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Old May 01, 2012, 01:05 AM
Registered User
San Diego
Joined Nov 2004
36 Posts
Question
Dimpled core after bagging - possible causes?

I'm fairly new to composite building techniques, and I've been working on composite wings for indoor free flight catapult gliders this spring. I've been experimenting with different foams for the core and different materials for the skins (carbon veil and .75 oz. glass).

This past weekend I finished my 4th wing, and it is full of dimples and/or gouges in the foam/skin, and I've not seen anything like this on the first three, although each has been different in terms of core material and skin materials. The pictures below show what it looks like just out of the vacuum bag. I'm trying to figure out what went wrong and hoping someone more experienced will have seen this before and can advise me.

I'm thinking perhaps it could be due to too much vacuum pressure or variation in vacuum pressure during the cure, too much heat, the type of wax used on the mylars, warming of epoxy in sink before mixing (moisture?). Here are some details of relevance:
  1. Vacuum pressure ranged from a low of 15 to a high of 25 hg - nothing new here as my previous three wings were bagged with similar fluctuations in vacuum pressure, and 3 of the 4 wings used the same foamular 250 foam with similar vacuum pressures.
  2. I use a picnic cooler for a hot box. Temperature during first 15 hours ranged from 70 to 75 degrees. Last 8 hours intended to be 110 -120 degrees. The thermometer near the top of the box (12 inches above the part in the vac bag) reached 130 degrees for a while, but the thermometer at the bottom next to the part in the bag never exceeded 110 degrees. I don't think it was any hotter in there than it was on the wing I made before this one, which had no such dimples and used the same foam core (albeit different glass - .6 oz from Hangar 9).
  3. The wax I used on the mylars is a car wax called Rally Cream Wax, and it is at least 10 years old. This is the only time I used that wax with higher curing temps. I'm not sure, but I may have used this same wax on the 2nd wing, but that one never exceeded 80 degrees during the cure - should have taken better notes.
  4. To warm the epoxy before mixing I put the cans of epoxy and hardener in the sink with hot water, but it only came up to about half the height of the can, and I dried the cans before opening them to pour the epoxy. I mention this mainly because it's the only time I've warmed up the epoxy a bit before mixing. - by the way I'm using proset laminating epoxy 125-226.
  5. The foam is foamular 250. These are small wings, 12 inches in projected span, 3 inch chord, and only about .14 thick at the root and .07 thick at the tip.
  6. The glass is .7 oz glass from CST. Fiber ratio (by weight) was about 63%

I appreciate any help on this mystery. This would have been my best attempt so far as it is very strong in torsion and weighs just 4 grams. I've spent lots of time reading various threads in the forum to educate myself on composite/vacuum bagging techniques, and I'd like to thank all the generous people that have shared such a wealth of information in here.

For the curious amongst you, I'm trying to improve upon the traditional balsa wings typically used on this type of glider. This wing is for a standard class category IV catapult glider. Over the past few years I've been chasing the record for this class, which requires a ceiling of over 128 feet. The Tustin blimp hangar is the place I've been making these attempts. The current duration record is aprox 1:58, held by Ralph Ray.
Tustin Hangar Record Trials - Ralph Ray Cat IV standard catapult glider flight (3 min 1 sec)


These gliders achieve speeds in excess of 120 mph at launch, and then circle down very slowly. The torsional loads at launch are significant and only the strongest balsa gliders with the right airfoil sections can cope with these loads without the wing twisting too much, resulting in increased drag and inconsistent launch trajectories. Ideal weight is around 7 grams. My goal is to create a wing of aprox 4 grams that is significantly stiffer in torsion than the best balsa wings that have been made for this class.

Paul
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Old May 01, 2012, 05:00 AM
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Paul, I had what seems like the same problem years ago bagging Apoge wings with hi-load 60 and .75 oz. cloth with West System epoxy. A very complete discussion can be found here:

http://www.rcgroups.com/forums/showthread.php?t=505848

I would add this about bagging .75 glass with West System epoxy. I achieved continual success by carefully controlling the amount of epoxy in the layup. It is very easy to suck too much epoxy from the cloth and getting too dry a layup and also easy to get too much epoxy in the layup . The secret for me was to let the epoxy begin to get a little thick or draggy on the roller before bagging.

1. Wet out the glass on the mylar.
2. Use paper towel to dry it out just enough.
3. Wait for the epoxy to just begin to thicken. ( Some drag when rolling )
4. Bag

I think you are seeing bubbles from trapped air in a layup that is too wet and not viscous enough.

The higher viscosity helps prevent the bubbles in wet layups because the epoxy canít be displaced to form a bubble but also helps a dry layup adhere better because itís stickier. I think you are looking for a dry sticky layup for less weight.
Also people have confirmed that sanding your foam after wire cutting lightens the foam but also sucks up more epoxy from the layup. All I would do was lay adhesive tape onto the surface of the new core and remove it to get rid of the hair. I did little or no sanding of the surface, Just shaping the leading edge.

I hope this helps,
Tom
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Old May 01, 2012, 08:07 AM
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Milwaukee G Mitchell, Wisconsin, United States
Joined Mar 2002
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Those don't look like bubbles to me. You can see by the reflection that it's a depression in the skin.

The first few Apogee wings I made used fomular250 and .75oz glass and West's. The surface was very soft. The 250 foam just doesn't have a lot of compressive strength.
What I found was that when I removed the mylars, if I ran my hand under the mylar to lift it, I'd leave those sort of indentations in the wing.
How do you remove the mylars? What thickness mylars are you using?

Also, with 250 foam, the vacuum you're drawing is way to high. I'd dial it back to 10in or less. While this doesn't produce the problems you're seeing it does compress the core to some extent.

To attempt the salvage those wings you could try steaming the dents out.
Lay a wet paper towel over the wing and use a very hot iron. As long as you hear it sizzling your ok. Don't let it get dry or you'll ruin the wing. And do it quickly...touch it for a second or two and check it. Repeat as necessary.

Another thing to do if you're going for ultimate strength is use a different resin. West works. Many use it. I used it when I stared. But it's a relatively soft resin. Your gliders probably won't see the sun but West gets soft when warm. The same manufacture of West makes ProSet resin. That has much better properties and benefits from a post cure where West does not.

Good luck.
davidk
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Old May 01, 2012, 10:35 AM
Composites guy
North OC, Ca.
Joined Jun 2005
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Looks like core crush(we get it on honeycomb sometimes), mostly along the leading edge where the mylar and bag tends to bridge. This increases the load in that region as the curved foam is the only material supporting the bag.
Try lower vacuum and make sure there is no bridging of the vacuum bag at the leading edge to the high point of the wing.
On honeycomb, we have found that the resin properties do play a roll as well so experiment with different viscosity ranges to see if the problem can be turned off and on.
Scott
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Old May 01, 2012, 11:42 AM
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heliwill10's Avatar
Greensburg, PA
Joined Feb 2008
558 Posts
Are you weighting the bag with anything?

If you are using the beds with weight that would be ok, but not just a block on top of the bag.

My gut tells me the vacuum is too high...
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Old May 01, 2012, 01:49 PM
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San Diego
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Thanks to everyone for all the input. Much appreciated.

I can't be sure yet, but I think David's theory is perhaps what might have happened. The mylars were a bit difficult to peel back from the first 30% of the wing. I did wedge my fingers in there a bit to help separate them from the surface of the wing. The dents are only on the forward part of the wing, so this correlates well with this theory. By the time I got the mylars peeled back to the high point, they came off much easier. While much about this theory correlates, I also find it hard to believe that i could have done so much damage with my fingers removing the mylars. Now I can't remember if I did this same thing with my fingers on the last wing, but that one did not have any such dents. After reading david's reply, I decided to make a dent intentionally by pressing my finger onto the skin. It did dent more easily than I thought it would. Not sure, I'm almost sold on this being the explanation, but part of me just can't believe my fingers pressed so hard on the wing when removing the mylars. By the way I am using proset epoxy, not west.

Tom, that thread you sent me to was excellent. I picked up some good ideas in there. What a saga. I'm glad you finally succeeded, and thanks for your thoughts on my problem.

Scott, core crush could be the problem, but somehow the pattern of the dents doesn't point to that. Not sure.

Helliwill, i did put about 20 pounds of bricks on top of the wing, but not directly on the wing. the weights are 15 inch bricks and I placed them on top of the top foam bed, so I think I'm good in that respect.

In a few days when i return from travel I'll try to steam out the dents and see what happens. Thanks again to all.

Paul
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Old May 01, 2012, 05:35 PM
working to the closest cm
jirvin_4505's Avatar
brisbane australia
Joined Nov 2002
2,119 Posts
damaged removing the mylar -use air

Nice work there Paul.

The Blimp hanger looks awesome

+1 on removal damage - i recognise the marks as similar to early damage I use to incurr. Use compressed air to blow the mylars off if you are having difficulty.

Not sure if you are looking for extra advice/analysis?
There is a thread on this forum for the composite HLG indoor glider that took the record a while back.

I don't fully understand you carbon spar with the gap out near the tip?

What disser technique are you using? What weight of tow?

cheers Jeff
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Old May 01, 2012, 06:24 PM
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Germantown, Maryland
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I also think the dents look like they were made during mylar removal, especially if you ran your hand under the mylars with your palm towards the mylar and your knuckles towards the wing.

The good news is that the dents will likely steam out since they were made after the epoxy cured. A better way to steam dents is to simply pour boiling water over the wing.

I'm guessing the gaps in the spar are where the wing will be cut and rejoined at polyhedral angles.
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Old May 02, 2012, 02:33 AM
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Quote:
Originally Posted by jirvin_4505 View Post
Nice work there Paul.

The Blimp hanger looks awesome

+1 on removal damage - i recognise the marks as similar to early damage I use to incurr. Use compressed air to blow the mylars off if you are having difficulty.

Not sure if you are looking for extra advice/analysis?
There is a thread on this forum for the composite HLG indoor glider that took the record a while back.

I don't fully understand you carbon spar with the gap out near the tip?

What disser technique are you using? What weight of tow?

cheers Jeff
Jeff, thanks for your vote on the cause of my dents. The blimp hangar is a truly special place to fly. It looks big until you start launching to altitudes approaching the top (165 feet), then it becomes very unforgiving if you make any mistakes.

I am very open and interested in advice on any aspect of this project, design, materials, construction techniques, etc. I've read the thread you mention from Jim about his record setting HLG, and the corresponding one on the small arts forum. They were very informative. I keep reading in this forum, and continue to pick up great nuggets of information that are helping me get closer to my goal.

Regarding the spar, as Phil mentioned in his reply, the break in the middle is where the wing will be cut in half and rejoined. Polyhedral wings seem to work best so far for this class of glider. All the carbon you see is unidirectional laminate. The top spar is .004 thick at the root tapered to .0025 at the tips and the bottom .003 thick at the root tapered to .002 at the tips. The carbon Xs on the inner panels are the same material about ranging from .002 to .003 thick. All unicarbon pieces together weighed .284 grams dry. I smeared epoxy on both sides of those pieces, then laid them on the core, then sandwhiched the core between the two mylars. This didn't work too well since the unicarbon pieces slid around a bit as I attempted to align the core properly in the mylars. On a different wing I placed the unicarbon pieces on the skins instead of the core, which worked a bit better.
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Old May 02, 2012, 02:40 AM
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Quote:
Originally Posted by Phil Barnes View Post
I also think the dents look like they were made during mylar removal, especially if you ran your hand under the mylars with your palm towards the mylar and your knuckles towards the wing.

The good news is that the dents will likely steam out since they were made after the epoxy cured. A better way to steam dents is to simply pour boiling water over the wing.

I'm guessing the gaps in the spar are where the wing will be cut and rejoined at polyhedral angles.
Thanks Phil. Now I'm wondering how I get some compressed air for the next one. I have those aerosol cans for blowing dust off of your keyboard/computer, but they don't seem too strong. Perhaps a bike pump?

Boiling water sounds scary, but I'll give it a try.

I've watched much of your video. Can't thank you enough for that! It certainly got me started on the right foot along with advice from a friend that's a few years ahead of me on this stuff.
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Old May 02, 2012, 06:11 PM
Arrarrar!
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Australia, NSW, Wagga Wagga
Joined Jan 2010
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Hey paul, you may find that a different wax might help your release. I use TR wax (not sure if you can get it where you are) and the mylars almost fall off on release. I guess if you take your wing straight out of the hot box and then remove the mylars the epoxy could still be a bit soft, making it easier to dent. Maybe leaving the wing in the mylars for another day or so could help. Ive never done this before because Im way too impatient!
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Old May 02, 2012, 06:15 PM
Just fly it!
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Cody, WY
Joined Nov 2007
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I really need to see videos of people who have mylars that fall off and parts that fall out of mold.
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Old May 02, 2012, 06:19 PM
Arrarrar!
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Australia, NSW, Wagga Wagga
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I said "almost"
They do come off pretty easy though
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Old May 02, 2012, 06:55 PM
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San Diego
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Quote:
Originally Posted by josh18 View Post
Hey paul, you may find that a different wax might help your release. I use TR wax (not sure if you can get it where you are) and the mylars almost fall off on release. I guess if you take your wing straight out of the hot box and then remove the mylars the epoxy could still be a bit soft, making it easier to dent. Maybe leaving the wing in the mylars for another day or so could help. Ive never done this before because Im way too impatient!
Yah Josh, that was my plan this time... to leave it in the vac bag under vacuum at room temperature for an extra day. I only lasted 3 hours. I suppose you guys that have done hundreds of these may be able to hold out, but it's going to be difficult for me to do so. Thanks for the wax recommendation, but I just ordered some meguires gold carnuba wax: http://www.amazon.com/Meguiars-G-701...6002797&sr=8-5
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Old May 02, 2012, 07:21 PM
Arrarrar!
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Australia, NSW, Wagga Wagga
Joined Jan 2010
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Hope it goes better for you. Ive never been able to last either! Its just coming to winter where I am so I have to make a hot box and I'll have to figure it out too.
Good luck with it
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