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Fired engineer calls 787's plastic fuselage unsafe

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Fired engineer calls 787's plastic fuselage unsafe

Old 19th Sep 2007, 22:31
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I like that 'blanket' analogy.

After my Pa contacted Terra Firma at well over 100 Kts at 30 degrees in his Carbon fibre glider fuse, the nose did indeed resemble a blanket if you poked it, but it held it's shape - almost eerily. But, he survived it, although he was also quite badly broken. There is no possible way that any non CF machine would have protected him to that extent. It was the "bit by bit" disintegration that saved him.

So, crash-wise, I think I'd prefer my chances in the Carbon composite.

Having said that, I'd only want to crash in a newish one. The inspection problems and the way that CRP reacts to electricity, fire, freezing, de-icer etc.. does worry me.
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Old 19th Sep 2007, 22:51
  #42 (permalink)  
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Every single frame on the A380 Main deck (98 of them) is Carbon fibre. and in the interests of accuracy they are manufactured completely differently and with far higher tolerances than boat masts ! (I had to laugh at that one) Furthermore only Titanium fixings are used to secure other structural items to them.
As a former professional engineer, and having worked in the aerospace industry, and in an airline, and having visited both Boeing composite manufacturing facilities and mast and hull builders, you are simply wrong to sneer about the manufacturing technology employed and the tolerance achieved. Furthermore the car industry uses about ten times smaller tolerances on their manufacturing processes and use far more sophisticated processes than do airframe builders, but in any case thats not the issue.

The issue is as Weldon succinctly puts it, the very low amount of linear strain before the composite material fails. In other words, it's main failure mode is a brittle fracture. My observation is that the considerable number of carbon hull and mast failures I've seen are consistent with this failure mode, and as a result, are "spectacular" compared to the failure of, say, an Aluminium structure.

What Weldon is alleging is that Boeing has avoided subjecting the 787 fuselage structure to a type of test (A drop test from 14 ft. producing a 30 ft/sec) velocity that in an Aluminium structure produced a maximum 20g deceleration - which is regarded as the maximum for passenger survivability.

The implication of what he is alleging is that the B787 fuselage is not going to behave as well as an Aluminum fuselage in a "Phuket" or "Yogyakarta" type of accident - and the behaviour is going to be worse, not better, with higher G loads on the Pax followed perhaps by the shattering of the fuselage, and possibly the ejection of the contents.

Whether this is likely to happen and whether this is a good or bad thing is beyond my competence. I will be interested to hear Boeing and the FAA's response.

The one thing we can all count on is that whether Boeing wishes to simulate such a situation or not, the B787 crash worthiness is going to be tested by nature, and as Feyneman famously said, nature will not be fooled.

As for his comments regarding Boeing's conservatism and "soul", I can testify that the Boeing I had dealings with twenty-five years ago certainly evinced the qualities of integrity and ethics he talks about, and it would be sad if that ethic has gone. I'm still not sure for example, if the aviation industry is using lithium/aluminium, and high strength low creep magnesium alloys, despite these having been around and tested for forty years.

To sum it up, if the allegations are correct, then this is the same sort of **** that got Douglas into trouble.
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Old 19th Sep 2007, 23:15
  #43 (permalink)  
 
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I shall look at the composite construction components on my aircraft now with abject terror. They have been twisted, bent, stretched and despite this they have been on the aircraft for God knows how long without rectification.
They're called helicopter rotor blades.
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Old 19th Sep 2007, 23:18
  #44 (permalink)  
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Just a thought, I think you will find as resistence increases, current decreases, Mr Ohm might say E=I x R, and P = V x I.
Yes, but the problem is that power dissipated is dependent on resistance and current.

It is true that P=V*I

However, V = I*R

so, P= I(squared)* R

Therefore, in order to get the same power dissipation you have to reduce the current by a factor of 4 if you increase the resistance by a factor of 2.

The only way to do that is to reduce the voltage.

Since the voltage for any serious electrical supply can be assumed to remain constant then current will also remain purely a factor of resistance from Ohms' law.

You will find that a higher resistance will increase the power dissipated through the fault given a constant voltage.
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Old 20th Sep 2007, 00:34
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DAN RATHER to present 787 safety concerns on tv
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Old 20th Sep 2007, 00:43
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Fareastdriver, you are missing the point. We are not talking about strength, we are talking about what happens when someone crash tests the B787.
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Old 20th Sep 2007, 04:55
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Firstly, the demanded comparison of exactly the same designs being tested against each other is, of course, not the point. One would design an aluminium structure to suit Al and a CF structure to suit that material.

Second, If one wants an example one might care to look at, say, Formula one, (often mentioned on these pages,) where CF has been used for quite a while. The energy absorbing qualities of Cf are well known and tried and are said to be superior to Al, and even fancy honeycomb double skinned stuctures, just their deformation style is radically different.
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Old 20th Sep 2007, 06:26
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things that make you go "hmm"

http://www.atwonline.com/news/story.html?storyID=10276
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Old 20th Sep 2007, 07:50
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You will find that a higher resistance will increase the power dissipated through the fault given a constant voltage.
No it won't. Power = V(squared)/R (the more appropriate form of the equation for constant volts) so if R is bigger, power goes down in the scenario you describe. You fail to cope with the fact that as you increase R, the current goes down.

The reality is more complex, because you're into impedance matching where maximum power is transferred when source and load impedances are the same, but as I don't know the source impedance of a lighting strike I can't work any numbers.
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Old 20th Sep 2007, 07:50
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with regard to absorbing impact energy... the vast majority of energy is absorbed during plastic-deformation
This is a quite generic statement, and not entirely true for crashworthiness of aircraft. Aircraft structure, highly optimized for low weight, often fails in a buckling mode, not in overstress. Therefore the plastic deformation is restricted to a very small part of the overall structure, and the energy dissipation is low, compared to the overall volume of material.
For composite structure you can generally state, that it could be designed to have superiour crash resistance. Using monolithic, thick walled, woven fabric, hybrid material design (Carbon/Aramid, or even better Carbon/Polyethylen), gives you crashworthiness unparalleled by any metal structure. Formula 1 demonstrates this impressively.
If you want to have a structure optimized for low weight, you will use unidirectional tapes, thin walled structure and carbon fibre only. Such design typically fails without much energy dissipation.
So the 787 could be both, superior to all metal competitors with regard to impact resistance, or a total nightmare, depending on the knowledge of the boeing engineers and the ethics of boeing management. Future will tell us.
Looking at the pictures of the crash tests, I am not too convinced, that crashworthiness was the primary reason for chosing composites. Looking at the boeing advertisments also looks more light low weight and economy were the driving factors.
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Old 20th Sep 2007, 08:17
  #51 (permalink)  
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Smile Yes!!! Finally a thread I can be an expert on!

Lightning and carbon fibre composites.

You ever seen a tree that has been struck, they explode.

Same with windmill blades.

Any water trapped in there flashes to steam and blows the structure apart.
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Old 20th Sep 2007, 08:40
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fish

The issue is as Weldon succinctly puts it, the very low amount of linear strain before the composite material fails. In other words, it's main failure mode is a brittle fracture. My observation is that the considerable number of carbon hull and mast failures I've seen are consistent with this failure mode, and as a result, are "spectacular" compared to the failure of, say, an Aluminium structure.
I shall look at the composite construction components on my aircraft now with abject terror. They have been twisted, bent, stretched and despite this they have been on the aircraft for God knows how long without rectification.
They're called helicopter rotor blades.
yes, but in rotor blade, and props, this Brittle Fracture Failure mode is desirable. They are less likely to dig in and cause more problems for a crew, who it must be said, have enough on their plates. assuming there isn't some poor sod under them, then having them shatter and nicely dissipate their excess energy in a ballistic arc is , while spectacular, generally safer for the a/c as a whole.

The question is, is it as desirable when we are talking about the fuselage itself, and indeed is it a more desirable set of traits than aluminium shows?

One question that is partly tangential, is the mass savings of moving to this material significant as opposed to aluminium, and if so, how significant or otherwise would that change be in terms of impact energy? It seems a stupid question, but I am not an engineer, and I am sure Boeing has information that I don't.
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Old 20th Sep 2007, 10:04
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As I mentioned earlier, I know sod all about modern composite structures. I am aware, though, that carbon fibre/epoxy resin structures do not suffer from many of the problems, such as osmosis, found in fibre glass reinforced polyester/acrylic resin ones.

I think many of us accept that a carbon fibre structure that hasn't received permanent strain will allow as good an impact survivability as an aluminium alloy one, if not better. Perhaps I'm being blind or thick (or both!) but I've not seen any explanation of how post "hangar rash" (credit to Capt Peacock), heavy landing, tail scrape/strike events will be investigated and evaluated. Did I miss it?
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Old 20th Sep 2007, 11:49
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Race cars seem to absorb some huge impacts and protect the driver.The Beech Starship did a lot to further the certification of composite structures. http://en.wikipedia.org/wiki/Beechcraft_Starship
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Old 20th Sep 2007, 12:27
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The race car experience in my opinion is directly applicable. Also not directly mentioned is the current experience with current carbon fibre aircraft bodies themselves. The F-22 comes to mind. This plane fuselage was broken into three large pieces and not shattered into dust
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Old 20th Sep 2007, 14:05
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As I understand it the lightning strike protection will be provided by either the usual flame spray type coating or an impregnated conductive mesh such that usual airframe conductive figures would be obtained.

Slight thread creep here but llondel and mocoman need to go back to school to re-assess the effect of a lightning strike on an aircraft. They have both been saying the voltage is constant whereas if fact it is NOT.

The aircraft acts as a resistor WITHIN AN EXTERNAL CIRCUIT which is from the ground to the PD source (or vice versa depending on whether you are conventionally or electron minded). SO. If the resistance within the airframe is high then the PD across the aircraft (i.e. the voltage) increases (V=IR) and therefore so does the current. As we are talking about thousands if not millions of volts then even a small resistance can result in HUGE power dissipation. This why the bonding of aircraft is so important.
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Old 20th Sep 2007, 21:55
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lomapaseo,
The thing about race cars (especially F1) is that although the body is made out of carbon fibre, it is bonded to an Aluminium honeycomb structure (at least in the nose cone). Its the honeycomb that helps absorb the energy, crumpling in a crash.
In other race cars like Nascar the driver is sitting in a huge steel cage onto which the body is attached.
Overall though I would rather be in an aircraft than a race car (parent of a 24 year old son who wants to take father round the nurburgring before son turns 25).
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Old 21st Sep 2007, 01:15
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The thing about race cars (especially F1) is that although the body is made out of carbon fibre, it is bonded to an Aluminium honeycomb structure (at least in the nose cone). Its the honeycomb that helps absorb the energy, crumpling in a crash.
In other race cars like Nascar the driver is sitting in a huge steel cage onto which the body is attached.
Overall though I would rather be in an aircraft than a race car (parent of a 24 year old son who wants to take father round the nurburgring before son turns 25).
I doubt that the honeycomb absorbs any significant energy. Crushing absorbs squat, it's deformation under the stress/strain curve that absorbs energy. You can take the honeycomb out lay it on a table and slap it flat with your hand, it's sole purpose is to provide stiffness to the supporting structure.

However I won't argue about the energy absorption capabilities of the race car composite cockpit shell. It simply provides lots of stiffness to distribute the g-loads evenly to the occupant. the real energy absorption takes place by the surrounding mangling of the frame and time extending bounces. One of the things that is applied is a basic crash recorder attached to the shell which records max G-loads. For the very serious crashes this is consulted before removing the occupant so as to lessen the internal trauma. (cutting out vs lifting the body out)

The argument about the composite was not it's excelent energy absorption but's it longevity throught the crash impact scenario.

So in simple opinion, in the same type of crash, both an aluminum airframe and a composite aircraft will protect or not protect the occupants relatively to the same level of survivability. in my experience I have seen both.
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Old 22nd Sep 2007, 12:34
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lomapaseo

The F-22 comes to mind. This plane fuselage was broken into three large pieces and not shattered into dust

were these 3 pieces the marry up joins?
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Old 22nd Sep 2007, 12:38
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Quote:
Every single frame on the A380 Main deck (98 of them) is Carbon fibre. and in the interests of accuracy they are manufactured completely differently and with far higher tolerances than boat masts ! (I had to laugh at that one) Furthermore only Titanium fixings are used to secure other structural items to them.

Actually the cargo deck uses metal beams, don't know if there are any plant to go CF with these............. I've only manages to count 95 frames on the main deck and the pressure bulkhead.
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