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  #1  
Old 07-07-2012, 12:45 PM
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Default Wrinkle in layup

I laid up the LG bulkheads yesterday, and when I checked them this morning, I found that two of them had cured with wrinkles running vertically from top to bottom. I'm not sure if this happened when I lifted the wet layups to move them to another part of the table, or because I loosened the layup while hitting it with the hair dryer. In any case, is this fixable or do I meed to scrap the part? I'm guessing that a fix would involve cutting out the wrinkle and lating a patch over the the gap, but how wide would I have to make the patch?
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Old 07-07-2012, 01:01 PM
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Default Re: Wrinkle in layup

Here are the photos:
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  #3  
Old 07-07-2012, 01:35 PM
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Default Re: Wrinkle in layup

If it were mine, I would scrap it and make a new one.

This is the cheap part of the construction so make it right.
As a footnote, I remanufactured two of my four bulkheads, my center section spar and my canard. In all cases, the second one was of a higher quality, lower weight and took a fraction of the time of the first.

....besides, that bulkhead could be curing while you are doing something else. I vacuum bagged mine but a piece of plastic and some 3/4" MDF would be good to get a nice flat cure.
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  #4  
Old 07-07-2012, 01:37 PM
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Default Re: Wrinkle in layup

That's why the plans have us do the small flat parts first - its a learning exercise. I'd say that has to be either fixed or replaced. Plans describe how to fix. IIRC they specify sanding for bond, then a 1 inch overlap on each side of the repair using the same number of plys as the original spec. I dont think the extra thickness would cause a problem, but I'd lean toward redoing the part. Its a fairly small part and you need to learn how to do this right.

How did you go about doing this layup? Did you lay the cloth on the part one ply at a time, or did you do the whole thing on the table, then lay it on the part. If the latter, then I'd suggest using the former method. When you get to curved parts the table layup approach wont work well anyway.
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Old 07-07-2012, 03:23 PM
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I laid the glass directly on the part. I laid plastic over the peel ply when I was done, and if there was a crease in the plastic it may have pulled the glass up. I also noticed that when I was using a hair dryer, the glass seemed to lossen up. I flattened it back down, but it may have lifted up again. Who knows?
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Old 07-07-2012, 03:41 PM
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Default Re: Wrinkle in layup

At the beginning when I was building smaller flat parts, I had two pieces of thick glass. After all the glass was wetted, I cover each side with 3-4 mil plastic and sandwich them between the glass, laying on the floor or other flat surface.
Load up on top all the weights and tool boxes I can. In the morning you have the most perfect flat piece you have seen. Plastic peels off without any effort.
On a part like the speed brake, one side has some uneven surfaces. Lay flat piece of cushion foam on top of the plastic and weigh it down with the glass or flat board. The finished product looks like it was vacuum bagged.
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Old 07-07-2012, 03:44 PM
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Default Re: Wrinkle in layup

I think plans suggest putting something flat and heavy on top of the layup for cure. I used a piece of 1/2 plywood I had laying around, with weights on top of that. I doubt that plastic would cause that wrinkle, unless its a very heavy gauge. 4ml is about the thickness as you need.

Looking at the pictures again, I wonder if you could get the epoxy soft (all the way through the layup) with a heat gun, inject some micro down under the wrinkle, then flatten it down and weight it for cure. Might be ok.

Quote:
cover each side with 3-4 mil plastic
Yes, then squeege through the plastic to get excess epoxy and air out. If the squeege doesnt slide well, wet the top of the plastic with a little epoxy.
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Old 07-07-2012, 04:37 PM
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Default Re: Wrinkle in layup

Quote:
Originally Posted by egstrong View Post
do I meed to scrap the part?
What bulkhead is that? The problem in the right pic is nothing. As for the wrinkle in left pic, it's likely just amounts to a cosmetic flaw. If it was a spar cap or upper wing skin in compression, that would be bad. But bulkheads are generally not high stress areas. Fix it if you want but if you cut out the wrinkle and put a patch over it, have you made it stronger? I doubt it.
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Old 07-07-2012, 05:34 PM
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Default Re: Wrinkle in layup

given where it is and what it will do, that spot if needed to hold you'd have much bigger fish to fry than that spot. there are a 1/4" of layups that cover most of that lg...both side.
that said, you will need the skill set at a later date so make shur you know how to do it right for later layup and move on.
if that is your worst part in your build you will have the nicest plane out there !
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Old 07-07-2012, 05:58 PM
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Default Re: Wrinkle in layup

Re: "I doubt that plastic would cause that wrinkle, unless its a very heavy gauge. 4ml is about the thickness as you need."

John, I was surprised at how easily a small crease in 4 mil plastic could cause a wrinkle in the glass. I had laid up 24 plies for the LG hardpoint over 4mil plastic and almost ended up with a ridge there as well:

http://edscozy.blogspot.com/2012/04/...up-part-i.html

O rdered 4 foot wide plastic from a supply house to avoid the creases, but it came folded. I'll just have to watch where I lay the creases from now on.
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Old 07-07-2012, 06:01 PM
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Default Re: Wrinkle in layup

Quote:
Originally Posted by kjashton View Post
What bulkhead is that?
Kent, it's the forward and aft LG bulkheads - hence my nervousness.
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Old 07-07-2012, 06:56 PM
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Default Re: Wrinkle in layup

Quote:
Originally Posted by egstrong View Post
Kent, it's the forward and aft LG bulkheads - hence my nervousness.
Those LG bulkheads have so many additional layups added and overlaps with the sides and bottom added layer that it doesn't change my opinion but if it's going to be visible then you might sand off the bump (left pic only) and apply a patch, peel ply. The patch won't be noticeable.

My point is that a lot of the airplane is likely overbuilt. Where the plans might call for three layers, perhaps actually only 2.2 layers is needed so the number is rounded up to three. Or where a bulkhead layup schedule is specified, it is likely that overlaps added later were not included in any calculations. No one really knows for sure. I have not heard of any weakness in the C-IV landing gear structure unlike the Long-EZ, which has had some failures at various points.

For example, some of my first Cozy layups were very dry--I could see air in the weave with a magnifying glass like looking at a wicker chair seat, and that airplane has 540 hours on it now.
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  #13  
Old 07-23-2012, 04:10 PM
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Default Re: Wrinkle in layup

John, did you mean heat gun or hair dryer? I used a hair dryer on it a few days ago and got it almost flat, but when I tried to get it flatter I ended up recreating the wrinkle. I tried to inject some epoxy in there, but I screwed up the resin to hardener ratio and what went under the layup may never cure. If you think a heat gun is safe, I'll try that next.

I also got the bulkhead flat simply by laying a bunch of old milk jugs filled with water on it and leaving it for a couple of days. The garage probably gets up to 140F in the summer, so that probably helped.
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Old 07-23-2012, 05:52 PM
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Default Re: Wrinkle in layup

You can use a heat gun, but use it carefully and heat the part gradually. I used a heat gun for a large part of my build - just don't point it full on and up close.
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Old 07-24-2012, 09:21 AM
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Default Re: Wrinkle in layup

Quote:
Originally Posted by egstrong View Post
Here are the photos:
The problem may be temperature control, or you have a very wet layup and the wrinkle formed during the cure. I had a similar problem with an early lay-up.

I discovered that covering with peelply, plastic (dont forget the plastic sheet), large flat 1/2" plywood, lots of old weights... well four or five 10 lbs weights work great. I got mine from the garage...or you can pick them up at yard sales. Go early Saturday morning and look for ol' guys with donuts and coffee in their hand. They will give them to you if you will load them in your car.

I agree this is a GREAT opportunity to do a repair.

If you are like the rest of us, you will have more learning and less money in the future. Enjoy the learning, it is expensive. Just not as expensive as not learning.

Hope this is some help.

Cheers!

Jeff
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