Daniel Saito

I have just built most of the tail with light CA. A friend of mine told me not to use it in the wing! Is that correct? I want it light and strong (yet flexible) so I can hi-start it!!

P.S. The plans tell me to use CA in everything.

Al Stein

Glue type, radio brand, CG setting, and choice of personnal savior are all religeous issues, not engineering decisions.

I built my Spirit and another one with all CA and another one and a half with all yellow wood glue. They were all (as the manufacturer says) great planes. There were no issues of weight or joint failure on any of the planes.

(Now for the personal preference based propaganda part of the post...) CA is great if you don't mind the burning in your eyes and lungs and aren't intending to use either (eyes or lungs, that is) for what might be considered a normal life span. It works a little too fast to allow precise positioning of parts, it doesn't grab plywood as well as wood glues, and it's got a fairly short usable life once opened. It's also way more expensive and a bit brittle by comparison to wood glues and sticks your skin almost instantly to the most amazingly inconvenient thing it can find in any environment.

If you don't mind that, CA's just as good as wood glue. I still use CA when I have to edge join balsa sheet.

JimTrainor

Here is some useful info, worth reading before you get too far along.

http://www.charlesriverrc.org/articl...spiritmods.htm

I've read opinions that CA edge joined sheets are difficult to sand. My own experience is similar... but I am not sure that I'd have the patience to use ambroid as is done here:

http://modelinghints.homestead.com/Sheeting.html

Daniel Saito

Sorry, but I don't think I know what Ambroid is made of! Does it have epoxi in its composition?

JimTrainor

Ambroid is a brand name. You should be able to find some at a hobby shop... I did.

In my brief test it does sand easily.

It has been around a long time. They have a web site: http://www.ambroid.com/Ambroid.html

... the "Original Liquid Cement" is the stuff being referred to in the link I refered to.

I found the link were I originaly read this. It is written by Mark Drela of Xfoil fame... so carries a bit more wieght that my opinion

http://www.charlesriverrc.org/articl...nstruction.htm

Al Stein

I use the same method with CA with a couple of exceptions

• I'm not sure whether the link shows this or not, but once taped, I smooth the tape down good and fold the sheets back over it and try creasing it so there's very little opportunity for CA to seep onto the taped side. That can make things way easier since that's the side that will be automatically aligned right if there's any differences in the thickness of the sheets.
• I don't have to weight it all down, I just flatten it out with the tape side down and the pressure causes the glue to do its thing, and
• Once the glue is applied, I use the edge of a razor blade (a dull one is fine) to remove any excess CA before it hardens. Whenever much CA builds up on the blade edge, I clean it off before continuing.

Done that way, the untaped side is usually free enough of CA to be sanded and the taped side is sometimes free of CA and often smooth enough to use without sanding anyway.

JimTrainor

Thanks Al... as with so many things, it's all technique!

Daniel Saito

Is it true that CA is very brittle, so it would actually break under stress, like if you hi-start a 2M sailplane?

Al Stein

No. Voyager made it around the world non-stop on the stuff -- that plane had stresses and flexes that scared the heck out of everyone invloved, but it didn't shatter and they all lived happily (but separately -- but that's another story) ever after. You are right to be concerned about sailplane launches, though -- they impose stress levels that are extremely rare in powered planes (except on impact). Many of us have used CA to build successful sailplanes that survive hi-starts and even winches.

HOWEVER, there are different ways of using CA... many people fit the parts together and drizzle on a couple drops of thin CA because it penetrates the wood... well, the thin stuff also has virtually no gap filling ability, so the surface area of the joint is very small and the added stiffness of the surrounding wood focuses all of the stress right at the joint... that type of joint is prone to failure.

CA is just like any other glue in that a strong joint relies on good technique. To me that means being sure that parts fit well (and this is most important... dry fit them and adjust them as needed before gluing), being sure that the glue covers the joint area (when I can, I use the slower types and apply them to the joint before joining), and reglue any joint that has to act under tension or shear (a little fillet adds a lot to the joint's ability to hold together).

All of that is way less important with wood glues like I normally use... but I still do it with wood glues anyway. Maybe that's why I don't really build much faster or slower because of using one glue type or another.