expandable foam

There's your answer. The only way to control it is to make a mold. Something the manufacturers could do, but not practical for anyone building one or even a few planes.

In the real world, manufacturers that use polyurethane foam have very controlled environments and equipment that heats and stirs the material consistently to yield a consistent product. Stepan is who I used to get my foam from when I was fooling with it. Here's a list of some of the foams they make

Those are just their rigid varieties. Besides the mold problem, you have to use a release agent of some kind. This must be washed off if you want anything to stick to it. And lastly, the material forms a sort of crust. For good adhesion, it should be sanded off.

The only way I'd consider it is if I used a foam strong enough where I wouldn't have to sheet it or put spars in it when I was done. Just paint it and go. That type of foam isn't cheap in the light density/high strength we need. From an engineering point of view, your strength should be in the skin, with the foam core only there to give a shape and prevent sheering.

My 2 cents. Keep the change.

Yes. It is good stuff. Dumb Thumbs resulted in a broken wing near and smashed around the joiner tube of a foam wing big bird.

I cleaned off the crunched stuff, jigged the wing back together, added some ply reinforcements where I thought they might help, and filled up the holes with the spray expandable foam. When dry, it carved down easily and sanded to finish very well. I did the bottom first, then the top. Then with tape and thinned white glue, I patched in the balsa sheet planking. Worked fine. Much stronger than original white foam. Maybe an ounce or two extra, but I don't know the difference. BTW, the can nozzle and plastic applicator can easily be cleaned with acetone for use again when you need it.

HC

Good stuff.I've used it many times.Especially when gluing a wing fairing block to the bottom of a wing.It's almost impossible to sand a block to fit the bottom of a wing from front to back perfectly so I just get it sanded to where I can use enough glue to stick it to the wing.After drying I squirt some of this foam,it expands and drys and it's a simple matter of using a sharp blade to cut it to shape,then sand a little. Holds up well,sandable and monocoate sticks to it.I no longer worry too much about a fairing block fitting flush against the bottom of a wing. I just foam it.......

I saw a thread on the World Models J3 cub that mentioned the group had run across a guy with a similar older model J3 that had the airframe stressed (crashed :) to the point all the stringers forward of the tail were shot, so he just injected the spray foam and has been flying it ever since........

Its not that light, and not that strong either.

It depends on the type. There is the gray stuff that is used for sealing around door frames during construction that is fairly dense and heavy. The other kind is the yellow urethane stuff that will turn brown and crumble in the sun. It is very light.

I have used the latter for repairs to fiberglass fuselages. Tape all the parts together then shoot some of the foam inside. It provides enough stiffness so that you can finish the repairs without the glass falling apart.

I have also used it in an ARF electric sailplane to hold the pushrod outers in place down deep in the fuselage.

yes, but compared wih epp or ep, its not as good and more expensive.

its fine for repairs, but not woth making a whole model out of.

You can't stop there, Jim. You have to tell us which advice you took and how it worked out. ;o)

Marty