Su-27 Flight Model Discussion

[IonicRipper]

The Flanker is known for the incredible AoA it can pull. The way i see it... It probably wouldnt be able to do so if it was just another standard layout plane. To be able to pull these crazy AoA, it has to be drastically different to the other fighters and what people are complaining about are just the unfortunate downsides of having such a different layout.

Edited April 7, 2015 by IonicRipper

[Valium]

Ok, so what's the best way to trim this plane right?

How to do it?

 

I usually use 'reset trim' and 'stick to trimmer control mod' but I'm not even sure if I do it well.

[Ironhand]

Ok, so what's the best way to trim this plane right?

How to do it?

 

I usually use 'reset trim' and 'stick to trimmer control mod' but I'm not even sure if I do it well.

Trim nose down before takeoff--I use about 4 or 5 "clicks". After takeoff, add more nose down trim as your speed increases. Once you are at a constant airspeed, trim for that airspeed (well, AoA, actually). That's how I get from point A to point B. In a fight, my trim is whatever it is--usually a significant amount of nose down trim--and I don't worry about it again until the flight home.

 

When slowing for landing, I just apply trim in the opposite direction. I have the trim functions mapped to a hat switch. So changing trim is fairly easy. Most of the time I don't even notice that I'm doing it.

 

[EDIT]Just want to add that, once you're at transonic airspeeds, trimming becomes unnecessary.[/EDIT]

Edited April 7, 2015 by Ironhand

[Valium]

Ok, thanks for these words. It's good to know it.

 

I understood that before taking off, I should use 'trimming down' about 4-5 times and after taking off, I should use it again while the speed will be increasing. I'll check it.

 

What should I do during constant air speed? Please, tell me what AoA means.

 

Now I have to check it and see how it works.

 

Would you be able to tell me what 'trim reset' ( yeah, I know it'll probably reset... :D ) and 'stick to trimmer control mod' mean and how to use them?

 

 

I used to play LOFC2 with VNAO mod. There was Super Hornet. I had problems with trimming but using 'trim reset' seemed to be good way. Now, I see that it looks completly different in DCS FC3.

 

I've never known how to do it in Su-27, so usually before take off I use 'stick to trimmer control mod' and later use 'trim reset' many times if only the air speed changed a little bit. I wondered how to do it well because flying for point A to B was too hard for me without autopilot - I couldn't stand holding joystick trough over 2 hours.

 

Btw, I don't understand how autopilot in Su-27 works.

I usually use Alt+1 at first, after that I use 'H' and it usually works fine. I use Alt+9 for turn it off. I don't know if you've ever noticed it but sometimes after switching autopilot on, the plane goes higher and higher ( 100 m to a few kilometers ) and starts to fall instantly.

 

 

 

Can you tell me what your joystick' settings for Su-27 looks like?

[Ironhand]

...

 

What should I do during constant air speed? Please, tell me what AoA means.

Once you have settled in to a constant airspeed, just trim either up or down until a very light pressure on the stick keeps you in level flight (even if you let go of the stick, you should only gain or lose altitude slowly). Because the FBW's airspeed control law mimics positive stability, you'll find that the aircraft slowly compensates for the gain or loss in altitude--it'll carve something similar to a sine wave through the air that grows increasingly smaller in amplitude as the aircraft slowly regains the original AoA (angle of attack) that you had dialed in.

 

 

...

 

Would you be able to tell me what 'trim reset' ( yeah, I know it'll probably reset... :D ) and 'stick to trimmer control mod' mean and how to use them?

I don't think that trim reset is even functional in FC3's Su-27. I could be wrong, though. For some reason I haven't tried to use it. I experimented with stick to trimmer and mentioned it earlier in this thread: Stick to Trimmer Control. It's not something I'd recommend using while in the air.

 

 

.....

 

Btw, I don't understand how autopilot in Su-27 works.

I usually use Alt+1 at first, after that I use 'H' and it usually works fine. I use Alt+9 for turn it off. I don't know if you've ever noticed it but sometimes after switching autopilot on, the plane goes higher and higher ( 100 m to a few kilometers ) and starts to fall instantly.

Since the Flanker got the Professional Flight Model, the autopilot has behaved oddly. I simply don't use it myself.

 

 

 

Can you tell me what your joystick' settings for Su-27 looks like?

I use a worn and abused X-52. My axes are set with a dead zone of 2 or 3 and a curve of around +20, IIRC, a least for the longitudinal axis. I think the curve for the other may be less.

Edited April 7, 2015 by Ironhand

[WildBillKelsoe]

It does function (you can try any trim and then trim reset, you'll see trim neutral lights reappear. In mig-21 however, I find that trim resetting is absent unless you manually set it so lights come on back.

[Rikus]

Here's URL to our site, where there is a screen with my settings axes for Cobra M5.

 

It is common settings such as airplanes: Su-25(T), Su-27/33, MiG-29A/C, F-15C, P-51D (TF-51) (this aircraft on which I fly aerobatics and combats (WVR/BVR)).

 

If the length of the stick <250 mm, for the precise control necessarily requires tuning curves.

 

- Hi, i have a Saitek X55, what curves should i use?

 

- Do you use neutral trim for aerobatics?

 

Thanks for the info!!

Edited April 7, 2015 by Rikus

[IonicRipper]

Rikus: My suggestion would be to use a curve of 15 for the pitch axis and neutral for roll axis.

[DarkFire]

I'm starting to wonder if the joysticks we all use are part of the problem. I'm guessing to some extent, but I can't think of a single consumer-grade joystic that doesn't have some sort of self-centring mechanism be it springs, rubber bands or whatever.

 

Look at videos of the real Su-27 that include the cockpit: the control colum doesn't appear to self-center. The way in which the trimmer appears to work also impinges on this: in the real Su-27 the trimmer appears to apply a given stick deflection in order to apply trim, something that's actually incredibly difficult to achieve in an intuitive way with a joystic that's self-centring.

 

I suspect that the same issue may be the root cause of so many issues that new pilots have with the Ka-50 and the way in which the 'autopilot' assists control inputs.

 

Thoughts?

[Ironhand]

I'm starting to wonder if the joysticks we all use are part of the problem. I'm guessing to some extent, but I can't think of a single consumer-grade joystic that doesn't have some sort of self-centring mechanism be it springs, rubber bands or whatever.

 

Look at videos of the real Su-27 that include the cockpit: the control colum doesn't appear to self-center. The way in which the trimmer appears to work also impinges on this: in the real Su-27 the trimmer appears to apply a given stick deflection in order to apply trim, something that's actually incredibly difficult to achieve in an intuitive way with a joystic that's self-centring.

 

I suspect that the same issue may be the root cause of so many issues that new pilots have with the Ka-50 and the way in which the 'autopilot' assists control inputs.

 

Thoughts?

Somewhere, if not in this thread, then in one of the many others concerning the flight model and need for constant trimming, someone noted that they had taken their dysfunctional joystick apart and removed the centering mechanism. They then, somehow, tightened things down so that friction held the stick's longitudinal axis steady. The stick no longer "centered" and the bird flew like a dream. Wish I could find it but it was a single post buried in a thread and it would be interesting to reread. Wondering if I'm remembering it correctly.

[Rikus]

Rikus: My suggestion would be to use a curve of 15 for the pitch axis and neutral for roll axis.

 

Ok thanks, i´ll try.

 

Cheers

[IonicRipper]

Actually, at least for manually controlled planes, there is a centering force caused by the airflow over the control surfaces. I believe the su27 recreates these forces artificially to give a feeling of control to the pilot.

[Bourrinopathe]

[…] BTW, the 200 km/hr (300 km/hr above 6000 m) figure you cite isn't the stall speed, it's the minimum speed for sustained level flight from the viewpoint of controllability. Below that airspeed, if you sneeze, bad things might happen.

Thanks for the correction, Rich. At first I was expecting the aircraft to vibrate before falling abruptly.

 

[…]

I've seen varying citations for the Su-27's critical AoA ranging from 23-33 degrees. Beyond the critical AoA, the wing stops producing lift and bad things happen. I do notice, however, that the real world manual cites 24 degrees as a permissible AoA at .5 Mach for aircraft with nothing hanging off her wings.[…]

 

The current PFM is deliciously close to the real behavior then. As it's not a scripted performance, I can only imagine how difficult it should be to achieve that level of accuracy with so many variables.

 

I have no doubt ED won't use the term "professional" lightly when it comes to the heart of an aircraft simulation (PFM). Even if it's still in beta, most of the flight envelope should be polished enough and close to reality.

Of course, we should not be able to pull a looping at Mach 1.2+ (using direct control) without being immediately smashed into a pulp and seeing the poor Flanker sprayed like a giant chaff, but it's certainly temporary as ED is still working on the supersonic part of the simulation (F/A-18C).

 

If anyone is interested, I tried to generate two T-O performance graphs from the Su-27SK Flight Manual. It's not 100% accurate as the source material is definitely "old-school":

 

 

Refreshed version (using pixels measurements to avoid too much "reinterpretation"):

Blue is for military thrust / Orange is for AB.

Not super-smooth, but it should be accurate enough to run some tests. ;)

 

(what I'd like to compare is the rotation performance in relation with the T-O run - no idea how useful those graphs could be for our current Su-27S)

[MrTUX]

Well i am joining this discussion as a Complete ignorant, but I have experienced weird things in the su27 PFM .

 

For example at higher altitudes(8/9km )if I try maneuver too much / dogfight i often end up in a inverted flat spin .

When i try the "instant action" mission for the su27 and dogfight the F5s guns only I'm always on the edge of a deadly stall (i recover 1 out 5 tries ).

 

I probably got used to the old FM, i agree

but when i switch the F15, which is also PFM , i rarely go into stall( when it does happen, it is more predictable) and i never found myself into inverted flat spins.

the f15 just feels more reliable (no heavy trim needed, sudden spins etc..) and i can kind of predict its behaviour in air.. in the sukhoi instead sometimes i feel like I get slapped out of nowhere :helpsmilie:

 

Now as I said I am no expert, I only have a vague idea on how these planes should fly IRL, probably an incorrect one :D

Edited April 8, 2015 by MrTUX
grammar

[esb77]

Somewhere, if not in this thread, then in one of the many others concerning the flight model and need for constant trimming, someone noted that they had taken their dysfunctional joystick apart and removed the centering mechanism. They then, somehow, tightened things down so that friction held the stick's longitudinal axis steady. The stick no longer "centered" and the bird flew like a dream. Wish I could find it but it was a single post buried in a thread and it would be interesting to reread. Wondering if I'm remembering it correctly.

 

I remember what you're talking about, no idea where it is though. There was also someone who built a simple hall effect stick out of some scrap wood and a large dowel that was friction based instead of sprung, and they found it very useful for some of the more trim intensive aircraft.

 

http://forums.eagle.ru/showthread.php?t=134618&highlight=wood+stick+hall is the wooden stick project. Couldn't find the other one.

[ShuRugal]

 

http://forums.eagle.ru/showthread.php?t=134618&highlight=wood+stick+hall is the wooden stick project. Couldn't find the other one.

 

http://forums.eagle.ru/showthread.php?t=50522

[Dell_Murrey-RUS]

- Hi, i have a Saitek X55, what curves should i use?

 

I don't know. :)

If this joystick linears axis, then you can use the values of 25-27 roll and pitch. For yaw axis values ~34.

If it's non-linear, then the values of the curves can be learned only by experience.

 

- Do you use neutral trim for aerobatics?

 

Su-27 (PFM) sets the balance for the speed 750 km/h (it's minus 2/5 pitch from the virtual stick travel).

Other aircraft from the Flaming Cliffs 3 with SFM neutral trim. :)

 

 

For example at higher altitudes(8/9km )if I try maneuver too much / dogfight i often end up in a inverted flat spin .

 

It's just your skill and knowledge.

At altitudes above 7000 m supposed to use tactics verticals.

Close combat maneuvers at high altitudes is not effective. Only verticals. Only high speed. Because 'low speed = death'.

 

When i try the "instant action" mission for the su27 and dogfight the F5s guns only I'm always on the edge of a deadly stall (i recover 1 out 5 tries ).

 

Close combat maneuvers at low speeds only says that you do not understand what is reality close combat on fighter Su-27.

Hi altitudes = high speed (not less than 0.85 M).

Low altitudes = maneuvers at high angular velocities (not to be confused with the angle of attack).

[Rikus]

If this joystick linears axis, then you can use the values of 25-27 roll and pitch. For yaw axis values ~34.

 

If it's non-linear, then the values of the curves can be learned only by experience.

 

Su-27 (PFM) sets the balance for the speed 750 km/h (it's minus 2/5 pitch from the virtual stick travel).

 

I don´t know if my joystick is linear or non-linear :(

 

For the pith, when you say minus 2/5 pitch, i understand you mean between 2 and 5 clicks in the POV to the front?

 

Thanks

[Valium]

Once you have settled in to a constant airspeed, just trim either up or down until a very light pressure on the stick keeps you in level flight (even if you let go of the stick, you should only gain or lose altitude slowly). Because the FBW's airspeed control law mimics positive stability, you'll find that the aircraft slowly compensates for the gain or loss in altitude--it'll carve something similar to a sine wave through the air that grows increasingly smaller in amplitude as the aircraft slowly regains the original AoA (angle of attack) that you had dialed in.

 

 

 

I don't think that trim reset is even functional in FC3's Su-27. I could be wrong, though. For some reason I haven't tried to use it. I experimented with stick to trimmer and mentioned it earlier in this thread: Stick to Trimmer Control. It's not something I'd recommend using while in the air.

 

 

 

Since the Flanker got the Professional Flight Model, the autopilot has behaved oddly. I simply don't use it myself.

 

 

 

 

I use a worn and abused X-52. My axes are set with a dead zone of 2 or 3 and a curve of around +20, IIRC, a least for the longitudinal axis. I think the curve for the other may be less.

 

 

I've got Logitech Attack 3 and I set dead zone on 10 and curve around 25.

 

I checked what you'd written before. I began to trim the nose down and it works well but I've got a few questions to you about it. What's more, it solved my problem with using auto pilot.

 

- when your airspeed is about 1000 kph, how many times you press the button which is responsible for trimming nose down and how HUD should look like ( I mean degrees )?

- if I use trimming properly, how many degrees should be on HUD ( less than 0 / more than 0 / just 0 )?

- if you use trimming, you still hold the joystick or you don't ?

- let's suppose that the air speed is about 1100 kph ( without afternurning ) and the plane is trimmed well and suppose that I begin to go to the top suddenly ( my plane becomes to be slower - let's say 600 kph ) - do you trim the nose up at this moment or do nothing ? I'm asking about it because I did something like this a few times, I had still nose trimmed down, so I did nothing and my plane starts to fall instantly and that made my plane lay on its back and it was outta control.

 

 

I checked trimming nose up - you wrote I should use it during landing.

Well, I'm not sure if it's safety. I approached with 320 kph and I damaged gears. I imply to use 'trim reset' after set the gears and flaps down.

 

 

 

Tell me if something's incomprehensible.

[Ironhand]

I've got Logitech Attack 3 and I set dead zone on 10 and curve around 25.

Have you flown with less of a dead zone? Mine is set to 2 or 3 because there's a slight flutter visible when I'm in that screen. The dead zone is simply to make sure the sim doesn't see and interpret that flutter.

 

I checked what you'd written before. I began to trim the nose down and it works well but I've got a few questions to you about it. What's more, it solved my problem with using auto pilot.

 

- when your airspeed is about 1000 kph, how many times you press the button which is responsible for trimming nose down and how HUD should look like ( I mean degrees )?

Glad trimming down before takeoff helped. :) By the time I reach something like 1000 km/hr I'm already fairly well trimmed. In my experience, after about 900 km/hr there's actually no need for further trimming because the aircraft seems to lock in with very little variation in pitch as speed increases. But that's my experience.

 

 

- if I use trimming properly, how many degrees should be on HUD ( less than 0 / more than 0 / just 0 )?

[EDIT]That'll depend on your airspeed. I don't normally pay much attention to it most times.[/EDIT] But, honestly, if you are trimmed properly, you'll know it because you are no longer fighting the stick. If you take you hand off of it, you will only very slowly change altitude.

 

 

- if you use trimming, you still hold the joystick or you don't ?

Depends on how well I'm trimmed and how stable my airspeed is. I've been known to walk into the kitchen and return with a fresh cup of coffee to find that I've only gained or lost about 100 meters altitude while I was gone. [EDIT] But I do hold onto the joystick while I'm trimming, if that's the question.[/EDIT]

 

 

- let's suppose that the air speed is about 1100 kph ( without afternurning ) and the plane is trimmed well and suppose that I begin to go to the top suddenly ( my plane becomes to be slower - let's say 600 kph ) - do you trim the nose up at this moment or do nothing ? I'm asking about it because I did something like this a few times, I had still nose trimmed down, so I did nothing and my plane starts to fall instantly and that made my plane lay on its back and it was outta control.

If I'm understanding the question correctly, I'd add some nose up trim if I was trimmed for the initial high airspeed. But, if you are climbing steeply, never, never, never, never (get the idea? :) ) simply push the stuck forward to level out. That puts high negative G's on the airframe and you will be on your back no matter how you're trimmed. Anytime you want to level off from a steep climb, roll inverted and pull the stick back. This allows you to make a quick transition to level flight under positive G's (which your aircraft will thank you for). Then roll upright into level flight. Anytime you push that stick forward, do it gently...very gently.

 

 

I checked trimming nose up - you wrote I should use it during landing.

Well, I'm not sure if it's safety. I approached with 320 kph and I damaged gears. I imply to use 'trim reset' after set the gears and flaps down.

 

By the time your wheels touch down, you should not be flying at more than 290 km/hr. If you are at about 50% fuel and no weapons left on the wings, you should probably be closer to 250 km/hr. That's what I aim for, anyway.

Edited April 9, 2015 by Ironhand

[Dell_Murrey-RUS]

I don´t know if my joystick is linear or non-linear :(

 

I don't use this joystick. I don't know its characteristics.

Use search in google or read the documentation to the joystick. :)

 

For the pith, when you say minus 2/5 pitch, i understand you mean between 2 and 5 clicks in the POV to the front?

 

Ehmm... no. This fractional calculus.

Tuning visual.

See the picture attached. ;)

[Ironhand]

I don't use this joystick. I don't know its characteristics.

Use search in google or read the documentation to the joystick. :)

 

 

 

Ehmm... no. This fractional calculus.

Tuning visual.

See the picture attached. ;)

Rikus, just in case you don't already know this, you pull up that screen Del=Murrey-Rus is using with the RCtrl+ENTER key combination.

 

Del=Murrey-RUS, that's helpful. I've never taken the time to figure out how much trim was required for what specific airspeed. And, if you're using the 750 km/hr setting, there's not much trimming required if your airspeed increases, since additional trimming seems unnecessary at speeds higher than about 900 km/hr.

[Valium]

By the time your wheels touch down, you should not be flying at more than 290 km/hr. If you are at about 50% fuel and no weapons left on the wings, you should probably be closer to 250 km/hr. That's what I aim for, anyway.

 

 

Thanks for help man.

 

I mean that I approached at 320 km/hr and I damaged gears but it was when gears touched the ground and it was because of the fact that the nose was too high.

 

I saw one video on you tube from real Su-27. They usually give the gears down even at 420 km/hr.

[Ironhand]

Thanks for help man.

 

I mean that I approached at 320 km/hr and I damaged gears but it was when gears touched the ground and it was because of the fact that the nose was too high.

 

I saw one video on you tube from real Su-27. They usually give the gears down even at 420 km/hr.

Ahh.. OK. If your nose is that high at 320 km/hr, then I think you must be coming in too shallow...or flaring too much just before touching down. IF you'd like, post a track of your landing and I'll take a look at it.

 

IIRC, the gear can go down anywhere under 500 km/hr. I think that's the real life cutoff point.

 

[EDIT]I'm very light in this example, but if you fast forward to the end, you'll see what things look like in my HUD (and from the outside as well) as I'm landing:

 

 

[/EDIT]

Edited April 9, 2015 by Ironhand

[IvanK]

The manual description talks about the SU27 AFCS also providing G limit protection (and AOA protection)

 

In DCS this does not happen and you can pull as much G as IAS permits.