Back in the days when we learned about RC by reading hobby magazines there was a guy named Chuck Cunningham who was a columnist for RC Modeler magazine.
Chuck also designed several very popular RC planes including the Lazy Ace, and he manufactured kits of some of his designs. One of the topics that Chuck liked to cover in his magazine column was float design. During the heyday of monthly magazines new guys were always coming along and asking the same old questions, so Chuck would simply re-run his old article on floats every so often. The article tells you how to calculate the dimensions of your own set of floats for whatever airplane you happen to have. Once you have the dimensions, you cut out the foam, you add a hard spine, and you finish the float to whatever extent you are satisfied with. Chuck liked things quick and easy, so he would sometimes reinforce the bottom of his floats with tape and leave it at that.
If you’re not into cutting foam, skip ahead to Part 3 of the Cunningham float series, which addresses floats made of wood.
Following Chuck Cunningham’s directions from the magazine (probably the last time it was reprinted in RCM) I built a couple of sets of foam floats for myself and a friend. I covered them with 1/16″ plywood on the forward part of the hull bottom and 1/16″ balsa everywhere else. Then I brushed on Balsarite (similar to Sig Stixit) to enhance film adhesion and waterproofing, then I finished the floats with yellow Super Monokote. The whole project was a resounding success, and I had a great time flying my plane on floats. Unfortunately my friend’s Cub got stuck in a tree about 40 feet off the ground, but I guess you can’t win them all.
I’ve been thinking for years that I should do some more float flying, so now I’m going to build a set of Cunningham foam floats, and I’m going to document the process here to show you how easy it is. First, here’s Chuck’s article. You can open it in a PDF reader and refer to it as we go along.
My new float plane is an Aerostar 40, so the first thing I need to know is that my floats are 3 inches wide. Calculating from Chuck’s guidelines I figured out that my total float length is 36″. From there I got the other dimensions and cut out the float sides from 1/16″ balsa.
You probably guessed that I cut these out on the laser cutter, and you would be right, but this whole project is very easy to cut out with a long straight edge and a hobby knife.
The next thing I did was I took out the big plank of white styrofoam I’ve been hanging onto for about 20 years, thinking I would build a set of foam floats some day. Well, today is some day and I’m building floats! I used a plain sheet of 3 x 48 balsa wood pinned to either side of the foam plank as a template for cutting out two 3 x 3 x 48 blocks of foam with a hot wire.
This is the view from the side, where the balsa skin is to be stuck on and then used as a hot wire template.
The cores were then passed through the table saw to make a half inch wide groove for the 1/4″ plywood spine. I want the top of the spine to be flush with the 1/16″ balsa skin, so I made the groove 3/16″ deep.
According to the article, Chuck drilled the plywood spine with 1/4″ holes in several places for dowels to anchor the spine into the foam. Since my laser cutter doesn’t like 1/4″ plywood I make all such pieces out of two layers of 1/8″. I made one layer with holes and one without, so the dowels will go through only the inner layer, and the outer layer will be smooth and pretty. Where Chuck made a few large holes, I opted to make a larger number of smaller holes. I ended up with eight holes 3/16″ in diameter.
First I cut eight pieces of dowel, each one long enough to anchor the spine but not long enough to protrude past the finished depth of the float. The dowels were numbered and lined up in order.
The plywood strip was taped into place in the groove to make sure it couldn’t move
A 3/16″ music wire rod was used to melt holes into the foam for the dowels.
The dowels were used to measure the correct depth on the hot wire, and the wire was gripped with a pair of pliers at the correct length, and then inserted into the holes to melt the foam underneath.
Here’s the foam core with the holes.
The holes were filled with glue, then the groove was coated, and the strip was laid in place. The dowels were inserted through the holes. I suppose I should have taken a photo of that step. Then more glue was added on top and the top strip was laid in place. The whole thing was weighted with heavy jars. Because the top is to be sheeted with 1/16″ balsa, the spine sticks up a bit above the surface of the foam. To keep the jars level, 1/16″ balsa shims were added on either side of the spine.
I eventually got the whole spine held down with weights, and now I’m waiting for it to dry.
In retrospect, it would have been a good idea, probably a better idea, to attach the top plate in one piece, as was done in the wood floats in Part 3. This is done by gluing the top balsa on either side of the hard spine to make a single top plate, as shown in the following photos.
Do it either way, and it will be a good float. For my own part, I would prefer attaching the top in one piece.
Check Part 2 for the rest of the Cunningham foam float project.