Su-27 Flight Model Discussion

Ironhand · January 22, 2015

The "override key" (S) function seems to be wrong.
I would bet all of my money that this key don't resett/override the trim function. This where stupid like hell. A harakiri/sepuko switch? Really?
If the DCS Su27 (s) key function is any near the original one, you must me pay several 100.000 dollars to be a Co on a Su27 hitting that switch to do a Cobra.
As I know, that switch only make the stick output more sensitive at the edges.
So around a normal input you cant feel any difference, you can only notice a difference if you push the stick to the limits. Thats why we always see the max way pull and push during a real Cobra.
So where does this "trim reset" coming from? Logic? Kill as many Pilots as an stupid engineer can?
I think it is more a change of center of gravity of the su27 to do the Cobra.
I can't believe that the ovveride key would act any near in RL like in DCS.

I'm 2000 miles from my computer and borrowed a friend's iPad for a minute to see what was going on here. Anyway, just want to mention that it doesn't override the actual trim--just the FBW revision. you can still manually trimm the aircraft. Should be back online early next week and hope to catch up with the discussions then.

Rich

bolek · January 22, 2015

The Su27 seems to be very/too tail heavy.

I wonder how much of that is the payload/fuel. A lot of people (myself included) have the dubious habit of loading up on 100% fuel and 10 missiles. This is bound to affect center of gravity.

Sgt_Baker · January 22, 2015

Preamble: I have not read this thread in full, since spending far too much time stuck to this keyboard/monitor/mouse as it is. :)

Statement of fact: None of the FC3 flight models feel in any way "real". This is to be expected. We pay $40 for a hypersim of any given aircraft, and cannot expect the same fidelity from the lesser cousins of those products.

An observation: If I'm not entirely mistaken, the F-16 had a "shuttlecock of death" mode of stall where it would fall from the sky the right way up. I recall (from FFAF) there being an "Override" button on the FBW which would allow the pilot to rock the aircraft out of the plunge.

Conclusion: Negative stability comes with a plethora of caveats. While the SU-27's flight model might not perform as desired, the modes of failure are closer to the "die or die trying" state of affairs in RL.

Bottom line: If every SU-27 pilot would part with $40, we'd have these problems fixed in no time at all.

Python · January 22, 2015

Statement of fact: None of the FC3 flight models feel in any way "real". This is to be expected. We pay $40 for a hypersim of any given aircraft, and cannot expect the same fidelity from the lesser cousins of those products.

I haven't flow the SU-27 enough to comment. I have however flown the F-15c an awful lot, more than any of the full on modules in fact. The flight dynamics feel pretty real to me.

Sgt_Baker · January 22, 2015

I haven't flow the SU-27 enough to comment. I have however flown the F-15c an awful lot, more than any of the full on modules in fact. The flight dynamics feel pretty real to me.

They feel sterile to me. Then again, I've never actually flown an A-10 nor an F-15.

Shepski · January 22, 2015

I have found out why the nose moves around so much with control stick inputs, which has been my main beef of the PFM...

When you roll the jet into a flaperon roll the rudder kicks the nose into the roll and when you stop the roll the rudder kicks the nose the opposite direction. flaperon/rudder interconnection is over done.

To see it in action just go to an outside view looking at the tail and move the stick from side to side and you will see large rudder deflections. Once this is fixed and the flaperons less affected from rudder deflection the jet will be much more stable about it's longitudinal axis!

Edited January 22, 2015 by Shepski

Shepski · January 22, 2015

Su-27 PFM Flight Model Longitudinal Axis Instability Bug

When you roll the jet the nose moves to much about it's longitudinal axis with a rubber band like motion.

The reason this happens is due to the stick/rudder interconnection. When you move the flight stick from side to side the rudders move with it. This causes the nose to move laterally to the left when you roll the jet to the left and then when you stop the roll with right stick movement the nose then moves laterally to the right then back left to re-center.

Watch in an external view and move the stick back and forth laterally and you can see the rudders moving too much.

The interconnection between the flight stick and rudders is over done at higher speeds as this system is designed for low speed and high alpha stability.

Edited January 22, 2015 by Shepski

Shepski · January 22, 2015

Not dismissing your arguments, but isn't that a bit difficult to base your expectations off on videos? For example, I am a fanatic Formula One fan...

As am I! :)

I have time behind the wheel of high performance go-carts and and single seaters and once you get used to the rapid acceleration/deceleration and cornering G they aren't all that hard to drive. After that it becomes all about timing with braking and turn in points.

I have found the cause of my biggest issue with the FM and it's because the flight stick interconnection with the rudders is over done which is causing all the nose bobbing and longitudinal instability.

Edited January 22, 2015 by Shepski

ShuRugal · January 23, 2015

If this would be the only Jet with PFM in DCS, I would say that your announcement could be right. But we have some planes we can compare with the Su27 (F15C, Mig21, A10, and so on). So I think, that there is much more around that "feeling" of the AC behavior.
One person in another thread said something very interesting about the "Pitch/Cobra" behavior.
The Su27 seems to be very/to tail heavy. I think that would explain most of the problems I have with the DCS Su27!
Perhaps we need this extrem heavy tail to do the Cobra?
This has nothing to do with, that a fighter jet is build aerodynamic unstable to be fast an agil. Even a fast and agil AC must have his point of the center of gravity at the right place.
I can land the DCS Su27 and hold and switch the nose hight of the DCS Su27 between an AoA of 4 and 12 even till 140 kph before the nose goes/falls down by herself. A real Su27 can do this too? I don't know, but compared to all other DCS AC with a PFM it feels very strange.

On the other side the DCS Su27 burns energie like hell at every turn, if I want to do that turn at the same speed and in the same time like the real plane! All other DCS planes feel more or less right and do not burn so much energie.
There is allways a very small point at which the butt of the DCS Su27 wants to come around. If you pull 0.001 degree to much, you do more or less a sudden air break maneuver. If this happens there is always an extrem speed loss, because the DCS Su27 pitches suddenly with the butt around with.
If I try the same with the F15C/Su25/F86F/A10C they all slowing down but they don't show any air breaking/drift car behavior, if you pull the turn the same way as with the DCS Su27.
The DCS Su27 wants to throw her butt around. She feels and acts like a drift car with not enough power.
It's a very tiny point at which this behavior will accour, and the point where this happend is not stabel and that would fit perfect with a wrong centerpoint of mass.
A wrong center of gravity could explain this and many other strange behaviors.

So more ore less, we can compare the DCS Su27 AC with those who are already insinde and if you fly them all and try to push them all to the limit, you can see that the Su27 acts very different to all other DCS AC at some points (where all planes should have to act more or less the same way).
A small difference at this point is good, an extreme different behavior not so!
They are all AC, so at some points they must/should act the same way!

For starters, you can tell just by looking at the bird that she's tail-heavy: The engines are entirely behind the center-of-lift on the wings, and over half of each engine is behind the trailing-edge of the wing. Further, the weapon stations are pushed back near the trailing edge of the wings. Finally, the aircraft has no fuselage to speak of, it is almost entirely wings and engine, with just enough space for the pilot tacked on to the nose. Any one of these features would push CG, aft. Tail-heavy behavior is definitely to be expected from this aircraft (would also explain the insane amounts of nose-down trim required).

As to comparing the SU-27 against other aircraft... you really can't.

The A-10 is in an entirely different class of aircraft, the only similarity it shares with the SU-27 is that it flies and has one pilot.

The F-15C, while superficially similar, has radically different design features and goals: The US Air Force philosophy on fighter design is dictated by the theory of 'energy maneuverability', which boils down to "faster == better". The Russian philosophy on fighter design calls for maintaining the ability to maneuver well at low airspeed. Where the F-15 practically falls like a brick below 200-250kts (370-460 kph), depending on loadout, the SU-27 is well-behaved at speeds as low as 250 kph (130 kts).

The Mig-21 is also a completely different animal, weighing less than half as much and having the same wing loading on a set of wings that are so small and steeply swept that calling them 'fins' seems more descriptive.

As far as making your turns match what you see in videos: how do you know that the pilot in that video is not burning as much energy as you are in the sim? The camera frame provides a very poor reference for estimating his speed, and you have no idea what his throttle positions through the turn are: for all you know, he could be burning 100kg of fuel every time he makes a turn in order to maintain airspeed.

Witchking · January 23, 2015

For starters, you can tell just by looking at the bird that she's tail-heavy: The engines are entirely behind the center-of-lift on the wings, and over half of each engine is behind the trailing-edge of the wing. Further, the weapon stations are pushed back near the trailing edge of the wings. Finally, the aircraft has no fuselage to speak of, it is almost entirely wings and engine, with just enough space for the pilot tacked on to the nose. Any one of these features would push CG, aft. Tail-heavy behavior is definitely to be expected from this aircraft (would also explain the insane amounts of nose-down trim required).

As to comparing the SU-27 against other aircraft... you really can't.
The A-10 is in an entirely different class of aircraft, the only similarity it shares with the SU-27 is that it flies and has one pilot.

The F-15C, while superficially similar, has radically different design features and goals: The US Air Force philosophy on fighter design is dictated by the theory of 'energy maneuverability', which boils down to "faster == better". The Russian philosophy on fighter design calls for maintaining the ability to maneuver well at low airspeed. Where the F-15 practically falls like a brick below 200-250kts (370-460 kph), depending on loadout, the SU-27 is well-behaved at speeds as low as 250 kph (130 kts).

The Mig-21 is also a completely different animal, weighing less than half as much and having the same wing loading on a set of wings that are so small and steeply swept that calling them 'fins' seems more descriptive.

As far as making your turns match what you see in videos: how do you know that the pilot in that video is not burning as much energy as you are in the sim? The camera frame provides a very poor reference for estimating his speed, and you have no idea what his throttle positions through the turn are: for all you know, he could be burning 100kg of fuel every time he makes a turn in order to maintain airspeed.

+1 :thumbup:

archer86 · January 23, 2015

For starters, you can tell just by looking at the bird that she's tail-heavy: The engines are entirely behind the center-of-lift on the wings, and over half of each engine is behind the trailing-edge of the wing. Further, the weapon stations are pushed back near the trailing edge of the wings. Finally, the aircraft has no fuselage to speak of, it is almost entirely wings and engine, with just enough space for the pilot tacked on to the nose. Any one of these features would push CG, aft. Tail-heavy behavior is definitely to be expected from this aircraft (would also explain the insane amounts of nose-down trim required).

As to comparing the SU-27 against other aircraft... you really can't.
The A-10 is in an entirely different class of aircraft, the only similarity it shares with the SU-27 is that it flies and has one pilot.

The F-15C, while superficially similar, has radically different design features and goals: The US Air Force philosophy on fighter design is dictated by the theory of 'energy maneuverability', which boils down to "faster == better". The Russian philosophy on fighter design calls for maintaining the ability to maneuver well at low airspeed. Where the F-15 practically falls like a brick below 200-250kts (370-460 kph), depending on loadout, the SU-27 is well-behaved at speeds as low as 250 kph (130 kts).

The Mig-21 is also a completely different animal, weighing less than half as much and having the same wing loading on a set of wings that are so small and steeply swept that calling them 'fins' seems more descriptive.

As far as making your turns match what you see in videos: how do you know that the pilot in that video is not burning as much energy as you are in the sim? The camera frame provides a very poor reference for estimating his speed, and you have no idea what his throttle positions through the turn are: for all you know, he could be burning 100kg of fuel every time he makes a turn in order to maintain airspeed.

+1 :thumbup:

+1:thumbup:

Bourrinopathe · January 23, 2015

Clever comment.

-----

Does anyone know if the Su-27 fuel system (and tanks location) is part of the PFM?

The automatic fuel management system should ensure that the centre of gravity stays within prescribed limits as fuel is burned.

I have no idea how it's modeled in the sim.

Based on ED's definition of the PFM, at least a part of the fuel system should be modeled.

Realistic simulation of Hydraulics, Fuel, Electrical, Engine and other systems influence flight characteristics.

(a few details are available in that post)

Edited January 23, 2015 by Bourrinopathe

Exorcet · January 23, 2015

I think the definition answers the question, it would be really surprising if the system wasn't modeled. Fuel weight is for sure, so as long as it's in the right place weight balance should be accurate and that would mean that fuel weight compensation would be necessary to fly the plane.

Bourrinopathe · January 23, 2015

Indeed. I assume, we could also test the roll inertia with the second warning light ON as the wing tanks are supposed to be empty at this stage. But I have no idea, how significant the difference should be.

*Rage* · January 23, 2015

Good observation. We noticed in one of our recent training exercises that the plane was yawing unnecessarily when observed as a third party in external view.

I hope ED will comment!

archer86 · January 23, 2015

not the same aircraft(same family:) but you can see how much rudder moving.

it can be seen clearly after 1:18

https://www.youtube.com/watch?v=BAcMm5cKYtg

Shepski · January 23, 2015

not the same aircraft(same family:) but you can see how much rudder moving.
it can be seen clearly after 1:18
https://www.youtube.com/watch?v=BAcMm5cKYtg

Definitely shows what it was designed for, low speed and higher alpha maneuvers!

In DCS I feel there is too much rudder movement at higher speeds where it really shouldn't be noticeable and this in turns moves the nose around too much with the rudder deflection and then rebound back to neutral.

archer86 · January 23, 2015

Definitely shows what it was designed for, low speed and higher alpha maneuvers!

In DCS I feel there is too much rudder movement at higher speeds where it really shouldn't be noticeable and this in turns moves the nose around too much with the rudder deflection and then rebound back to neutral.

From the day it has released, this is known problem but the didnt fix flanker high speed behaviour yet. hopefully on future updates

related thread:

http://forums.eagle.ru/showthread.php?t=134869

There is no friction force limit on flanker controls.

combatace · January 24, 2015

not the same aircraft(same family:) but you can see how much rudder moving.
it can be seen clearly after 1:18
https://www.youtube.com/watch?v=BAcMm5cKYtg

This guy isn't doing rolls. Looks like his is doing barrel rolls, which surely requires moving nose out of your longitudinal axis.

Nedum · January 24, 2015

Can't see a barrel roll!

All he was going was a aileron roll... a very big difference as you can see.

But you a right.. not one "roll only" i could see.

But the OP is right. The Su27 is overdoing the rudder input as you can see if you activating the autopilot under 600kph.

Edited January 24, 2015 by Nedum

DarkFire · January 24, 2015

Well well, this discussion grew a bit! As my early posts will show to begin with I had a real problem grappling with the new flight model for the Su-27. Once I learned that key "S" = END OF WORLD my life bace easier.

I later did a test at high altitude and genuinely failed to get the Su-27 to depart from controlled flight. Since then I've been observing the conditions under which I experience most trouble and can offer the following observations:

1. I agree with Shepski that I think the cross-control between flaperons and rudder is both over-exaggerated and is probably the cause of the 'wallowing' in longitudinal stability. Doesn't really bother me per-se but it does feel... untidy.

2. I'm suspicious of the accuracy of the aerodynamic and kinematic modelling of missiles. Yes, that is a very different discussion but I do suspect that weapons loaded on the Su-27 are causing an exaggerated amount of parasitic drag. At present the change in the way the aircraft handles between an empty condition and even having say 4 x AA missiles is astounding. I think this could benefit from further review by the dev team. As could the missiles. I'm looking at you, utterly-useless-huge-pile-of-steaming-fail R-27ER :mad:

3. I think we can all agree that the full waypoint-following autopilot on the Su-27 is painfully bad. If it's realistic, so be it but I suspect that it is not.

Shepski · January 24, 2015

3. I think we can all agree that the full waypoint-following autopilot on the Su-27 is painfully bad. If it's realistic, so be it but I suspect that it is not.

If you keep your speed it up and out of the wallowing range it seems to work fine in holding a course and following a VNAV(Vertical Navigation) profile. Where it is suspect is in the amount of intercept angle it uses to acquire the course between 2 waypoints. It is overly aggressive so get on course then engage route following mode.

I first engage route following mode followed by "h" baro altitude hold for enroute VNAV and it will fly quite accurately.

Evilducky · January 25, 2015

FYI... If you stall out this aircraft and it goes on its back, you mind as well eject because its impossible to flip it back over and recover it.

flankerted · January 25, 2015

2. I'm suspicious of the accuracy of the aerodynamic and kinematic modelling of missiles. Yes, that is a very different discussion but I do suspect that weapons loaded on the Su-27 are causing an exaggerated amount of parasitic drag. At present the change in the way the aircraft handles between an empty condition and even having say 4 x AA missiles is astounding. I think this could benefit from further review by the dev team. As could the missiles. I'm looking at you, utterly-useless-huge-pile-of-steaming-fail R-27ER :mad:

Well, I think the aerodynamic of both aircrafts and missiles is of high accuracy, however, the control systems ED simulated still has some problems. For example, the logic of missiles guidence seems to be wrong (inertial guidence systems), resulting in insufficient energy to intercept bandits.

bolek · January 25, 2015

FYI... If you stall out this aircraft and it goes on its back, you mind as well eject because its impossible to flip it back over and recover it.

Actually, as long as you are conscious you can usually recover. Deploying the air brake helps a lot. It's when you completely black out and loose consciousness then you can't recover (understandably so :)).

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