Su-27. Extreme G-loads... G=?

[DarkFire]

As Bushmanni previously stated, bending moments is the cause of wings falling off at high load factors. This means that weapons hanging off the 6 wing pylons actually reduces the bending moments at the root, allowing for higher load factors (if we assume that failures occur at the wing root). This means that the increased gross weight might actually work in favor of higher load factors, depending on where it is located.

 

Has anyone seen that this would be the case in the sim (only carrying missiles/bombs on wing pylons to increase load factor before failure)?

 

P.S.

One note on this is that while bending moments are reduced, shear stresses in the wing might be increased and could probably also be a cause of failure. And then also the case of the weapon attachments failure as happens with the MiG-21, but I guess this is not implemented on the SU-27?

 

 

Interesting theory. For what it's worth, every excess G-load failure that I've experienced without weapons loaded has resulted in the outer ~1/3 of the wing shearing off, and interestingly it's always been the right wing that's failed. Whilst carrying a full complement of A2A missiles I typically experience total loss of both wings, i.e. both shear off at or near the wing roots. I've only deliberately done it 3 times in each configuration though so that's certainly not enough data points to definitively tell whether or not there's a difference in failure modelling between loaded & unloaded wing stations. I can't tell whether it's dynamically calculating wing stress/strain or whether the failures are entirely scripted.

 

From the very useful table that Ironhand posted I'd be inclined to believe that the sim is doing more than simply scripting a failure beyond a certain G load, but I'm not sure about any of this at the moment. There's also the apparent 'problem' of Tacview not recording accurate G loads just before the point of failure, which makes deliberate reproduction and data recording difficult.

[Ironhand]

Interesting theory. For what it's worth, every excess G-load failure that I've experienced without weapons loaded has resulted in the outer ~1/3 of the wing shearing off, and interestingly it's always been the right wing that's failed...

I haven't been paying close attention but I do know of at least one instance in which the outer third of the wing--left wing in my case--sheared off and I was carrying weapons. In that instance I was both pulling Gs and rolling the aircraft simultaneously.

[SinusoidDelta]

Which makes sense IMHO. Pure logitudinal maneuvering distributes load across the wings spanwise. Simulataneously rolling the aircraft, the distribution is no longer symetrical and the wing producing the most lift will experience the highest load. Net wing load would be concentrated in to point loads at the edge of the flaperon. If the load exceeds structural limit, I think it's reasonable that the wing failure would occur at the edge of the flaperon i.e. the outer 1/3 of the wing. Should that failure mode always result? No.

 

I highly doubt stress/strain or the multitude of other material strength properties are being calculated dynamically.

[DarkFire]

I highly doubt stress/strain or the multitude of other material strength properties are being calculated dynamically.

 

Agreed. I would imagine that would simply be too much for a CPU to calculate along with all the other things it has to do in the sim.

[pewbeef]

Which makes sense IMHO. Pure logitudinal maneuvering distributes load across the wings spanwise. Simulataneously rolling the aircraft, the distribution is no longer symetrical and the wing producing the most lift will experience the highest load. Net wing load would be concentrated in to point loads at the edge of the flaperon. If the load exceeds structural limit, I think it's reasonable that the wing failure would occur at the edge of the flaperon i.e. the outer 1/3 of the wing. Should that failure mode always result? No.

 

I highly doubt stress/strain or the multitude of other material strength properties are being calculated dynamically.

 

 

You are definitely right about the rolling adding load to the wing going upwards. Not sure about where the break should occur though, since the overall bending moments will still be at its largest at the root, but however reduced by the rolling state of the aircraft. Without knowing the difference between "normal" wing lift and the added lift coming from aileron deflection, or the wing spar dimensions/material properties at spanwise sections, it is hard to estimate where the wing would actually break. Things tend to get a bit tricky when you consider all factors.

 

I also agree in that doing structural analysis of the wings in realtime would be a bit too CPU demanding... :P

 

But I was under the impression that the PFM of the Su 27 would be complex enough to produce a realistic spanwise lift distribution? I'm thinking that in this case, scripting how the wing breaks would not be that difficult? Might be wrong on this...

[Svend_Dellepude]

If I understand correctly, the wing is divided into two sections (EDIT: 3 sections), inner and outer and possibly divided at the MAC (it's a long time since i looked in the FM folder so i might be wrong).

You can have a look in the folder containing the FM for the SU-27, that you give you an idea on how it's put together. It's in …mods/aircrafts/... dont remember the entire path, but you'll find it.

Edited April 20, 2016 by Svend_Dellepude

[esb77]

As Bushmanni previously stated, bending moments is the cause of wings falling off at high load factors. This means that weapons hanging off the 6 wing pylons actually reduces the bending moments at the root, allowing for higher load factors (if we assume that failures occur at the wing root). This means that the increased gross weight might actually work in favor of higher load factors, depending on where it is located.

 

Has anyone seen that this would be the case in the sim (only carrying missiles/bombs on wing pylons to increase load factor before failure)?

 

P.S.

One note on this is that while bending moments are reduced, shear stresses in the wing might be increased and could probably also be a cause of failure. And then also the case of the weapon attachments failure as happens with the MiG-21, but I guess this is not implemented on the SU-27?

 

I've seen this behavior for sure in the Su-25T, and I think maybe also in the A-10C. I would guess that the model for the Su-27 works similarly, though it generally flies with much lighter munitions and pulls harder turns than the ground pounders, so the effect might be harder to detect with it.

[OldE24]

So. had the wings come off with 8G's at 1300T with 2 R73's on the wings.......umm i understand they Over g...but 8??????

[GGTharos]

What's 1300T? And what's the over-g history of that airframe?

[OldE24]

the yellow needel was AT 8 as i was falling to the ground....i was high and fast at the time making a turn out.......when it hit 8 there went the wings.....dont remember fuel load but i was down Some as i had burned to my location, HADNT even warmed my pilot up and he didnt black out......but the airframe did. 1300T is what indicated in BVR mode on the SU...somewhere around mach 1.1 ish.

 

 

i can go low and MAX the needle out all day but up high it comes off at 8G's on the first tug. seems alittle overdone.

Edited April 26, 2016 by OldE24

[Esac_mirmidon]

8 G at 1300 TAS? is a LOT over the maximum.

[GGTharos]

Yup. The chart clearly indicates that the M0.85 -> M1.25 region is very vulnerable to high g, and there's no telling what your GW was. 'I was down some' is not an argument, it's lack of information ... so by 'down some' I'll assume you burned a tonne maybe, and you were sitting in and area where your maximum safe g is 5g.

[OldE24]

the scaling is off or something......on the deck it bitches over g over g maxed out off the gauge...then get up and try to turn out and and it breaks but the computer never has a word to say about it... not bad....

 

i played the other day and broke it once...but today thing just fell apart on smoth turnouts.

[GGTharos]

It still doesn't mean anything at all without detailed descriptions, tracks, or at least tacviews with a GW listing.

'The scaling is off' is completely meaningless without numbers - it's just feelings :)

[DarkFire]

I still genuinely don't see that the airframe G limitations are a real issue. At cruise speed (~85% RPM) with 80% fuel and a full A2A missile load it's still perfectly possible to slow to corner speed with a near-maximum STR turn and then perform a near-maximum STR turn without breaking the aircraft.

 

I have broken it under these conditions but it was entirely deliberate and I had to repeatedly exceed 9.5G to do it.

[OldE24]

well go up high and go fast then make a glancing turn and about the time you hit 8G's before you even start to greay out. it's not an issue with breaking but there should be more Ques to your G load on the airframe i guess is my problem, down low it bitches but doesnt break but you go up and it doesnt bitch it just breaks even before the Glock it seemed, i dont fly it enough to get the total feel i flew it a week ago and didnt have much issue broke it once, yesterday i hop in it and broke it SUPER easy.

[probad]

structural fatigue has no correlation with your pilot's g limits so im not sure what you're trying to imply when you keep bringing that up

[Ironhand]

well go up high and go fast then make a glancing turn and about the time you hit 8G's before you even start to greay out. it's not an issue with breaking but there should be more Ques to your G load on the airframe i guess is my problem, down low it bitches but doesnt break but you go up and it doesnt bitch it just breaks even before the Glock it seemed, i dont fly it enough to get the total feel i flew it a week ago and didnt have much issue broke it once, yesterday i hop in it and broke it SUPER easy.

Whether correctly modelled or not, Betty's bitching seems to be based on a weight of 21,400 kg at a max speed of M .85. That should get her complaining at 8 Gs. If you are either heavier or faster, bad things can start happening before any warning is given. In other words, that audible warning system is hard coded for those parameters and doesn't take your current values into consideration.

 

So don't feel that you're safe until your visions starts to fade. You're not. At high speeds and heavier weights, you'll have problems long before things go black.

Edited April 26, 2016 by Ironhand

[GGTharos]

No, I won't. It's really not that hard to produce a quick track to demonstrate the problem. 'Go up high and fast' means nothing to me - I can pick the most vulnerable region or I can blow right past it. I can fly a really light flanker, too.

 

So make that track. I don't care about your description of what you did unless you have very, very concrete parameters. If you don't want to do that, provide a short track demonstrating the problem.

 

well go up high and go fast then make a glancing turn and about the time you hit 8G's before you even start to greay out. it's not an issue with breaking but there should be more Ques to your G load on the airframe i guess is my problem, down low it bitches but doesnt break but you go up and it doesnt bitch it just breaks even before the Glock it seemed, i dont fly it enough to get the total feel i flew it a week ago and didnt have much issue broke it once, yesterday i hop in it and broke it SUPER easy.