Building the Stinson SR-9 (22)

The first step is just the same as what I did in covering the wing…I chose the best place to start (in this case the fuselage bottom, or belly) and cut a piece of Stits Polyfiber fabric big enough to cover the entire structure with a generous overhang all around.
Stits Polyfiber, like any other aircraft covering, is not cheap, but trying to save a few dollars by trimming your fabric margins so narrow that you can’t grab-and-pull-and-stretch properly is false economy. Here the cut-out section of fabric is resting loose on the fuselage bottom.

The last image showed you the tail. Here’s a look at the underside of the nose before I used any adhesive. You can see where I have made shallow cutouts in the fabric to permit it to fit neatly around the main landing gear legs.

I used Stits PolyTak to stick the covering around the tailwheel mounting plate and now I’m doing the same thing around the bottom center of the firewall/front former. I’ll go on to adhere the covering all along the lower fuselage longerons and then trim away all the overhanging loose edges to get ready for covering the fuselage sides.

I’m going to cover the fuselage sides with one piece of fabric each, which will run all the way from nose to tail, with the upper and lower edges defined by the upper and lower longerons in turn. Since the structural design of the model Stinson calls for the horizontal stabilizer to be assembled in place before covering, we have to cover around it. The kit design made sure there would be a wide enough margin of flat sheet balsa surface all around the stabilizer root so all I have to do is to make a cutout that permits the working piece of fabric to fit snugly around it and lie flat.

Here’s the same fabric-around-the-tail arrangement as seen from the other side. I have pulled the rest of the working piece of fabric loosely but smoothly along the fuselage and over the nose to be sure it’s going to fit exactly where I want it to.

Here I have brushed PolyTak adhesive all around the outer edges of the fuselage side and the stabilizer root.

I want the PolyTak to be WET for maximum adhesion so I immediately pulled the PolyFiber fabric smooth over my work area and pressed it into place.

It takes PolyTak only a few moments to “grab” hard enough to permit you to pull gently against the bond, so I was able to move directly to the nose and secure the front end of the fabric around the center of the firewall (F-1) former.

Just as I did with the bottom piece of fabric, I’m cutting out a “relief” section from the side panel so it can fit around the main landing gear strut with no chance of catching or wrinkling. Because this strut-to-fuselage junction will be covered/concealed by a wide fairing that goes in place after covering and finishing there’s no need to make a “fussy” fit of the fabric around the strut. The amount of structural strength that might be gained by doing so would be negligible at best.

Same place with the fabric stuck down with PolyTak around the strut and along the fabric seam that runs toward the nose. Notice that I’ve left a broad overlap of the fabric edges. This will create a seam, or raised edge, that will need hiding with surfacing primer and sanding, but in this case the extra strength will count and I want it.

This is still the left fuselage side fabric panel. It’s adhered at the tail, the nose, and along the lower fuselage edge ahead of the landing gear strut. Now I’m closing up the seam that’s formed along the lower longeron. You can see from the shine that I’m being generous with the PolyTak. (I’ll brush that adhesive along about HALF the length of the open seam, position the fabric, then close up the rest.

Here’s the remainder of the lower longeron seam closed and pressed into place. (Yes, this happens to be the RIGHT fuselage side.)

Back to the left side, different view. We’re looking down at the left fuselage side…that’s the still-open fuselage top in front. The curved widget is part of the fuselage structure that becomes the left wing trailing edge root fairing. I’ve cut a slot in the fabric to permit it to lie flat all along the top longeron (beneath my hand at the left) and fit into the corner created by the protruding wing root fairing.

Same game as seen from in front and below. I’m using “calibrated thumbnail pressure” to seat the fabric edge into the PolyTak I previously brushed into the structural “corner” above the cabin window and door openings that you can see through the fabric.

I worked the fabric tightly into this “corner” all the way along the wing root base to the leading edge. Where the door frame portion of the structure meets the wing root base here, there is only 1/8″ of “overhang”, but it’s critical that the fabric gets stuck down tight to follow the structural surface everywhere. Not doing that would result in “bridging” , or unattached bubbles in the finished fabric surface and a generally sloppy appearance.

Now I’m closing up the left fuselage side panel by sticking the fabric in place all along the edge created by the firewall.

I finished the left side covering by adhering the PolyFiber all along the remaining open edges along the nose and rear edges of the windshield opening.

This is the other (right) side of the fuselage with the same job finished.

I’m trimming the excess fabric overhang along the left lower longeron using a (sharp) single edge razor blade. Notice that I’m holding the blade at an angle. What you can’t see is that I’m “slicing” through the fabric, not “pushing” the cutting edge. PolyFiber permits you to make very clean cuts if you do it this way.

Now we get to the good part! I begin the fabric fairing between the vertical tail and the rear fuselage top by cutting a single piece of fabric that covers the entire area defined by the upper longeron, the top center fuselage stringer, and the top/back of the windshield cutout. I have draped the cut-out section of fabric loosely over the entire area I’m going to cover.

Just checking… I’m pushing the fabric panel into place to be sure it’s going to fit along the top longeron seam and reach out past all the other edges I want it to stick to.

Now we can get to work. I have used PolyTak to stick the fabric in place along the vertical fin trailing edge and the narrow balsa sheet “shelf” above the horizontal stabilizer root, along with about a foot of the top longeron. Don’t allow ANY puckers or folds here. Very soon the “open” area of fabric above where I’m working is going to get pulled and stretched REALLY TIGHT.

Here’s the fabric panel at the same stage of the process as seen from the front.

Now I’m getting close, but there’s still a little bit too much fabric bridging that triangular area to permit a close fit along the dorsal edge without gathering and puckering. You can see from the tension creases in the material that I am PULLING on it.

Now we’re getting there. You can see how closely I had to slit the fabric up to the dorsal fin edge to get it to lie smoothly all along the seam. So far there’s no PolyTak on the fin leading edge.

It’s stuck! At this point you’ll get your fingers sticky…there’s no alternative to pressing and holding until the PolyTak dries enough to grab.

From the front it looks like this. I have yet to pull the fabric smooth all the way along the top center stringer, but you can see where this is going.

Now the left half of the job is done. You can see that there are still some slack, wavy areas of fabric, but that all of the edges are stuck down smooth and tight around the entire perimeter of the fabric panel. Heat-tauting of the PolyFiber per the Stits manual will take care of the rest of that job, but NOT YET. I have to close up the other side before I put any shrinking tension on the structure.

One last check! Here’s the same part of the job from the still-unfinished side. I will cover this side of the fuselage just the same way as the first, with a generous overlap along the top stringer and fin leading edge…BUT…I’m gonna make you wait until next time to see how the finished, closed covering job looks after it’s heat-tautened tight and smooth.