Building the Stinson SR-9 (18)

An old No. 11 blade is still tough enough to rough cut the openings in the aluminum panel. With that done I shaped and finished them with a narrow circular file, lined up the holes with the ends of the rudder and elevator control cable guide tube exits, and used ZAP-A-GAP to finish the job.

Now we can get started doing wheelpants. They are provided as half-moldings of ABS plastic. The right and the left panels are symmetric, but mirror-imaged…that is, the inner half of the left and right assembly in turn has a molded-in recess to fit the lower ends of the left and right LG struts respectively. The result of this is that all four “halves” are different and you need to keep track of which is which. Here’s the outer half of the right assembly still in the molding matrix.

I used a pair of heavy offset metal snips for this job…overkill, maybe, but I wanted to be sure the curved plastic did not “grab” the tool and spoil the cut.

This is the right/inner half shell getting the same treatment.

Snips and shears can take you only so far toward a properly true, smooth joining edge. I fixed a full sheet of 100-grit production paper to the bench and worked each wheelpant half in turn against it…flat…until I had an accurate mating edge.

This part is easier. I’m cleaning up the inside edge with a loose piece of 100-grit.

I taped each pair of halves together, joined the inside seam with a strip of 2 oz. fiberglass cloth and 30-minute Z-POXY, then removed the tape and cleaned up the outer face of each seam when the epoxy had cured hard. This is the assembled left pant, ready for fiberglass cloth reinforcement.

The kit instructions suggest “glassing” the assembled wheelpants for added durability. If you plan on flying your SR-9 regularly you NEED to do this or risk beating the finished ABS plastic pants to an early death with rough fields and less-than-perfect landings. I’m adding a piece of 1.5 oz. glass cloth large enough to wrap entirely around one side of the assembled wheelpant with a generous overlap to the opposite side. Z-POXY finishing resin, thinned about 25% with denatured alcohol, is the best adhesive I have found for jobs like this. Here I’m using a short, stiff brush to work a wet coat of epoxy into the fiberglass weave as I pull and press the cloth smoothly into place. NOTE: This shot is of the INSIDE surface of the pant, where I’m adding extra reinforcement.

Here’s a look at the assembled wheelpants with both sides glassed and the overlapping edges trimmed and sanded smooth. You can see the kit-supplied 1/8″ plywood insert in the right assembly…this forms a load-bearing base that mounts against the landing gear strut.

This is the right pant “dry-fitted” over the axle assembly that inserts from the inside.

I’m going to bed this threaded end of the axle (the part that assembles into the LG strut) firmly into epoxy resin, against the inset plywood face in order to define and lock in the angle at which it rests. I want to be able to remove the axle after the epoxy cures, so I’m coating the threaded surface with machine oil before fitting it into place.

This is a liberal glob of 30-minute Z-POXY…still wet… that thoroughly fills the cavity where the LG strut is going to fit.

This is the good part. I have covered the wet epoxy surface with a sheet of plastic wrap pushed over the threaded axle end and pressed the LG strut into place over that. Now I’m threading on the heavy fiberlock nut that will hold the whole thing together. I’ll pull this assembly together tight…the molded-in cavity and axle will align the wheelpant with the LG strut…and let it cure overnight.With a little careful tapping and prodding the cured epoxy assembly will pop apart at the plastic wrap and I’ll clean up any remaining rough edges.

I elected to give the entire forward fuselage (the part that’s sheet balsa covered) a reinforcement of more of that 1.5 oz. glass cloth I used on the wheelpants, applied with the same Z-POXY and alcohol. I have trimmed all the edges and overlaps and here I’m doing a preliminary sanding of the fuselage belly with a sheet of 100-grit paper.

Same deal on the fuselage side. You can see where the left LG strut fairing stops short of joining the fuselage.There’s a separate fairing coming along to cover and close that opening.

As with the wheelpants, the upper LG strut fairings get cut out of ABS plastic matrix sheets.

I’m using 80-grit production paper on a block to square off the lower end of the fairing where it’s going to slip-fit over the LG strut fairing.

I have done the fit-and-trim-and-check procedure to get the upper face of the fairing to mate neatly with the curvature of the fuselage.

I’m going to fiberglass these upper fairings for durability just like the wheelpants. Here I have them placed on temporary stand-off holders (odd cans and bottles) so I can work all the way around each piece with the epoxy-glass process.

I’ve been talking about my coating-laminating medium of Z-POXY Finishing Resin thinned with one part denatured alcohol to four parts mixed epoxy. This is what it looks like. The alcohol makes it easy to brush onto and through the fiberglass without dragging or pulling the cloth and gives complete penetration of the cloth weave through to the underlying structure, which in this case is the ABS plastic molding.

Here I have positioned a piece of glass cloth to cover approximately half of the fairing and wrap neatly around the curvature at either end.

The thinned Z-POXY mixture has exactly the right “grab” to make the cloth lie neatly in place when I “pat” it into place with the brush.
BTW: Those marks in the ferrule happened when I had cleaned the brush and put it away after a previous job. My cats decided to check to see whether it was actually prey in disguise, but then gave it back to me.

This is a similar piece of glass cloth cut to fit around the opposite side of the fairing and wrap into a generous overlap.

I brushed the second piece into place just like the first.  At this point you don’t worry about the overhangs…just let ’em be.