Su-27 trim issue is in need of explanation

[Ironhand]

...

 

Why would you design the aircraft to demand that much extra trimming? It's adding pilot workload nothing else.

To make sure the pilot is paying attention? :) I don't know the answer to that or whether the amount of trimming is "extreme". I know that you feel it is. I thought so myself when I first experienced it but, now, feel much less strongly about it. I hardly notice the trimming any more unless I decide to be prefectionistic about it. Then I have to pay attention...and only then.

 

Another possible issue is that we do not have a fully functional cockpit. There may be switches that fine tune the experience. I have no idea. I do know that in the real aircraft you have more control over how the FBW system behaves. But none of that is currently modeled. In short, we'll have to wait and see what develops, if anything.

 

Regarding the horizontal stab position, I mentioned this as I noticed no change switching between S key on and S key off, which makes the change in behavior even stranger and aerodynamically impossible. But perhaps that was a bug on my end I dunno, will have to check again.

Have another look. It seems to me that I once reported something similar but no one could reproduce it and, when I checked again, I didn't see it either. I wrote it off as an error on my part. But perhaps it wasn't and it's a graphics representation glitch that turns up from time to time.

 

Rich

[LJQCN101]

Try flying at 780~900km/h and trim for that speed where the airspeed trimming law doesn't interfere with the stabilizer anymore and also the aircraft is well before supersonic.

 

At that point you'll feel as if you won't go pitch up and down even with rapid throttle input or accel/decel.

[Hummingbird]

Not more than the L39.

 

I hardly need to trim the L-39, so it definitely does.

[Hummingbird]

To make sure the pilot is paying attention? :) I don't know the answer to that or whether the amount of trimming is "extreme". I know that you feel it is. I thought so myself when I first experienced it but, now, feel much less strongly about it. I hardly notice the trimming any more unless I decide to be prefectionistic about it. Then I have to pay attention...and only then.

 

Another possible issue is that we do not have a fully functional cockpit. There may be switches that fine tune the experience. I have no idea. I do know that in the real aircraft you have more control over how the FBW system behaves. But none of that is currently modeled. In short, we'll have to wait and see what develops, if anything.

 

What really bugs me is the loss of full pitch authority once speeds drops, something the real pilot doesn't experience as his stick simply moves with the trim.

[Ironhand]

What really bugs me is the loss of full pitch authority once speeds drops, something the real pilot doesn't experience as his stick simply moves with the trim.

Maybe I'm misunderstanding but I've never noticed any real loss of pitch authority--at least not the way I fly which might not be as adventurous as some. :) Or do you mean that you can't shove the stick forward with enough authority to induce higher negative G's?

[Nedum]

What really bugs me is the loss of full pitch authority once speeds drops, something the real pilot doesn't experience as his stick simply moves with the trim.

 

Same here.

I call this the rubber band behavior.

In the direction you pull the stick you’ll get an aggressive stick behavior at a special point and at the other direction you will get a sloppy feeling... the plane drops down without no feeling for this behavior and for me with no logical reason why a small release of the stick let the nose drop down.

I know this is because of how the game translates the stick input of the player, but it gives me a total wrong feeling of the behavior of any Plane in DCS right now.

My problem is more the way the control feeling is simulated and not so the physic models itself.

Most of us will not use a FFB flight stick and so the steering of the plane feels wrong because of the actually “very simple” translation of the player stick position too the in game stick position.

For us the stick center is always at the same point but in RL this will change with every trim input and this should get a deeper view. ED should “simulate” this behavior too.

The controls should get another system so the "player pilot" will get a better response and the player flight stick center position is always the position of a trimmed in game flight stick (like we have in the KA50 module.. press a button for trim and release it and at this release point all new stick inputs will sart at a new "virtual center point…. perhaps in such a way?).

Would be nice if ED would take a deeper look at this behavior!

:thumbup:

Edited October 30, 2015 by Nedum

[Hummingbird]

Just bought the MiG-21 module and took it for a spin, and unsurprisingly this bird requires only a fraction of the trim in pitch with speed that the ingame Su-27 does, infact the MiG-21 hardly needs any trim in pitch from take off to 1,000 km/h.

 

So again I am very curious where this extreme upwards pitch of the Su-27 comes from, I've never experienced anything like it.

[Hummingbird]

Same here.

I call this the rubber band behavior.

In the direction you pull the stick you’ll get an aggressive stick behavior at a special point and at the other direction you will get a sloppy feeling... the plane drops down without no feeling for this behavior and for me with no logical reason why a small release of the stick let the nose drop down.

I know this is because of how the game translates the stick input of the player, but it gives me a total wrong feeling of the behavior of any Plane in DCS right now.

My problem is more the way the control feeling is simulated and not so the physic models itself.

Most of us will not use a FFB flight stick and so the steering of the plane feels wrong because of the actually “very simple” translation of the player stick position too the in game stick position.

For us the stick center is always at the same point but in RL this will change with every trim input and this should get a deeper view. ED should “simulate” this behavior too.

The controls should get another system so the "player pilot" will get a better response and the player flight stick center position is always the position of a trimmed in game flight stick (like we have in the KA50 module.. press a button for trim and release it and at this release point all new stick inputs will sart at a new "virtual center point…. perhaps in such a way?).

Would be nice if ED would take a deeper look at this behavior!

:thumbup:

 

I agree, however we only really feel this because of how severe the Su-27's trim changes are with even small increases in speed.

 

The funny thing is that with stabs level in S mode the ingame Su-27 experiences a VERY normal amount of speed related upwards pitch from 400-1,000 km/h - i.e. it already feels and behaves like a longditudally stable aircraft, yet put on the stability system and the aircraft suddenly wants to pitch up like it was a WW1 biplane with a rocket strapped to its back.

[Nedum]

Just bought the MiG-21 module and took it for a spin, and unsurprisingly this bird requires only a fraction of the trim in pitch with speed that the ingame Su-27 does, infact the MiG-21 hardly needs any trim in pitch from take off to 1,000 km/h.

 

So again I am very curious where this extreme upwards pitch of the Su-27 comes from, I've never experienced anything like it.

 

I would really like to hear the explanation for this behavior.

It doesn't feel right for any logic!

Such a high lift so you really don't need any flaps but at the same time you got a trim movement which is even higher as for a Boing 747.

Perhaps the physics are right but then the "stick simulation" is not right.

As I've said, ED should give this behavior a deeper look.

 

I agree, however we only really feel this because of how severe the Su-27's trim changes are with even small increases in speed.

 

The funny thing is that with stabs level in S mode the ingame Su-27 experiences a VERY normal amount of speed related upwards pitch from 400-1,000 km/h - i.e. it already feels and behaves like a longditudally stable aircraft, yet put on the stability system and the aircraft suddenly wants to pitch up like it was a WW1 biplane with a rocket strapped to its back.

 

That's exact how I feel it.

I don't know exactly where it comes from, but if feels wrong as hell.

Fly the Mig15 or any other fast mover with trim, after that the Su27 and you can clearly see/feel there is a extreme difference for lift and trim and not in a good way for the more modern fighter! ;-)

Why is the Su27 build in such a way that they want to perform loops by itself and produce too many lift at low speed so she must stall because you can't fine control the AoA anymore...?

Edited October 30, 2015 by Nedum

[Ironhand]

Just bought the MiG-21 module and took it for a spin, and unsurprisingly this bird requires only a fraction of the trim in pitch with speed that the ingame Su-27 does, infact the MiG-21 hardly needs any trim in pitch from take off to 1,000 km/h.

 

So again I am very curious where this extreme upwards pitch of the Su-27 comes from, I've never experienced anything like it.

A few times now you've indicated that you don't understand where this pitch behavior comes from. Perhaps you've read all of what follows when Darkfire pointed you to it in this thread's 2nd post but just in case...

 

From the DCS Su-27 manual:

The fly-by-wire system is a trajectory control system. In other words, if the control stick remains in the same position, the system will hold the set flight path. This is why the reduction of airspeed (for example) and resulting decreased lift and increasing angle of attack, will lead to the aircraft attempting to hold the initial flight path and prevent the aircraft from descending. This will result in the airspeed stability degradation up to neutral stability.

This is the underlying FBW system in the aircraft. If left here, you would have an aircraft that flies much like the F-15 without the need to trim. However:

 

Further from the DCS Su-27 manual:

The longitudinal control system includes an airspeed trimming law that generates a signal proportional to the ram air. When the indicated airspeed is increased, the FCS causes the stabilizers deflection (up to 5 degrees) nose down. When decelerating, it causes the nose to come up. This imitates the airspeed stability of the aircraft, which is neutral in the presence of a g loading feedback signal. The airspeed stability imitation allows the pilot to use the control stick like he or she would with a stable aircraft.

This added trimming law is what causes the nose to pitch up or down as airspeed increases or decreases. The pitching is artificially induced by this law if there is no G-loading and the landing gear are up. I also need to put this last quote into clearer English, since the translation is a bit messed up. Where the last quote refers to "nose down" it's referring to the nose (leading edge) of the horizontal stabilizer.

 

Why have it? Speed stability, the condition of an airplane returning to its initial trimmed airspeed after a disturbance, is familiar to most pilots and might be the main reason for the airspeed trimming law being added to the underlying system in the first place. Another important side effect of stability is tactile feedback to the pilot. If the you're holding a sustained rearward force on the stick after trimming, then your speed is probably less than the last trim speed. Those are a couple of possible reasons that it was designed into the FBW system.

 

Anyway, that's where it comes from, if that's actually your question. Should it be as sensitive as it is to changes in airspeed? I don't know. I personally wouldn't mind it being a bit more relaxed. But, again, I don't find it especially problematic in its current state.

Edited October 30, 2015 by Ironhand

[Hummingbird]

I understand where the pitch behavior comes from (the FCS), I just don't understand the severity of it ingame. In short I am having a hard time believing that the real life aircraft experiences said behavior to the same degree, or in other words I really doubt that the real life stability system induces such a dramatic increase in upwards pitch, esp. since much less would be needed to simulate a longitudinally stable aircraft.

 

Either way something needs to be done about the lost pitch authority due to a loss in max stab deflection, something the real pilot does not deal with as his whole stick moves with trim changes.

Edited October 31, 2015 by Hummingbird

[Ironhand]

I understand where the pitch behavior comes from (the FCS), I just don't understand the severity of it ingame. In short I am having a hard time believing that the real life aircraft experiences said behavior to the same degree, or in other words I really doubt that the real life stability system induces such a dramatic increase in upwards pitch, esp. since much less would be needed to simulate a longitudinally stable aircraft.[/Quote]

I don't know if anyone has ever submitted a bug report on it. But, if you feel that strongly about it, perhaps you should. If nothing else you will find out whether they have reason to believe that the current implementation is correct.

 

Either way something needs to be done about the lost pitch authority due to a loss in max stab deflection, something the real pilot does not deal with as his whole stick moves with trim changes.

Here you are going to have to educate me. I'm not aware of any loss of pitch control when I fly due to trimming requirements. What airspeeds are we talking about? I ask because the aircraft always seems to do what I want it to do assuming I have the airspeed to do it.

[Hummingbird]

Maybe I'm misunderstanding but I've never noticed any real loss of pitch authority--at least not the way I fly which might not be as adventurous as some. :) Or do you mean that you can't shove the stick forward with enough authority to induce higher negative G's?

 

Overlooked this.

 

No I mean mostly in upwards pitch authority, i.e you loose pitch authority in the opposite direction of the trim direction: Trim down and you loose full up pitch and vice versa (albeit losing max down pitch is less important)

 

The real pilot faces no such issues has his stick simply moves with changes in trim, thus he retains full pitch authority in both directions. Our sticks however don't move with the trim (obviously), thus we loose authority opposite what'ever direction we trim.

[Weta43]

That's not a problem with modelling of the Su-27 trimming as such though is it (?), more a deficiency in the hardware most people use, and will be more or less of a problem to any simulated aircraft where the sim alllows the joystick to return to the center after trimming (& so is not an issue with FFB sticks) - the 'more' being for aircraft that require trimming with speed changes .

Do you know of an alternate approach that doesn't involve non-linear responses for people that don't fly with 'curves' on their setups ?

[Hummingbird]

That's not a problem with modelling of the Su-27 trimming as such though is it (?), more a deficiency in the hardware most people use, and will be more or less of a problem to any simulated aircraft where the sim alllows the joystick to return to the center after trimming (& so is not an issue with FFB sticks) - the 'more' being for aircraft that require trimming with speed changes .

Do you know of an alternate approach that doesn't involve non-linear responses for people that don't fly with 'curves' on their setups ?

 

Well the only way to alleviate such an issue I can see would be to compress & elongate the curves as the virtual center of the stick is moved via trim.

 

However none of this would really be an issue if the aircraft exhibited normal amounts of speed related trim changes in the first place. The Su27 is the only aircraft where it becomes a problem quite simpy due to the unusually large amount of trim required.

[Weta43]

And now we've come full circle, because as we've discussed, E.D. implemented the currect representation of trimming in the Su-27 because they believe it accurately reproduces the behaviour of the aircraft IRL, so this is 'Normal' for this aircraft.

 

Leaving aside actual SME's being involved in the development, don't you think that there will be enough people 'in the know' on the Russian side of the boards to be able to say 'trimmed to level at this speed under these conditions the stick will be this far forward of center', and if it were as far out as you're suggesting there's be Russian fans on here backng you up in loud voices..?

Lord knows there are enough F-15 experts on this side to loudly correct issues with the FCS on that machine, and many changes have been made as a result...

[NineLine]

Well the only way to alleviate such an issue I can see would be to compress & elongate the curves as the virtual center of the stick is moved via trim.

 

However none of this would really be an issue if the aircraft exhibited normal amounts of speed related trim changes in the first place. The Su27 is the only aircraft where it becomes a problem quite simpy due to the unusually large amount of trim required.

 

You mean you want unrealistic amounts of trim then as the Su-27's trim was based off real data.

[Hummingbird]

You mean you want unrealistic amounts of trim then as the Su-27's trim was based off real data.

 

No I mean that I want realistic stick deflection authority simulated, i.e. no loss of max stab deflection with trim changes.

 

As for the amount of trim being based off real data, that I would certainly like to see. It's easy to test too by leaving trim in neutral from take off and then noting the G pulled at specific speeds after releasing the stick.

 

I'll do that later today.

[OnlyforDCS]

...and the circle is complete. Care for another go-around?

[NineLine]

As for the amount of trim being based off real data, that I would certainly like to see.

 

I doubt ED is going to share their data, nor should they have to. They dont have to prove anything based of someones 'feelings' or 'opinion'. Show the FM that is wrong with your own data, then we will talk.

Edited November 3, 2015 by NineLine

[Yo-Yo]

Overlooked this.

 

No I mean mostly in upwards pitch authority, i.e you loose pitch authority in the opposite direction of the trim direction: Trim down and you loose full up pitch and vice versa (albeit losing max down pitch is less important)

 

The real pilot faces no such issues has his stick simply moves with changes in trim, thus he retains full pitch authority in both directions. Our sticks however don't move with the trim (obviously), thus we loose authority opposite what'ever direction we trim.

 

I really can not understand, why pitch down neutral point shift can reduce pitch authority for positive G???

The plane is almost neutral and (if you take a look at the stab itself) requires almost nothing in the stab deflection. It's all about physical ability of the plane to create AoA. Even with the full pitch down trim the pilot can pull the stick up to limiter to get the desired G. What's a problem?

[Ironhand]

Overlooked this.

 

No I mean mostly in upwards pitch authority, i.e you loose pitch authority in the opposite direction of the trim direction: Trim down and you loose full up pitch and vice versa (albeit losing max down pitch is less important)

 

The real pilot faces no such issues has his stick simply moves with changes in trim, thus he retains full pitch authority in both directions. Our sticks however don't move with the trim (obviously), thus we loose authority opposite what'ever direction we trim.

 

Damn. I wish it hadn't taken me this long to respond...

 

But I waited until I had some time to fly so that I could check out what you were saying. How much of a difference have you measured? Or are you just assuming there must be a difference? I ask because I'm not seeing any significant difference on my end. Nor, based on the sim's manual, should I expect to. The trimmer law is left out of the equation as soon as you put Gs on the airframe. And that's what seems to happen.

 

I flew several times both trimmed for my initial airspeed (780 TAS) at 2000 m and with neutral trim. I started from level flight and pulled the stick into my lap and left it there until I had come over the top of the loop and was entering my recovery dive. On all occasions (trimmed and neutral) the initial horizontal stabilizer moved to 4 degrees. As I held the stick in my lap, the stabilizer moved through 7* and ended up at 11* once my speed bled to about 190 TAS. By that point I was over the top and moving into a dive to recover.

 

There were variations between trimmed and neutral trim as my airspeed dropped but not in a single direction. Sometimes the trimmed position would be a bit more at a given airspeed, sometimes a bit less. The differences seemed tied more to differences in G loading at that particular point than anything else.

 

What were you seeing, when you tested?

 

EDIT: Oops. I see Yo-Yo has responded since I began writing this. Guess what he says is what I'm seeing.

 

Rich

[Teknetinium]

YoYo,

Since our stick is centered when we go autopilot off we need to find the position of the trim. Could it be possible to have a option to save the trim setting for us who don't use FFB sticks?

Edited November 3, 2015 by Teknetinium

[Ironhand]

...It's easy to test too by leaving trim in neutral from take off and then noting the G pulled at specific speeds after releasing the stick.

Unfortunately, that would only be true if the system was purely mechanical, wouldn't it?

Edited November 3, 2015 by Ironhand

[Fox One]

I really can not understand, why pitch down neutral point shift can reduce pitch authority for positive G???

The plane is almost neutral and (if you take a look at the stab itself) requires almost nothing in the stab deflection. It's all about physical ability of the plane to create AoA. Even with the full pitch down trim the pilot can pull the stick up to limiter to get the desired G. What's a problem?

 

Yo-Yo, please do the following experiment:

After takeoff with 56% fuel, accelerate at low level to 1000Km/h, trimming for horizontal flight. Reduce speed to 400Km/h, leaving the trim where it was. Engage full AB and pull the stick fully back and perform consecutive slow loops, maintaining the stick full aft. The AoA will oscillate around 19-21 deg, the only way to reach 24 deg AoA again is by applying trim aft. Is that behaviour correct or it is an error?