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Long term saftey of Composite Structures


c-17guy

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Hello all,

 

I am very new to the composite process and have completed a practice kit. The composite structure seems to be very strong. My question has to due with the temperature breakdown or Tg, I think. I have noticed that most room cured epoxy's have a Tg of 130-150. I am in the AF and travel quite a bit, and my concern is that I would park my new Cozy on a tarmac on a summer day that can radiate temperatures in this range. I have also noticed that post curing raises the Tg, however, where can you go to post cure an airplane? I guess what I am looking for is someone to give me the confidence that the plane will be structually sound even on a hot day. Thanks again.

 

ivan

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These things have been sitting in the hot desert sun for years. I commuted from San Jose to Fresno every day for about a year. With an OAT of 115, I often seen my temperature guage hitting 130-140.

 

 

I post cured all my parts simply by setting them out in the sun. Just keep an eye on the temperature, not to exceed 160.

 

Waiter

F16 performance on a Piper Cub budget

LongEZ, 160hp, MT CS Prop, Downdraft cooling, Full retract

visit: www.iflyez.com

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Ideally you post-cure the parts still in the mould (when working with moulds), by slowly raising the temperature and keeping it there for a few hours. If you don't have a big enough temperature controlled curing chamber, you can build a tent out of plastic, place the parts in a big cardboard box, surround it with styrofoam, or build a box out of sheetrock or similar material and put a heater or a bunch of light bulbs inside.

Most homebuilders don't seem to bother though, which I find kind of weird. Then again building a plane takes years, and by that time the epoxies will have cured to a sufficient degree and the sun will take care of the rest.

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Ideally you post-cure the parts still in the mould (when working with moulds), by slowly raising the temperature and keeping it there for a few hours. If you don't have a big enough temperature controlled curing chamber, you can build a tent out of plastic, place the parts in a big cardboard box, surround it with styrofoam, or build a box out of sheetrock or similar material and put a heater or a bunch of light bulbs inside.

Most homebuilders don't seem to bother though, which I find kind of weird. Then again building a plane takes years, and by that time the epoxies will have cured to a sufficient degree and the sun will take care of the rest.

If you are going to post cure, either by oven, plastic (black in the colder climates), watercoloring the part black etc. Make sure that you support the thing you are curing. It may take a set.

 

The original dragonfly had it's wheels at the outside of the 22' canard. It was noticed, on the prototype, that the propeller got closer and closer to the ground as time went on. The cure was to heat the wings near the root and jig it so that the anhedral was restored.

I Canardly contain myself!

Rich :D

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> If you are going to post cure,

 

It kind of bothers me that people are obviously using epoxies like the L28X range without post curing. L285 with hardener 285 will only develop its great mechanical properties and high Tg when post-cured, and H286 and H287 definitely need heating when making parts in moulds for demoulding because they end up very brittle without it. Actually every epoxy with a higher Tg than West system should be post cured, they all benefit from heat treatment.

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> If you are going to post cure,

 

It kind of bothers me that people are obviously using epoxies like the L28X range without post curing. L285 with hardener 285 will only develop its great mechanical properties and high Tg when post-cured, and H286 and H287 definitely need heating when making parts in moulds for demoulding because they end up very brittle without it. Actually every epoxy with a higher Tg than West system should be post cured, they all benefit from heat treatment.

 

Ya, what you said.

 

However the real question is, "is the un-postcured structure strong enough for our application and is post-curing guilding the beautiful lilly?"

 

Those of you flying these birds can do us all a service by sharing some info such as:

 

1. Did you post cure your bird and if so, what parts,how did you do it

2. # of hours and years your aircraft has been flying

3. Normal ambient temperature in which your plane is kept

4. Hangered or non-hangered

5. If not post-cured have you noticed any problems due to the structure

 

My uneducated guess is that most of the planes flying have not been post cured. Is this a case of numerical one-upsmanship or is this a real concern.

(Is it important that a tweeter can give 25K Hz when the human hearing limit is much less). If it is a concern, why do we not see these planes falling from the sky, in gooy pieces:confused: On the other hand, do we all need to be very concerned about post-curing of specific or all of the parts:sad:

I Canardly contain myself!

Rich :D

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Rich;

 

Back in the late 80's, it was easy to see who was post curing and who wasn't.

 

Look down the top surface of a wing, particularly in the area of the spar caps. If the plane was finished to a high degree, then this would be a very nice flat, slightly curved surface with no imperfections, perfect. After several years, if you could see any signs of ripples in the finish, odds are, the plane was built in the north and/or was not post cured.

 

My EZ has 2600 hours on it and is 12 years old, the finish is as good as the day I painted it, there are no ripples in any of the surfaces.

 

I built my EZ while living in Calif (San Jose / San Diego), all the parts were post cured, simply by placing them out in the sun for a couple days. I monitored the temperature with my hand. If it was to hot to leave my hand on the part, then it was to hot and I'd throw a tarp over it.

 

Planes that were built in the northern climate and not post cured may suffer from "creep". This is what causes the "ripples".

 

If the parts are post cured before finishing, you won't get this "creep".

 

Keep in mind that this "creep" that I described is not unsafe. The epoxies just take on a new set when its exposed to a higher temperature. The idea of post curing is to expose the epoxies to the highest temperature it will see in its life, before the finishing process, not after.

 

Waiter

F16 performance on a Piper Cub budget

LongEZ, 160hp, MT CS Prop, Downdraft cooling, Full retract

visit: www.iflyez.com

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You could probably use someone's facilities. Andy Millin (a velocity builder) did his postcure at what looks like a body-shop, although he says it was in there with other airplanes. http://www.kal-soft.com/velocity/weblog.asp?pic=137

 

I would think that you could probably rent time from a car body shop that has the facilities, assuming the temps are right.

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Seriously, setting the item out in the summer sun will do the job, even in the northern latitudes.

 

If you can get the surface temperature up to 120-140, thats more than adaquate to prevent any future creeping.

 

Enclosed are photos of my landing gear doors going through post cure. Even in March with an OAT of 70 degF, the doors still got up to almost 100 degF.

 

Waiter

post-706-141090161351_thumb.jpg

post-706-141090161357_thumb.jpg

F16 performance on a Piper Cub budget

LongEZ, 160hp, MT CS Prop, Downdraft cooling, Full retract

visit: www.iflyez.com

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MGS charts show postcure temperatures ranging from 50C to 80C (120F-175F). The melt temperature of polyethylene is supposed to be around 100C (212F). So in theory, a poor man's postcure chamber could be constructed using a 6 mil polyethylene vapour barrier bubble and a couple of hair dryers.

 

I've done this in the past when the garage got too cold for epoxy. Garage temperature was 14C (57F) while the temperature inside the enclosure was 40C (104F). Cold garage air was heated by the hair dryers and blown in to inflate the bubble, and simply leaked out where it could. If this was done in a space where ambient temperature is warmer (i.e. room temperature), the enclosure temperature might be in the postcure range.

 

For even hotter enclosure temperature, warm exhaust air can be ducted back into one of the hair dryers, provided that it doesn't over-heat and trip out. Also, the air needs to circulate well to prevent hot spots, on both the part as well as the bubble. (I've tinkered with that and it could be done if set up correctly.)

 

The photos below show two set-ups. The larger one is 6 mil vapour barrier with 2 hair dryers. The smaller one is made of garbage bags and uses one hair dryer. Similar results for both.

 

I've only used this method for "room temperature" cure, and not for actual postcuring, but I intend to give it a whirl when the canard is done.

 

 

Joe Polenek

post-2447-141090161359_thumb.jpg

post-2447-141090161365_thumb.jpg

Joe

Cozy Mk IV #1550

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Waiter-

This must be one of those post-cure facilities where as your part cures, you chase a ball around with several sticks in a bag and play cards afterwards while drinking iced tea...

Works for me..[except me not a tea drinker]. What style!...You are livin' the life brother! =]

Self confessed Wingnut.

Now think about it...wouldn't you rather LIVE your life, rather than watch someone else's, on Reality T.V.?

Get up off that couch!!! =)

 

Progress; Fuselage on all three, with outside and inside nearly complete. 8 inch extended nose. FHC done. Canard finished. ERacer wings done with blended winglets. IO540 starting rebuild. Mounting Spar. Starting strake ribs.

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  • 1 year later...

Not being an expert on composites, I have listened to many discussions. This post curing of composites in the sun seems to "fly" in the face of other things that I have heard about UV deterioration of epoxy structures.

 

Do you put sunscreen on the unfinished components prior to placing them in the direct sunlight? Or maybe; is this limited time in the direct sunlight not enough to be concerned about?

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Do you put sunscreen on the unfinished components prior to placing them in the direct sunlight? Or maybe; is this limited time in the direct sunlight not enough to be concerned about?

Typically to get the most out of this process, the builder will usually cover the part with black plastic to get the most solar gain while still providing UV protection (as unintentional as it might be.)

 

Because of the limited amount of time, I would say it is a non issue.

What would concern me more would be the improper storage of material to provide UV protection.

T Mann - Loooong-EZ/20B Infinity R/G Chpts 18

Velocity/RG N951TM

Mann's Airplane Factory

We add rocket's to everything!

4, 5, 6, 7, 8. 9, 10, 14, 19, 20 Done

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UV exposure brakes down the molecular structure of the epoxy.

 

This is a long term process that takes years in the sun to have an impact.

 

The UV exposure for a couple days will have neglegable effect.

 

The challange I see associated with the "Bake it in the Sun" process is being able to monitor/control the heat. Remember, you need to stay below 160F or damage to the foam could result.

 

My thoughts on a tent are, its an overkill. You could trap heat and raise the temperature higher than you need to go (140F should be OK.)

 

I lay the components out in the sun with no protection, then use the "Hand Test" to monitor the heat. If I can't keep my hand on the component, its to hot. (about 140F)

 

Remember, the idea behind "Post Cure" is to cure the component at a higher temperature that it will be exposed to during its life cycle.

 

About the highest I've seen is on a black tarmac on a hot fall day in Fresno Calif. The tarmac air temperature easily reaches 130 F.

 

On the subject, I did some temperature testing here in NW Ohio. Look at my web site;

 

http://www.iflyez.com/LongEZ_Retrofit_JUN_07.shtml

 

Scroll down to the 9 JUNE 2007 entery

 

Waiter

F16 performance on a Piper Cub budget

LongEZ, 160hp, MT CS Prop, Downdraft cooling, Full retract

visit: www.iflyez.com

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Not being an expert on composites, I have listened to many discussions. This post curing of composites in the sun seems to "fly" in the face of other things that I have heard about UV deterioration of epoxy structures.

 

Do you put sunscreen on the unfinished components prior to placing them in the direct sunlight? Or maybe; is this limited time in the direct sunlight not enough to be concerned about?

most of the aircraft built are not post cured. I believe Burt stopped recommending it very early in the program.

Evolultion Eze RG -a two place side by side-200 Knots on 200 HP. A&P / pilot for over 30 years

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> most of the aircraft built are not post cured.

 

Maybe the homebuilt canard variety isn't. Every commercially built composite airplane most certainly is post-cured.

 

> My question has to due with the temperature breakdown or Tg, I think

 

Tg has nothing to do with breakdown. It's a temperature where some of the mechanical properties of the composite start to change. Many polymers (rubbers for example) are constantly well above their Tg and they do not break down.

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Post cure also makes the part a little more rigid (harder). For this reason, I wouldn't post cure cowlings. Keep them as flexable as you can until after they are mated up to the fuselage/wing combination. Cowlings will naturally cure after a few flights from the hot engine.

 

The only draw back to postcure would be if you need to make an adjustment on something, i.e. a wing or panel that is slightly warped.

 

you can sometimes heat he part up and tweek it a little. I made some pretty large changes to my main landing gear doors with this method

 

Waiter

F16 performance on a Piper Cub budget

LongEZ, 160hp, MT CS Prop, Downdraft cooling, Full retract

visit: www.iflyez.com

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> most of the aircraft built are not post cured.

 

Maybe the homebuilt canard variety isn't. Every commercially built composite airplane most certainly is post-cured.

 

 

I was referring to our canard aircraft and that is a pretty bold statement that you know that they all are most certianly post cured. should I start naming the ones that I know are not. there are plenty of commercially built aircraft parts that are hand layups and are not post cured.

Evolultion Eze RG -a two place side by side-200 Knots on 200 HP. A&P / pilot for over 30 years

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> that is a pretty bold statement that you know that they all are most

> certianly post cured.

 

At least all the manufacturers I know and all the shops I have seen on this side of the big pond use post curing, for a variety of damn good reasons. One of the reasons being that epoxies with reasonably long working times REQUIRE it for a full cure. Another one has something to do with epoxies being approved for aviation use and legalities. If the regulations say: heat it, then the manufacturers better not cut any corners and get caught doing so.

 

> should I start naming the ones that I know are not.

 

Please do.

 

> there are plenty of commercially built aircraft parts that are hand layups and

> are not post cured.

 

Scary. Please don't tell me they use West System, too...

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Do you put sunscreen on the unfinished components prior to placing them in the direct sunlight? Or maybe; is this limited time in the direct sunlight not enough to be concerned about?

Here's a data point: My Velocity project was parked in my carport in a place where the morning sun would hit the top of the right spar for a few hours every day. I didn't realize that it was getting any direct sunlight (Doh!) until I noticed after about 8 months that the top of the spar was getting a little chalky. So after about 600 hours of direct sunlight the spar was damaged enough that I needed to reinforce it. After 50 hours I'll bet the damage was so slight that no repair would have been necessary, and after 5 or 10 hours I'll bet it was neglible.

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