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Composite aircraft may hide dangerous flaws


Allan
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Arrr....scary....

 

The next question is how does this apply to composite aircraft like the Jabiru and, say, Flysynthesis aircraft like the Texan ?

 

What inspection techniques do they use to inspect these types of planes ?

 

Is there a LAME that could enlighten us perhaps please?

 

Regards

 

Phil

 

 

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i dont thing the danger is as bas as stated, the Boeing 707 hs composite components that have never failed in their 50 yrs service, the composites these days are in a different league with advances in polymers, and adhesive qualities.

 

707 flight controls were bonded metal honeycomb, even the floor panels were made of balsa wood in fibreglass.

 

not only that, NDt techniques and technology has improved dramatically in relation to composites. the Tap test is only a quick test, not unlike looking for bulging paint to find corrosion in a metal part. i have seen the use of hand held ultrasonic devices that find the smallest flaw in composite parts.

 

the biggest killer to composite integrity is water ingestion, it wil freeze at altitude and cause thurther delamination to the point of rapid failure. though water detection instruments are readily available.

 

 

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As an apprentice we used to make the 707 floor panels using honey comb fiber boards with a couple of layers of fiberglass mating using two part epoxy resins so as to keep them light.Stilettos used to puncture holes in them. That was 40yrs ago as I just recall I think I do well maybe. Cheers T87

 

 

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I think the term composite is to broad - there are many different types of composite - are we talking fibreglass, and then there is carbon fibre coated alluminium and finally there is pure carbon fibre of which there are 3 different types of that.

 

One of the dangerouse ones I feel is the carbon fibre coated allumninium as the last properties of that can be a worry - you have two different materials rubbing against each other and if moister gets in there then big problems.

 

The absolute best carbon fibre - as I have just recently found out with my trip to Italy, is the carbon fibre that is impregnated with resin and stored at about -20 degrees - this is the best, strongest and less prone to human error in the combining of the resin with the carbon fibre. I will put a note in here that the Millennium Master is built this way.

 

So as I said I think that composite is a very broad term and people may get the wrong idea whenever this word is used - just my thoughts!

 

 

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Using pre-preg is not necessarily the 'only' avenue for constructing aircraft. Gliders have been built in composite since the 60's and the only ones that have 'exploded' due to voids in the laminate are the very few that have been struck by lightning.

 

Taking that into account, airspeed and (lack of) air pressure will definitely have a roll in determining the durability of a composite structure, mainly due to friction. The Concord supposedly grows 1 foot when its cruising at supersonic speeds and the skin temp is around +100C. Whilst common jet liners do no fly at supersonic speeds yet, the temperature due to friction will certainly be increased. Naturally, they can cope with this using different resin systems, as Burt Rutan had the issue with his Starship one engine which needed to cope with +2800C temps without melting, so saying that the rudder exploded because of voids in the laminate, leaves the outside airpressure to be the determining factor possibly?

 

By the sounds of it someone forgot to stir the resin pot when they made the rudder or the autoclave wasn't working properly and instead of throwing the rudder away they put it on anyway.

 

My 2c worth. :-)

 

Chris

 

 

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Voids in Composites

 

I think that moisture entering into the composite, and subsequently freezing,( and therefore expanding) may be part of the problem. This is with jet high flyers, where the OAT is about minus 55.(although it only expands at the point of freezing), There is the effect of cycles. (accumulative). Incidently, the temp. rise at a leasurely Mach .8 is about 33 degrees c..Re the earlier Jets.B707, B727, DC8 etc. I don't think that they used composites in the basic structure itself. The floor panels are a particular application where they were damaged by stilletto heels (as mentioned), a particularly ridiculous concept that applied very high pressures to floor surfaces, by virtue of the small area of the heel. Nev.

 

 

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Guest rocketman

If we get into 787's and A380's in recreational aviation, there might be a problem. In airline use, yes, there's some scary things about the use of composites, but good old aluminum also has it's flaws out there. Airlines love carrying seafood as cargo, good money in the freight business, but if the seafood containers leaked, then salt water inevitably wound up under the floor, were it would slowly turn aluminum to dust. We have a 737 parked up near here with the belly skins rotted out. Then there's mercury (broken thermometers and other industrial uses), fortunately everyone's pretty aware of it now, but it will eat aluminum for breakfast then turn around and want more. As far as composites go for recreational aircraft, the "tap" test is a good way for checking delamination. Use a 20 cent (or equivalent) coin and lightly tap all around the suspect area. If the structure is good, it will sound sharp and clear, if it's delaminated, it will have sound dull and indistinct. But it takes practice sometimes to determine the difference. Neale.

 

 

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Carbon fibre should never be in contact with aluminum, the carbon will cause very rapid corrosion in the aluminum, and this corrosion residue will de-laminate the structure, if carbon is to be bonded with an aluminum part, the plies that will make the bond must be Kevlar or Fibreglass. same goes for fasteners, Steel fasteners only in carbon.

 

Pre preg fibreglass and carbon fibre is inherently stronger, not only through the temperature and pressure used to cure, but also in machine accurately and thoroughly mixed resins.

 

the higher the cure temp of the resin and the greater the pressure the stronger the bonding.

 

Latest techniques being used now involve curing the parts in Hot oil baths, at extremely hight temps, but at pressures that cannot be achieved in autoclaves and vacuum bags.

 

 

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  • 7 months later...

Composite report

 

Attached is a link re a Research and Analysis on Composite Materials undertaken by the Australian Government ATSB … if nothing else read the conclusion but there is quite a bit of useful info on safety for composite LSA a/c

 

http://www.atsb.gov.au/publications/2008/pdf/AR2007021.pdf

 

Regards

 

Jack

 

 

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