Building the Stinson SR-9 (21)

All the good stuff…  Stits Poly Fiber fabric comes in a clear package like this one. In the rear is a smaller piece of fabric I’ve saved from earlier jobs that’s just right to use on this one. Along with the fabric you can see the right wing panel, the rudder, a medium and LARGE pair of sharp scissors, and cans of Stits Poly Tak (adhesive) and reducer. I’ve also shown you my copy of the REALLY COMPLETE Stits Process manual.

As with just about any covering, I begin by cutting a piece of PolyFiber large enough to cover the entire surface I’m working on (the bottom surface of the right wing panel) with an overhang of several inches all around. Here I’m double checking that it fits right. IMPORTANT NOTE: Every other fabric covering I’ve used has a “grain” which must be oriented with the long axis of the surface for best tautening/shrinking. Stits PolyFiber features a bi-directional weave that eliminates that requirement…you can cut your fabric for the best yield without worrying about the grain.

With the Poly Fiber aligned where I want it to be, I’ve folded back an edge along the root rib where I’m going to start adhering the covering to the structure.

By folding the covering like this, I can guarantee that the entire workpiece will stay in alignment while I brush a generous bead of Poly Tak fabric adhesive along the edge of the root rib and “just a bit” out onto the leading and trailing edge structure.

I have pulled the PolyFiber gently tight…just smooth…along the rib. Here I am pressing it down lightly to ensure that it contacts the PolyTak all along the joint.

Now I’m repeating the fold back-brush PolyTak – press down operation along eight to ten inches of the leading edge. PolyTak begins to dry fast, so you need to work in short segments you can complete in a minute or so to be sure the PolyFiber gets pushed against REALLY WET adhesive.

Pull, stretch and press…this is where it’s up to you to get the fabric smooth across the area you’re sticking down and work it around the leading edge with NO folds or creases.

Sometimes “pull” means just that. I have to be sure the fabric extends around past the center of the leading edge radius in order to guarantee an overlap (for structural strength) when I cover the top surface. You don’t need to worry about tearing the PolyFiber when you do this.

Now it’s time for the same operation on the inboard end of the trailing edge.

And…back to the leading edge, a foot or so at a time. Here you can see that I’ve gotten the balsa WET with PolyTak before pressing the covering down onto it.

Once I’m satisfied that the covering is going onto the inner end of the panel straight and smooth, I’ll lock it down into a permanent position by attaching what has so far been the loose end at the wing tip. There’s PolyTak along six inches or so of the wingtip structure beneath the PolyFiber.

With that done I’ll work along the remaining sections of the leading and trailing edges to get all the fabric edges attached.

The wing tip structure extends back beyond the aileron well/trailing edge to form a rectangular cutout where the aileron will go. We have to trim ANY covering to lie smoothly along the various surfaces it contacts in a place like this. I have cut a diagonal slit in the covering all the way up to that right angle corner to make sure that can happen.

I cut the covering at the inboard end of the aileron well in the same way. You can see how that has created an overhang that will lie flat against the trailing edge with a simple fold.

The next step…again…is to brush a wet bead of PolyTak onto the structure where the folded over covering will adhere.

…and then I press and pull to get the fabric to stick where I want it to be.

Here’s a quick look back at how I worked on that same part of the structure on the left wing. Don’t be afraid to PULL enough to get out any deep sags and ALL “crinkles”.

Finishing the tip. I have trimmed the overhang back to about an inch all along the tip. Here I’m doing the “wet PolyTak” thing on the opposite (top) surface.

Then…just like before…I fold and press the overhang into place. By trimming it back I ensure that I can fold the entire edge of the fabric around the radius without gathering or bunching.

Now I’m starting to cover the upper surface of the right wing. The bottom is already covered and this operation will “close up” the structure and provide double overlaps all around the perimeter of the wing panel. At this point I have laid the previously cut-out sheet of PolyFiber loosely in place on the wing and I’m folding back a few inches at the root rib prior to brushing on the first strip of PolyTak.

The wing panel is upside down here…you’re looking “up” at the top surface. the covering is smooth but still slack as I wrap it around and over the leading edge to overlap the fabric that is already in place.

I’m doing the same thing at the wing tip. This is the top of the right wing.

When I trim off the excess PolyFiber and turn the panel bottom side up, it looks like this.

The next step is to brush a generous bead of PolyTak into the area that will become the overlapped fabric joint.

This part is where there’s no substitute for getting stuff on your hands…I’m using the palm of my hand to fold-stretch-press the fabric into place.

I finished up the other wing panel the same way off-camera and now I’m starting the job of covering the horizontal stabilizer. Here you’re looking at the top of the right panel with the working sheet of PolyFiber resting loosely in place.

This is the inboard end of that same panel. I have stretched and pressed one end of the fabric into a bead of PolyTak along the base (root) of the stabilizer. It’s easy to tell the difference between where the fabric is stuck down and where it’s not.

Same as with the wing panels, I’m pulling the fabric panel smooth spanwise and sticking it in place along the stabilizer tip with more PolyTak.

Again, just like the wing, I’m working alternately along the leading and trailing edges to get the entire fabric panel adhered neatly in place with no puckers or creases.

Working from the underside of the same panel, I have trimmed off the excess fabric and now I’m folding the remaining overhang onto the trailing edge. I brushed PolyTak onto the structure before making the fold you see here.

The leading edge is next (this happens to be the left stabilizer panel). I’m making sure to get a generous, wet bead of PolyTak brushed onto the seam area. This leading edge seam is just about as long a section as I want to work on at a time.

Same old fold-pull-stretch. There’s no substitute for educated fingertips when you’re doing this part of a covering job. You’ll learn to tell just when the PolyTak is about to “grab”.

When that’s done it looks like this. Next I’ll use a sharp razor blade to trim that overhang even with the inside rear face of the leading edge.

Now I’m doing the opposite side of the panel. I’ll repeat all the steps of folding edges and pressing and trimming until the structure is entirely covered.

I’m doing the same covering sequence on the rudder. At this point I have both sides covered with all the overlapping seams sealed. You can see what I mean by having the fabric “smooth, but not taut”. NO CREASES is the important thing now.

Here’s a look directly at the leading edge of the rudder to reinforce what I’m talking about.

The rest of the control surfaces get the same treatment. This is the right wing flap at the same step on the covering process…all stuck down, ready to shrink.

Follow the Stits Lite manual faithfully at this stage of your covering job. I’m beginning the “heat-tauting” part of the process by pressing and smoothing all of those fabric overlaps at 250 degrees. Don’t trust the markings on your covering iron to be reliable…use a good thermometer to be sure your iron’s actually operating at the proper temperature before you put it anywhere near your nice new covering job.

Here’s another look at the same process…I’m sealing and smoothing the leading edge of the rudder.

With all the edges, overlaps and seams ironed down smooth and tight, I’m going over the entire fabric job with the iron still set at 250. This “pre-shrink” pulls the entire covering up smooth and evens out potential stress areas. In the next step I’ll pull it TIGHT at 300 degrees.

That looks like THIS. Next time we’ll work on the fuselage and you’ll get to see more shrinking operations.