Building the Stinson SR-9 (6)

The vertical tail assembly…the fixed vertical fin and the rudder…are built in the same way as the horizontal tail. That building task also presents me with a chance to talk about some of the reasons those parts are designed, or engineered, the way they are. You could build all the tail surfaces on this airplane using simple flat sheets of balsa. There are several good reasons why we aren’t doing it that way. Since this is a scale model, perhaps the most obvious is that tail surface made from a sheet of balsa will always look like just that…a sheet of balsa…no matter how carefully we might finish and decorate it. The full scale Stinson had thick, contoured (airfoiled ) tail surfaces, and nothing other than an acccurate reproduction of those shapes would ever look right. We could use really thick sheets of balsa, but a simple sheet surface, even one shaped to the right contour, would be WAY heavier than a built-up structure, and if we ducked the weight issue by using very light balsa, the structure would be too flimsy to stay together in the air. Beyond that, balsa is expensive, and doing the job right with small amounts of carefully chosen wood costs significantly less.

What about those little models that use plain sheet balsa tails and even wings? If we ignore the issue of scale accuracy (appearance) we can get by doing it that way, although we will still be using more material and making a model that is heavier than it needs to be. The point at which the trade-off between cost and simplicity, strength and weight, and appearance all come together for a specific model airplane will always be an elusive, moving target. For this Stinson, though, the choice is easy. Let’s build a tail…the right way.

We are off to a running start. The vertical tail/fin is built just like the horizontal stabilizer, using integral tabs to hold the ribs in position while the leading and trailing edges as well as the balsa sheet covering on what is for the moment the top surface are all glued in place. Here I have removed the preliminary assembly from the building board and am reinforcing a joint between the rib and the sheeting. For this job I'm using thin (fast) cyanoacrylate. Those of you are are really paying attention will remember that not so long ago I mentioned that I prefer to use traditional wood glue (Titebond) for most primary structural joints. That was BEFORE I discovered that I have a severe contact dermatitis allergic reaction to formaldehyde,, which happens to be an important ingredient in just such adhesive products. For assemblies that demand slow drying wood glue, I get to "suit up" with nitrile gloves to prevent getting any trace of it on my skin.

With the completed fin sub-assembly jigged in position on the board, I have glued the die-cut rudder core down the centerline of the rudder leading edge and likewise set that assembly up in correct alignment with the fin.

The rudder ribs are cut from strips of 1/2″ x 3/32″ balsa trimmed to the required taper, and glued to the rudder core sheet. The other side is done the same way. The blocks against the leading edge are 1/4″ x 1/2″ balsa hinge mounting points.

Capstrips are cut from 1/16″ x 1/4″ balsa, just as I did with the vertical fin. Note that the ribs are recessed by 1/16″ to allow the caps to fit flush with the surface.

The trailing edge of the rudder is laminated from two strips of 1/16" x 1/4" balsa. The first step was to sand the ends of all the capstrips flush with the core. Here I am putting the first, or inside, strip in place, using medium cyanoacrylate as an adhesive. My kit included enough strips soft enough to take this bend without breaking that I was able to do the laminate "dry".The alternative would be to wet each strip with water before attaching it. Wet or dry, the trick here is to keep PRESSURE on the strip as you roll the core assembly over it and into place.

Both trailing edge laminate layers are in place, and I have added the 5/8" sq. basswood tailwheel mount reinforcement as well as balsa blocks on the bottom and at the upper tip that will be shaped to produce the finished surface outline. The other side is done the same way.

Here I have sanded the basswood reinforcing block flush with the surface of the rudder and cut the finished part outline into the bottom blocks.

I am using a sanding block with 80-grit (coarse) paper to finish shaping the rudder as defined by the outline shown on the plan. On an assembly like this, where you are sanding structural material of varying hardnesses, coarse paper on a firm block is the best way to blend all the various parts into a smooth surface. 80-grit paper will cut soft balsa REALLY FAST. Don't be afraid of it...use it with a fine touch to remove material without having to push, press or strain to do the job.

This is the top block that forms the outline of the rudder AND the counterbalance that extends forward over the fin. I have rough shaped it with the 80-grit block. This does NOT count as finish sanding.

This is the top block seen from the other side after sanding with 120-grit and 320-grit paper in turn to get it ready for covering, when that time comes.