Su-27 Flight Model Discussion

Had time for a few attempts this morning with the numbers and loading that you cited, OB1, in your original post. Also added some crosswind. I can't make your numbers work. At 220k/h my AoA is about 8. At 210 it sits squarely on 10*. I played around for a bit to see where I lost pitch control and it was just below 170k/h. A that point things happened very rapidly.

Here is my entry :-). Very simple: fully loaded plane, push the stick all the way forward. No recovery.

Yup. That works.

EDIT: Looks like you encounter -4.7Gs in the initial pitchover and, then, a continuous -1.5 to -2.5 or so for awhile before it eases off a bit. I don't know if that's enough to sustain indefnite LOC. Seems to be very little research on the subject. Nothing I've found so far even mentions LOC, just redout. I suppose significant -Gs can induce a stoke. Maybe that's what's doing us in--we're stroking out. :)

Yup. That works.

 

EDIT: Looks like you encounter -4.7Gs in the initial pitchover and, then, a continuous -1.5 to -2.5 or so for awhile before it eases off a bit. I don't know if that's enough to sustain indefnite LOC. Seems to be very little research on the subject. Nothing I've found so far even mentions LOC, just redout. I suppose significant -Gs can induce a stoke. Maybe that's what's doing us in--we're stroking out. :)

If the stroke don't get ya, the big ball of rock and iron will!

If the stroke don't get ya, the big ball of rock and iron will!

:D It'd be nice to know if the blackout is by design and belongs there or is something that ought to be fixed. The same, I suppose, for that pitchover, too.

The real world manual specifies a number of "don't"s. But the negative AoA references refer mainly to the impact on structural strength.

Had time for a few attempts this morning with the numbers and loading that you cited, OB1, in your original post. Also added some crosswind. I can't make your numbers work. At 220k/h my AoA is about 8. At 210 it sits squarely on 10*. I played around for a bit to see where I lost pitch control and it was just below 170k/h. A that point things happened very rapidly.

Interesting. I still don't have time to try but if you do - Just theorising here but perhaps the crosswind was also a tad on the tail too. TacView indicates GS right ? maybe my IAS was actually allot lower which caused the problem earlier. In navigation mode what speed measurement is displayed on the HUD ?

...In navigation mode what speed measurement is displayed on the HUD ?

IAS on the HUD, TAS on the HDD. I'll recheck your landing in TacView. I hadn't noticed a tailwind component but, then again, I wasn't particularly looking for it either.

EDIT: Double checked. TAS was always higher than IAS. More importantly TAS and GS were almost identical except that GS was always 0.1 k/h less than TAS. So nothing to indicate a tailwind component. Noticed something interesting this time that I hadn't seen the other time through. This time I had slowed playback down. Your airspeed jumped from 194 k/h to 203 at the instant you hit the runway the first time and you went airborne again. You then instantly slowed down and "floated" for a bit.

Rich

I went through the in-game track and may have found a contributing factor. As a matter of habit on the first touch on the runway I transitioned from pitch down to pitch up for a snap moment as I was probably expecting the nose to come down hard after the rear wheels touch, and as you can see in the screenies I took the first touch shows the stabilatiors half way in that transition in the neutral pos. After that is was all manner of ugly.

Prepare to see some shots of the most ugly landing ever. You can see after liftoff again I was pitch down again all the way until it dropped out of the sky and I was all pitch up in an attempt to stop becoming a lawn dart. The alpha doesn't look too bad on TacView but it sure looks bad in those screenshots.

I think this can all be avoided with a tad higher landing speed. Not sure if this is a flight model problem anymore unless you consider what happened after the first touch a problem.

27.zip

I went through the in-game track and may have found a contributing factor. As a matter of habit on the first touch on the runway I transitioned from pitch down to pitch up for a snap moment as I was probably expecting the nose to come down hard after the rear wheels touch, and as you can see in the screenies I took the first touch shows the stabilatiors half way in that transition in the neutral pos. After that is was all manner of ugly.

 

Prepare to see some shots of the most ugly landing ever. You can see after liftoff again I was pitch down again all the way until it dropped out of the sky and I was all pitch up in an attempt to stop becoming a lawn dart. The alpha doesn't look too bad on TacView but it sure looks bad in those screenshots.

 

I think this can all be avoided with a tad higher landing speed. Not sure if this is a flight model problem anymore unless you consider what happened after the first touch a problem.

Minimum speed is supposed to be 200k/h but I'd stay above it. 240 would probably be the slowest I'd want to go. :)

EDIT: Minimum speed for level flight below 6000m, that is. That's really the stuff of airs hows-- low speed, high alpha passes.

has anyone else noticed that the Su-27 is a bit harder to land now?

After flying the Mig-21 which is a very stable platform the Su-27 is too pitch happy and the nose bounces around all over. The PFM still needs work to stabilize it. They really should bring the stability of the Mig-21 to the Su-27.

has anyone else noticed that the Su-27 is a bit harder to land now?

I wouldn't say it was much harder to land. What I do find almost impossible is lining it up accurately with the runway, though I guess that solving this issue is a combination of an accurately flown glide slope and correcting for wind over the runway.

That being said, it has become necessary to watch the sink rate very carefully. I try to make full use of the ground effect to cushion my landing so that the wheels don't make contact until the sink rate is down to 1-2 m/s. I find that this avoids popped tyres and damaged undercarriage.

After flying the Mig-21 which is a very stable platform the Su-27 is too pitch happy and the nose bounces around all over. The PFM still needs work to stabilize it. They really should bring the stability of the Mig-21 to the Su-27.

Apparently stability increases, and the pitch 'bouncing' decreases, as flight speed increases. Just for the hell of it I tried a strafing run at an airfield the other day and happened to be in a shallow dive at nearly Vmax (1500m, ~1500 Kph) and found that at this sort of speed the pitch axis is rock steady. I was able to slide the gun reticle over the target vehicle with total precision.

The fact that I hadn't loaded any gun ammunition is a totally different matter :music_whistling:

I wonder whether the pitch bouncing is an artefact of the game engine or is genuine behaviour for the -27. Anecdotally, back in the days of Flanker 2.5 I ended up speaking via email to a chap who claimed to work for Sukhoi in some capacity. I genuinely can't remember how or who he was, but I do remember asking him about the pitch bouncing and whether this was realistic. His reply was that the Su-27 doesn't exhibit this sort of behaviour but that otherwise the simulation was, for that time, very accurate. Obviously 75% of everything on the internet is B-S (probably less so for this web site, the signal-to-noise ratio here is astonishingly high for a web forum) but it did make me wonder at the time. Still do.

After flying the Mig-21 which is a very stable platform the Su-27 is too pitch happy and the nose bounces around all over. The PFM still needs work to stabilize it. They really should bring the stability of the Mig-21 to the Su-27.

According to Yo-Yo, the aircraft isn't as "point and shoot" as you'd expect because of an added control law that mimics positive speed stability in an aircraft. Increase your speed and the nose pitches up. Decrease it and your nose pitches down. I can understand where that would be a plus in the landing and takeoff regimens but the rest of the time...It seems like it ought to be damped out as airspeed increases (my opinion). But that certainly isn't happening in the sim, if it's supposed to. So, unless your speed is absolutely steady, your nose is always rising or falling.

Increase your speed and the nose pitches up. Decrease it and your nose pitches up.

fixed it for you :lol:

After flying the Mig-21 which is a very stable platform the Su-27 is too pitch happy and the nose bounces around all over. The PFM still needs work to stabilize it. They really should bring the stability of the Mig-21 to the Su-27.

Amen brother!

Till today there is no physical law that could explain why the Su27 gets more agil around pitch at low speeds.

Yaw becomes like jelly but pitch is sensetive like a micrometer clock. Your are near stall but one hard pull at the stick an the Su27 goes inverted/flips over. Because? :huh:

So which physical law is made for this behaviour?

Dark matter?

All I know is, that this DCS Su27 is at some points against all physical laws!

Try to fly a barrel roll with a good steady speed of 300 mph and you will get some big eyes. Ok.. not that hard, fly a good barrel roll with the DCS Su27.

A simple barrel roll becomes a master piece with this plane. But why? :huh:

Just because it is difficult for you to fly it doesn't mean that it is not realistic. This is among the first versions of the Su27 with its FBW system that is represented at a lot more realistic level than you are aware of. ED know what they are doing and have their sources. Just because its harder to fly doesn't mean it needs to be made easier. You just have to adapt to its quirks.

The bouncy nose is not accurate for any aircraft be it prop or jet. I think adverse yaw is over modeled and really shouldn't even be present with a modern aircraft with flight control computers. It should be much easier to trim in pitch for stable flight. I'm hoping it is still very much a work in progress with tuning to come.

I'm using the curves at 5 FWIW since YoYo recommends none. I'll play with more curve and see what happens.

I'm very, very impressed with the Mig-21 and feel it may be the best of the DCS flight models yet.

Just because it is difficult for you to fly it doesn't mean that it is not realistic. This is among the first versions of the Su27 with its FBW system that is represented at a lot more realistic level than you are aware of. ED know what they are doing and have their sources. Just because its harder to fly doesn't mean it needs to be made easier. You just have to adapt to its quirks.

If this was how it was designed IRL it never would have been approved by Sukhoi to go into production. It feels like there are rubber bands attached to the nose and when you pitch down the band wants to pull it back up. When you roll the nose wants to bob up and down like it's being pulled by elastics.

Try the single mission "River Run" and see how stable the jet is when doing simple bank and pulls to get through the gates. Note: Increase your altitude in the mission editor prior to running the mission. I set 600m and and increased speed to 420.

Like I said I will play with the axis curves.

Just because it is difficult for you to fly it doesn't mean that it is not realistic. This is among the first versions of the Su27 with its FBW system that is represented at a lot more realistic level than you are aware of. ED know what they are doing and have their sources. Just because its harder to fly doesn't mean it needs to be made easier. You just have to adapt to its quirks.

But it also does not mean that if a/c is difficult to fly then it is modeled correctly..

I don't know what the whole fuss is about when people talk about landing Su-27.

My first attempt at landing her was almost perfect. Airplane reacted to my input as it should (read: as I expected). For the first time I could almost feel the aircraft under my fingertips, unlike the old flanker "on rails" which felt wrong. I'm flying with modified cougar with curves set for MORE sensitivity, to shorten the throw needed for medium to high control surface deflection... counter intuitive but works for me.

Having said that, I had my fair share of inverted-butt-flat-spins and stalls and what not.

But it also does not mean that if a/c is difficult to fly then it is modeled correctly..

But asking for a PFM+ aircraft model to be tweaked so that it flies "as expected" for desktop simmers is against the whole concept of a realistic flight model. At this point, most of us have never flown in a flanker or talked to flanker pilots. With their track record, I am confident ED has access to pilots and have more inside information than us. The flanker is a very unstable aircraft with its centre of gravity close to the rear, the aircraft has a natural tendency to pitch up in any airflow. This is what the FBW corrects. Trim is necessary to keep it neutrally in one line. The FBW on the other hand in F-16 is very stable...a lot more advanced than the early Russian FBW and hence that aircraft feels on rails compared to the flanker.

If this was how it was designed IRL it never would have been approved by Sukhoi to go into production. It feels like there are rubber bands attached to the nose and when you pitch down the band wants to pull it back up. When you roll the nose wants to bob up and down like it's being pulled by elastics.

 

Try the single mission "River Run" and see how stable the jet is when doing simple bank and pulls to get through the gates. Note: Increase your altitude in the mission editor prior to running the mission. I set 600m and and increased speed to 420.

 

Like I said I will play with the axis curves.

I remember reading that the FBW is very rudimentary on the modelled flanker version. this is remember the C version which is a 1989 version of the aircraft. The FBW is designed to bring the aircraft back to neutral position, which is why it could explain the aircraft trying to resist and reverse every move you make. However, I also remember yoyo mentioning, the flanker needs to be constantly trimmed to fly properly. The moment your altitude, speed changes, the trim has to be changed as well. Anyways, I will check out that mission and see if I face those same issues.

Just because it is difficult for you to fly it doesn't mean that it is not realistic. This is among the first versions of the Su27 with its FBW system that is represented at a lot more realistic level than you are aware of. ED know what they are doing and have their sources. Just because its harder to fly doesn't mean it needs to be made easier. You just have to adapt to its quirks.

this pretty much echos what was said to pilots of the F-15C that were so used to SFM and how it reacted to certain situations. etc.

A bit long video, but it shows the problems of rear COG flying. It shows some of the problems you have here. Violent pitch, too much pitch down will flip the plane, good high alpha characteristics...

It is a RC video, but it is very good explanation of pros and cons of rear COG.

Here's another example of a slow high alt loop to show how the system saves you and then what happens if you smash the controls with it off and don't bother saving it.

I have recovered a couple of these. I don't understand why the engines stop too.

flyingsaucer.trk

this pretty much echos what was said to pilots of the F-15C that were so used to SFM and how it reacted to certain situations. etc.

A bit long video, but it shows the problems of rear COG flying. It shows some of the problems you have here. Violent pitch, too much pitch down will flip the plane, good high alpha characteristics...

 

It is a RC video, but it is very good explanation of pros and cons of rear COG.

 

Thanks. Its good to get some validation on the aspect of rear COG and how that affects flight. My fav flight documentary is the Su-27: the best fighter in the world. Check it out on youtube and you will notice this same issue of the pitch up tendency of the flanker and the evolution of the FBW system on how it corrects it. Its dumbed down for the public, but gives an idea.

Here's another example of a slow high alt loop to show how the system saves you and then what happens if you smash the controls with it off and don't bother saving it.

 

I have recovered a couple of these. I don't understand why the engines stop too.

I know there are certain parts of the flight envelope where the engines just shut down. It could be negative pressure of the fuel (sorry...don't quote me on it...Its something I am guessing), but that is a realistic simulation of the engines. However, I know something where it has been said that at the time the flanker was introduced, none of the other western jets could do something like the tail slide because the engines would shut down. I heard that in a documentary somewhere.