Let’s Build a Real Old-Time Balsa Model Airplane – Part 3

Let’s get started building a wing for our Comet Cub. Here’s a better look at that first rib. Can you figure out what I’ve done to it already, and what still needs to be done? This piece of balsa began its new life as part of a model airplane just a while back when I cut it out of the balsa printwood matrix sheet the same way as the tail surface parts we saw in Part 1.

Once the part is cut out we still have to cut out any slots, notches or other openings that are shown on the plan. When all the cutting is done, good model builders will sand both faces of any sheet balsa part, both to remove any surface imperfections and to provide a neat surface. This looks good, but it also serves to trim away a bit of extra weight that the finished model shouldn’t have to carry. We saw some of this already during the assembly of the tail surface parts. Here we are working on the version of the wing rib pattern of which there are the most in this model…Rib B. Learning to cut accurate notches that are going to fit together with other parts…in this case the rear bottom wing spar…is going to test your skill. Here I am pressing straight down with the corner of a sharp (new) single edge razor blade for this straight across-the-grain cut.

It’s easier to control the pointed No. 11 blade making this with-the-grain cut. Small cut-out pieces like this often split free and separate by themselves. Don’t count on that…use your blade and make the cut precise.

Sometimes these little scraps even fall out on their own. When they don’t, pushing them free with that No. 11 blade is a good way to avoid trying to get your fingers to work the way you’d like at a very small-scale task like this. It always takes new model builders a while to learn to handle small balsa parts like this without having them split, crack or otherwise break. It takes lots of practice.

This is how the getting smooth part works. We talked about those various grades of abrasive paper in Part 2. I am using a 220-grit sanding block to true-up and smooth both surfaces/faces of this rib. Holding the balsa part between two abrasive surfaces permits us to crate a really even, uniform surface. At the same time it supports the entire workpiece so we don’t risk breaking it by holding it in one place and sanding (pressing) in another. I have discovered that by sanding away just enough wood to remove any obvious bumps or rough grain and then lightly erase any printed markings .., and stopping right there…I can avoid the risk of sanding away too much material and spoiling the part.

We also talked about stripwood, which is supposed to be cut from balsa for models like this and why poplar as a wartime substitute led to lots of unhappiness. We have already used some of the good stuff that I custom-cut from extra balsa sheet stock. This happens to be a spray of 3/32” sq. balsa…left over from some past project…that demonstrates what good balsa should look like. This piece of wood is straight-grained and uniform, which is good, but it also betrays a slightly rough surface, which is something we can improve.

Here’s how I did that. I cut several of those 3/32” sq. strips loose and then used the same two-sided sanding technique to give each of them a proper smooth surface. Can you compare the single balsa strip in this photo to the wood in the previous one and distinguish the difference in the surface quality?

All the wing ribs have been cut out and are ready for assembly, but there’s one more set of wings parts we need to work on before starting to actually put a wing panel together. On most full scale fabric covered light airplanes (including the Piper J-5) each round wingtip outline consists of a laminated wood (usually spruce) bow. A majority of traditionally designed small balsa kits for models like the J-5 use wingtips made from several cut-out balsa sheet parts like these. (Yes, we could replicate the scale tip bows in the interest of realism, but that would demand more advanced model building skills. One of the design criteria for classic kits like this one was to leave out such more complicated building challenges that might intimidate beginners.) I’m going to follow the design by using these printwood wingtip outline parts, but I’m also going to include a change in the way they are assembled that will make a serious difference in the appearance of the completed model.

Rather than leave the two wingtip sub-assemblies pinned flat to the workboard the way I did with the tail surfaces I’m removing them from the plan as soon as the glue has set in order to be able to sand them to the exact shape and surface finish I want before assembling them to the main wing panel structure. You’ll see why.

I’m cutting/shaping the outer edge of this tip assembly with some careful sanding, so that it will match exactly with the outline shape shown on the plan.

Because this model, like a majority of light weight rubber-powered jobs, will have a see-through covering I chose to finish-trim even those edges and outlines that might be partly hidden on the interior of the model.

Remember that the wingtip bow assembly is supposed to be assembled flat on the plan. We’re not going to do that, so I have left those balsa parts aside while I begin the actual construction of the left wing panel. The 1/16” x 1/8” front and rear bottom wing spars, which I have made from some of that custom-stripped wood, get pinned into position exactly per the plan. I am assembling the wing ribs beginning with Rib C, which is slightly shorter than the rest of the ribs. For this part of the assembly I have chosen to use Deluxe Materials Roket Hot, which sets very quickly.

Here’s a close-up of the Rib C assembly. If you look closely you should be able to see the precise, tight fit between parts that I have tried to achieve. The better you get at this, the better the models you build will look.

The rest of the ribs go in like this. I have also assembled the 1/16” sq. balsa top spar into the notches on the top edge of each rib. This part follows the plan as well.

Now it’s time to add that wingtip bow sub-assembly. I mentioned earlier how the plan shows it flat on the board, parallel to the bottom surface of the wing. What’s going to be different here is that I am assembling the tip bow at an angle up from where it joins the leading and trailing edges so that it lines up with the top surface of the wing. All the full scale planes we have talked about do it this way.

All assembled, with the necessary pins and spacer blocks to hold it in place, the wingtip bow a6ttached at the scale angle looks like this. We’ll trim away that extra trailing edge material later.

This is the other end of the wing panel…the wing root. You can see how much the 1/8” thick Rib A differs from the rest. This will provide extra strength where the wing root is going to attach to the fuselage.
Next time I have some more details of the wing construction to share, and then we’ll get into some serious finish sanding.