Building the Stinson SR-9 (8)

The next step is the addition of the leading edge sheeting and the capstrips to the wing structure. All of the assembly up to this point has been done with the wing flat on the building board, kept in alignment by various built-in jigs and spacers and held in place by those big lead building weights. The upper leading edge sheeting will go on with all of this structure still fixed in place. Lots of experience has shown that adding a sheet balsa leading edge is one of the best ways to lock everything up tight and keep it that way, so it’s important to be sure it’s all aligned the way we want before grabbing the glue bottle.

The upper leading edge sheeting, which is 3/32" inch balsa, is supplied in 3" x 36" sheets and must be trimmed to fit the structure. For this assembly I chose to wet the outside surface of the sheet with water to make it easier to bend, and to use Titebond glue for assembly to make certain I had enough time to get everything lined up perfectly before the adhesive started to "grab". I have used ordinary masking tape to hold the glued lay-up in place. It would be very difficult to use pins or clamps to do this job without compressing or tearing up the wood.

Now the wing structure is off the board, turned upside down, and ready for adding the lower leading edge sheet. The rectangular notches just behind the leading edge are for the 1/4" x 3/8" basswood leading edge spar, which I still have to add before going on with the sheeting operation.

This is the underside of the left wingtip (the top side was done the same way) The rear edge of the leading edge sheet on the full scale Stinson follows a unique curvature. To reproduce this accurately, the plan provides a paper template to use as a cutting pattern for this section. I have attached the sheet after wetting the outer surface, using medium fast cyanoacrylate because the piece is small enough that I can press and hold it in place with my hands while the adhesive cures.

I used Titebond to adhere the main section of the lower leading edge sheet because the workpiece was too big to hold in place by hand. As with the upper surface, I used masking tape. Tape will not stick to wet balsa, so I wrapped it all the way around the structure.

This is the leading edge sheet shown from behind so you can see how I used enough tape to extend to dry wood. You can also see how tightly the rear edge of the sheet should fit against the auxiliary spar.

Here you can see how the large section of sheet balsa fits right up against the smaller tip piece I installer earlier.

All the exposed edges of the wing ribs, top and bottom, are capstripped, and more capstrip material (1/4" x 1/16" balsa) is added along the adjacent edges of the trailing edge parts to create a smooth surface. Here the caps have been added to the trailing edge sections as well as ribs one through three, and the fourth capstrip is ready to be glued in place.

: This is the underside of the left wingtip. All the tip structure doublers, leading edge sheet and capstrips are in place, ready for the sanding block to do the final shaping.