Let’s Build a Real Old-Time Balsa Model Airplane

Let’s work on that stick-and-tissue Piper Cub some more. Here are the fully assembled tail surfaces. No, they don’t come out of the box that way! You already know that because we left off last time with the bare beginnings of the horizontal tail laid out on the building board. There was a lot left to do to get them to look like this. If you haven’t been around these things before you might be tempted to think it’s finished and ready to attach to the rest of the model. Not so! To understand how we got to this stage of construction, let’s have a closer look as the vertical tail goes together. Later in this series we’ll get a closer look at all the things there are still left to do.

Just as with the horizontal tail surface, we built the “rudder” (vertical tail) over a sheet of plastic wrap. All these “cut-out” shaped pieces, in this case the vertical tail outline parts, depend on the same razor blade/craft knife techniques to get them ready to assemble. The vertical tail outline parts are all 1/16” balsa sheet. On this airplane the tail surfaces are “flat” (of constant thickness) so that straight piece of balsa that becomes the leading edge of the moveable rudder is 1/8” wide by that same 1/16” thick to march the outline parts. At this point he rudder trailing edge (R-2) is already pinned and glued in place. I have trimmed R-3 for an exact fit between R-2 and the rudder leading edge, and you can see the beads of aliphatic resin glue on the joining surfaces at each end. Using that pointy blade as a tool to hold and position a balsa part like this is a good alternative to using your fingers when the structure is so small.

Now is a good time to get my fingers involved, so I can feel just the right pressure to endure that R-3 is aligned and seated against the building surface.

The rest of the outline structure goes together just the same way. You can see how each of the shaped/cut-out pieces fits over a designated spot on the plan. While the straight “sticks” get cut from strip balsa stock (in this case 1/8” x 1/16”) to fit the various lengths defined by the plan. All the joints you can see have been assembled with aliphatic resin glue, and you can see how I have worked to keep those pins outside each piece of balsa as I described earlier.

You could probably figure out by examining the previous photo that there is still some more balsa left to assemble to complete the vertical tail. These 1/16” sq. “sticks” reinforce the outline structure and provide support “in the middle” so the tissue covering will lie flat and not sag. It’s OK to call these “ribs” because they share some of the function of the “real” ribs in the wing by providing the structure that defines the outer surface. I cut them from some more of that custom-stripped balsa I made to replace the poplar that came in the kit. I also made a significant change in the design/assembly by insetting each of the 1/16” sq. ends into the corresponding outline part with a carefully cut 1/16” notch. The little extra work this entails pays off with a serious difference in the overall strength and
stiffness (resistance to twisting) of the finished component. You can go back and check…I did this on the horizontal tail as well.

Take another look at that last photo. Al the glued joints are so closely fitted that it’s hard to find any gaps…but the surface of the assembly is so rough and uneven that you can’t miss noticing it. Here’s how we fix that. More sanding…very careful and controlled sanding…gets that surface so smooth and level that there will be NO bumps to bulge through the tissue and spoil the finished surface. So what, exactly, is going on here? It’s not hard to figure out that a piece of balsa model airplane structure like this is going to break the moment we try to hold it with one hand and scrub on it with some sandpaper held in the other hand. What happens is what leads to words like flimsy and fragile. Balsa model airplane parts like this have to be supported, or backed up, before we can apply enough sanding pressure to do any useful shaping or smoothing. Working on a stable surface is a good way to provide that support, and facing that with an appropriate grade of abrasive paper large enough to move the entire part across is even better. With some more sandpaper on a smaller block like this it’s easy to cut down all those uneven spots, glue bumps, etc. with precise control of the pressure we apply to the work. Depending on the grit (coarseness) on the moving block relative to the base sheet the base may hold the part stationary while the block moves across it, or the other way around. All this will make a lot more sense once you have actually done it.

Here’s a variation that that technique. If you need to put sanding pressure on a small, specific portion of a part without cutting away the balsa around it, direct finger pressure like this lets you fine-tune your work.

After the surfaces of a part like this horizontal tail are as smooth as you can get them, free-handing around the outer edges with a smaller sanding block gives you a defined edge/shape to match the plan as closely as you want.

What’s that red stuff? Sandpaper is a generic term. Actually, abrasive paper of the quality we want to use on model airplanes contains no “sand” at all. This brick-red-colored sheet is production paper made with aluminum oxide or a similar material that you can get in various grits (grades) at your local hardware store. This is a sheet of a 220-grit proprietary abrasive paper sold at Lowe’s. The tan-colored stuff in the other photo is another brand of 220-grit. In the photo that shows my fingers pressing directly on the balsa part 120-grit, which is coarser. I chose that sheet to do the initial smoothing as the more aggressive abrasive cuts away unwanted material with less pressure.

There’s a lot of work left to do on there tail surfaces, but I’m going to wait until we have a fuselage structure to attach them to before going any further. Before we get to that, though, I am going to build the wing panels. We’ll get to that next time.