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a question about foam core wings

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a question about foam core wings

Old 08-25-2004, 05:28 PM
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Default a question about foam core wings

can i use elmers white glue to sheet foam core wings . if not, what should i use. thanks in advance.
Old 08-25-2004, 05:41 PM
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Default RE: a question about foam core wings

I prefer to use Elmer's Probond water based contact glue. Just make sure everything is lined up properly before you join two pieces together, cos' when they are stuck, they are STUCK!!
Old 08-29-2004, 11:27 AM
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Default RE: a question about foam core wings

I like some stuff called Southern Sorgham. You get it at the hobby shop and its $5 but it works awesome. Probably one of the best glues I have ever used.

Mike
Old 08-29-2004, 11:53 AM
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Default RE: a question about foam core wings

Elmers glue works great but you must weigh it down and let it cure for several hours. It also can be quite heavy. Contact cement is quicker but is not as secure.
I like to use a combination of both. Put on the contact cement as per usual and add white glue around the edges and several big X's internally. Then put it together and the contact cement will hold it while the white glue cures.
Old 08-29-2004, 12:10 PM
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Default RE: a question about foam core wings

Well if your not worried about how soon you will have the bird up in the air and want to take your time to think out all control serfaces I would recomend Gorrila glue. You don't have to put on a thick layer and just lightly dampen the wood and in about 4 to 6 hrs. there you have a bueatiful wing ready for the leading edges and sand. I use it all the time and it is also sandable unlike elmers which becomes to hard and will ball up or plug the sander block. Titebond works very well too. The 3M spray adhesive Im not inpressed with cause I've had it cut loose after you have spent the time to sand and then cover[:@]. Just my 0.02 worth.
Old 08-29-2004, 01:47 PM
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Default RE: a question about foam core wings

I have used Southern Sorgham on a 10' span 4 engine plane and had delamination of the foam and sheeting. Gorilla glue works well.

I repaired my bird by slitting the trailing edge and pouring thinned epoxy and then taping it back on. Ugh the wing has not failed since the epoxy was used, but I would have lost the plane had I not found the delamination.

And I did follow instructions for Southern Sorgham but perhaps I just got a bad batch,, or the 10' put too much shear on the sheeting during landings.

In either the gorilla will work well. Titebond will work but must air dry. I have heard of mixing Titebond and Gorilla glue 1/2 and 1/2 to make a really tight bond.

Theory,, Titebond gives up moisture to cure and gorilla glue adsorbs moisture to cure, so a mixture of both makes a strong glue. I only know of one big bird built this way and it has never had a delamination.

Best of luck on your choices.
Old 08-29-2004, 03:15 PM
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Default RE: a question about foam core wings

I have used Titebond. It took 3 days to cure, but I was happy with the result.

Elmer's has a Polyurethane glue that used to be called Probond (not the one Irish used). It's cheaper than Gorilla glue and seems to be the same stuff. I'm not sure what their polyu is now called (maybe someone can chime in?).

My experience with contact cements (Southern Sorghum and 3m spray) is that they remain gummy after completely cured. This makes it difficult to sand. Although I didn't experience delamination, I always worried about it. Works for some, though.

Other popular glues/methods for sheeting is epoxy (easy to put on too much and wind up heavy), and elmer's as a heat activated contact glue. Paint the cores, paint the inside of the sheeting, let dry, then iron on. I haven't tried this last method yet, but it sounds promising.

So far, I prefer Elmer's polyurethane glue.
Old 08-29-2004, 03:19 PM
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Default RE: a question about foam core wings

Hi!
White glue work, Southern Sorgum contact glue work, 24 hour epoxy work better...but best and lightes is Polyurethan glue (think one brand in USA is called "Gorilla glue").

Regards!
Jan K
Sweden
Old 08-29-2004, 10:06 PM
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Default RE: a question about foam core wings

I have tried them all and the best is to use polyurethane glue with lots of pressure. The popular brands of ployurethane wood glues are "Gorilla Glue" and "Elmers Ultimate Glue - Polyurethane". Elmers has dropped the "probond" label, but many of us still refer to the "probond" glue. Ployurethane glue is found in hardware stores, not hobby shops. It is not terribly expensive. Walmart sells it here (St. Louis). It looks like maple syrup. Spread it very thin on the balsa sheet and then put as much pressure as you can on the wing without crushing the foam. I use a vacuum pump, but you can also use bags of sand, old car batteries, etc. I use 5-7 pounds of pressure per square inch. More than that and the foam will crush.
Old 08-30-2004, 06:52 AM
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Default RE: a question about foam core wings

For many years I have used Z-Poxy finishing resin.
In the areas that will see high stress, (retracts) I will
layer in 2 oz cloth between the sheeting and foam .
Then put the "sandwich" into a wing press.

Regards
Roby
Old 08-30-2004, 08:22 AM
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Default RE: a question about foam core wings

I use 5-7 pounds of pressure per square inch. More than that and the foam will crush.
DM,
Are you sure you have that right? I am thinking of a 500 sq in wing say 50" span x 10" chord. At 7 #/sq in that is 3,500 pounds on the wing.

Ed S
Old 08-30-2004, 12:07 PM
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Default RE: a question about foam core wings

5-7#/sq" is about right for 1# eps. Blue and pink will tolerate more.

This is for Vacuum Bagging.

If you plan on curing the wings in the cradles with weights on top, you can't really achive this kind of pressure unless you have a couple of cars you can drive on top of the wings, and then you have to worry about distributing the pressure evenly. That's really the tricky part. So when you hear someone say "heap all the weight you can get on top of the wings", they really mean it.

Also, since you probably will be doing wing halves separately, then for a 500 sq in wing @ 250 sq in per half, it's 1250-1750 lbs.

DMcQuinn,

You're the first person I've heard vacuum bag wings using Polyurethane glue. I've heard people speculate that it won't work because of the foaming, not enough moisture in a vacuum bag, etc. I've tried it myself and have not had any issues with the cure. I think this is the way to go since it's lighter than epoxy, you don't need to mix anything and it penetrates the foam (unlike epoxy). But i've had intermittent problems with warpage. After a 24 hour cure, the wing comes out of the bag straight as an arrow, and after 20 minutes, a warp develops.

Have you had similar issues? I've now stopped misting my cores and sheeting before using the Polyu. Ambient moisture is more than sufficient.
Old 08-30-2004, 12:33 PM
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Default RE: a question about foam core wings

5-7#/sq" is about right for 1# eps. Blue and pink will tolerate more.

This is for Vacuum Bagging.
I have to queston this again. I actualy do skin my wings in one piece. Are you sure you are not confusing pounds/sq in with inches of mercury on the vacuum gauge. When vacuuming the pressure is on both sides of the wing as opposed to one side with weights. I am sure that with 7,000 pounds of pressure I would end up with a very thin wing!

Ed S
Old 08-30-2004, 01:29 PM
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Default RE: a question about foam core wings

Hi Ed,

Your 7000 lbs sounds like a pretty high figure, and if you were to use 7k lbs on top of your wing, I would be surprised if it didn't flatten out like a pancake. This is what I do know.

I use a gauge that reads 12"-14" hg in typical use. That does not crush the 1# eps.

I have not used 1750 lbs on top of a foam wing half, so in experience, I can't say for certain it will work.

With Vacuuming, pressure is on both sides of the wing, but in calculating an equivalent weight to produce an equivalent pressure, I'm not certain adding the numbers up to 7000 gives the right result.

Typically, those who vacuum bag wings say to use 5-7 PSI or 10-15" hg for 1# eps. 10 psi or 20" hg for blue foam.

Like I said before, the tricky part is to evenly distribute the pressure. If you actually are going to use 3500 lbs of weight on top, any part of the cradle not in contact with the wing means more pressure somewhere else, and probably will result in some sort of deformation. So in practice you might have more problems achieving this pressure with weights.


In the past, I have tried weights on top of the wing (ab 250 sq in wing half) in cradles (with epoxy) using well over 100 lbs and and detected some spots that did not adhere. Just behind the high point of the airfoil. After opening it up, I detected epoxy on the sheeting, but none on the foam. From that experience I concluded for that size wing, I could use wayyy more weight.

Ever since I went to the vacuum bagging, I have not had any adhesion/delamination problems.

Ed, if you decide to test 3500 or even the 7000 lb on a piece of foam I would be interested to know your results.
Old 08-30-2004, 08:47 PM
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Default RE: a question about foam core wings

Using the weights for me is a great way of putting sheeting on wings. This also puts all those AMA mags to use in my house instead of collecting dust and something to read when bored[&:]. This ia also a way of putting even wieght on the full wing half.
Old 08-30-2004, 09:44 PM
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Default RE: a question about foam core wings

I have tested a 4x8 inch piece of white beaded foam (32 sq in) with 170 pounds of weight on it. It does not crush and the adhesion of the skin to the foam is good. I have never scaled this experiment up to a 500 square inch wing and 2500 pounds of weight because it is impractical to do so. But I believe that if you could accomplish this, you would get good adhesion with the poly glue.

Ed smith - I took a close at my regulator dial. It does indeed show inches (and mm) of mercury, not PSF. So i am using 10 inches of mercury which is about 5 PSI. (about 30 inches of mercury equals about 15 psi, correct?). Basically, using some test foam, I cranked up the regulator until the foam shows some crushing and then I back off some. I think the foam began to crush at about 15 or 20 inches or so.

one time, I cut a 1/2 inch hole in the foam for the servo wires before I bagged it. the foam crushed into this void. So you must drill the servo access holes after the wings are skinned.

patternwannebe - I, too, have read about the "polyurethane in a bag" issue, but have had no problem myself. I live in a humid area (St. Louis). Las Vegas may be different. I put the entire bagged wing into the original shucks and then put some weight on the shucks. Never have any warps. All my experience is with large, thick wings (35%) and tails. Never tried this on sailplane wings, etc, although this is pretty common.

If anyone wants to do some more testing, do this: Take a 3x3 chunk of foam and glue a 3x3 balsa sheet on it with polyurethane glue. put a 9 pound weight on it. Repeat the test on another 3x3 foam this time with 50 pounds of weight. When I did this, the higher pressure test had better adhesion.
Old 09-02-2004, 05:41 PM
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Default RE: a question about foam core wings

Just a side note, one of the best sources of weight is to go to your local sun store that caters to reloaders and pick up some 25lb bags of lead shot. The smaller shot (bird shot, etc, #9 or close, not "buckshot") works best. The bags nicely conform to the curves on the wings, and you can stack them rather high.
Old 09-02-2004, 06:15 PM
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Default RE: a question about foam core wings

I have sheeted a lot of wings. Use Probond or Gorilla Glue - there is nothing better. White / Aliphatic glue is OK, but it is heavy, takes a lot to time to cure and can weaken in humdity. Poly glues are much more stable and very lightweight.

DP

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