What are pros and cons of carbon fibre aircraft, specifically in hot humid North Queensland

[SSCBD]

Hi all what are the problems of a carbon fibre aircraft in hot humid conditions in nth Qld, and if you put a ding in the wing or leading edge how much of a problem is it to get repaired compared to metal wing.

 

Any special maintenance things that one should be aware of. Any structural problems over time etc

 

And the last question will UV cause any major problems with carbon fibre over time say 3 to 5 years.

 

What are the do’s and don’ts compared with metal aircraft

 

Thanks in advance

 

 

[DrZoos]

Biggest con in any climate is de laminating n, or invisible structural failure

 

Biggest pro is lack of corrosion

 

Be cautious with max ambient temps as certain glues in any aircraft go soft or lose strenght with heat

 

 

[Bruce Tuncks]

A big con is the difficulty of repairs compared with glass. And carbon fibre is much more brittle, so you can get damage transmitted by shock wave to places well away from the impact.

 

But the strength allows you to have no struts and still not too heavy a wing.

 

You still need a coating to protect from UV . The coating should be white for the reason DrZoos pointed out, to protect from extreme temperatures.

 

 

[Pearo]

As carbon fibre user in sailing, the stuff should not delaminate. Delamination is a builder issue, not a product issue. Repair is much easier (stronger are more long lasting) than metals, but you need someone that knows what they are doing.

 

Just be warned, carbon fibre corrodes everything (galvanic corrosion) so make sure the carbon itself never makes contact with any metal parts.

 

UV problem has nothing to do with carbon, but everything to do with the resin. UV stabilised resins with a good UV stable paint and you will never have an issue. Almost none of the carbon fibre parts on my boat have paint, and the boat lives in the sun and never has a problem. I also know may other sailors that use carbon masts that are 15-20 years old with no issues.

 

Dont be scared of carbon fiber, used well its much better than any metal. Then again, we yachties run it on the edge of strength to save weight and when it fails it sends a million spears of stuff harder than any metal toward you. Aluminium on the other hand just buckles and bends!

 

 

[DrZoos]

agree Pearo but, unless your the actual builder with many many many years of personal experience the biggest issue is still delamination..... As we don't know the quality till it lives on or fails....and even within great companies, quality of components varies....as two aircraft may have different tradesman or even two wings or parts of wings etc....

 

learn how to check and check regularly, especially on structural or critical parts...do not trust reputations or rumours of quality and strength when it comes to checking continued airworthiness of any component

 

When i made the choice to buy carbon , i did so on the assumption i would not fly it in extrme heat or leave it outdoors except when travelling

 

 

[M61A1]

What sort of resins are the manufacturers using? Most of the ones I'm familiar with are happy at temperatures over 100 degC, some of them use an accelerated cure around 180 degC. The machines I work on have a lot of composite parts, mostly carbon/glass hybrid, and most of it dark green/ matte black. Sometimes the rotor blades get so hot you can't hold them.

 

Composites require different NDI techniques than metal, but I don't think it's any worse than wood. Repairs are not particularly difficult, just a different process.

 

 

[Pearo]

agree Pearo but, unless your the actual builder with many many many years of personal experience the biggest issue is still delamination..... As we don't know the quality till it lives on or fails....and even within great companies, quality of components varies....as two aircraft may have different tradesman or even two wings or parts of wings etc....learn how to check and check regularly, especially on structural or critical parts...do not trust reputations or rumours of quality and strength when it comes to checking continued airworthiness of any component

 

When i made the choice to buy carbon , i did so on the assumption i would not fly it in extrme heat or leave it outdoors except when travelling

No offence, but the quality of any aircraft is dependent on the builder regardless of the material. Your suggestion about regular checking of components is not restricted to aircraft built with composite materials.

 

Your choice about when to fly a carbon aircraft is extremely flawed.

 

 

[bexrbetter]

Hi all what are the problems of a carbon fibre aircraft in hot humid conditions in nth Qld, and if you put a ding in the wing or leading edge how much of a problem is it to get repaired compared to metal wing.

Not a single person can answer your question without knowing the layup.

 

CF can so soft it can be folded over onto itself, if that's the schedule used to make it, the stiff schedule layup that shatters that people mention (and which is nonsensical internet lore), also fail to mention how much more force is required over aluminium before it yields (shatters), aprox 3 times more and as much as 10 times the tensile strength, but again, impossible to determine without know the layup schedule.

 

You would be able to hit an aluminium wing with a sledgehammer and %^&* it but the hammer would merely bounce off CF - depending on, wait for it .... yup, the layup.

 

 

[M61A1]

Not a single person can answer your question without knowing the layup.CF can so soft it can be folded over onto itself, if that's the schedule used to make it, the stiff schedule layup that shatters that people mention (and which is nonsensical internet lore), also fail to mention how much more force is required over aluminium before it yields (shatters), aprox 3 times more and as much as 10 times the tensile strength, but again, impossible to determine without know the layup schedule.

 

You would be able to hit an aluminium wing with a sledgehammer and %^&* it but the hammer would merely bounce off CF - depending on, wait for it .... yup, the layup.

Agreed that the layup is of utmost importance, but, generally that wonderful stiffness come at the cost of brittle when dealing any impact. Hitting a carbon panel with a hammer ( hard), will not bounce off, it usually results in a hammer shaped hole. If the panel is flexible enough to give, it will probably give under flight loads. ( a panel usually of sandwich construction).

 

 

[bexrbetter]

Hitting a carbon panel with a hammer ( hard), will not bounce off, it usually results in a hammer shaped hole.

Exactly what I pointed out above, you have failed to offer it in context.

 

You have to hit that CF around 10 times harder (hammer speed tripled) than you would to the equivalent aluminium to cause the hole (tensile strength). You have to hit it 3 times harder (hammer speed almost double) to make the CF give and stay give'd (yield, eg; make a dent).

 

The greater tensile and yield strengths (at equal weights) are why parts can be made so much lighter while retaining the same strength.

 

 

 

[jetjr]

Possibly an answer well beneath your request but many "plastic" aircraft are fibreglass composite not carbon fibre. Plenty use the wrong name.

 

Although not as strong, much cheaper and easier to work on and own

 

 

[Bruce Tuncks]

Yep glass is much easier to repair. I repaired a glass glider from a write-off and I have been around enough carbon repair jobs to be pleased they were not my problem. It's hard enough to do a 50:1 scarf in glass where you can see everything. With carbon, the layer is much thinner and everything gets obscured with soot-like powder when you sand. And that black powder is highly toxic.

 

You can get a light wing using glass if you use struts... I wonder what aircraft is that smart.

 

With gliders, you have to use carbon on the big wingspan jobs, as glass is too flexible and the tips would rotate just enough to muck up the aerodynamics.

 

 

[M61A1]

Exactly what I pointed out above, you have failed to offer it in context.You have to hit that CF around 10 times harder (hammer speed tripled) than you would to the equivalent aluminium to cause the hole (tensile strength). You have to hit it 3 times harder (hammer speed almost double) to make the CF give and stay give'd (yield, eg; make a dent).

 

The greater tensile and yield strengths (at equal weights) are why parts can be made so much lighter while retaining the same strength.

 

All correct, the point is or was though that when the CF panels are made they are normally sandwich construction, which has very thin skins with a foam or honeycomb centre. Under normal loads it exceptionally strong for it's weight, but being struck is not a normal load, CF does not do well in shear, it comes down to design as you say. Usually CF will be used with glass, or sometimes aramid to improve it's durability. I have seen perfectly good panels holed from mistreatment, where aluminium of similar strength would have bounced it off or at worst dented. Bear in mind that I work on military stuff that has been engineered to within an inch of it's life, and carries no extra fat..

 

 

[Old Koreelah]

My ideal is a carbon-Kevlar composite; the carbon keeps it rigid and, after being over stressed, the Kevlar pops it back into shape. Might get you home, but it's a throw away job because the carbon will have broken.

 

 

[cscotthendry]

Also, WRT operating temps, CF components that are structural are usually autoclaved at 80C or thereabouts to cure and harden. This is why CF aircraft must be painted white in structural areas. If those components were painted dark colours it might be possible to get them to a temp that would soften the resin.

 

My Legend is completely CF with an extra layer of Kevlar in the cabin area. It would be interesting to have a temp probe inside the wings and a gauge that reads max temps.

 

 

[Bruce Tuncks]

Does anybody know what was wrong with the carbon fibre blades Rolls-Royce were going to use on their engines but which nearly sent them broke?

 

 

[Oscar]

In my training to become a C of A inspector for gliders, we were taught to look very hard for the way in which forces applied at one part of the structure would be carried out to the rest of the structure. For example, in the case of damage to a wing l/e, almost the first place to inspect was the wing root attachments to the fuselage: quite small damage well out on the wing applies very considerable force to the wing attachment 'hard points' at the wing root/fuselage structure.

 

NDT of carbon-fibre structural components for internal cracking in particular requires very high-tech equipment and a lot of operator training, and some of that is only possible in laboratory conditions. With complicated shapes of structural components, it is sometimes impossible to 'test' unless the design loads can be applied exactly as for the proof testing - and that may be impossible for a component that is now surrounded by other parts of the airframe.

 

In 2009, Ducati introduced a form of 'chassis' ( of patented design) to its motoGP bikes, that had the c/f sub-frame bolted to the engine and incorporating the headstock and airbox passages. Ducati intended this to be the 'test' programme for a new production technique for its top-end road bikes - but did NOT use c/f as the material to be used in the road bikes, rather substituting aluminium. It is not difficult to imagine why: in the event of a crash, for product safety reasons, Ducati would have had to mandate at the minimum 'return to factory' of the subframe for testing. Ducati abandoned the c/f subframe idea completely in 2011.

 

 

[Pearo]

Also, WRT operating temps, CF components that are structural are usually autoclaved at 80C or thereabouts to cure and harden. This is why CF aircraft must be painted white in structural areas. If those components were painted dark colours it might be possible to get them to a temp that would soften the resin.My Legend is completely CF with an extra layer of Kevlar in the cabin area. It would be interesting to have a temp probe inside the wings and a gauge that reads max temps.

This is not true also. Although it the resin may be cured at lower temps, once cured it requires much higher temps to fail. Infact, using heat to cure epoxy it actually increases the temperature at where failure occurs.

 

For aerospace, its my understanding that the you must use resins that have a high failure temp (Tg) and from memory I think the actual decomposition temperatures are high than the melting point of aluminium (although I would not like to be quoted on that one!)

 

My ideal is a carbon-Kevlar composite; the carbon keeps it rigid and, after being over stressed, the Kevlar pops it back into shape. Might get you home, but it's a throw away job because the carbon will have broken.

This is fairly typical for hull construction in the maritime industry.

 

 

[Oscar]

Some vinyl ester resins have softening temps up around 130C, maybe a bit more after autoclaving. I am absolutely not aware of any 'resins' with the sort of softening temps that Pearo suggests. lc3600 Araldite, as used by Jabiru, starts to soften at around 80C .

 

 

[Oscar]

As a side issue, the occupant safety of a c/f cockpit IF it fails under impact, is very bad. I have inspected one such ( the Goulburn Sting crash) and virtually all of the cockpit area back to the mainspar was shattered into small pieces. As M61A1 says, it is extremely brittle when designed for maximum stiffness for weight.

 

 

[M61A1]

To get an idea of the temp/strength of some adhesives try googling the Tech Data sheets for some Hysol products...

 

9396 is one we use regularly for low temp applications (not around engine/exhaust) is begins to lose strength around 82degC.

 

There is another we use that is good up to around 150 degC , but has the disadvantage that it must be heat cured.

 

 

[Bruce Tuncks]

I've never heard of a mishap from epoxy softening with heat, but most glider handbooks say to keep them white, except for bits of color in the extremities.

 

On a hot day, you can certainly feel the difference with the non-white. I can imagine close to 80 degrees might just be reached.

 

 

[Oscar]

Bruce, similarly I have never heard or seen any incidents due to softening, but there isn't a huge margin of safety in extreme Australian sun conditions. One can use a standard domestic steam iron to 'smooth' thin epoxy composite skins (probably better not to ask how I know, but I was taught that trick by an aero-engineer who is well and truly up on composite construction..) A reasonably well-polished ( i.e. reflective) white skin shouldn't get above about 60C even on a stinker-hot day, but any dark area in the direct sunlight might cause local softening and if then stressed highly, could cause a localised high-stress point.

 

Your Libelle and our Jabs. have the 'advantage' over lighter but stiffer c/f composite aircraft, of the glass matrix being 'over-strong' in order to achieve stiffness. The cost there is weight; one of the residual benefits is a great ability to absorb damage stresses gradually, with the matrix failing ultimately by a tearing action which dissipates energy as it happens.

 

For those who cannot visualise the difference in failure mode between a 'stiff' and a 'strong' material (without getting into the Young's modulus area): try bending a high-carbon steel rod ( e.g. a Grade 12 bolt) vs a mild steel bolt of larger diameter. I don't have the required maths knowledge to suggest the exact equivalents, but if you get a piece of 'tool steel' rod of 1/4" diameter and a standard black steel bolt of say 1/2" diameter, I suspect that both will support the same weight applied on say a one-metre long bar without distorting. Then add weight to that bar until the tool steel rod breaks - which it will do dramatically ( wear eye protection..) Then repeat the experiment to the black steel bolt shank: it will bend and distort greatly, but it won't fail with the 'bang' and bits of metal being shot out across your workshop.

 

THIS is what happens when a high-stressed c/f structure fails:

 

 

and THIS is what happens when a low-tech composite (non-c/f) aircraft flies into trees following an EFATO:

 

https://www.google.com.au/search?q=plane+crash+wedderburn&biw=1513&bih=722&tbm=isch&tbo=u&source=univ&sa=X&ved=0ahUKEwjpxvD-lOfKAhVGVZQKHe8sB8gQ7AkIJQ&dpr=1.25#imgrc=SNqffmYa0dJhbM:

 

 

[Pearo]

Some vinyl ester resins have softening temps up around 130C, maybe a bit more after autoclaving. I am absolutely not aware of any 'resins' with the sort of softening temps that Pearo suggests. lc3600 Araldite, as used by Jabiru, starts to soften at around 80C .

I said decomposition temperature, which is where the material breaks down. I am well aware the the glass transition is below that.

 

 

[Oscar]

Surely, the relevance to this thread is the temperature that the aircraft structure starts to fail? Once the resin gets above the glass transition temperature, the structural integrity of the component is breached.

 

I suggest that you are thinking about the decomposition of c/f tows rather than c/f structures; while some aluminium alloys will start to lose strength at above around 220-230C (Jabiru head material being a prime example in aircraft use), most resins used in basic c/f structures will be molten at that temperature, if not aflame.

 

There are many c/f applications that have amazing temperature handling capabilities: brakes for aircraft and racing vehicles regularly get to around 1100F without failing - but those are NOT resin-compounded structures. They are 'grown' by extreme heat over a considerable period of time to a chemical mixture where the carbon is created in molecular chains - not physical strands - within the compound. High pressure is usually a feature of this process.