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New Composite Airliners, Why No Paint Issues?  
User currently offlineNorthwest727 From United States of America, joined Jul 2005, 491 posts, RR: 0
Posted (3 years 2 months 3 days 3 hours ago) and read 4307 times:

Hey everyone. I just completed my Cirrus CSIP training, and one interesting thing to note, is that there is a limitation to the SR-20/SR-22 that states that the composite skin must not exceed 150ºF in temperature. Therefore, that is why you don't see dark-colored Cirrus'. The newer models thanks to advancements in the composite technology in recent years, are allowed to have a darker color, provided it is reflective (metallic).

So this makes me wonder, is there a difference in technology between the composite on Cirrus, a general aviation aircraft, and airliners, such as the Boeing 787? The Boeing "house" color has the dark bottom, which based on photos, does not seem reflective.

8 replies: All unread, jump to last
 
User currently offlineFly2HMO From , joined Dec 1969, posts, RR:
Reply 1, posted (3 years 2 months 2 days 23 hours ago) and read 4243 times:

Quoting Northwest727 (Thread starter):
I just completed my Cirrus CSIP training, and one interesting thing to note, is that there is a limitation to the SR-20/SR-22 that states that the composite skin must not exceed 150ºF in temperature.

Is this a straight up limitation listed in the POH?

Not doubting you but that seems like an awfully low temp. There's no way in hell a Cirrus parked out in the sun in a sunny PHX day when its hitting 120F in the shade will be below that, even if its white.

Quoting Northwest727 (Thread starter):

So this makes me wonder, is there a difference in technology between the composite on Cirrus, a general aviation aircraft, and airliners, such as the Boeing 787? The Boeing "house" color has the dark bottom, which based on photos, does not seem reflective.

Well as you said yourself, it is the bottom only, so logic would dictate it is not nearly as directly exposed as the top half of the fuselage, therefore that area must remain a bit cooler overall.

And I think its pretty safe to say that the stuff used on these "plastic" airliners is much MUCH more resilient than what's used on those expensive toys for doctors/lawyers with a big fan up front  


User currently offlineGST From United Kingdom, joined Jun 2008, 932 posts, RR: 0
Reply 2, posted (3 years 2 months 2 days 18 hours ago) and read 4166 times:

Quoting Fly2HMO (Reply 1):
it is the bottom only, so logic would dictate it is not nearly as directly exposed as the top half of the fuselage, therefore that area must remain a bit cooler overall.

If you're parked on concrete or asphalt the difference is not nearly so great as you might imagine, they do have a habit of reflecting heat rather well.

The problem is the resin used in composites, which is usually a form of epoxy. Epoxies never quite set, they go dry and hard but if you warm them up they soften and are able to flow a little and anything they are supporting (i.e. the fibers in the composite) can move. Have you ever looked at a 10 year old composite sports car? Odds are the skin panels aren't aligned quite so cleanly as when the vehicle was new. You get around this problem by curing (or post curing, putting it in the oven after the epoxy has initially set but being sure the part is properly supported so it can't warp and re-set into a shape you don't want) your composite at a higher temperature. This raises the thermal gradient (TG) that defines how soft the material will go at a given temperature, so your composite will not start to warp at less than the temperature you cured and post cured it at.

You quote the Cirrus' limit of 150ºF (66ºC) which suggests that some critical parts of the aircraft are temperature limited to that temperature as they may have only been cured at perhaps 70-80ºC (158-176ºF). You would expect the part to be good until the temperature it was cured at, but as skin temperature across the entire aircraft is not something you can continually accurately measure practically, they impose a lower limit to give a safety margin.

Another possibility with the Cirrus temp limit is that some components may be made around a foam core that is liable to warp or turn to mush over a given temperature.

Now to put this in the context of larger aircraft such as the upcoming A350, the important or high temperature composite parts are being cured at 180ºC (356ºF) as standard, so their TG is more than an order of magnitude higher. All components are molded also, so no foam cores that are sensitive to temperature either.


User currently offlinedynamicsguy From Australia, joined Jul 2008, 873 posts, RR: 9
Reply 3, posted (3 years 2 months 2 days 18 hours ago) and read 4152 times:

Quoting GST (Reply 2):
This raises the thermal gradient (TG)

Sure you don't mean T subscript G which is the glass transition temperature, and which affects the temperature which the composite can be safely exposed to?

Another consideration is that many composite material allowables are reduced at higher temperatures.


User currently offlineraptor1090 From United Arab Emirates, joined May 2011, 82 posts, RR: 0
Reply 4, posted (3 years 2 months 2 days 17 hours ago) and read 4120 times:

Quoting dynamicsguy (Reply 3):
Sure you don't mean T subscript G which is the glass transition temperature

That's surely what he meant. Thermal gradient doesn't define anything, it's just the difference in temperatures across the material.

Quoting dynamicsguy (Reply 3):
Another consideration is that many composite material allowables are reduced at higher temperatures.

That is because of Tg.


User currently offlineNorthwest727 From United States of America, joined Jul 2005, 491 posts, RR: 0
Reply 5, posted (3 years 2 months 2 days 16 hours ago) and read 4106 times:

Quoting Fly2HMO (Reply 1):
Is this a straight up limitation listed in the POH?

Not doubting you but that seems like an awfully low temp. There's no way in hell a Cirrus parked out in the sun in a sunny PHX day when its hitting 120F in the shade will be below that, even if its white.

Yes, this is straight out of the Cirrus POH:

Quote:
To ensure that the temperature of the composite structure does not exceed 150ºF (66ºC), the outer surface of the airplane must be painted with an approved white paint, except for areas of registration marks, placards, and minor trim. Refer to SR20 Airplane Maintenance Manual (AMM), Chapter 51, for specific paint requirements.
Quoting GST (Reply 2):

Now to put this in the context of larger aircraft such as the upcoming A350, the important or high temperature composite parts are being cured at 180ºC (356ºF) as standard, so their TG is more than an order of magnitude higher. All components are molded also, so no foam cores that are sensitive to temperature either.

Your post makes good sense, my question is, why is the Cirrus then cured at a lower temperature? Is there a higher cost involved with curing at higher temperatures?

[Edited 2011-07-18 04:09:26]

User currently offlinedynamicsguy From Australia, joined Jul 2008, 873 posts, RR: 9
Reply 6, posted (3 years 2 months 2 days 15 hours ago) and read 4064 times:

Quoting raptor1090 (Reply 4):
Quoting dynamicsguy (Reply 3):
Another consideration is that many composite material allowables are reduced at higher temperatures.

That is because of Tg.

The reduction in allowables happens way below Tg. For analysis our of certain prepregs hot/wet allowables are at 108 degrees F below Tg.


User currently offlineGST From United Kingdom, joined Jun 2008, 932 posts, RR: 0
Reply 7, posted (3 years 2 months 2 days 8 hours ago) and read 3910 times:

Quoting dynamicsguy (Reply 3):
Quoting raptor1090 (Reply 4):

Thanks for correcting me, seems I had left my common sense and experience in bed this morning.

Quoting Northwest727 (Reply 5):
why is the Cirrus then cured at a lower temperature? Is there a higher cost involved with curing at higher temperatures?

Cost is almost certainly it. If you're curing much above normal room temperature you need an oven. You can make a curing room that will go to high temperatures when no one is inside but that would be inefficient as hell. Aircraft parts are often chunky, awkward shaped, or both so for production you generally need a big oven (especially to cure many parts in a batch). A 120ºC (248ºF) oven is about the hottest that can be made on the cheap (i.e. timber frame with plenty of insulation foam), but you also need heating elements, fans inside to move the air around, thermocouples to measure the surface temperatures of your parts in as many places as possible, and some sort of control system (as there is a maximum rate that you can ramp up or down the temperature without risking losing bond strength or warping the parts). In addition you need some form of vacuum pump as you bag your part and its mould, before removing the air so that the atmospheric pressure around it acts to consolidate the part. All this can be done for a few thousand dollars. This is the type of setup I use on a day to day basis making aircraft parts, albeit for a prototype.

More expensive ovens can go to higher temperatures, but where cost is less of a problem you may go for an autoclave, which is basically an oven combined with a pressure vessel. As you suck the air out of your vacuum bag in an autoclave, you can have the pressure of several atmospheres acting on your part improving the compression between layers of fibre as you set the resin thus guaranteeing a more consistently strong bond. They cost a lot to build, run, and maintain mind.

Electricity will be the big cost in serial production, you have to maintain the temperature for a long while (the lower the cure temperature the longer you need to hold it) to set the resin. The hotter you go the less effective your oven's insulation will be so you may see something approaching an exponential rise in electricity required to hold increasing temperatures. Therefore running the oven overnight for a 15 hours (an arbitrary example, this will change from one resin brand & type to another) at 70ºC (176ºF) may cost less than a 120ºC (248ºF) 90 minute cure. Also don't forget the limit in ramp up or down rates for your resin that must not be exceeded, it takes far less time to get from ambient to the low temperature cure and vice versa than it does to get to the high temperature, and this time must be added to the time at full temperature required to run the cure.

Basically it all comes down to cost, at some point they asked what value it would add to the aircraft to let people paint it any colour they wanted or to leave it outside in the hottest place on earth, and came to the conclusion that this did not sufficiently offset the cost of producing a structure that could take the associated maximum temperatures. Airlines place a big value on painting their aircraft distinctively to mark their bran and to fly anywhere any time, private owners and syndicates generally see it as an interesting benefit, but not worth massive additional cost.


User currently offlinebikerthai From United States of America, joined Apr 2010, 2130 posts, RR: 4
Reply 8, posted (3 years 2 months 2 days 7 hours ago) and read 3882 times:

Coming from a different perspective.

Consider that most likely the paint on a commercial airline is an Epoxy base paint, the compatibility of the paint to stick to the Epoxy skin system is much greater.

If you are to paint on to Aluminum, you will need a primer base. And guess where the failure mode will occur . . .

The thing about composite skin vs. a metal skin on the Tarmac is that the metal skin will be able to conduct the heat away from the surface much better than a composite skin.

But then they have been painting on composite for some times now (trailing edges, 777 tail, engine nacelle) so presumably they have found a paint (epoxy based or other wise) that will meet the thermal requirements.

Quoting dynamicsguy (Reply 6):
For analysis our of certain prepregs hot/wet allowables are at 108 degrees F below Tg.

We use 180F as a standard max operation for a 250F Cure system, though allowables would be diminished (as you said).

bikerthai



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