Building the Stinson SR-9 (23)

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Now we get to one of those places where “doing scale” starts to get a higher weird factor.  I can guarantee you that the many hours you may spend doing what I’ll describe this time will never make your model airplane fly better.  What a well done stitch-and-tape job will do is guarantee that your model will grab and hold the attention of anybody who gets close enough to notice it. Why?

All certificated (legally flyable) fabric covered full scale airplanes that I’m aware of use some mechanical arrangement to hold the fabric tightly against the wing ribs and corresponding fore-and-aft members of the tail and control surfaces. (These are also referred to collectively as the flying surfaces…their purpose is to generate aerodynamic forces). The strength of whatever adhesive has been used is not sufficient by itself to keep the covering from pulling away from the aircraft structure. As a rule we don’t we don’t have to do this with our models (with the possible exception of the very biggest ones) because at the low levels of mass and force we’re dealing with, the bonds generated by our covering adhesives and the structure are sufficient to keep it all together.

The collected bits and pieces that go together to make up this 1/5 scale Stinson just don’t need that added reinforcement. The truth is that the rib stitching I’m adding, as is the case with nearly all RC scale models, is cosmetic and adds no strength. However, if the “RC model” part of the project doesn’t care about rib stitching, the “scale” part most certainly does. Most anybody with an appreciation of aviation history and for sure any scale judge will notice whether the rib stitching is there and whether it’s done right. I probably won’t ever compete with this model, but I’m building it to that standard. There are a number of ways of getting it done right. What I’m about to show you is probably the most common, and it has worked well enough for me to make it through some pretty heavy-duty competitions.

 

We left off the last session with the entire airframe covered with Stits PolyFiber using PolyTak adhesive and then heat-tauted to a uniformly smooth tension. Whether or not any rib stitching detail is going to follow, the next step in the Stits process is to give the entire covering job a generous coat of PolyBrush sealer. Like clear dope on a traditional fabric covering job, PolyBrush seals the weave of the fabric. However, because the properly heat-tauted PolyFiber fabric is as tight as we will ever want it to be...and will stay that way...we don't need any shrinking effect from anything we subsequently brush or spray onto it. PolyBrush doesn't shrink at all...we don't want it to. One of the advantages of the Stits process is that once the fabric is heat-tauted to exactly the degree we want it, it stays that way throughout the rest of the finishing job. Right now I'm brushing the first PolyBrush coat onto the "raw fabric" of the tail assembly. The goal here is to fill the fabric weave...to get as much PolyBrush onto/into the material as it will hold without running or sagging...and them go away and let it dry.

We left off the last session with the entire airframe covered with Stits PolyFiber using PolyTak adhesive and then heat-tauted to a uniformly smooth tension. Whether or not any rib stitching detail is going to follow, the next step in the Stits process is to give the entire covering job a generous coat of PolyBrush sealer. Like clear dope on a traditional fabric covering job, PolyBrush seals the weave of the fabric. However, because the properly heat-tauted PolyFiber fabric is as tight as we will ever want it to be…and will stay that way…we don’t need any shrinking effect from anything we subsequently brush or spray onto it. PolyBrush doesn’t shrink at all…we don’t want it to. One of the advantages of the Stits process is that once the fabric is heat-tauted to exactly the degree we want it, it stays that way throughout the rest of the finishing job.
Right now I’m brushing the first PolyBrush coat onto the “raw fabric” of the tail assembly. The goal here is to fill the fabric weave…to get as much PolyBrush onto/into the material as it will hold without running or sagging…and them go away and let it dry.

 

If I were not going to represent stitches I would now simply add another PolyBrush coat, but...there's more to do before that happens on this airplane. I'm starting with the rudder. No matter where, the technique is the same anywhere on the airplane. If this were a full scale airplane, rib stitching cord would be passed through a tiny hole in the fabric beside each rib, THROUGH the structure, out around the opposite surface, and then back through and out again. The stitching cord will run the entire length of each rib in one piece. It gets knotted at each crossover, drawn back to the next "stitch interval" position, and then the stitch is repeated over and over all the way from the leading to the trailing edge. The stitch interval for each airplane is determined by FAA specs and it can vary from one part of the airplane to another. On this model I'm representing an interval of 2", which is 1/2" at 1/5 scale. You can see where I have drawn spanwise lines along the rudder surface to denote 1/2" intervals where each line crosses each rib. Each intersection denotes the location of a stitch.

If I were not going to represent stitches I would now simply add another PolyBrush coat, but…there’s more to do before that happens on this airplane. I’m starting with the rudder. No matter where, the technique is the same anywhere on the airplane. If this were a full scale airplane, rib stitching cord would be passed through a tiny hole in the fabric beside each rib, THROUGH the structure, out around the opposite surface, and then back through and out again. The stitching cord will run the entire length of each rib in one piece. It gets knotted at each crossover, drawn back to the next “stitch interval” position, and then the stitch is repeated over and over all the way from the leading to the trailing edge. The stitch interval for each airplane is determined by FAA specs and it can vary from one part of the airplane to another. On this model I’m representing an interval of 2″, which is 1/2″ at 1/5 scale. You can see where I have drawn spanwise lines along the rudder surface to denote 1/2″ intervals where each line crosses each rib. Each intersection denotes the location of a stitch.

 

No matter what I might or might not put inside the structure, only the short sections of cord that remain outside the fabric where they cross each rib are going to show...so...those are all I really need to represent. One of the best ways that's been developed to represent those "crossovers" is to place short "lines" of aliphatic glue (Titebond) along each crossover using a syringe of some sort. There area lot of ways to rig one up. Mine is an ordinary veterinary syringe with a piece of 1/16" O.D. aluminum tubing ZAP'd into the tip where the needle is supposed to go. You have to experiment and practice to learn how hard to press, how fast to move thee tip, how much the glue is going to shrink when it dries and so on. there's a lot to it, but once you get the technique dialed in, it's not hard to do. What you will need is patience.

No matter what I might or might not put inside the structure, only the short sections of cord that remain outside the fabric where they cross each rib are going to show…so…those are all I really need to represent. One of the best ways that’s been developed to represent those “crossovers” is to place short “lines” of aliphatic glue (Titebond) along each crossover using a syringe of some sort. There are a lot of ways to rig one up. Mine is an ordinary veterinary syringe with a piece of 1/16″ O.D. aluminum tubing ZAP’d into the tip where the needle is supposed to go. You have to experiment and practice to learn how hard to press, how fast to move the tip, how much the glue is going to shrink when it dries and so on. There’s a lot to it, but once you get the technique dialed in, it’s not hard to do. What you will need is patience.

 

This is the bottom surface of the right wing with all the "glue line stitches" in place and dry. The next step will be to cover up all the work you've done to this point without making it disappear.

This is the bottom surface of the right wing with all the “glue line stitches” in place and dry. The next step will be to cover up all the work you’ve done to this point without making it disappear.

 

In real life the row of locking cross-overs along each rib get covered, both for reinforcement and for streamlining, with a pinked-edge cover tape made from the same material as the primary covering. It would be nice to be able to make the little bumps created in the tape by each stitch disappear, but that would require too much sanding base/primer in the paint job (too much weight), so the typical full scale fabric job includes "semi-blended-in" tapes with rows of little bumps where the stitches are. What we are trying to do is to make the painted fabric surface of our model look as it that's what's happening. Over the years model builders have come up with many way to represent "rib tape" in scale sizes. Stits Lite has the best solution...they offer PolyFiber tapes pre-cut to several useful sizes especially for models. On this full scale Stinson the rib tapes are 2" wide. This is a 1/5 scale model, so ideally our tapes would be 0.4" wide. So far Stits doesn't make that, but they do offer 0.5", which is 2" for 1/4 scale models and that's so close that only the most determined scale judges might notice (or care). So...her we are with several ribs/rows of stitching in the foreground all taped and another ready to start. I have cut a length of rib tape long enough to reach from leading to trailing edge with a little "working overhang". Notice that while there are no rib stitches near the leading edge where the fabric covered sheet balsa surface represent aluminum on the full scale airplane, and rib tape extends all the way up to and around the leading edge. We NEVER leave the blunt, open end of a tape exposed where airflow could catch it and tear it loose. More on this in a moment.

In real life the row of locking cross-overs along each rib gets covered, both for reinforcement and for streamlining, with a pinked-edge cover tape made from the same material as the primary covering. It would be nice to be able to make the little bumps created in the tape by each stitch disappear, but that would require too much sanding base/primer in the paint job (too much weight), so the typical full scale fabric job includes “semi-blended-in” tapes with rows of little bumps where the stitches are. What we are trying to do is to make the painted fabric surface of our model look as if that’s what’s happening.
Over the years model builders have come up with many ways to represent “rib tape” in scale sizes. Stits Lite has the best solution…they offer PolyFiber tapes pre-cut to several useful sizes especially for models. On this full scale Stinson the rib tapes are 2″ wide. This is a 1/5 scale model, so ideally our tapes would be 0.4″ wide. So far Stits doesn’t make that, but they do offer 0.5″, which is 2″ for 1/4 scale models and that’s so close that only the most determined scale judges might notice (or care). So…here we are with several ribs/rows of stitching in the foreground all taped and another ready to start. I have cut a length of rib tape long enough to reach from leading to trailing edge with a little “working overhang”. Notice that while there are no rib stitches near the leading edge where the fabric covered sheet balsa surface represents aluminum on the full scale airplane, the rib tape extends all the way up to and around the leading edge. We NEVER leave the blunt, open end of a tape exposed where airflow could catch it and tear it loose. More on this soon.

 

I'm brushing a generous wet coat of PolyBrush in a line along the rib where the next tape will go.

I’m brushing a generous wet coat of PolyBrush in a line along the rib where the next tape will go.

 

The next step is to stretch the tape just enough for it to lie flat and press it into the wet PolyBrush a few inches at a time.

The next step is to stretch the tape just enough for it to lie flat and press it into the wet PolyBrush a few inches at a time.

 

Here's another look at that same process from another angle. What needs to happen is that each tape must lie flat and smooth against the surface with no gaps or bumps and be completely saturated with PolyBrush.

Here’s another look at that same process from another angle. What needs to happen is that each tape must lie flat and smooth against the surface with no gaps or bumps and be completely saturated with PolyBrush.

 

Where the initial coat of PolyBrush is beginning to stick well enough that it won't let the tape slip, I give each new rape an additional wet coat that flows out and blends with the surrounding fabric surface.

When the initial coat of PolyBrush is beginning to stick well enough that it won’t let the tape slip, I give each new tape an additional wet coat that flows out and blends with the surrounding fabric surface.

 

Off camera I have finished taping both wing panels along with the other flying surfaces. Here you see the rib tapes in place on the horizontal stabilizer and vertical fin, all dry and ready for the next step...pinked edge cover tapes.

Off camera I have finished taping both wing panels along with the other flying surfaces. Here you see the rib tapes in place on the horizontal stabilizer and vertical fin, all dry and ready for the next step…pinked edge cover tapes.

 

Before I get around to that, though, I have to add more of the same tape...which would be 2" wide on the big plane...along and over every place where fabric contacts a fuselage stringer or other fore-and-aft structural component. On the full scale airplane these reinforcing tapes prevent flexing and cracking under flight loads. Here I'm working along a stringer on the right side of the fuselage, just as I did on the wing but without any stitches to cover.

Before I get around to that, though, I have to add more of the same tape…which would be 2″ wide on the big plane…along and over every place where fabric contacts a fuselage stringer or other fore-and-aft structural component. On the full scale airplane these reinforcing tapes prevent flexing and cracking under flight loads. Here I’m working along a stringer on the right side of the fuselage, just as I did on the wing but without any stitches to cover.

 

Here's another look at the same process. This is one of those places where it's really important to check by sighting along each tape, fore and aft, to be sure it's straight, not up-and-down wavy.

Here’s another look at the same process. This is one of those places where it’s really important to check by sighting along each tape, fore and aft, to be sure it’s straight, not up-and-down wavy.

 

Now that tape is firmly stuck down. Notice that I've got a few runs in the initial PolyBrush coat. For now I'll leave them...the fix for that comes later.

Now that tape is firmly stuck down. Notice that I’ve got a few runs in the initial PolyBrush coat. For now I’ll leave them…the fix for that comes later.

 

As with the wing tapes, I'm going over each tape here with a second coat of PolyBrush.

As with the wing tapes, I’m going over each tape here with a second coat of PolyBrush.

 

Where the longerons define the corners of the fuselage and I have significant fabric seam/overlaps, I'm using 1" edge tape (that would be 4" on the big plane). This wider tape does a better job of reinforcing and concealing that fabric seam.

Where the longerons define the corners of the fuselage and I have significant fabric seam overlaps, I’m using 1″ edge tape (that would be 4″ on the big plane). This wider tape does a better job of reinforcing and concealing the seam.

 

Same deal along the dorsal stringer,where I had to do all that fussing to complete the dorsal fabric fairing. This is an excellent way to smooth out any bumpy spots left in that hard-to-get-right seam.

Same deal along the dorsal stringer, where I had to do all that fussing to complete the dorsal fabric fairing. This is an excellent way to smooth out any bumpy spots left in that hard-to-get-right seam.

 

This is where you get to stretch-and-pull-and-press with the edge tape. This part takes patience...and you probably won't get every last pucker out. Like the PolyBrush runs, the fix for that is coming soon.

This is where you get to stretch-and-pull-and-press with the edge tape. This part takes patience…and you probably won’t get every last pucker out. Like the PolyBrush runs, the fix for that is coming soon.

 

Here's some more 1" tape actually being used on an edge. This is the horizontal stabilizer trailing edge, and I have just laid the tape down smoothly into a wet bead of PolyBrush.

Here’s some more 1″ tape actually being used on an edge. This is the horizontal stabilizer trailing edge, and I have just laid the tape down smoothly into a wet bead of PolyBrush.

 

Now I'm pressing the tape firmly into the PolyBrush. You can see where it's already been bonded over at the right. Don't try to fold over that sharp edge yet...let it dry.

Now I’m pressing the tape firmly into the PolyBrush. You can see where it’s already been bonded over at the right. Don’t try to fold over that sharp edge yet…let it dry.

 

That takes only a few minutes...then I can add more PolyBrush under the open edge and press that part of the tape into place.

That takes only a few minutes…then I can add more PolyBrush under the open edge and press that part of the tape into place.

 

Same deal on the trailing edge of the vertical tail.

Same deal on the trailing edge of the vertical tail.

 

This tape also gets an extra sealing coat of PolyBrush.

This tape also gets an extra sealing coat of PolyBrush.

 

This is the left wing aileron well area. I'm laying down the initial wet bead of PolyBrush for the edge tape. Remember what I said about not leaving "blunt ends" of rib tapes to catch the airflow? This is the fix...we seal all those loose ends under edge/cover tapes that run all the way around the wing and tail surfaces. All that starts here. I'll work my way around to the leading edge with wide (1") tape, which represents 4" tape on the big airplane.

This is the left wing aileron well area. I’m laying down the initial wet bead of PolyBrush for the edge tape. Remember what I said about not leaving “blunt ends” of rib tapes to catch the airflow? This is the fix…we seal all those loose ends under edge/cover tapes that run all the way around the wing and tail surfaces. All that starts here. I’ll work my way around to the leading edge with wide (1″) tape, which represents 4″ tape on the big airplane.

 

Same as on the tail, I'm laying out a strip of cover tape FLAT...

Same as on the tail, I’m laying out a strip of cover tape FLAT…

 

and pressing it down smooth and tight.

and pressing it down smooth and tight.

 

In about the time it took me to do the same step on the other wing, this PolyBrush-and-tape application dried enough for me to fold the loose half of the tape over and stick it down.

In about the time it took me to do the same step on the other wing, this PolyBrush-and-tape application dried enough for me to fold the loose half of the tape over and stick it down.

 

Now the leading edge gets its turn.

Now the leading edge gets its turn.

 

These pre-cut, pinked-edge Stits tapes will STRETCH and curl around some pretty serious convex curved surfaces if you learn to PULL on them. This is that curve of the inboard leading edge that's characteristic of the "Stinson Gull Wing". The trick here is to pull, stretch, and then hold on and wait for a minute or two while the PolyBrush grabs.

These pre-cut, pinked-edge Stits tapes will STRETCH and curl around some pretty serious convex curved surfaces if you learn to PULL on them. This is that curve of the inboard leading edge that’s characteristic of the “Stinson Gull Wing”. The trick here is to pull, stretch, and then hold on and wait for a minute or two while the PolyBrush grabs.

 

Remember those navigation/position lights I put in a few sessions back? I want those leading edge cover tapes to be seamless and overlap-free all around the leading edge and tip, so I have cut out a small hole to permit the tape to slip over the masked-off light housing where it protrudes.

Remember those navigation/position lights I put in a few sessions back? I want those leading edge cover tapes to be seamless and overlap-free all around the leading edge and tip, so I have cut out a small hole to permit the tape to slip over the masked-off light housing where it protrudes.

 

This is where I get to do some SERIOUS pulling and stretching. This is the top surface of the left wingtip. You can see that the tape has permitted itself to be pulled down ALMOST perfectly flat. I'll address those last few imperfections soon. (See how neatly the masked-off light fits through the tape?)

This is where I get to do some SERIOUS pulling and stretching. This is the top surface of the left wingtip. You can see that the tape has permitted itself to be pulled down ALMOST perfectly flat. I’ll address those last few imperfections soon. (See how neatly the masked-off light fits through the tape?)

 

Finishing up the wing trailing edge/right aileron well is easier since I don't have to worry about pulling any curves here. I'm brushing on the wet/fill coat of PolyBrush.

Finishing up the wing trailing edge/right aileron well is easier since I don’t have to worry about pulling any curves here. I’m brushing on the wet/fill coat of PolyBrush.

 

Same thing on the leading edge.

Same thing on the leading edge.

 

I did the same thing on ALL the tapes, everywhere on the airplane. Every inch of cover/edge/reinforcing tape gets that second, extra brushload of PolyBrush to seal the weave.

I did the same thing on ALL the tapes, everywhere on the airplane. Every inch of cover/edge/reinforcing tape gets that second, extra brushload of PolyBrush to seal the weave.

 

OK...now we get to deal with those little "fuzzies" and all persistent edge wrinkles that just won't go away. The best way to find 'em all is the calibrated fingertip method. You are going to find that EVERY tape edge will demand attention.

OK…now we get to deal with those little “fuzzies” and all persistent edge wrinkles that just won’t go away. The best way to find ’em all is the calibrated fingertip method. You are going to find that EVERY tape edge will demand attention.

 

The traditional fix for fabric fuzzies isto fine-sand them out. With Stits products, DON'T DO IT! You'll break the PolyBrush film and make it worse. Use the tip of your iron at about 250 degrees and press and smooth out every last one of those imperfections. This is one step in the Stits covering process that may seem counter intuitive. Take al the time you need and go over every inch of tape on the airplane. Never mind how long you think it's going to take. Lots of experience (mine included) has proven that this is the only way to get the kind of smooth finish you're looking forward to.

The traditional fix for fabric fuzzies is to fine-sand them out. With Stits products, DON’T DO IT! You’ll break the PolyBrush film and make it worse. Use the tip of your iron at about 250 degrees and press and smooth out every last one of those imperfections. This is one step in the Stits covering process that may seem counter intuitive. Take all the time you need and go over every inch of tape on the airplane. Never mind how long you think it’s going to take. Lots of experience (mine included) has proven that this is the only way to get the kind of smooth finish you’re looking forward to.

 

Same deal on the wing. Next time I'll show you how to finish preparing the base/primer surface and we'll get to see some color.

Same deal on the wing. Next time I’ll show you how to finish preparing the base/primer surface and we’ll get to see some color.

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