I have a Wing Mfg. A-26 Invader short kit and wood pack that's been sitting in the shop for several years now, mostly due to the fact that I've never built a model with sheeted foam wings.
In doing some web searching, I found plenty of discussion on choices of adhesives, but I wasn't able to find a really good tutorial on exactly how to go about the task. I don't know whether I need to cut out all the servo holes, channels for the Gold-n-rods, install the leading and trailing edge pieces, etc., BEFORE I sheet, or if this is all best done later.
Does anyone have a favorite thorough reference on foam wing construction (book or website) that would be a good primer for me? I really want to put this model together at some point. ;-)
On Sun, 20 Dec 2009 17:19:10 -0500, "Robert Scott" wrote in :
I see lots of tutorials here:
I don't have a favorite among them.
I use vacuum bagging myself. I've only done a couple of wings, but it is a nice system. I'm using epoxy resin for now. When it runs out, I may consider some of the alternatives.
Marty's way is probably the best when used with ProBond, but it presupposes equipment, knowledge of what you are doing, and some friends to help you learn new and different tricks. :)
That being said, the first part of the direct answer to your question is that this link probably gives the closest to what you want and need:
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To resolve the question about adhesives, I will report the substantive differences between epoxy and ProBond that I have noted in use.
EPOXY:
I have had epoxy wings delaminate (probably because I was in a hurry and did something wrong) but more importantly the adhesive bond between epoxy and the foam substrate is weaker than the bond between ProBond and the foam. With the epoxy the only adhesion is where it actually touches the foam bead and it does not penetrate the foam (which allows delamination under extreme conditions). Many try to improve that bond by adding enough epoxy to get down between the surface layer of foam beads, which works to a point but also adds unnecessary weight.
ProBond (or any urethane glue):
I have managed to break a couple of wing sheeted with this stuff and what I found was very interesting. As the link I provided suggests, edge gluing is not needed and when the wing broke (don't even ask what I was doing to cause that to happen!) I got a good look at how urethane glues work in our environment and was impressed. The glue penetrated almost 1/2 inch INTO the foam. When you add the fact that ProBond sheeted wings weighed substantially less than wings skinned using epoxy there is a winner. I love the results, less work, better adhesion, less weight!
Now for the secondary parts of your question.
Cut all the servo wire channels (fill them with waxpaper wrapped rods) and servo holes (measure their location and write it down) before you sheet and install leading and trailing edges after sheeting. Trying to install those prior to sheeting only complicates the process for me, others may have a different approach.
You do not need to vacuum bag with probond or the other poly glues but be sure to use wax paper between the outside surface of the wing and the shucks that you will eventually throw away... (dont ask me how I know this....) As the glue expands it finds any microscopic holes in the balsa and can also bond the shucks back onto the wing... use lots of weight to keep the foam from expanding the sheeting and foam apart. Probond is not a good glue if you cannot clamp the parts together somehow. Make a test setup with some scrap foam and balsa to get the feel of what you are doing. Also it takes very very little glue to get a good bond... I scrape almost all of it off the wood (till there is only a slight sheen) and that has always been more than enough. Bob Furr
PS If you ever decide to cut your own wings do a google search on "foam wing cutting". It really is much easier than most folks realize.
You guys are the greatest! My fear is gone. Instead of being nervous about the process, I'm now very interested and looking forward to trying it. I can put the model into the building line-up at some point. :-)
-> This is about the best tutorial I have run across.
Great stuff! Lots of good info and lots of photos; very thorough. He has a neat way to install the aileron hinge points and cut out the ailerons. Thanks for that link, Rob.
Do NOT use any of the poly glues as a purely structural adhesive. You know, bulkhead to skin or framing up things. None of them have the small point strength of any of the adhesives we use but almost all of them are the best for very large surfaces. The link I provided from Marty's list is one of the best write ups I have seen about how to use a poly glue to skin wings.
I had an acquaintance that build up one of those giant scale Ace Seamaster's with a poly glue. When the forward bulkhead slipped out of the glue the bird went in. It was recovered and you could see where the glue had foamed up all around the missing bulkhead.
Poly glues approach the strength of epoxy but ONLY if you can clamp the surfaces together till the glue completely cures and have a good mechanical joint to start with.
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