Building the Stinson SR-9 (22)

Before we get started on this month’s installment of our Building the Stinson story, I want to invite any and all of you who are reading my rcmodel.com blogs to participate in deciding where they will be going next. I have been working on the big Top Flite/Great Planes Stinson SR-9 for over two years, and as you can guess by the material I’m writing about now I’m getting very close to having the airplane finished and flown….and yes, you will get plenty of completed-model photos and a flight report. (I’m even working on a plan to have some flight video available, but that’s a story for another time.) What I need your help with today is deciding what the next Old Time Model Airplane Workshop project is going to be.

What would YOU like to see me spending the next year or so working on right here? We are talking about devoting a lot of time and effort to sharing my experience at building model airplanes, so whatever I end up with has to be based on a kit. Whether it is a traditional full kit or a short kit to which I’ll add standard strip and sheet stock is a good question…there are good reasons for going with either option.  As you have probably already figured out if you have been reading my stuff is that whatever I decide on will be SCALE and it’s going to be electric powered, but within those limits there are a whole lot of airplanes out there waiting to be chosen. Please share your thoughts with me via email.

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This time we get to work on one of the parts of the covering  and finishing of a classic fabric covered light airplane that I have to admit to really looking forward to. The SR-9, like many other Stinsons, as well as WACO’s and Aeronca’s and Pipers and quite a few others, uses a fabric fairing, or faired fabric fillet, to turn what would otherwise be a very linear (and aerodynamically less-than-efficient) right-angle joint between the vertical fin and the horizontal stabilizer along with the surrounding fuselage into a long smooth, flowing complex curved surface. When airplanes like this one were being designed and built…in an age without computer generated complex surfaces… the only practical way to achieve that characteristic contour was to let a piece of aircraft fabric under tension span the opening between the defining edges of the fin leading and trailing edges and, usually, the upper rear fuselage longeron. It’s pretty well agreed that at the speeds we’re talking about (100 – 150 mph) the feature reduced drag measurably…and…it adds class to the airplane’s appearance beyond measure. What’s the down side? Doing a fabric fairing right demands some extra effort and attention to detail. Is all that necessary on an RC scale model airplane? I’ve seen a whole lot of models finished by builders who didn’t think so. If you understand what the airplane in question is supposed to look like, in my opinion the simplified, easy treatment looks like —–. The Top Flite designers appear to agree with me. They include the fabric fairing feature in the covering instructions that come with the kit. However, perhaps to save a bit of covering material they modified the full scale technique with an extra fabric joint/seam just ahead of the dorsal fairing. This works, but it will show, and you’ll know. Follow along and I’ll show you how to get it right. If you’re reading this I know you want to learn how, and if you take your time and work carefully, you can do it.

Let’s cover a Stinson fuselage.

 

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 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 have used any adhesive. You can see where I have cut short slits in the fabric to permit it to fit neatly around the main landing gear legs.

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 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 al I have to do is to make a cutout that permits the working  piece of fabric to fit snugly around it and lie flat.

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’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.

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.

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.

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.

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.

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.

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.)

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.

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 he PolyTak I previously brushed into the structural "corner" above the cabin window and door openings that you can see through the fabric.

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.

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.

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.

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.

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.

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 rail 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.

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 besure it's going to fit along the top longeron seam and reach out to all the other edges I want it to stick to.

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.

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.

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

Stinson 22-25

Now it’s time for the serious stuff. I have cut a slit into the fabric “down” into the triangular area formed by the fin leading edge and the top center stringer. This is necessary to permit pulling the covering tightly up to the edge formed by the leading edge-dorsal fairing-stringer. Don’t cut too far…anything PAST the dorsal ruins the job and you’ll have to start over. Make a shallow cut, pull and stretch and snip, and sneak up on a good fit.

 

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 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 slot 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.

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.

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.

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.

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.

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.

What’s the down-side?  There’s no way to fake it, and doing a fabric fairing right demands some extra effort and attention to detail.

2 Comments

  1. I built this kit several years ago and I’ve been avidly following your blog ( rcgroups, Hey_You )remembering all the mistakes I made. The nose in the kit is designed for a gas engine and I would think some weight might be saved there due to the electric motor’s smoothness – whatcha think (?)
    Bob

    • Bob,

      You are correct. The load bearing structure in this airplane’s nose that’s intended to deal with engine vibration could have been lighter if it had been intended for E-power from the outset.

      That’s an excellent observation and I had to give it some thought…it’d be too easy to oversimplify my answer. Once upon a time the “correct” first response would have been to say, “Of course, just drill a bunch of lightening holes and perhaps leave out some sheet covering…” and so on. Not so now. State of the art E-power equals or exceeds the power to weight ratio of traditional glow/gas engine set-ups. It simply is no longer necessary to “lighten everything” just to get the model to fly as designed. With that said, I agree that the smoothness of an electric setup compared to an engine DOES offer the option of using less structure to deal with vibration, and that’s what I’d do if I were working with a new design. In this case, thought, the TF/GP Stinson already exists as a well-thought-out kit, and I did not want to get into what would have been a heavy duty redesign of the structure just to show how it might be done. Having worked on several large scale models built from kits and converted from gas to electric (check out the WACO SRE scale-out in FLY RC a couple of years ago) I have discovered that I ALWAYS needed nose weight to achieve the design longitudinal balance point (CG) when using LiPo battery packs. Most of these kit-based scale jobs need added nose weight with an engine installation even if the manufacturer won’t admit it, and if the E-power setup is no heavier than “gas” would have been, well…

      Moreover, I wanted to make the point with the Stinson article series that with contemporary E-power structural redesign and/or compulsory weight reduction just isn’t necessary any more. (That doesn’t mean I don’t always look for ways to reduce weight when creating anything new.)

      Bob

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