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Jun 11, 2019, 12:30 AM
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Soft Wing - What went wrong?


Iíve had some limited success (done it only 6 times including this one) building vacuum bagged wings for free flight catapult and hand launched gliders over the years, thanks to the Phil Barnes videos and all the great information in this forum. Thanks to all of you in advance for that!

However, my last one (just pulled it out of the bag today) was a complete failure and Iím hoping some of you more experienced builders can shed some light on what went wrong. I suspect my epoxy is bad or I am using it incorrectly, or perhaps the carbon cloth is not getting wet out fully, but I donít want to assume too much considering my limited experience in general and with this particular combination of carbon cloth and epoxy.

I knew something was wrong as I tried to pull off the mylars and they were very difficult to remove. This was after 36 hours in the hot box, at 105 degrees for the first 6 hours, then between 80-90 degrees for the rest. The mylars took a lot of force to peel away....were stuck to the skin to the extent that as I removed the mylar, it lifted the majority of the skin from the foam core, and now the wing is more like a soft flexible sailboat sail than a stiff wing as intended. The weird thing is that the skin is dry, but not stiff. It remains flexible like cloth, not much stiffer than it was in its raw state before the layup. The mylars were waxed exactly like all other vac bag efforts Iíve made before, with two coats of Meguiars Gold Class Carnuba Plus wax. Iíve had no problems at all getting mylars to release with this wax up until now, with curing temps up to 120 degrees.

Some details:
  • Epoxy: Adtech 820 with 824 hardener, brand new. Note that when I got it a few weeks ago, it was milky white and kind of thicker than I expected. Adtech support was very helpful, told me to heat it for a few hours and that it would return to normal. I did this a few weeks ago, and since then the epoxy has remained clear and of what I imagine to be its normal viscosity. For this wing, I mixed 20 grams of epoxy with 3.6 grams of hardener (100:18 ratio per instructions, mixing thoroughly for several minutes). The left over epoxy in the mixing cup, which I put in the hotbox along with the wing, seems to have cured nice and hard (although it has a bit of a waxy feeling on the surface). All my previous efforts in vacuum bagging wings have been with Pro-set 125/226, but itís almost 10 years old so I figured it was time to get some fresh epoxy and had read great things about Adtech 820 in this forum.
  • Skin: first 25% of chord is 2.4 oz unicarbon, and the rest is carboweave 30. First time using these cloths. Previous carbon wing used carboline 26g for the skin.
  • Core: pink foamular 250, probably 10 years old, but I havenít heard of foam going bad. I prepared the foam as I always have, removed the hairs from hot wire, light sanding and vacuum the dust away.
  • Process - rolled thin film of epoxy on mylars, laid down carbon cloth, rolled more epoxy onto cloth, doing so for much longer than seems necessary to wet the cloth thoroughly, blotted with seam roller and toilet paper until I achieved a 60/40 fiber/resin ratio.
  • Curing: 36 hours at elevated temperatures as mentioned above, with vacuum at 20 inches of mercury.

In retrospect, there may be one other potentially helpful clue. My first test of the Adtech 820 was on a very thin (1/8 inch max thickness) stab last week, using carboweave 11 gram skin over 1 lb density white foam. This too turned out kind of soft after a similar curing cycle temperatures to the one above, but at the time I attributed it to perhaps the cloth was too light (my first try with such a light weight cloth), or the foam core was too thin and weak. This was done using 4 mil drop cloth (not waxed) instead of mylars, and there was no problem with this material sticking to the epoxy when I removed the part from the bag.

Any insights on where this wing went bad would be greatly appreciated. That Christmas feeling I was supposed to get today ended up more like getting a bag of coal

Paul
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Jun 11, 2019, 06:07 AM
Entropy is happening!
Jim.Thompson's Avatar
Paul,

You have documented a very thoroughly detailed step by step construction method. I cannot find anything that provides us readers with a suspicion that anything in your procedure could be the cause of this failure.
From where I am reading, I have to say, it looks like a resin problem.
How about you do one or two small tests with it?
Can you totally rule out a mixing error?
Some more (several) tests will eliminate, or confirm either way. If they all fail, we are looking at a resin failure.

By the way, there is one thing I will comment on that is not related to the failure. That is the temperature sequence. The temp should be progressively ramped up over some hours, then left at the desired optimum for the most of the duration of the post cure.
Then, progressively ramp down.
Once you have cured at 105 deg.(F?), it is of little use curing for any extended time at a lower temperature. It is best the other way.

I would get back to the suppliers and at least inform them of your dilemma. You are clearly experienced enough to know there is something wrong other than your workshop procedure. (Given that the tests I mention above reveal the same result).

I would like to see some pics of your gliders, after you sort this out.

Keep us posted.

Jim.
Last edited by Jim.Thompson; Jun 11, 2019 at 04:14 PM.
Jun 11, 2019, 09:38 AM
Scott
Pylonracr's Avatar
I agree with Jim. Sounds like measuring error to me. You state that this is your first time with this resin? Double check with Adtech and make sure that they provided you with the correct mixture ratios. As suggested, do some test mixtures with resin and inexpensive glass fabric on some mylar or glass and see what it does. It sure sounds like the resin did not cure.

Scott
Jun 11, 2019, 01:44 PM
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Thread OP
Thanks for the input and encouragement guys. Much appreciated. I will run some tests against my other epoxy as you suggest and get in touch with Adtech again as well. Assuming the mixing ratio I have is correct, I don't think it was a mixing ratio error because I triple check that, but anything is possible. I will update this thread as I go in case we can all learn something.

I forgot to mention it last night, but there is one part I built with the Adtech that seems to have turned out fine, that being a throwing blade made with a wooden mold and vacuum bag. Pic below. The blade is very solid...can't even come close to breaking it with my bare hands. It was built with thick tows from some very heavy weight woven carbon fabric, saturated by sliding the tows through my gloved fingers coated with epoxy. If in fact the resin in the blade has cured as it should, that adds a little complexity to this mystery.

Jim, thank you for the advice your on curing temperatures, I did ramp gradually up to 105 degrees over about 3 hours. Adtech support told me that 4 hours at 105 should be good for the initial cure, equivalent to a 24 hour room temperature cure at 77 degrees. They also said that if I leave it longer at that temperature, even up to 24 hours, it's not a problem. The higher temp post cure was intended to be a second step, he suggested around 150 degrees for 4 hours.

Also, a few pics of my composite free flight gliders are below., and here's a video link to one of the first launches with the most recent.
040910 4k60 ctlg#1 second launch closeup from behind IMG 3103 TRIM (0 min 3 sec)


Back to the garage
Jun 11, 2019, 02:30 PM
Registered User
I agree, you have documented this very well and your procedures sound correct as well.

I switched to 820/824 about a year ago and have not had any problems.
Your mixture ratio looks right.
The hardened material in your mixing cup is adequate proof that the material worked and that you mixed it properly.

Except for the winter time when I use a hot box, I do mine at ambient temperature. In Arizona, that can be a tad warm. Regardless, all mine have turned out fine (...one ready to come out today, hope I donít jinx myself...).

The only thing I can think of from your description is that the layup was not fully wetted out, so when it stuck to the mylar, the loss of material led to a less rigid structure.

Partall 2 is the wax Iíve been using for years but just last week I had a part of the paint stick to the mylar. An RCG member suggested that I may have rushed the job as my paint might not have fully cured. Maybe something similar happened to you ?


Again - everything you did sounds right. If it were me, I would do some scrap pieces as a test and see what happens.

Please let us know what you find.
Thanks,
-Keith
Jun 11, 2019, 04:22 PM
Entropy is happening!
Jim.Thompson's Avatar
Quote:
Originally Posted by plove891
...............................
I forgot to mention it last night, but there is one part I built with the Adtech that seems to have turned out fine, that being a throwing blade made with a wooden mold and vacuum bag. ..........................
Was this done with the same mix of epoxy?
If not, then it is fairly conclusive that the problem was a mixing error.
If it was the same mix, then the problem is in the layup or the procedure somewhere.
Jun 11, 2019, 04:54 PM
Scott
Pylonracr's Avatar
Agreed. Was the mix by weight or by volume?

Scott
Jun 11, 2019, 05:43 PM
Registered User
Thread OP
Quote:
Originally Posted by Jim.Thompson
Was this done with the same mix of epoxy?
If not, then it is fairly conclusive that the problem was a mixing error.
If it was the same mix, then the problem is in the layup or the procedure somewhere.
The blade was made with a separate batch of epoxy. All three items were each made with their own batch of epoxy, wing, stab, and blade.

I took video of the wing build, and can confirm that I used 20 grams of epoxy and 3.6 grams of hardener, and mixed for almost 2 minutes. The mix was measured by weight, corresponding to the 100:18 ratio.

Below is a picture of the resin alone. The hardener is clear. I checked the resin again, and it is slightly milky this morning, so i'm sure this was how it was when i made the wing. It was much more milky looking when it arrived, before i heated it as mentioned in the original post. Keith, is this how yours looks?

I'm wondering if the epoxy is curing ok in concentrated masses like the bottom of the mixing cup and on the blade, but having difficulty in thin films like would be in the skin of the wing or stab. when you have a piece of cured epoxy the size about a quarter of an inch thick like in the bottom of the mixing cup, it's going to feel solid if at all cured, but it could be rubbery when in thin films....not sure. The solid mass from the bottom of the cup has a dull thud when i knock it against a hard surface, not a sharp crack sound like when I take a piece of hard plastic and do the same.

My upcoming tests and a reply from adtech support should hopefully help to figure this out.

If the epoxy is fine, and for some reason the wax didn't want to release, that could certainly explain how the skins were pulled from the cores. The fact that those skins are so flexible instead of stiff will be examined against how stiff a similar piece of cloth is when wetted out by itself on a mylar. That should provide some useful info.
Last edited by plove891; Jun 12, 2019 at 02:40 AM. Reason: add a detail
Jun 11, 2019, 07:43 PM
Registered User
Quote:
Originally Posted by plove891
Below is a picture of the epoxy. I checked it again, and it is slightly milky this morning, so i'm sure this was how it was when i made the wing. It was much more milky looking when it arrived, before i heated it as mentioned in the original post. Keith, is this how yours looks?
Mine was clear (mostly) but it does turn a touch whitish when mixing - but no where near as much as shown in your picture.
-Keith
Jun 11, 2019, 08:40 PM
It's time for me to fly
JimZinVT's Avatar
Quote:
Originally Posted by plove891
If the epoxy is fine, and for some reason the wax didn't want to release, that could certainly explain how the skins were pulled from the cores..
I'm far from an expert on this stuff, I have about exactly the same amount of experience as you with bagging wings. Just a guess here, since as the others have said, your procedure sounds correct: Maybe too much epoxy was removed when debulking the cloth, resulting in a poor bond to the foam, and maybe your new brand of epoxy is not entirely compatible with the Merguires wax (difficult release), the two could combine to cause the skin pulling from the foam. How is the bond and stiffness in the leading edge area where you used heavier carbon?
I find carbon, especially the unidirectional, much harder to wet out than glass.
Jun 11, 2019, 08:57 PM
Registered User
I agree with others: well documented and nothing obviously wrong with your technique. Here are some random thoughts that may or may not be relevant.

Leftovers in the mixing cup subjected to same heat treatment as the actual part is done exactly for this kind of a situation. Do you have a hardness meter? If not, a scraping test is also useful: scrape the surface with a sharp, pointy thing (corner of a chisel, tip of an x-acto blade, etc.). Don't try to cut into the epoxy, but scrape it by pulling the blade sideways at 90į to the surface. It should produce a nice, crisp 'crystalline' feel and sound, and the swarf should be loose, almost powdery. If it feels rubbery, with soft sound, and the swarf forms discrete balls, it hasn't cured properly.
Cut the test block in half, and test it along the cut surface to see if it is uniformly cured or if there are any differences between surface and cup bottom or side walls.

Curing in thin film vs a massive block could be different if the mixing cup is narrow enough to prevent heat radiating away and exothermic reaction accelerating the cure in the cup. But your heat treatment procedure would keep also the laminate at elevated temperature, so I don't see how this could be an issue. The only other explanation for differences of cure between cup and laminate is an uneven mix, but you sound too experienced to fall for that trap.

Cloudy epoxy (hardener in particular) that clears up when heated is usually an indication of expired product. Using it in repairs of full-size aircraft is a big no-no, but as long as heating clears it up, I have not had issues using it in non-critical applications.

Any possibility of contamination of your materials or tools (waxed cardboard cup used for mixing, somebody using silicone spray in the same room, etc.)?
Jun 11, 2019, 08:57 PM
Everything's A Compromise
Larrikin's Avatar
So, you're absolutely certain that the epoxy and wax are compatible?
With such light weight skin material, the release agent must be exceptional IMO ... like semi-perm, 'fall off the mylar' kind of exceptional.

Quote:
Originally Posted by plove891
I knew something was wrong as I tried to pull off the mylars and they were very difficult to remove. The mylars took a lot of force to peel away....were stuck to the skin to the extent that as I removed the mylar, it lifted the majority of the skin from the foam core, and now the wing is more like a soft flexible sailboat sail than a stiff wing as intended.
What has my attention is that there's a uniform layer of foam that has adhered to the skin. Because the skin material is so light, pulling on the mylar has peeled the skin from the foam before the mylar released from the skin. The foam has failed because the mylar is stiffer than the skin material. IMO.

Quote:
Originally Posted by plove891
The weird thing is that the skin is dry, but not stiff. It remains flexible like cloth, not much stiffer than it was in its raw state before the layup.
I'd be confirming this with another batch of resin, too. Use a scrap to test that this ultra light material creates the stiffness by being laminated to a substrate and that's it's not flimsy when it's not laminated to anything.

D.
Jun 11, 2019, 10:15 PM
Scott
Pylonracr's Avatar
The cloudy hardener worries me the most. Is there any chance that it has separated? If some of the binders have fallen out of suspension you will get a wildly inaccurate mixture even though your measurements are correct. I still feel that the resin did not cure properly. Are the bottles dated? As stated above, it does indeed sound like your produce is expired. That does not explain why some batches have properly cured, hence my comment about the hardener separating.

Scott
Jun 11, 2019, 10:21 PM
Entropy is happening!
Jim.Thompson's Avatar
Quote:
Originally Posted by Pylonracr
The cloudy hardener worries me the most...................
Scott

Me too!
Jun 12, 2019, 01:24 AM
Registered User
Thread OP
Quote:
Originally Posted by kablair
Mine was clear (mostly) but it does turn a touch whitish when mixing - but no where near as much as shown in your picture.
-Keith
Keith, thanks for chiming in on this. Just to be clear, the picture I posted showing the milky epoxy is just the epoxy, not the mix of epoxy and hardener. My hardener is clear. I see a few of the most recent replies reference milky hardener, which is not the case for me.


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