learn to read what is written before you start attacking other people.

 

In my very first post I said that it was normal that trimming was required with speed, but that the amount of upwards pitch the Su27 experiences seems extremely exaggerated, which is made even more strange by the fact that it requires much less trimming with the S key activated. You ignore this and then claim I have changed my position: See now THAT is trolling.

 

Finally you haven't provided a single shred of evidence yourself, so don't start preaching about a lack of evidence when you have none yourself, esp. when you don't even understand the OP.

Edited October 23, 2015 by Hummingbird

[OnlyforDCS]

It's physics. An aircraft that has positive stability needs to be trimmed differently than one that has relaxed stability. I dont see how I can make this more clear than I and many others have already tried to. As for providing evidence, Im not the one here who is claiming its supposed to act differently in any other way.

 

Sent from my HTC One M8s using Tapatalk

Edited October 23, 2015 by OnlyforDCS

[Ironhand]

Hummingbird, unfortunately I'm 300 miles from my copy of the sim and my reference materials, including the manual, with a tenuous internet connection and a rapidly dying iPad battery. So, this will be necessarily brief. Concerning the pitch up after takeoff (and you may already know this): 1) using flaps on takeoff. Extended flaps put a nose down force on the aircraft. Pulling them in causes the nose to pitch up. Only part of the FBW system is engaged while the landing gear are extended. The full system engages (including the additional trimmer law that is overplayed on the underlying system), when the gear are raised. So both raising flaps and gear gives you a double whammy. As a matter of practice, I don't use flaps on takeoff and already have the nose trimmed down before my takeoff roll.

 

There should be a difference in the horizontal stabilizer setting with FBW engaged vs not engaged (S key), especially at lower airspeeds. But that's not something I can check into now. More at another time. Sorry because this is an interesting topic and we will probably not know all the details of how the system actually works. But I choose to believe that it's reasonably accurate for the degree of cockpit modeling we currently have. Again more at another time but probably not until Monday when I'm back with the sim.

 

[EDIT]OK. I have a few more minutes of life left--battery life. If you read the section in the manual that Darkfire referenced, you'll realize that there are two basic parts to the FBW system. There's the part that makes it behave as an aircraft with (more or less) neutral stability. This what what most people expect to see in a modern day jet fighter. Where you point the nose is where the nose continues to point regardless of changes in airspeed, etc. For whatever reason--I have my own ideas as to why--someone or some group during the aircraft's development with enough authority decided that they wanted the aircraft to mimic the flight characteristics of an aircraft with positive stability because that is the type of aircraft most pilots learned to fly on. This is the trimmer law that is somewhat layered on top of the underlying system. It's the part that's giving you and a number of others fits. I know my first experience of it was definitely a WTF moment.

 

But it's not a bug. It's there on the real aircraft or it wouldn't have been included. Exactly how it works might be an issue that will be better resolved if we ever get a fully modelled cockpit. But, as a matter of practice, I no longer find it problematic at all. One thing it defintitely does--and this might be why it was included in the real aircraft--is to keep you constantly aware of even small changes in your airspeed. And, in my experience at least, it becomes a non factor at airspeeds over roughly 850 k/hr when the aircraft behaves more neutrally stable anyway.

Edited October 24, 2015 by Ironhand