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ScotCan
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Platinum
Re: Its fair shair of problems
ScotCan   6/26/2012 4:50:32 PM
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Fair comment, I suppose, except that small aircraft are generally way over designed and military aircaft are more so. Large passenger aircraft are designed for reduced weight and maximum payload and as a result end up with using exotic materials about which there is little history...that's the problem. If the cracks are in AlLi (and the location suggests that they may be) preload and other fit up factors could stress the material. I mentioned fitup factors because nothing fits together perfectly...despite the use of CAD. Most aircraft parts are designed to nominal sizes and when the parts are made tolerances have to be provided...a classic example is a machined part which nests into a formed sheet metal part....sometimes it'll fit other times it won't. Since the machined part carries tighter tolerances, the sheet metal part is deemed to be under or over size as the case may be. On other occasion a wee bit of persuasion gets the undersized sheet metal part to fit albeit with some preload which is OK for 2024-T3, marginal for 7075-T6 and totally unacceptable for ANY AlLi.....as you can see here the rules change quite a bit for AlLi BUT it is not always conveyed explicitly to the shop floor....there will be marginal conditions which the shop floor experience deems it knows how to work around and which work admirably for the above mentioned 2024 and 7075 material.This is liaison engineering talking, and we're the guys that return the aircraft to the design intent...a bellwether indication of how troublesome a new aircraft is is to do a count of the number of liaison engineers a company hires for the production run...the Boeing Dreamliner had one of the highest number ever, so, we'd best stay tuned for upcoming events concerning that aircraft....the A 380 has more flight hours than the Dreamliner and more landings and take-offs...when the Dreamliner matches that number things'll start showing up....it's the name of the game!

ScotCan
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Platinum
Re: Its fair shair of problems
ScotCan   6/26/2012 3:45:31 PM
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The EH 101 helicopter Lower Forward Fuselage built as an industrial offset in Canada was almost exclusively AlLi. The shop had a whale of a time working the stuff since it does not behave the same way as even 7075 material. The shop had "tribal knowledge" on how to fix discrepancies in the standard alloys, but came up against all sorts of stumbling blocks with the AlLi. It has an enormous strength and as a consequence the cross sections (and the weight) were correspondingly less than the conventional material. Stringers for example were somewhat forgivable in preload, but could develop large internal stresses unknown to the operator tweaking the fit...all it needed was a glancing blow to crack the stuff. Eventually the shop got used to its eccentricities and we did OK towards the end of the contract. The weight savings with AlLi are remarkable. The lower fuselage slighter larger in planform than the Dash 8 lower flight compartment weighed in at 350 lbs, so as a material it is here to stay.

Ann R. Thryft
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Blogger
Re: Its fair shair of problems
Ann R. Thryft   6/26/2012 1:12:32 PM
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ScotCan, I also find your comments on AlLi intriguing, since this is being positioned by aluminum makers as the answer to the commercial aircraft OEMs' composite woes. Interestingly, the Airbus wing cracks occurred in the unnamed aluminum alloy. I wonder if it's AlLi?

Ann R. Thryft
User Rank
Blogger
Re: Its fair shair of problems
Ann R. Thryft   6/26/2012 1:11:10 PM
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ScotCan, I have also noticed that some commenters draw conclusions for large aircraft from the use of composites in smaller aircraft. My understanding, like yours, is that scale matters and that a divide has been crossed with Airbus and Boeing. That said, the comparisons with the use of composites in military aircraft have been made by nearly everyone involved in using composites on these large aircraft, including the OEMs and the suppliers, such as in this DN article: http://www.designnews.com/document.asp?doc_id=235863

ChasChas
User Rank
Platinum
failure rating
ChasChas   6/26/2012 9:56:02 AM
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Seems strange with all the engineers posting here that very few understand failure rating.

Design errors are always rated from potential failure to catastrophic failure.

These cracks a rated as potential long term failure - it is completely safe for the duration of the planned fix.

We can't just trust in an engineers' design ability (if it works) and then lose faith in the engineers' failure rating ability and cause mass hysteria.

sensor pro
User Rank
Gold
Re: Wrong testing procedures...
sensor pro   6/25/2012 9:32:28 PM
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Thank you for putting is so correctly. Great.

ScotCan
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Platinum
Re: Its fair shair of problems
ScotCan   6/25/2012 4:53:59 PM
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The Dreamliner's next in line for complications. Both Boeing and Airbus bit off more than they could chew, both with the use of composites and Aluminum-Lithium (AlLi)

AlLi is great stuff EXCEPT it does not behave like standard Al Alloy material. Generally,percussion rivetting is avoided because AlLi has been known to crack under impact...one project used blind fasteners all over the assembly to avoid cracking AlLi components. The shop couldn't fix mistakes as readily as they could with traditional Al Alloys, and etching AlLi prior to adhesive bonding did not work out at all well.

ScotCan
User Rank
Platinum
Re: Wrong testing procedures...
ScotCan   6/25/2012 4:43:40 PM
NO RATINGS
The last sentence can be turned around...engineers don't have a PR problem...it's the system that doesn't listen well enough to the engineers!

ScotCan
User Rank
Platinum
Re: Its fair shair of problems
ScotCan   6/25/2012 4:36:24 PM
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Ann, it would be helpful if commenters would NOT try to draw conclusions from smaller aircraft and military aircraft using composites

DeHavilland in Canada is a past master at glueing aircraft together right from the days of the Mosquito all the way up to the Dash 8 and it's a lot trickier working with composites. Both Boeing and Airbus lost their way going hell bent into composite construction with both of them barging ahead with insufficient testing and arguments around military aircraft experience and smaller aircraft are not valid since neither of them carry large passenger payloads.

In earlier days when Boeing bought DeHavilland it found out very quickly that building smaller aircraft was not the same as Boeing's normal expertise...there is a magical divide when the transition is made from one size of aircraft to the other.

Jerry dycus
User Rank
Gold
Re: Its fair shair of problems
Jerry dycus   6/20/2012 3:23:02 PM
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Ann wrote,  

,Jerry, we've reported several times on on the use of composites in aircraft over the last few decades. We've also done features on fasteners and adhesives for structural apps, including aerospace.

----------How about a link to the tape adheasive one?

 What I've been told is that composites have been adapted to component designs originally engineered for metal without rethinking those shapes, and that fasteners for composites have not been re-thought thoroughly. This was all in the rush to get composites into commercial aircraft.

-------- Basically what I just said, they can't change easily because of the way they think.  I started in composites 45 yrs ago now so I've alway thought that way and saw others do just as you say.  This cuts across almost all industries.

It's the 50% and greater proportions of composites to metal that are new--as well as all this being somewhat newer in commercial than in military aircraft--which means joints are being made between dissimilar materials where they weren't before.

-------------  The problem is one material or the other take the main share of the load. Otherwise the weaker one will just be destroyed.  It has nothing to do with the materials, it's the designer, engineer who is at fault for not designing within specs, good engineering practice.  There is little difference in materials/metals used then and now.

----------------BTW this is the same problem CF has.  It  has to take all the load because it will.  If only 50% on CF and 50% on FG because the FG elongates more, the CF takes the whole load until it breaks which then transfers it to the FG which also breaks then.  Had it  been in CF or FG it would have been fine.

 Also, adhesives are simply not strong/durable/reliable/etc enough for the extreme stresses involved

------------  Again military jets are under far heavier loads and most of the 70's jets had many composite parts including many of mixed metal, composite close to equal amounts  and are still flying.  And modern adheasives are better we hope.  Only time will tell.

and, as kfw2qd points out, it's tough to tell when they've failed.

---------------  Maybe kfw, some others have a problem finding debondings but those who do it for a living don't have a problem.  If there was the FAA wouldn't let them fly.  It's not easy to get something by the FAA/NTSB like that more than once.


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