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787: Why Test To Destruction The Weak Wingbox?  
User currently offlineA380900 From France, joined Dec 2003, 1091 posts, RR: 1
Posted (6 years 1 week 1 day ago) and read 9650 times:

It looks like Boeing intends to perform the airframe tests with two of the first six "strengthened" wingboxes. I don't understand why you would perform these tests with a non final design. Will the FAA/EAA like that?

Similarly what guaranty will they have that the final wingbox behaves as expected?

[Edited 2008-04-12 06:01:02]

24 replies: All unread, jump to last
 
User currently offlineOsiris30 From Barbados, joined Sep 2006, 3186 posts, RR: 26
Reply 1, posted (6 years 1 week 1 day ago) and read 9643 times:



Quoting A380900 (Thread starter):
It looks like Boeing intends to perform the airframe tests with two of the first six "strengthened" wingbox. I don't understand why you would perform these tests with a non final design. Will the FAA/EAA like that?

Similarly what guaranty will they have that the final wingbox behaves as expected?

See the wing test on your namesake for more information. The 380 failed the ultimate load test on it's original wing, which was then reinforced and certified without a retest.



I don't care what you think of my opinion. It's my opinion, so have a nice day :)
User currently offlineA380900 From France, joined Dec 2003, 1091 posts, RR: 1
Reply 2, posted (6 years 1 week 1 day ago) and read 9570 times:

Quoting Osiris30 (Reply 1):
The 380 failed the ultimate load test on it's original wing, which was then reinforced and certified without a retest.

Granted. It is still different. In one case you test the final wingbox and decide on modifications. In the other you test a modified wingbox and assumed results apply to the final thing.

To me, this is a big difference. You know for sure that the original wingbox of the A380 breaks at 147% and some reinforcement can help make up for the 3%.

What will you know about the final wingbox of the 787 when you know where a first version with some aluminium fixes breaks?
And let's add some complexity: let's assume that Boeing (I know they are infaillible but, hey, you never know !) gets 147% ultimate workload as well. How do you validate fixes based on this reinforced wingbox, different from the final one? Aren't we getting in murky waters then?


If I were the FAA, I would insist on seeing the test performed on the final design. And then fine tune it, or not if it's not necessary.

[Edited 2008-04-12 06:07:49]

User currently offlineBoeingBus From United States of America, joined May 2004, 1596 posts, RR: 18
Reply 3, posted (6 years 1 week 23 hours ago) and read 9530 times:

The A380 had a similar design issue with the wing failing a load test. Airbus proved via simulation that the design change is sufficient. Boeing is going to have to do the same.


Airbus or Boeing - it's all good to me!
User currently offlineOsiris30 From Barbados, joined Sep 2006, 3186 posts, RR: 26
Reply 4, posted (6 years 1 week 23 hours ago) and read 9518 times:



Quoting A380900 (Reply 2):
Granted. It is still slightly different. In one case you test the final wingbox and decide on modification. In the other you test a modified wingbox and assumed results apply to the final thing.

Well obviously Boeing isn't redesigning it on a hunch. So they must have some feedback from the FEM or other simulators that said 'whoops, you were a tad too aggressive'. As such they are choosing to brace the wingbox. It's different from the 380 situation only in two regards:

1) They are making the fixes before the test is failed and
2) If the FEM is off, it is off in the direction of too pessimistic, unlike Airbus's original computer model which was apparently about 2% too generous.

Boeing may well test a new wing box in isolation of everything else down the road when one is built. My guess is the failure mode wouldn't turn up in any normal testing anyway (i.e. fatigue frame, ultimate load, etc.) and occurs only in some weird case (or else it would have been caught sooner IMHO). As such it may never be truly tested one way or the other, and all these decisions are being made on a simulator's output.



I don't care what you think of my opinion. It's my opinion, so have a nice day :)
User currently offlineNomadd22 From United States of America, joined Feb 2008, 1764 posts, RR: 0
Reply 5, posted (6 years 1 week 23 hours ago) and read 9502 times:

They aren't testing two of the first six strengthened wingboxes. The six modified boxes are going on the first six production planes. The two test articles are separate. There was no mention of if those two were modified. The modification might not be needed for the wing break test. If it is, Boeing wouldn't have any trouble convincing the FCC that the final design would equal the modified one. That's why Airbus didn't have to do another test after they strengthened the 380 wing.


Andy Goetsch
User currently offlineBuyantUkhaa From Mongolia, joined May 2004, 2828 posts, RR: 3
Reply 6, posted (6 years 1 week 22 hours ago) and read 9268 times:



Quoting Osiris30 (Reply 4):
2) If the FEM is off, it is off in the direction of too pessimistic, unlike Airbus's original computer model which was apparently about 2% too generous.

Having worked with FEM models, I have to say it's not that simple. The same model can be either too conservative or too generous depending on a zillion factors and the strength/weight relation is normally highly non-linear.

Quoting A380900 (Reply 2):
How do you validate fixes based on this reinforced wingbox, different from the final one? Aren't we getting in murky waters then?

If you make sure the final one is not different, it should be OK.



I scratch my head, therefore I am.
User currently offlineOsiris30 From Barbados, joined Sep 2006, 3186 posts, RR: 26
Reply 7, posted (6 years 1 week 22 hours ago) and read 9264 times:



Quoting BuyantUkhaa (Reply 6):

Having worked with FEM models, I have to say it's not that simple. The same model can be either too conservative or too generous depending on a zillion factors and the strength/weight relation is normally highly non-linear.

I've worked with a ton of FEM as well, and written some FEM EM solvers in the past. I know that.. the point was this error would have been caught by the FEM model since no actual destructive testing was done to find it.. As opposed to the Airbus FEM model which gave a pass to something that ultimately failed..



I don't care what you think of my opinion. It's my opinion, so have a nice day :)
User currently offlineTdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 8, posted (6 years 1 week 19 hours ago) and read 8913 times:



Quoting A380900 (Thread starter):
It looks like Boeing intends to perform the airframe tests with two of the first six "strengthened" wingboxes. I don't understand why you would perform these tests with a non final design. Will the FAA/EAA like that?

The OEM's work constantly with the FAA/EASA to make sure that their certification plans are acceptable. The mere fact that Boeing is going ahead with the test as it is very strong evidence that FAA and EASA have already accepted that test method. To test first, then ask the regulators if the test is valid, would be grossly irresponsible.

Quoting A380900 (Thread starter):
Similarly what guaranty will they have that the final wingbox behaves as expected?

There are no guarantees in any aviation analysis. What guarantee do you have that the A320 coming out of the factory today is as strong as the one they tested when the plane was certified? It's all probabilities. And the combination of test results, analysis, and conservatism gives you a very high probability that the as-delivered product does what you expect.

Also keep in mind that we're talking about ultimate load tests here, which are 50% higher than anything the airplane will ever see in service (outside a crash, which is moot). Even if your testing and analysis are off by a few percent, it doesn't have a meaningful effect on the actual safety of the aircraft.

Quoting A380900 (Reply 2):
It is still different. In one case you test the final wingbox and decide on modifications. In the other you test a modified wingbox and assumed results apply to the final thing.

To me, this is a big difference.

Why is it a big difference? In both cases you have real test data for a known structure that's very close to your final structure. Provided you can analyze the differences between the tested structure and final structure (which is a lot easier than analyzing the whole structure) the two cases are basically identical.

Tom.


User currently offlineKELPkid From United States of America, joined Nov 2005, 6264 posts, RR: 3
Reply 9, posted (6 years 1 week 18 hours ago) and read 8794 times:



Quoting A380900 (Thread starter):
Will the FAA/EAA like that?

What say does the Experimental Aircraft Association, the non-governmental organization that sanctions mostly homebuilt aircraft in the United States, have in the testing of an aircraft that is intended for type production?  eyebrow 

If you're looking for the European certification authority, isn't that now the JAA (Joint Airworthiness Authority)?



Celebrating the birth of KELPkidJR on August 5, 2009 :-)
User currently offlineRheinbote From Germany, joined May 2006, 1968 posts, RR: 52
Reply 10, posted (6 years 1 week 18 hours ago) and read 8782 times:



Quoting Nomadd22 (Reply 5):
They aren't testing two of the first six strengthened wingboxes. The six modified boxes are going on the first six production planes. The two test articles are separate.

AFAIK the six strengthened wing boxes are CENTER wing boxes - section 11 in Boeing parlance - which are buried in the fuselage.
AFAIK the test wing box (only ONE existing AFAIK) which has been announced to be tested to destruction well BEYOND ulitmate load is a full-scale, 2/3-span section 12, i.e. the 'visible wing', not the one buried in the fuselage.

Once a test successfully passes ultimate, that's what is required, you want to know at which point the structure brakes to determine how much 'excessive' strength = excessive weight has been designed into it.

Airbus did it the other way round and 'intercepted' the ultimate load criterion from below. The A380 wing broke at 147% and they just added the additional 3% required to meet 150% of limit load.


User currently offlineOldAeroGuy From United States of America, joined Dec 2004, 3423 posts, RR: 67
Reply 11, posted (6 years 1 week 18 hours ago) and read 8738 times:



Quoting KELPkid (Reply 9):
If you're looking for the European certification authority, isn't that now the JAA (Joint Airworthiness Authority)?

No, it's EASA (European Aviation Safety Agency)

http://www.easa.eu.int/ws_prod/index.html



Airplane design is easy, the difficulty is getting them to fly - Barnes Wallis
User currently offlineZvezda From Lithuania, joined Aug 2004, 10511 posts, RR: 64
Reply 12, posted (6 years 1 week 18 hours ago) and read 8624 times:

An important reason for destructive testing is to calibrate/verify the FEM.

User currently offlineRheinbote From Germany, joined May 2006, 1968 posts, RR: 52
Reply 13, posted (6 years 1 week 17 hours ago) and read 8564 times:



Quoting Zvezda (Reply 12):
An important reason for destructive testing is to calibrate/verify the FEM.

Why that? I'd guess that obtaining from tests the stress/strain data and deformation is all you need. I don't think FEM is suitable to reliably predict the characteristics of strucutral failure.


User currently offlineZvezda From Lithuania, joined Aug 2004, 10511 posts, RR: 64
Reply 14, posted (6 years 1 week 17 hours ago) and read 8494 times:



Quoting Rheinbote (Reply 13):
Why that? I'd guess that obtaining from tests the stress/strain data and deformation is all you need.

How else would you know whether or not your FEMs had any relationship to reality?


User currently offlineRheinbote From Germany, joined May 2006, 1968 posts, RR: 52
Reply 15, posted (6 years 1 week 17 hours ago) and read 8382 times:



Quoting Zvezda (Reply 14):
How else would you know whether or not your FEMs had any relationship to reality?

 confused  Again, you don't have to break something to calibrate/validate your FEM.


User currently offlineNomadd22 From United States of America, joined Feb 2008, 1764 posts, RR: 0
Reply 16, posted (6 years 1 week 15 hours ago) and read 7800 times:

I've been having so much fun with the FCC I can't get away from them. I do actually know the difference between them and the FAA.
What usually gives first on the wing break test on the static frame? Or, if they designed perfectly, does everything sort of fail at once?



Andy Goetsch
User currently offlineFsnuffer From United States of America, joined Jun 2007, 246 posts, RR: 0
Reply 17, posted (6 years 1 week 14 hours ago) and read 7560 times:

It is simple. They are not testing the strength of the weak wingboxes, they are testing the accuracy of the computer models used to predict the strength of the wingboxes. By improving the accuracy of the computer models, they will be able to improve their accuracy. That in the long run will save them millions and let them turn out a much better product

User currently offlineB777fan From United States of America, joined Oct 2004, 167 posts, RR: 2
Reply 18, posted (6 years 1 week 14 hours ago) and read 7165 times:



Quoting KELPkid (Reply 9):
What say does the Experimental Aircraft Association, the non-governmental organization that sanctions mostly homebuilt aircraft in the United States

As pointed out, they didn't mean EAA, but the EAA doesn't 'sanction' homebuilts or any experimental aircraft. That is an FAA function - which the FAA may delgate to qualified individuals called DAR's. Many DAR's happen to belong to the EAA but the EAA plays no role in what they do.


User currently offlineTdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 19, posted (6 years 1 week 12 hours ago) and read 6622 times:



Quoting KELPkid (Reply 9):

If you're looking for the European certification authority, isn't that now the JAA (Joint Airworthiness Authority)?

Used to be. Now it's EASA.

Quoting Zvezda (Reply 12):
An important reason for destructive testing is to calibrate/verify the FEM.

Destructive testing is a lousy way to calibrate/verify FEM because FEM isn't very good at predicting the failure point. It's much better at predicting the stress/strain distribution. Thanks to statistical variance in the actual strength of materials, even a perfect FEM model won't correspond exactly with the actual failure point. The testing prior to destruction is a good way to calibrate FEM because it lets you compare the stress and strain on the test article to the FEM predictions.

Quoting Nomadd22 (Reply 16):
What usually gives first on the wing break test on the static frame? Or, if they designed perfectly, does everything sort of fail at once?

In a perfect world, it would all go at once. On the 777 test, I believe it was the upper wing stringers that either buckled or crippled. Most modern wings are probably compression (i.e. buckling) critical because you need so much fatigue resistance in the lower wing that ultimate strength probably isn't a problem there.

Tom.


User currently offlineOsiris30 From Barbados, joined Sep 2006, 3186 posts, RR: 26
Reply 20, posted (6 years 1 week 12 hours ago) and read 6542 times:



Quoting Rheinbote (Reply 15):
confused Again, you don't have to break something to calibrate/validate your FEM.

You're both right and wrong.. Rheinbote is right in that you don't necessarily need to break something and partial damage (i.e. stress tests) can provide useful insight. Zvezda is right in that breaking something and calibrating the FEM to that allows you to be certain the FEM is accurate over the full range.



I don't care what you think of my opinion. It's my opinion, so have a nice day :)
User currently offlineZvezda From Lithuania, joined Aug 2004, 10511 posts, RR: 64
Reply 21, posted (6 years 1 week 10 hours ago) and read 5710 times:



Quoting Tdscanuck (Reply 19):
The testing prior to destruction is a good way to calibrate FEM because it lets you compare the stress and strain on the test article to the FEM predictions.

... which is why testing is done to destruction, to get the full range of data available.


User currently offlineTdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 22, posted (6 years 1 week 9 hours ago) and read 5407 times:



Quoting Zvezda (Reply 21):

Quoting Tdscanuck (Reply 19):
The testing prior to destruction is a good way to calibrate FEM because it lets you compare the stress and strain on the test article to the FEM predictions.

... which is why testing is done to destruction, to get the full range of data available.

But it's the wide range, not the destruction, that gives you useful data as far as calibrating the FEM goes. The actual destruction point is of limited value in calibrating the FEM, although useful for other things.

Tom.


User currently offlineZvezda From Lithuania, joined Aug 2004, 10511 posts, RR: 64
Reply 23, posted (6 years 1 week 9 hours ago) and read 5322 times:



Quoting Tdscanuck (Reply 22):
But it's the wide range, not the destruction, that gives you useful data as far as calibrating the FEM goes. The actual destruction point is of limited value in calibrating the FEM, although useful for other things.

Yes. The question was: Why test to destruction? The answer is to get data because those data are useful for calibrating the FEMs. The destruction point is a tiny fraction of the total data collected.


User currently offlineLightsaber From United States of America, joined Jan 2005, 12420 posts, RR: 100
Reply 24, posted (6 years 1 week 6 hours ago) and read 4621 times:
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Quoting Rheinbote (Reply 13):

Why that? I'd guess that obtaining from tests the stress/strain data and deformation is all you need. I don't think FEM is suitable to reliably predict the characteristics of strucutral failure.

The FEM predicts when the stress/strain is sufficient for a high likelihood of failure. I agree, the exact point of failure has uncertainty, but to a few percent.

As to why TTD? There is an old history in aerospace about knowing when a part ultimately fails. Designs are built to TWO safety factors. Generally a pretty low safety factor to 'yield.' Thus why the desire to better understand the true safety factor to failure.

An easier to understand case is the hydraulic systems (e.g., in the 787 there are localized ones to deploy the landing gear). First, the parts must not leak at 1.5X their maximum rated pressure. Then the parts must prove they do not explode at 2.5X their maximum operating pressure (but the part can deform under this pressure). Boeing also enforces a 4x analytical safety factor to burst (FEA or via testing. Usually FEA.). By understanding if the part fails or not, one gains a greater confidence in the design.

The same is true of the wingbox. Remember that the test isn't just testing the wingbox, but the full wing. From a structural design standpoint, the changes to the wingbox are relatively low risk. Thus the test will show that the loads and deflections from the wing impart a certain level of stress and strain. Is the failure point 100% representative? No. But that is such a minor part of the data collected, the FAA will easily be able to understand the impact of the design changes.

Lightsaber



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