RC FAQ, Cutting foam wings and turtledecks for your model airplane

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What can you tell me about cutting foam wings and turtledecks?

(If you want, you can skip straight to the foam cutting links. There are links including suppliers, CNC foam cutters, foam wing manufacturers, foam wing vacuum bagging, general construction tips, as well as other sources.) Also, here's a good page that answers the question "Why Foam Wings?" And finally, How do I Glue the Sheeting to the Wing?

For starters, this link has a good description of Foam Wing Construction, and making a foam cutting bow. For my bow, I used 3/4 x 1 1/2" hardwood. I picked out the stiffest type of wood that I could find, but after working with it, I see that just about any kind will work--there isn't that much force on the foam bow that it makes a difference. While I was getting the wood at Home Depot, I bought some 1/4" steel rod. That works fine for tensioning the wire. To make your bow, just drill a couple 1/4" holes in the bottom of the board, so they angle out. Notch the rod, and insert it into the holes. Now you can wrap the wire around it.

One important aspect that I will emphasize for a good finish is the quality of the foam. I bought some foam at a local hardware store, and it was poor, insulation quality foam. When I cut it, there were hard lumps where the foam was more dense. That made for a very ugly finish, which would require a lot of sanding.

I recently cut a foam turtledeck. (That's the top curved part of the fuselage behind the pilot, between the back of the cockpit and the stabs. It's basically shaped like a long cone that's cut in half lengthwise.) The plane I'm building, a Stinger 1.20, comes with a plastic one, which is kind of heavy and ugly. I cut one out of styrofoam, to be sheeted with balsa. Not only is the foam one a little lighter (but not a lot) but it's also structural, so it will add rigidity to the plane. Also, it won't be cracking on me, like plastic can.

First, I had to splice two pieces of foam together, because the pieces I had were too short. (O/A length is about 27", and mine were under 24".) I use 3M-77 spray adhesive for joining them. Then I cut out a template for the front of the turtledeck, which is the back of the cockpit. I cut ten degrees off of the front of the foam, so it will slant back, making wing installation easier. To do this, and to square up the ends, I used spray adhesive on two sticks, and glued them square on opposite sides, then cut it with my hot wire bow. The parts held with 3M-77 spray adhesive should pull off if you do it right.

Now comes the fun part. I took a 2x4 and cut it to length. I figured that my fuselage has a 1:8 ratio--that is, for every 8" in length, the fuse will widen or narrow 1". Since the fuse is 1" wide at the rudder hinge-line, I cut the 2x4 8" longer than the 27" turtledeck. Then I sprayed glue to the foam to stick it to the 2x4. I put a screw into the end of the 2x4, with a notch on the corner of the wood to guide the wire centered. I wrapped my cutting wire around the screw, and attached the battery charger clip. On the other end, I wore leather gloves and put a handle on the end of the wire, and held the 2x4 with my left hand. To cut the foam, I turned the charger on, pulled the wire tight, and slowly guided it along the front template. (The reason I used the 2x4 instead of the workbench was so I could go under the foam at the start, to sneak up on it. Also, it kept the glue off of the workbench.)

Voila! A foam turtledeck. I was impressed with how well it turned out--especially considering how mediocre the wings that I cut looked last year--I'm betting the foam everything to do with it, the last time I tried the foam was from the hardware store, and had hard spots in it. Anyway, with this turtledeck, a little sanding to reduce the high spots where I jumped the wire a little quicker, and it's ready to be sheeted with balsa.

One thing that I learned was about the tail end thickness. Because the wire is dwelling at the back end for so much longer--it's about 3-4" wide at the front, but only 1" at the rear, so if I move the wire 1/2" up front, the rear hasn't moved much at all--the amount of foam that the wire melted was significantly greater at the rear. With this one it doesn't matter much, but I'll have to remember that for next time. Maybe a template would help, or a longer pivot point, or some other solution.

For sheeting, I used 3/32 balsa. To get it to curve into the tight radius of the tail end (about 1/2 radius), I had to soak it with ammonia--the first attempt at soaking it with plain water rewarded me with a cracked sheet of balsa. Before I applied the adhesive, I let the sheeting sit in the foam cradle for an hour to let it take shape. Don't be tight with the balsa--it's better to trim a little balsa off than it is to start over. Then I used ProBond, and spread it on the foam turtledeck as thin as I could get it and still see that there was some there. That seemed like enough, because a tiny bit oozed out at the edges. Make sure your balsa is still wet when you lay it on the foam, because the moisture helps to activate the urethane adhesive in ProBond. Press the foam turtledeck and sheeting into the foam cradle that was left over, and clamp/weight it tight, and let the adhesive cure. The ProBond actually foams up and helps to fill voids, so you get full adhesion.

Anyway, it was a fun project. The finished weight of the turtledeck ended up at about 1.6oz lighter than the 3.9oz plastic one...and a LOT better looking. I just thought some of you might like to read about it, and hear a success story so you know it's not all that difficult. You an apply these same techniques to making foam wings.

Power Supplies

I used my 10 Amp auto battery charger for cutting, and it worked fine. What's more important than current output is the voltage at that current. With low voltage, you could have a 100amp supply, but the wire still wouldn't get hot enough, because the overall wattage isn't high enough. You need a good balance of power--wattage, which equals volts X amps. Just try it, and see what happens. If you aren't comfortable with experimenting, then only use a tried and true method. I won't be held responsible for your decisions.

Foam Wing Sheeting

I though I would share a little information with all of you. I'm in the process of skinning some 40% wings and have found an alternative to both vacuum bagging, and piling a whole bunch of weights on top of the wing chucks. I have made up a press to do the job. I have taken and cut two pieces of 3/4" plywood, and sized them a bit bigger than the wings by about 2 " all the way around. Next I proceeded to make a wooden frame that will go on one side of each piece of plywood. The frame would best described as a "header and floor joist" set up. That is one piece of 2x4 down each side, and shorter pieces running across between them at about 8 to 10 " on center. Obviously all of these 2x4`s must be straight. The assembles are then glued and nailed together. I then proceeded to glue the skins, and put them in the chucks. Now instead of lifting heavy bricks, or patio stones, or putting the whole thing in a vacuum bag, I put the chucks between the two frames. Then I put a series of clamps all around the outer edges of the frames. I used bar clamps, but F clamps or C clamps would do fine too. I plan on using the same jigs for the tail feathers too. You would be amazed at the pressure you can generate, without all that lifting! For those of you with bad backs, or do not have access to heavy weights, you might want to give this a try. Mike Clemmens

Mike. Be careful with this. A friend of mine did that and it didn't work for a 40%er. The edges were fine but it didn't bond in the center of the wing. It bowed just a few thousandths and there was no adhesion in the center. He had a similar support system. FYI. Maybe you have a better support system. Hope your works fine. Gene

Thanks for pointing that out. I should have made it more clear in my original post about the importance of getting the 2x4`s perfectly straight. I don't mean just by eye. I ran mine over a jointer, which I have in my shop. The wood should also be kiln dried so it does not warp. Actually this is not the first time I have done this, and it has worked out well in the past. I am using ProBond too, which may also contribute to the success I have had. Mike Clemmens

FOAM CUTTING LINKS

Why Foam Wings?

CNC foam cutters, and similar machines

(Foam Wing Cutter Greg's Diversions sorry, out of order at this time.)

Quick & Dirty foam cutting jig

http://www.distrito.comjuanchi/

hobby CNC router

Router Plans

Computerized foam cutters! - Professional hot wire EPS foam cutters

Links for How To's regarding foam wings:

Here's a good site on RIBBED foam wing construction!

Foam wing lightening techniques used on Richard Biron's Sukhoi SU31M NEW