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ORIGINAL: LunaSea

I am a strong proponent of polyurethane glue (Gorilla Glue and other brands)... It has some unique characteristics, and takes a bit of getting used to because of that. Chief among those is that fact that it "foams" or expands as it is curing. That has the positive effect of filling gaps (of which there are usually many is our wood structures) resulting in a more complete (and, presumably, stronger) glue joint. This expansion can be a bit of a problem until you get used to using this material. Only a minimal amount is required. I can build a quarter-scale sized all-wood plane with about 2-3 ounces of this glue (some of which gets wiped away as waste). Note that I am a "heavy builder" using probably more materials and glue than really required. I think that is a very reasonable volume (and weight) of glue to add to an airframe of such size. I have found it to be very strong and somewhat flexible (not prone to cracking). The joints are VERY MUCH stronger than the surrounding wood. For laminations, if you use a "woodpecker" or some other technique to make small perforations in the surface of the material, the glue expands into these and really gets "teeth" in to the material. The glue also adheres well to most other materials (except for some "slick" plastics such as polypropylene, and because of that SARAN wrap, or knockoffs, make a good covering for plans as the glue will release from it fairly easily in case it gets glued to the work).

I use mostly Robart hinge points for attaching control surfaces. I use the P-U glue for those, as well. Simply place a TINY amount of oil or melted vaseline on the hinge portion of the hinge-point. Wipe away any excess. After compleing drilling and fitting the holes for the hinges, start the effort by placing the hingepoints into the control surface component. You can simply use a thin stick (I use bamboo barbeque skewers) to work a SMALL amount of P-U glue into the hole for the hinge (put it well down into the hole, not hear the opening. Use a Q-tip to remove any excess that is near the opening. Then place the hingepoint into the hole (make sure it in the proper orientation and depth). Let it set up for a couple hours (maybe a bit longer if you are in a very low humidity environment, as it is the humidity the "sets" the glue). If you have done it right, there should be very little, if any, expansion into the hinge area. If there some, just trim it away w/ your Xacto blade. Once you have the hinges set in the control surface and set up, the perform the like operation on the flying surface attachment points and attach the control surface. The nice thing about this technique is the expansion of the glue into the "barbs" of the hinge point. Believe me, you can't pull it out without destroying the surrounding wood.

P-U glue is also excellent for skinning foam wing cores. After edge glueing the sheeting (for which I usually use PVA "white" or "yellow carpenters glue" , place a small amount of P-U glue on the core side of the skin, spread it out THIN using an old credit card, use a spray bottle and mist the OUTSIDE of the skin (the side opposite the glue/core, this will accelerate the expansion and curing of the glue (due to the moisture penetrating thru the skin to the glued side) and will also help to "curve" the sheeting around the core), place the sheeted core back into the core "shuck" and weight it with something heavy. The P-U glue will expand to attach the skin to the foam with a complete joint, filling any gaps our imperfections in the core. Give the technique a try w/ some scap balsa and foam before commiting a set of cores. I think you will like the results if you are conservative on the amount of glue that you use. If you use too much, you are going to have a LOT of expansion around the edges to trim away after it sets up. If you place spars (or gear mounts, etc.) into the core prior to skinning, P-U is a good choice for those as well, again due to the expansion and gap filling nature of it.

It is NOT as good for any glue joint that will be exposed and visible (not painted) after construction, as it will turn increasingly brown upon exposure to sunlight. I am not sure that it results in any meaningful loss of strength, but it is not cosmetically acceptable (unless painted).

P-U glue is a bit pricey. As noted above, it is "triggered" by humidity. It is, therefore, ESSENTIAL to keep it tightly capped when not in use, keep it away from sources of moisture, otherwise it will begin to "kick" in the bottle, turning into an unusable gel... Your first indication that this is happening will be that it clouds a bit (normally a pretty much clear amber color).

BTW - it sands pretty well, too...

I know a lot of folks prefer CA glues because of the instant setup. I have found that CA causes me to have respiratory problems (sort of like having a head cold), so I use very little of it (mostly only to "harden" threaded holes like wing mounts). I am also a bit "ham handed" so instant setup not really a good thing for me. Some time to adjust parts fit after applying glue a big plus... Harbor Freight has soem nice little plastic spring-loaded clamps that are CHEAP that as great for building. I have a whole shoebox full of them on the building table...