Su-27 trim issue is in need of explanation

[Hummingbird]

I really think its about time an explanation is given regarding the insane amount of change in upwards trim that the ingame Su-27 experiences with increases in speed.

 

Now that an aircraft wants to pitch up more and more with increases speed is not unusual, but the amount at which the ingame Su-27 wants to do so seems extremely exaggerated, and I really doubt it's the case with the real thing - esp. considering the by comparison very small changes in trim that other jets experience from take off and right up to the transonic region.

 

Even more strange it becomes when in the Su-27 you reset to zero trim at around 800 km/h, experiencing a large amount of upwards pitch as a result, only to see it completely disappear the moment you press the S key(!)

In short you can somehow magically make the upwards pitch disappear by pressing the S key eventhough that the horizontal stabs stay at the exact same zero trim setting as before (!). Aerodynamically that makes zero sense and I'm not sure how ED overlooked that.

 

Finally the real Su-27 comes equipped with a stability system which helps stabilize the aircraft in flight as well as make sure the pilot doesn't overload the aircraft. Ingame however the system only seems to really do one thing, and that is to limit the amount of upwards pitch that the pilot can apply as well as some minor rudder corrections to stabilize a roll, but strangely enough it doesn't prevent the pilot from applying too much down pitch to send the aircraft into an irrecoverable upside down flat spin (the only TRULY dangerous maneuver, one you would think a stability system would want to prevent) and ofcourse neither does it help combat the ever so strange extreme changes in pitch the aircaft exhibits with changes in speed.

 

I find it very hard to believe that the Russians would handicap their pilots this much and install a stability system that essentially does nothing but limit positive G loads and applies minor rudder corrections yet completely neglects to prevent real and potentially fatal departures, and I definitely don't buy the "it's simply a difference in philosophy" excuse.

 

No I tend to belive that the real aircraft actually requires minor trim changes, and I even think the stability system is there to help somewhat with this, even if it isn't a true "1 G auto trim system", in addition to of course preventing the pilot from overloading the airframe and/or sending the aircraft into that irrecoverable upside down flat spin.

 

An explanation is due IMHO.

Edited October 20, 2015 by Hummingbird

[DarkFire]

Some of the points you mention are covered in the Su-27 flight manual that comes with the game, specifically on pages 110 - 117. The reason for the way in which the ACS behaves is described on pages 111 & 112, that reason being that the Sukhoi designers wanted the Su-27 to mimic the behaviour of an aircraft that's inherently stable in forward flight (i.e. the centre of pressure is to the rear of the centre of gravity) whereas in actual fact the Su-27 is designed to be dynamically unstable, i.e. the centre of pressure is actually forwards of the centre of gravity. Real aircraft that are dynamically stable do behave in this way - as speed decreases the nose has a tendency to pitch up and vice versa.

 

We can question that rate at which the change happens, but given the history of ED modelling the Su-27 I can only imagine that the DCS version must have been tested by real Su-27 pilot(s) and signed off as being mostly accurate. I don't know that for sure of course, but if they managed to find a couple of F-15C pilots I'm sure they managed to find at least a few VVS Flanker pilots to test the game.

 

The major issue I have with the trim system is the rate of change of the trim with trimmer inputs. I think the rate of change is too high. When even the slightest milliseconds-long touch of the trim controls produce trim changes that are nearly impossible to zero-out, I feel that there's a minor issue. I don't have an issue with the way in which it behaves, it's a matter of getting used to flying the thing. It certainly isn't as carefree as the F-15C but you do get used to it. After some 32 hours flying the PFM Flanker I barely notice it any more, but I do wish that the trimmer input was made slightly less sensitive.

 

As for the -ve G issue, yes the Flanker really, really hates negative G but then watch videos of real Russian Su-27 pilots: they never, never pull negative G, instead preferring to roll inverted and pull down. It's probably why the Su-27 has a cache tank installed in the fuel system.

 

So ED, if you read this please, please consider reducing the input rate of the trim controls. Otherwise I think the Flanker is fairly easy to fly. It definitely takes a lot more time to master than the other aircraft but you get there eventually...

Edited October 21, 2015 by DarkFire

[Teknetinium]

That you need to trim the aircraft to the speed is quite natural. Autopilot is a different story, If you use autopilot in real Su-27 the hydraulics would move the stick to the position autopilot is trimming it to. So if you would go autopilot off you would hold the stick in same position and only compensate for acceleration. While most of us in DCS dont have stick moved by hydraulics accordingly to autopilot, that make us to compensate for finding the right position of autopilot and at the same time compensate for acceleration. ED made it right only that we dont have sticks that move with the autopilot:(

Edited October 20, 2015 by Teknetinium

[Hummingbird]

Some of the points you mention are covered in the Su-27 flight manual that comes with the game, specifically on pages 110 - 117. The reason for the way in which the ACS behaves is described on pages 111 & 112, that reason being that the Sukhoi designers wanted the Su-27 to mimic the behaviour of an aircraft that's inherently stable in forward flight (i.e. the centre of pressure is forward of the centre of gravity) whereas in actual fact the Su-27 is designed to be dynamically unstable, i.e. the centre of pressure is actually to the rear of the centre of gravity. Real aircraft that are dynamically stable do behave in this way - as speed decreases the nose has a tendency to pitch up and vice versa.

 

We can question that rate at which the change happens, but given the history of ED modelling the Su-27 I can only imagine that the DCS version must have been tested by real Su-27 pilot(s) and signed off as being mostly accurate. I don't know that for sure of course, but if they managed to find a couple of F-15C pilots I'm sure they managed to find at least a few VVS Flanker pilots to test the game.

 

The major issue I have with the trim system is the rate of change of the trim with trimmer inputs. I think the rate of change is too high. When even the slightest milliseconds-long touch of the trim controls produce trim changes that are nearly impossible to zero-out, I feel that there's a minor issue. I don't have an issue with the way in which it behaves, it's a matter of getting used to flying the thing. It certainly isn't as carefree as the F-15C but you do get used to it. After some 32 hours flying the PFM Flanker I barely notice it any more, but I do wish that the trimmer input was made slightly less sensitive.

 

As for the -ve G issue, yes the Flanker really, really hates negative G but then watch videos of real Russian Su-27 pilots: they never, never pull negative G, instead preferring to roll inverted and pull down. It's probably why the Su-27 has a cache tank installed in the fuel system.

 

So ED, if you read this please, please consider reducing the input rate of the trim controls. Otherwise I think the Flanker is fairly easy to fly. It definitely takes a lot more time to master than the other aircraft but you get there eventually...

 

Problem with this is that none of it explains why the upwards pitch suddenly disappears the moment you press the S key at the same zero trim setting.

 

In short pressing the S key at zero trim and there's no movement of the horizontal stab yet there's a sudden change in pitch.

[OnlyforDCS]

Oh boy here we go again....Hummingbird did you really need to open another thread when this entire argument was discussed to death already?

 

Here: http://forums.eagle.ru/showthread.php?t=137234

 

The DCS Su27 flies very well, IMO exactly like it should. You seem to be misunderstanding both how the trim, ACS & CAS (FBW) systems work in tandem in this plane. It's all very well explained by a number of people in that thread.

Edited October 21, 2015 by OnlyforDCS

[Svend_Dellepude]

an aircraft that's inherently stable in forward flight (i.e. the centre of pressure is forward of the centre of gravity) whereasi in actual fact the Su-27 is designed to be dynamically unstable, i.e. the centre of pressure is actually to the rear of the centre of gravity.

 

Actually it's the other way around.

Instability comes from center of pressure being in front of center of gravity.

[Hummingbird]

Oh boy here we go again....Hummingbird did you really need to open another thread when this entire argument was discussed to death already?

 

Here: http://forums.eagle.ru/showthread.php?t=137234

 

The DCS Su27 flies very well, IMO exactly like it should. You seem to be misunderstanding both how the trim, ACS & CAS (FBW) systems work in tandem in this plane. It's all very well explained by a number of people in that thread.

 

No, they are not answered there. Read my post again.

[Santi871]

I don't see how it's exaggerated. Maybe you are used to the F15C? The Mig21bis, L39C and even any plane in other games such as FSX show the exact same speed/pitch-up response. This response is deliberately allowed by the plane's ACS, it's not like the F15C's SAS that trims the plane automatically.

[Hummingbird]

I don't see how it's exaggerated. Maybe you are used to the F15C? The Mig21bis, L39C and even any plane in other games such as FSX show the exact same speed/pitch-up response. This response is deliberately allowed by the plane's ACS, it's not like the F15C's SAS that trims the plane automatically.

 

No, that is simply not the case and it is easily proven by simply pressing the S key at neutral trim and accelerating the aircraft from 400 to 1000 km/h. You will notice that the aircraft then needs only a very small amount of trim with speed, very similar to the amount needed in for example the F-86, MiG-15 or L-39.

[DarkFire]

Actually it's the other way around.

Instability comes from center of pressure being in front of center of gravity.

 

Ah, yes you're absolutely correct. This is what happens when I try to write coherent things when I'm too tired :( I'll correct my original post.

[DarkFire]

That you need to trim the aircraft to the speed is quite natural. Autopilot is a different story, If you use autopilot in real Su-27 the hydraulics would move the stick to the position autopilot is trimming it to. So if you would go autopilot off you would hold the stick in same position and only compensate for acceleration. While most of us in DCS dont have stick moved by hydraulics accordingly to autopilot, that make us to compensate for finding the right position of autopilot and at the same time compensate for acceleration. ED made it right only that we dont have sticks that move with the autopilot:(

 

A very good point. I often wonder if most of the issues that people have with flying things like the Su-27 and also the Ka-50 are largely due to the fact that neither aircraft in reality has a self-centring control column, whereas all commercial joysticks do self-centre. If we had available a non-centring stick then things like the stick-to-trimmer mode on both the Su-27 and Ka-50 would probably feel a lot more natural to use.

[Svend_Dellepude]

Ah, yes you're absolutely correct. This is what happens when I try to write coherent things when I'm too tired :( I'll correct my original post.

 

I thought that, no worries! :)

 

A very good point. I often wonder if most of the issues that people have with flying things like the Su-27 and also the Ka-50 are largely due to the fact that neither aircraft in reality has a self-centring control column, whereas all commercial joysticks do self-centre. If we had available a non-centring stick then things like the stick-to-trimmer mode on both the Su-27 and Ka-50 would probably feel a lot more natural to use.

 

If you had a non-centering stick like the ones from komodosim http://www.komodosimulations.co.uk/Controls.html, you wouldn't have to touch the trimmer at all. You just had to move the stick to the required position. For the fixed wings at least, the KA-50 is a different story because of the autopilot coupled to the controls.

Both aircraft in fact do have selfcentering controls but they center on the trimmed position and not in a fixed center like a joystick does.

[OnlyforDCS]

No, they are not answered there. Read my post again.

 

I am quoting Ironhand, verbatim:

 

"The thing to keep in mind is that, in essence, there are two control systems on this aircraft. There's the mechanical control system and, also, the FBW control system that overlays it. The purpose of the latter is to help you better control the aircraft in all situations.

 

That being said, when you take direct control of the aircraft (press the "S" key) you are turning off the "computerized" control which alters your control inputs to keep you out of trouble. So, when you take direct control, the trim setting you get is the mechanical one you dialed in, not the altered setting you just turned off. When flying at higher speeds, hit the "S" key and your trim goes to just about full nose down which is what you actually have it set at. Not the FBW version of it. And over you go. "

 

Or in other words, flying with the FBW system "on" and your trim set to "0", the actual mechanical trim tabs will be set for a positive trim setting, I.E. pitch up. As soon as you turn off the FBW system, they will default to this setting and you will experience a pitching up moment in the plane. Despite the horizontal stabilizers not moving.

 

The Su27 is not meant to be flown with the FBW or CAS system turned off. The air-frame and its inherent negative instability, allow it to pull much more than 9g at speed, enough to instantly kill a careless pilot. Two test pilots were killed during it's design and testing precisely when this system failed or was being tested.

Edited October 22, 2015 by OnlyforDCS

[Santi871]

I am quoting Ironhand, verbatim:

 

"The thing to keep in mind is that, in essence, there are two control systems on this aircraft. There's the mechanical control system and, also, the FBW control system that overlays it. The purpose of the latter is to help you better control the aircraft in all situations.

 

That being said, when you take direct control of the aircraft (press the "S" key) you are turning off the "computerized" control which alters your control inputs to keep you out of trouble. So, when you take direct control, the trim setting you get is the mechanical one you dialed in, not the altered setting you just turned off. When flying at higher speeds, hit the "S" key and your trim goes to just about full nose down which is what you actually have it set at. Not the FBW version of it. And over you go. "

 

Or in other words, flying with the FBW system "on" and your trim set to "0", the actual mechanical trim tabs will be set for a positive trim setting, I.E. pitch up. As soon as you turn off the FBW system, they will default to this setting and you will experience a pitching up moment in the plane. Despite the horizontal stabilizers not moving.

 

The Su27 is not meant to be flown with the FBW or CAS system turned off. The air-frame and its inherent negative instability, allow it to pull much more than 9g at speed, enough to instantly kill a careless pilot. Two test pilots were killed during it's design and testing precisely when this system failed or was being tested.

 

The su27 doesn't have trim tabs.

[Hummingbird]

I am quoting Ironhand, verbatim:

 

"The thing to keep in mind is that, in essence, there are two control systems on this aircraft. There's the mechanical control system and, also, the FBW control system that overlays it. The purpose of the latter is to help you better control the aircraft in all situations.

 

That being said, when you take direct control of the aircraft (press the "S" key) you are turning off the "computerized" control which alters your control inputs to keep you out of trouble. So, when you take direct control, the trim setting you get is the mechanical one you dialed in, not the altered setting you just turned off. When flying at higher speeds, hit the "S" key and your trim goes to just about full nose down which is what you actually have it set at. Not the FBW version of it. And over you go. "

 

Or in other words, flying with the FBW system "on" and your trim set to "0", the actual mechanical trim tabs will be set for a positive trim setting, I.E. pitch up. As soon as you turn off the FBW system, they will default to this setting and you will experience a pitching up moment in the plane. Despite the horizontal stabilizers not moving.

 

The Su27 is not meant to be flown with the FBW or CAS system turned off. The air-frame and its inherent negative instability, allow it to pull much more than 9g at speed, enough to instantly kill a careless pilot. Two test pilots were killed during it's design and testing precisely when this system failed or was being tested.

 

Again please read my post carefully, none of it is addressed by your quote which btw is completely incorrect as the Su-27 like any modern jet doesn't feature any trim tabs.

Edited October 22, 2015 by Hummingbird

[OnlyforDCS]

Problem with this is that none of it explains why the upwards pitch suddenly disappears the moment you press the S key at the same zero trim setting.

 

In short pressing the S key at zero trim and there's no movement of the horizontal stab yet there's a sudden change in pitch.

 

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.

Edited October 22, 2015 by OnlyforDCS

[Hummingbird]

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.

 

Yet it is... like I said, press the S key before taking off and then when airborne accelerate to 1000 km/h and notice how little trim the aircraft needs during said acceleration as compared to when the S key hasnt been pushed.

 

The question obviously is: Why does the ingame Su-27 magically need significantly more trim with the stability system on than with it off.

 

Hence why I mention the zero movement of the horizontal stab, as in real life no movement here equals no change in pitch.

[Tucano_uy]

If we had available a non-centring stick then things like the stick-to-trimmer mode on both the Su-27 and Ka-50 would probably feel a lot more natural to use.

 

That's why, in my opinion, a reset trim button makes sense, even if it doesn't exist in the real plane. Center the stick and reset trim. Somehow the shortcomings of simulating a real aircraft have to be addressed. We're not feeling the forces on the stick and the position of the stick doesn't correspond to the position of our joystick unless there's zero trim.

[ZaltysZ]

Yet it is... like I said, press the S key before taking off and then when airborne accelerate to 1000 km/h and notice how little trim the aircraft needs during said acceleration as compared to when the S key hasnt been pushed.

 

The question obviously is: Why does the ingame Su-27 magically need significantly more trim with the stability system on than with it off.

 

It is by design of Sukhoi. Stability system "fakes" forces to give feedback to pilot.

[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.

 

Secondly the way that the FBW system works is it corrects the pilot joystick input after mechanical trim is considered, and moves the horizontal stabilizers so that it simulates a plane with positive stability. (Think WW2 birds stability). So as airspeed increases, plane will want to pitch up, and vice-versa as airspeed decreases plane will want to pitch down.

 

So you are flying along with FBW engaged, because of the airspeed trimming law (providing the feel of a positively stable aircraft) but the amount that you "turn the trim wheel", or trim nose down so to speak, to fly straight and level does not correspond directly to the amount of mechanical trim you dialed in, the FBW will counteract to keep the airplane in an envelope of positive stability. It's much less. In that case when you hit the "s" switch and turn FBW off, suddenly you have the full mechanical nose down trim engaged, and your plane will start to pitch down into a nose dive.

Edited October 22, 2015 by OnlyforDCS

[OnlyforDCS]

Y

 

The question obviously is: Why does the ingame Su-27 magically need significantly more trim with the stability system on than with it off.

 

 

Remember, that the FBW will simulate an aircraft with positive stability. As airspeed increases you need more trim down than in an aircraft with neutral stability. When you disengage the FBW the aircraft loses this simulated stability, it becomes neutral or even unstable so much less (or even reverse trim) is required.

[Hummingbird]

Remember, that the FBW will simulate an aircraft with positive stability. As airspeed increases you need more trim down than in an aircraft with neutral stability. When you disengage the FBW the aircraft loses this simulated stability, it becomes neutral or even unstable so much less (or even reverse trim) is required.

 

What you're saying is that in order to be stable the aircraft has to experience a higher than normal increase in upwards pitch with speed... i.e. the stability system actually applies downwards horizontal stab deflection for increased upwards pitch with speed. That makes very little sense from an end user perspective...

 

If the idea is to prevent the pilot from not imparting too much negative G on the airframe you simply program the FBW system to limit upward stab deflection for downward pitch at certain airspeeds. To instead make the system gradually increase the upwards pitch over the norm with speed seems very illogical, esp. since the only result is that the pilot will just trim the stab right back to the neutral position in order to stay level. In short all you've then achieved is an increased pilot workload and nothing else.

 

I really cannot believe the Russians are that backwards...

Edited October 22, 2015 by Hummingbird

[OnlyforDCS]

What you're saying is that in order to be stable the aircraft has to experience a higher than normal increase in upwards pitch with speed... i.e. the stability system actually applies downwards horizontal stab deflection for increased upwards pitch with speed. That makes very little sense from an end user perspective...

 

If the idea is to prevent the pilot from not imparting too much negative G on the airframe you simply program the FBW system to limit upward stab deflection for downward pitch at certain airspeeds. To instead increase the upwards pitch seems very illogical, esp. since the only result is that the pilot will just trim the stab right back to the neutral position. In short all you've then achieved is an increased pilot workload and nothing else.

 

Im not the one who designed the system. Just trying to explain how it works. Unlike it's western counterpart (the F15C) the FBW in the Su27 works to give the effect of a positively stable aircraft, not a neutrally stable aircraft. Yes, the end result would be more trimming from the pilot, but it also allows pilots who have trained on such aircraft to easily switch to the Su27. It's very intuitive for example, for someone who has only flown the L39 trainer (or piston engined aircraft) for example to switch to the Su27. I have a FFB joystick, and the trim settings actually move the centerposition of the stick itself. You don't need to think about trim after a while, and only need to set it when cruising at a constant speed.

 

It's purpose is not to prevent the pilot from imparting negative g's, it's purpose is to allow the plane to still be very controllable even at very low speeds and yet not kill themselves or destroy the plane when doing high G maneuvering. Most importantly the plane is not meant to be flown with the FBW system turned off.

 

The Russians are not backwards. The pilot workload you are talking about is minimal.

Edited October 22, 2015 by OnlyforDCS

[Hummingbird]

Sorry but I really find it extremely hard to believe the theory that this heavy artificially applied upwards pitch is to make the transition for new pilots easier. I have litterally NEVER heard of anyone having problems transitioning to an aircraft that needs less trimming, never. It's the other way round.

 

If the goal is to prevent overstressing the airframe you simply have the FBW limit stab deflection at certain speeds, that's it. There's no point in forcing the pilot to constantly retrim to achieve this cause in the end that's not the issue, the issue is the amount of deflection past the neutral point which is applied at certain speeds - hence why western aircraft use the 1G trimming system, even in their most naturally unstable designs.

 

So the system you describe is infact very backwards, esp. since despite what you say the need for constant retrimming is a significant extra workload for a pilot with as many systems to tend to that a modern fighter demands. i.e. it is one finger or even one hand less for other applications.

[OnlyforDCS]

Sorry but I really find it extremely hard to believe the theory that this heavy artificially applied upwards pitch is to make the transition for new pilots easier. I have litterally NEVER heard of anyone having problems transitioning to an aircraft that needs less trimming, never. It's the other way round.

 

If the goal is to prevent overstressing the airframe you simply have the FBW limit stab deflection at certain speeds, that's it. There's no point in forcing the pilot to constantly retrim to achieve this cause in the end that's not the issue, the issue is the amount of deflection past the neutral point which is applied at certain speeds - hence why western aircraft use the 1G trimming system, even in their most naturally unstable designs.

 

So the system you describe is infact very backwards, esp. since despite what you say the need for constant retrimming is a significant extra workload for a pilot with as many systems to tend to that a modern fighter demands. i.e. it is one finger or even one hand less for other applications.

 

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.

Edited October 22, 2015 by OnlyforDCS