Building the Stinson SR-9 (13)

I cut two pieces of 1/2″ balsa tri-stock – one for each side of the horizontal stabilizer saddle – and made several transverse cuts with a razor saw to permit bending them to match the curve of the light ply saddle. (You MIGHT be able to bend the tri-stock against the grain to fit the curve, but that would introduce stresses that could deform the tail assembly.) The cuts don’t compromise anything because you are depending on the tri-stock as a glueing base…not for structural strength.

With the cut and bent tri-stock held in place I used fast ZAP to glue up the assembly.

With that done, the fit for the horizontal stabilizer will be close, but not perfect. I used a short sanding block with coarse (40 grit) production paper to smooth off the joining surface.

This is the saddle assembly ready to accept the horizontal stabilizer. Note that I have left an overhang of the top center stringer behind F-9. This will get trimmed off.

Here the horizontal stabilizer is dry-fitted in place and pinned to prevent shifting while I use a strip of 1/4″ sq. balsa pinned on the fuselage centerline at F-3 to trammel (square off) the assembly. The plans suggest using string…but string can stretch. I prefer a measuring stick.

Here’s a closer look at the working end of that measuring stick. When my pencil mark meets the left rear corner exactly like this when I swing the stick across to the other side,the stabilizer is squared.

The vertical tail gets aligned and glued in pretty much the same way. You can’t see it because I put it away before taking the picture, but I used a 90 degree square to align the horizontal and vertical trailing edges with each other.

Here I have added a 1/4″ balsa tri-stock gusset along the base of the vertical fin, trimmed and tapered the two top/side stringers, and added a stab fairing made from leftover 1/4″ balsa sheet. This has been trimmed and sanded to flow from the top corner stringer at F-9 and taper to nothing where the trailing edge of the horizontal stabilizer meets the fin.

Here’s the same assembly as seen from the opposite side. Look carefully and you can see that I have added the vertical fin fillet of 1/4″ balsa and sanded it to blend with the fin leading edge.

The next part of the job is mounting the wing. To begin I built up the top and bottom halves of the joiner box, each of them from two pre-cut 1/8″ x 1 11/16″ x 10 9/16″ plywood sheets along with some 1/4″ x 3/8″ basswood. These are the parts for the bottom of the box…the top goes together in exactly the same way.

This is the bottom joiner box half installed in place on the 1/4″ x 1/2″ top longerons at F-4 and tight against the front face of F4C. Note that I have trimmed and sanded the outer ends of the joiner box structure flush with the outer faces of the longerons.

I got ahead of the camera again…the cardboard joiner tube and the joiner box top have been ZAP’d in place and now I am lining up the left wing saddle base with several clothespin clamps to hold it in position for the next step.

This is the W1-F fuselage root rib clamped in place against the left wing root rib with the rear basswood wing bolt block in place. The “other” block with the hole is one of the flap hinge blocks.

Both the left and right root ribs are in place, along with the aluminum wing joiner tube. NOTE: These ribs, along with the top wing saddle bases, are NOT YET glued to anything. They’re still “floating” in place to help with alignment of the wing panels with the fuselage. What you see here is a test-fit.

I have drilled 5/32″ holes through the front and rear of each W1F and through the wing bolt blocks, then seated a 6/32 blind nut on the face of each block that that will be exposed inside the completed wing.

Here’s the root of the left wing with the W1F rib held in place by the 6-32 bolts. The next step will be to re-assemble both wing panels – with the W1F’s bolted in place – onto the fuselage and the aluminum joiner tube.

I have the fuselage blocked up level on the building board ( as defined by the top longerons) and now I’m using an incidence meter to confirm that both wing panels rest on the wing saddle bases at exactly the same angle of incidence. (There are in fact multiple detailed steps in this assembly sequence, all of which are carefully spelled out by the instruction manual.)

When the entire wing attachment sequence was complete, I removed the 6-32 wing bolts and pulled each wing panel free of the aluminum joiner tube. What’s left is the W1F fuselage root ribs securely glued in place to the wing saddle bases. The next step is adding the top wing fillet sections along with some extra 1/4″ sq. balsa reinforcements. (The camera crew was goofing off when I was doing that, but you’ll get a better look at that part of the center section later on.)

Now I’m going to mount the rudder and elevator servos. The instructions and plans refer to an alternative mounting of these two servos in the tail as an aid to balancing the airplane if a really heavy gas-ignition engine is installed. With electric power any such extra weight is not an issue, so I’m using the primary location with the servos at the rear of the cabin (where they can be concealed by the rear seat). Electric power also means I don’t need a throttle servo. Here I have assembled two 1/4″ x 3/8″ basswood rails and two 1/8″ plywood servo rail spacers. The kit-standard assembly accepts my Airtronics 94757 Digital servos without any modification.

The bottom edges of the servo rail spacers are ZAP’d to the cabin floor and the ends of the rear rail to F-5. NOTE: I have trimmed the outer ends of both rails to fit snug against the 1/8″ balsa side sheeting that will be added later.

In order to install the outer pushrod guide tubes for both the rudder and the elevator accurately, we need to have both control surfaces temporarily mounted in place to serve as a basis for measurement. The first step in doing this is to drill 7/64″ holes in both elevator control horn blocks, then tap them for a 6-32 thread.

I have threaded a 1  1/2″ 6-32 rod into that hole and screwed on a nylon torque rod horn.

This is how the elevator section looks when it is seated into place.

I want to guarantee that the elevator stays in the NEUTRAL position while I’m cutting and connecting the control rod. This little jig made from a clothespin clamp and two scraps of balsa sheet does that job for me.

I centered the other elevator half as well as the rudder the same way, then installed the outer pushrod guide tubes through the several plywood guide tube holders that were already in place within the tail framework.  Here you can see two dark gray outer guide tubes that will carry smaller inner push-pull control tubes to each half of the elevator AND two lengths of the light gray smaller diameter tube that will carry pull-pull cables to either side of the rudder horn. I have left just enough free space between the tube ends and the servos to fit and install the various cables, inner tubes, and clevis fittings that will complete the job.

This is how the outer tubes look in place in the tail structure.

When that part of the control installation is finished, it looks like this.