Su27 Wing Thing

[ShuRugal]

Just passing through... I guess I don't understand this fascination with AoA. Max AoA has nothing to do with whether or not your wings tear off. It has everything to do with whether or not the wing stalls and you fall out of the sky like a rock with your wings still attached.

 

the higher the AoA, the more surface area the wings present to the airflow, resulting in greater force from the airflow on the wings. 1000 kph at 0 AoA, the only vertical force experienced by the wings come from airfoil shape altering the pressure above the wing. At 90 degrees AoA, the wings are now subject to several hundred tons of drag force (until they break, that is)

[jackmckay]

@lunaticfringe FAR 25 Aircraft components Ultimate FOS is between 1.5 - 2.5 depending on load margin and position. Killing airplane with FCS off is not possible or easy at least because no designer will ever allow that. By the way, operational margin avoids plastic deformation on structural components.

 

@Ironhand stalling wing at high mach is possible but its more likely that airflow drag force vector vertical component will induce high loads on structural components of wing first, relative to AoA.

[GGTharos]

Killing airplane with FCS off is not possible or easy at least because no designer will ever allow that. By the way, operational margin avoids plastic deformation on structural components.

 

That first statement is so un-true it's funny. There are a lot of aircraft out there that can be flown to destruction quite easily. Let's choose a very obvious one: U-2.

 

There are other aircraft that are very capable of taking very high g-loads, eg. F-16, F-15.

 

The second statement - your margin in DCS, IIRC, is about 1.3x. So if you're flying an aircraft that has a maximum safe load factor if 139/26=5.3g, your margin before deformation is 6.95g (in DCS, you probably just start losing 'hit points' as well as safe g limit, or both), and the breaking point at 1.5x is 8.0g.

[jackmckay]

That first statement is so un-true it's funny. There are a lot of aircraft out there that can be flown to destruction quite easily. Let's choose a very obvious one: U-2.

 

There are other aircraft that are very capable of taking very high g-loads, eg. F-16, F-15.

 

The second statement - your margin in DCS, IIRC, is about 1.3x. So if you're flying an aircraft that has a maximum safe load factor if 139/26=5.3g, your margin before deformation is 6.95g (in DCS, you probably just start losing 'hit points' as well as safe g limit, or both), and the breaking point at 1.5x is 8.0g.

 

U-2 has fuselage of F104 and very long wings with high L/D ratio because it is designed for HA Recon. It's not high maneuverability fighter plane and not same class. Su27 is G9+ fighter capable of very high G loads too as F15/F16, structural limit is G9*1,5FOS=13,5G BEFORE plastic deformation. Other factors like hardpoint weights inertia, wing fuel sloshing, elevator deflection increase loads on wingbox in respect to AoA and indicated airspeed and flying down the gravity vector decrease it. Its not that simple formula as you think and no G limit is reduced only load factors increased. You used formula for variation of the load factor with the bank angle during a coordinated turn, but case here is vertical turn down the gravity vector with turn radius indicated by AoA value as tangential deflection vector.

 

Guy in Eagle pulls so much Gs(30-40) but his plane remained in one piece:

 

You're telling me that Su27 wings are made of Swiss cheese and that they disintegrate at less than 10G. That's bullshit. Period.

 

[GGTharos]

U-2 has fuselage of F104 and very long wings with high L/D ratio

 

Hey, what do you know. Could there be a difference between Su-27 wings and say, F-15 wings? Not to mention GWs?

 

It's not high maneuverability fighter plane and not same class. Su27 is G9+ fighter capable of very high G loads too as F15/F16, structural limit is G9*1,5FOS=13,5G BEFORE plastic deformation.

 

No it isn't, and the proof is directly in the Su-27 flight manual. That's what was used to tune the damage model.

The F-15 doesn't do 13.5G before deformation either - it has been taken to 12.5G causing deformation that required writing off the airframe.

 

Its not that simple formula as you think and no G limit is reduced only load factors increased. You used formula for variation of the load factor with the bank angle during a coordinated turn, but case here is vertical turn down the gravity vector with turn radius indicated by AoA value as tangential deflection vector.

 

Not relevant. All you need is the load factor.

 

Guy in Eagle pulls so much Gs(30-40) but his plane remained in one piece:

 

12.5G for several seconds, because I read the actual report. Reaching 30-40G is not even physically possible. That aircraft was flown back but the airframe was deformed to the point that it was not possible to maintain, so it was written off. It's also not a Su-27 :)

 

You're telling me that Su27 wings are made of Swiss cheese and that they disintegrate at less than 10G. That's bullshit. Period.

 

I'm telling you're having a serious disconnect between your theory and something that's at least backed up by real design limitations.

[Ironhand]

the higher the AoA, the more surface area the wings present to the airflow, resulting in greater force from the airflow on the wings. 1000 kph at 0 AoA, the only vertical force experienced by the wings come from airfoil shape altering the pressure above the wing. At 90 degrees AoA, the wings are now subject to several hundred tons of drag force (until they break, that is)

 

:) Yes, I know that. And max force (lift) occurs at the critical AoA assuming the wings are still attached. OTOH, that chart that jackmckay cited from the Flanker manual states that it is in reference to stability and maneuverability rather than aircraft integrity as the 1) Max speed/Mach, 2) Max operational overload, and 3) Minimum negative G-force limitations are.

 

Of course, the manual further seems to confound things by noting that the stability and maneuverability in question is in reference ВПК (Взлётно-Посадочная Kонфигурация--Takeoff-Landing Configuration). Why you would be landing at Mach 2, I don't know. :) Unless, of course, "ВПК" is the abbreviation for something entirely different.

[probad]

this is getting ridiculous.

 

this thread is (has been) a personal issue, and only op himself can resolve it.

[jackmckay]

@GGTharos Then why overloaded su-27 didn't get chance to be written off at ground instead of instantly destroyed airborne?

 

30-40Gs was applied on airframe for a fraction of second (as mentioned in vid) and even to me it is still suspicious. USAF had flight recorders to check G spikes after all.

 

So how we can test it away from ED's (at my point of view) suspicious DCS su-27 coding? Should I make 3d detailed model and make some FEM/CFD structural test to see who's right or wrong in this case?

[GGTharos]

Then why overloaded su-27 didn't get chance to be written off at ground instead of instantly destroyed airborne?

 

Because it exceeded the breaking point.

 

30-40Gs was applied on airframe for a fraction of second (as mentioned in vid) and even to me it is still suspicious. USAF had flight recorders to check G spikes after all.

 

I know what the video says. The sensors read 12.5g for several seconds.

 

So how we can test it away from ED's (at my point of view) suspicious DCS su-27 coding? Should I make 3d detailed model and make some FEM/CFD structural test to see who's right or wrong in this case?

 

There's nothing suspicious about it. It breaks exactly where it should break according to certain assumptions made based on the safe overload limit formulae provided in the real Su-27SK manual.

 

The only suspicious things that I saw is g rising when the stick pressure was released. Without going back in time we cannot tell why this happened, we can only speculate.

 

As for FEM/CFD ... who's going to verify the results? The only CFD work I've seen here was backed up by comparison to empirical data of a similar object.

 

What we've got to go by is just some industry rules of thumb.

 

https://www.youtube.com/watch?v=rak2HldVp9M

[lunaticfringe]

@lunaticfringe FAR 25 Aircraft components Ultimate FOS is between 1.5 - 2.5 depending on load margin and position. Killing airplane with FCS off is not possible or easy at least because no designer will ever allow that. By the way, operational margin avoids plastic deformation on structural components.

 

I'm sorry- why are you quoting US FAA regulatory rules for the construction of Russian fighter aircraft? You should consult the FAVT.

 

And at the end of the day, why not quote the manufacturer's limitation sfrom three pages ago?

 

What's 130% of 6.5G?

 

Its where you broke the aircraft.

[DarkFire]

 

The stress / strain graph for high tensile steel and high-tensile aluminium alloys doesn't look like that. There's barely any plastic deformation before failure occurs. Insta-breaking wings is materially accurate.

[Ironhand]

There may be an issue still though - the G was increasing despite the stick having been moved forward.

 

So, the question here is what caused that?

 

The G and wing breakage is correct. The AoA is fine, too.

 

The only remaining issue is the increase in G. Is it an FM issue, is it a real aircraft issue, was it, as someone mentioned, a game freeze where the game continued interpreting the input incorrectly? (but then, why did it record the input correctly? :P )

Ummm...so I just got around to unlocking the external views on his track (SATAC 2016 mission-20160924-193008.zip) and the stick isn't released until just after the wings are beginning to break. Or are you referring to a different track?

[GGTharos]

Same track, and I posted screenshots from it in this thread. I turned on the controls display indicator - externals were not needed for me at least.

 

To clarify, you're right - the stick isn't released, but stick position is reduced by half or so well before the wings break IMHO.

[lunaticfringe]

Same track, and I posted screenshots from it in this thread. I turned on the controls display indicator - externals were not needed for me at least.

 

To clarify, you're right - the stick isn't released, but stick position is reduced by half or so well before the wings break IMHO.

 

Desaturate the Pitch Y channel to 65% (ie, 65% maximum stick deflection). Repeat the maneuver with a max pull, making certain to zero out roll once inverted. Repeat the maneuver in the horizontal, again, zeroing roll.

 

Breaks. 10 AoA. 9.2-9.5G. And it breaks at 55% deflection with any amount of roll still in play.

 

Take that as you will regarding the original track.

[Ironhand]

Same track...

...the stick... position is reduced by half or so well before the wings break IMHO.

Not in the track I viewed. He pulled back on the stick and started loading Gs. Then pulled back a bit more and held that position right up until the wings snap. An instant after they do, the stick snaps forward to the new position and G's reduce to 4.3. Not sure why the stick snaps forward but I'm assuming the G reduction is due to loss of lift from the wings as they snap. But, anyway, I'm not seeing any unwarranted buildup of Gs.

 

EDIT: BTW, the F2 view shows the reduction in Gs an instant before the G-meter in the cockpit does :EDIT ENDS

 

The attached images show the sequence. The external and immediate cockpit view were taken with the sim paused so that they show the same instant in time.

Edited October 14, 2016 by Ironhand

[DarkFire]

Not in the track I viewed. He pulled back on the stick and started loading Gs. Then pulled back a bit more and held that position right up until the wings snap. An instant after they do, the stick snaps forward to the new position and G's reduce to 4.3. Not sure why the stick snaps forward but I'm assuming the G reduction is due to loss of lift from the wings as they snap. But, anyway, I'm not seeing any unwarranted buildup of Gs.

 

EDIT: BTW, the F2 view shows the reduction in Gs an instant before the G-meter in the cockpit does :EDIT ENDS

 

The attached images show the sequence. The external and immediate cockpit view were taken with the sim paused so that they show the same instant in time.

 

Agree. Irrespective of stick movement (reduction in G is probably due to loss of wing surfaces) the pictures clarify nicely what the cause of the accident was:

 

Screen shot 4: At that speed 7.5G is deep in to the danger zone.

Screen shot 6: 8G. Aircraft is doomed.

 

Clear case of over-G resulting from too much pitch control input.

 

Edited to add:

 

The problem, and destruction of the wings, is easily reproducible - see the attached track files. In attempts 1-4 the wings broke at nearly identical G values: between 8.5 & 9. It is not however possible to break the wings without a continued pull once the nose has fallen through the horizon. Increasing the Gs via stick input is the only way to break the wings. With stick neutral the maximum observed G during the dive was 1.3 - 1.7.

 

Keeping the G at a maximum of not more than 8.0 makes the inverted dive & pull through survivable as shown in track 5.

 

Again, cause of crash: deliberate pilot input resulting in over-G condition.

 

Further edited to add: The observed conditions at the point of failure agree completely with the contents of the actual Sukhoi Su-27S manual, together with the 1.3-1.5 x safety margin that the design engineers appear to have built in, as shown in Ironhand's tests for another thread.

 

Conclusion: nothing wrong with the aircraft and the DCS Su-27 is empirically accurate to the real one.

 

/thread.

Edited October 14, 2016 by DarkFire

[lunaticfringe]

Concur with DarkFire

[Ironhand]

Concur with DarkFire

 

Ditto

 

EDIT: Given the various conversation points, I was expecting to see something anomalous somewhere. But nothing seemed out of line.

Edited October 14, 2016 by Ironhand

[SinusoidDelta]

Ditto

 

EDIT: Given the various conversation points, I was expecting to see something anomalous somewhere. But nothing seemed out of line.

 

Tritto...if that's a word.

[jackmckay]

Agree. Irrespective of stick movement (reduction in G is probably due to loss of wing surfaces) the pictures clarify nicely what the cause of the accident was:

 

Screen shot 4: At that speed 7.5G is deep in to the danger zone.

Screen shot 6: 8G. Aircraft is doomed.

 

Clear case of over-G resulting from too much pitch control input.

 

Edited to add:

 

The problem, and destruction of the wings, is easily reproducible - see the attached track files. In attempts 1-4 the wings broke at nearly identical G values: between 8.5 & 9. It is not however possible to break the wings without a continued pull once the nose has fallen through the horizon. Increasing the Gs via stick input is the only way to break the wings. With stick neutral the maximum observed G during the dive was 1.3 - 1.7.

 

Keeping the G at a maximum of not more than 8.0 makes the inverted dive & pull through survivable as shown in track 5.

 

Again, cause of crash: deliberate pilot input resulting in over-G condition.

 

Further edited to add: The observed conditions at the point of failure agree completely with the contents of the actual Sukhoi Su-27S manual, together with the 1.3-1.5 x safety margin that the design engineers appear to have built in, as shown in Ironhand's tests for another thread.

 

Conclusion: nothing wrong with the aircraft and the DCS Su-27 is empirically accurate to the real one.

 

/thread.

 

 

You got to kidding me. Your speed was (litlle bit) over 950 kmh IAS every time but I can live with that. Second, my planned maneuver was high speed descend at -30(-35)deg elevation (roll -180, pull -30deg, reduce trust, roll leveled, pull 30deg) without much energy build (I was entering combat engagement zone plunging at the deck) NOT SPLIT-S maneuver at 1350 kmh IAS on gravity vector G build at lower point of recovery. That's very much inconsistent, sorry.

[lunaticfringe]

You got to kidding me. Your speed was (litlle bit) over 950 kmh IAS every time but I can live with that. Second, my planned maneuver was high speed descend at -30(-35)deg elevation (roll -180, pull -30deg, reduce trust, roll leveled, pull 30deg) without much energy build

 

What you planned is an absolute nonfactor to this conversation; what matters is what you did- and what you did, as explicitly evidenced by the track you provided, was to pull in excess of the fatigue threshold for the airframe at your stated speed, as evidenced by both gauges and their intrinsic relationship with the airspeed and weight at which you were currently flying.

 

As a refresher to this conversation:

 

You have been shown, repeatedly, the same data direct from the manufacturer showing that you well exceeded the capability of the Su-27 to withstand overstress at the airspeed (M 1.26) and weight (in excess of 21,400 kg).

 

The track you provided, as outlined by Ironhand, showed explicitly that you still had the stick well aft of 65% at the point of the wings coming off- the exact point that just prior to him I showed that in *any* relationship between inertial coupling- gross, slight, or none, the airframe will disintegrate- in specific accordance to the relationship of AoA and G that was previously outlined- that 10 units of AoA at your stated speed and weight well exceeds the G limit as outlined by Sukhoi (6.5).

 

You were accurately refuted as to your claim that ED fails to calculate G correctly in relation with AoA at the speed given.

 

You were shown, repeatedly, the charts concerning your weight and load being well in excess of what the aircraft was designed for at the speed in question.

 

You were refuted as to the intent and translation of the charts you mentioned being in relation to AoA limits regarding stall, rather than structural limitations. And the specific relationship between Mach, altitude, AoA, and G was outlined. Twice.

 

You present US FAR regulations, rather than nation of regulations regarding the aircraft in question- regulations that Sukhoi, in fact, does not have to meet, because they're not under the auspices of the FAA for the construction of tactical aircraft.

 

You present material deformation data that, as previously stated, has absolutely nothing to do with the construction of the Su-27.

 

The only thing consistent in this conversation at this point is your continued denial of the facts at hand. Fault for this failure does not lay with ED. Not the laws of physics. Not Sukhoi. And not anyone here presenting the correct evidence to refute your wholly disjointed and entirely mediocre claims to the contrary. You broke the aircraft. Anything you say from this point forward in opposition to this amounts to the grasping of straws.

 

And as far as anyone with any sort of knowledge of the subject should be concerned, this matter is closed.

[jackmckay]

Take a look at these tracks.

Tracks.zip

[Frostie]

The stress / strain graph for high tensile steel and high-tensile aluminium alloys doesn't look like that. There's barely any plastic deformation before failure occurs. Insta-breaking wings is materially accurate.

 

[DarkFire]

Take a look at these tracks.

 

Track 1: During the descent you briefly went just over 8G --> airframe was fatigued. At the moment the wings broke the G meter spiked to over 9. Mach was about 1.1 --> well within the 'mach notch' on the safe G diagram.

 

Track 2: Just before the wing broke the stick input maxed out in positive pitch. Seconds later the G meter hit >9 which was again the reason for the airframe destruction. Speed was again over the Mach which puts it within the mach notch on the G safety margins diagram.

 

Track 3: Just after rolling inverted the G meter spiked to 8, at I think around 1.25M --> airframe was already weakened. When you pulled up the G meter spiked past 9G --> destruction of airframe. Speed was again well over 1.1M putting it in the danger zone of the G chart.

 

Track 4: Again just after you rolled inverted & began the descent the G meter spiked to 8. Couldn't see the mach meter but by the IAS it looked like it could have been in the region of 1.1M. During the pull up at ~1.25M the G meter spiked to 9 --> destruction of airframe.

 

Track 5: Again just after you rolled & pulled the G meter spiked at 8, at 1.2M. When you pulled up at just over 1.3M the G meter spiked at 9 and the wings broke at about 8.5. Again completely as expected.

 

Track 6: G meter didn't go in to the danger zone, hence no broken wings.

 

OK, let's try to fix this for you. I think I see what the problem is.

 

From watching the stick input v the in-game control column movement at 1/4 speed, I'm guessing that you don't use control axis curves? You need to start using them. In DCS the liner 1:1 input, with no curvature, is calibrated to the real aircraft. Thing is the real aircraft has a control column that's probably some 50cm long, whereas most of the joysticks we use don't, they're obviously much shorter.

 

Because of this short, sharp stick inputs result in disproportionately large excursions of the control surfaces, which in turn produces the large G spikes that are causing your wings to break. Try using curvature values of between 15 & 25 for both roll and pitch. This will produce much smoother inputs for you. Slow is smooth, smooth is fast.

 

I'm sure you also noticed the 'swaying' that happened at high speed with short, sharp roll inputs? That's also a product of not using input curvature. Personally I think the cross-talk between yaw and roll channels of the ACS is much too high, but that's really nothing more than a personal opinion. This will also go away if you start using curvature for your roll input.

 

Last thing, I didn't see any rudder movement but if you are using rudders, feet off! The one and only time you need them flying the Su-27 is for cross-wind landings.

 

At any rate, the in-game aircraft is behaving exactly as per spec. Try using input curves for pitch & roll and see if it helps. I think it'll solve the problem for you.

[ShuRugal]

The attached images show the sequence. The external and immediate cockpit view were taken with the sim paused so that they show the same instant in time.

 

Anybody else notice that for the duration of the pull, control indicator shows the commanded input riding the edge of the soft limiter? The soft limiter actually move considerably rearward throughout the maneuver.

 

It looks to me like the player was holding a stead stick position, around 60% deflection, and that the soft limit had kicked in around 55%, then ramped up to commanded input, and that this is what broke the plane.

 

So, to the player, it would look like an uncommanded pull.