Pitching up at slow speeds

[karambiatos]

I understand the aircrafts FBW system mimics a stable aircraft, but why does the 27 keep pitching up even as its about to stall, where a stable aircraft would start pitching downward.

 

Track shows what happens at slow speed (recorded in 1.2.12)

27Pitch.trk

[Thumper1606688436]

Different airframes have different static & dynamic stability, some positive, some negative and the rest neutral.

 

The SU-27 PFM implies positive static stability.

 

But modern fighter jets are inherently unstable and FBW only helps to make the aircraft more stable in flight but it doesn't completely remove it.

[Weta43]

Different airframes have different static & dynamic stability, some positive, some negative and the rest neutral.

 

The SU-27 PFM implies positive static stability.

 

But modern fighter jets are inherently unstable and FBW only helps to make the aircraft more stable in flight but it doesn't completely remove it.

 

IRL the Su-27 is not inherently unstable, it just has relaxed stability ...

[Ironhand]

... (recorded in 1.2.12)

I have V1.2.11 installed and, in viewing the track, saw what I expected to see, once I saw you input some trim up. The nose rises and, as the airspeed drops, the critical AoA is exceeded and the wing begins to stall. The nose drops, airspeed increases and, once the airspeed is more or less correct for the AoA you set, the nose starts to rise and you end up in the expected phugoid oscillation. Given enough time and altitude, things would probably even out. But perhaps this plays differently viewed in 1.2.12.

[Nedum]

The new Su27 feels plain wrong for me.

There is always a littel bit "Corba" in every maneuver.

Even if the Su27 is stalling, she is doing a little cobra.

Fly a Loop and wait till the "low speed" warning is coming and the Su27 stalls with the nose down. Now pull the stick a little bit to you and the Su27 gets more speed but if you hold this little stick input a little bit to long, the Su27 gets another stall in miliseconds with the nose full up to the sky.

I can't understand why this little nose can produce so much drag to flip the whole AC around.

The Su 27 eats speed with ever tiny little stick input. I can't believe that this "modern" AC is such a beast.

The engines are the other weak point I have to fight with. It feels like full power or no power, nothing between. With every little stick input the Su27 lost more speed than evey other AC in DC and the engines do nothing to compensate this. All or nothing.. very strange.

As I said before... the Su27 feels wrong for me.

Edited December 3, 2014 by Nedum

[laugen]

To me it feels just right.

 

Speaking of my nonexistent real experience of course.

[theropod]

i think it feels right for me too.

flanker's this behaviour more like delta wing behaviour.

AoA position increases on low speeds and can be deeper if you are carrying weapons and that makes us not to fly on low speed with weapons

su-27 have tendance to pitchup and perform cobra but asc blocks it not to do.

maybe similar behaviour for f-18e and suchlike aircrafts because of their shape.

just feelings:)

Edited December 3, 2014 by theropod

[OnlyforDCS]

To me it feels just right.

 

Speaking of my nonexistent real experience of course.

 

Well said :lol:

[karambiatos]

I have V1.2.11 installed and, in viewing the track, saw what I expected to see, once I saw you input some trim up. The nose rises and, as the airspeed drops, the critical AoA is exceeded and the wing begins to stall. The nose drops, airspeed increases and, once the airspeed is more or less correct for the AoA you set, the nose starts to rise and you end up in the expected phugoid oscillation. Given enough time and altitude, things would probably even out. But perhaps this plays differently viewed in 1.2.12.

I just centered the trim.

yes thats whats pretty much happening, however, the oscillation wont even out if i give it some throttle, the only reason it may seem like it is because its not regaining as much speed as in the beginning of the track.

 

It keeps pitching up and increasing AoA at really slow speeds, and with that gaining altitude until it stalls, then goes back down, and the process repeats it self, until it falls out of the sky.

 

if you look at the FBW pitch stability, (rctrl + enter) it doesn't vary the amount of push its giving the aircraft depending on speed, its linear and on the virtual joystick display (rctrl + enter ) it's always set at one spot, irregardless of the aircraft's speed.

[Ironhand]

I just centered the trim.

yes thats whats pretty much happening, however, the oscillation wont even out if i give it some throttle, the only reason it may seem like it is because its not regaining as much speed as in the beginning of the track.

 

It keeps pitching up and increasing AoA at really slow speeds, and with that gaining altitude until it stalls, then goes back down, and the process repeats it self, until it falls out of the sky.

 

if you look at the FBW pitch stability, (rctrl + enter) it doesn't vary the amount of push its giving the aircraft depending on speed, its linear and on the virtual joystick display (rctrl + enter ) it's always set at one spot, irregardless of the aircraft's speed.

I'll have to watch the track again when I have a chance but...the only time you stalled the wing was in the initial pitch up. You never exceeded the critical AoA again. In fact, the AoA remained at 8 or 9 degrees throughout the oscillations and never changed. That was the trim you entered and it should have been trying to settle in on the corresponding airspeed for level flight with the given AoA.

 

As for the rest, I'm not sure what it should be doing. I've read and reread the corresponding section in the the manual and, in some respects, I think it matches exactly what I see in the sim. In other instances, not. Just haven't had the time to explore it further.

 

Rich

[Kwiatek]

The new Su27 feels plain wrong for me.

There is always a littel bit "Corba" in every maneuver.

Even if the Su27 is stalling, she is doing a little cobra.

Fly a Loop and wait till the "low speed" warning is coming and the Su27 stalls with the nose down. Now pull the stick a little bit to you and the Su27 gets more speed but if you hold this little stick input a little bit to long, the Su27 gets another stall in miliseconds with the nose full up to the sky.

I can't understand why this little nose can produce so much drag to flip the whole AC around.

The Su 27 eats speed with ever tiny little stick input. I can't believe that this "modern" AC is such a beast.

The engines are the other weak point I have to fight with. It feels like full power or no power, nothing between. With every little stick input the Su27 lost more speed than evey other AC in DC and the engines do nothing to compensate this. All or nothing.. very strange.

As I said before... the Su27 feels wrong for me.

 

 

For me Su27 also feels strange. She is doing little cobra also with negative angle of attack - when you push stick forward. Plane is making negative G cobra then go on its back and get inverted depart.

 

Tried the same with F-15 but it stall in classical way or automatic stabilisation don;t allow such things.

 

Su27 behave strange in such sitution like automatic stabilisation dont work correctly.

[chuck04]

- when you push stick forward. Plane is making negative G cobra then go on its back and get inverted depart.

 

 

Very true, i killed myself many times with this behavior, unbelievable, a very small input at low to medium speed forward and bang !! As if the fly by wire control loop system had suddenly disengage.

It doesn't fall very quickly in that position but to get out of it you must pull the stick to gain speed toward the ground... need to be at a certain altitude when it happend. Needless to say in a fight you're dead.

[Nedum]

Try to do a simple barrel roll.. .you will get thick eyes.... there is always a little bit "Cobra"... !

With no other plane in DCS I can get G warnings and a stall with doing a simple barrel roll.

I would like to see this simple maneuver in a track from the "test pilots" at low speed. A simple clear barrel roll. With the F15c I can do this with ease and fully loaded weapons and tanks. With the Su27 it's a hard thing even without any load.

There is always this "Cobra" behavior... EXTREM strange for me.

Every WWII bird is more easy to fly as the Su27 now and I can't believe that this is like it should be. The F15C in DCS is lightyears ahead of the Su27.

I've had never before so much stalls with an airplane, like with the Su27 yet.

Every manuever must be flown with milimeter inputs at the controls, a milimeter to much and you stall. That can't be right! And the strange engine behavior is the point over all of this.

If you was only one time in idle postion you have to go at Edit: (89 were false) 80% thrust and have then to reduce in between 72% and 80% for sensitive inputs. If you are under 72% you must go above 78% to get some thrust, If not you will not accelerate even you are at 78% thrust after your thrust was a second on idle. An extrem strange engine behavior too.

Edited December 4, 2014 by Nedum

[theropod]

Try to do a simple barrel roll.. .you will get thick eyes.... there is always a little bit "Cobra"... !

With no other plane in DCS I can get G warnings and a stall with doing a simple barrel roll.

I would like to see this simple maneuver in a track from the "test pilots" at low speed. A simple clear barrel roll. With the F15c I can do this with ease and fully loaded weapons and tanks. With the Su27 it's a hard thing even without any load.

There is always this "Cobra" behavior... EXTREM strange for me.

Every WWII bird is more easy to fly as the Su27 now and I can't believe that this is like it should be. The F15C in DCS is lightyears ahead of the Su27.

I've had never before so much stalls with an airplane, like with the Su27 yet.

Every manuever must be flown with milimeter inputs at the controls, a milimeter to much and you stall. That can't be right! And the strange engine behavior is the point over all of this.

If you was only one time in idle postion you have to go at 89% thrust and have then to reduce in between 72% and 80% for sensitive inputs. If you are under 72% you must go above 78% to get some thrust, If not you will not accelerate even you are at 78% thrust after your thrust was a second on idle. An extrem strange engine behavior too.

 

it is still in beta and aircraft's stabilization is getting better and better with updates.

there is no comparable dcs aircraft with su-27 which was released before.

if you are comparing with f-15, i think they are not comparable because of their differenet airframes and different flight Dynamics.

i guess ,only their duty is same or similar.

su-27 has unseen Flight Dynamics for a sim and we are still too stranger for this.

for example we learned to fly with using trim very often:)

we will see when it is finished.

Edited December 4, 2014 by theropod

[Beagle One]

I simply can't get the Su-27 in 1.12 to fly straight. It lacks trim authority to keep the nose down. its always rising and even the autopilot cant keept it straight and I do not believe this is intended and the way a Su-27 flies....to keep it straight at 1000 km/h I have to push the stick full forward all the time.

[Ironhand]

I simply can't get the Su-27 in 1.12 to fly straight. It lacks trim authority to keep the nose down. its always rising and even the autopilot cant keept it straight and I do not believe this is intended and the way a Su-27 flies....to keep it straight at 1000 km/h I have to push the stick full forward all the time.

Interesting. I just completed some test flights and had no trouble trimming from low speeds up through 1000+ IAS. One thing I noticed that's now different is that, if you start on the runway in V1.2.12, your flaps are extended and need to be retracted. That wasn't by any chance what was going on? But, if they were out all that time, there should be some damage modeling issues. So probably not.

 

Rich

[Kwiatek]

I simply can't get the Su-27 in 1.12 to fly straight. It lacks trim authority to keep the nose down. its always rising and even the autopilot cant keept it straight and I do not believe this is intended and the way a Su-27 flies....to keep it straight at 1000 km/h I have to push the stick full forward all the time.

 

Turn off force fedback in game options.

 

Unfortunaley from unknown reason trim in jet planes ( Su27 or F-15) dont work if force fedback is on.

[Beagle One]

Problem was in my side, the centering of the HOTAS went wrong after a reboot.

Edited December 4, 2014 by Beagle One

[theropod]

Turn off force fedback in game options.

 

Unfortunaley from unknown reason trim in jet planes ( Su27 or F-15) dont work if force fedback is on.

 

i had this observation when su-27 first released.

i didnt close ffb from menu but i unchecked swap axis and decreased trimmer force to %30 and aircraft became very stable on flight.

before that correction my flight experience was only pushing the stick forward to gain speed:)

[Kwiatek]

i had this observation when su-27 first released.

i didnt close ffb from menu but i unchecked swap axis and decreased trimmer force to %30 and aircraft became very stable on flight.

before that correction my flight experience was only pushing the stick forward to gain speed:)

 

How exacly you did it? Casue i tried different things with FF ON and trim dont work anyway. Only disable FF casue trim to work :(

[theropod]

How exacly you did it? Casue i tried different things with FF ON and trim dont work anyway. Only disable FF casue trim to work :(

i did it from options/su-27/conrols/ axis commands. select pitch and you can change it from FF tune.

i tried different stettings but unchecking swap axis and decreasing trimmer force worked for me.

this settings solved other aircrafts stabilization too.

for example su-25:

after i take off ,aircraft always wanted to turn left because of trim force(aileron or rudder) and this settings corrected that problem.

Edited December 4, 2014 by theropod

[Kwiatek]

I tried your settings but it dont work for me :(

 

I tried trim force 30% or even 0 %, got unchecked swap axis but still trim dont work in Su27.

 

Only with FF off trim work ok.

[Weta43]

What sort of stick ? I have a MSFFB II & it works correctly.

If it's a different stick, it may be a bug, same stick - probably settings...

[Kwiatek]

Saitek Evo Froce

[LJQCN101]

Let me clarify something about the AoA limiter.

 

The AoA limiter of Su-27 will only limit the amount of stick input to a lower value. It doesn't have the authority to command nose down (stabilizer trailing edge down). The aircraft can still overshoot the 28-deg AoA limit at low speeds with stick in neutral position.

 

 

 

It's easy to conduct an 1g-stall test to evaluate the performance of AoA limiter. Here's the test result of F-16 at an initial flight condition of 0.45Mach and 9144m. (Tests conducted by NASA)

 

 

(δh>0 means stabilizer trailing edge down)

 

As we can tell from the graph, AoA is gently limited to 25 degs even with a full stick-back command. (See gcom curve)

 

The F-16 does pretty good in the test and the flight control system have the authority to command nose down. However Su-27 doesn't, and uncommanded pitch up is expected due to negative static stability. (Pitching moments increase with AoA builds up)

Edited December 5, 2014 by LJQCN101