Building the (Old) FLYLINE Great Lakes 2T-1A Kit (20)

It’s time to put some covering on this airplane. I am using materials/products on this project with which you may not be familiar, so I’ll describe each of them in turn and share my sources with you. The covering material itself is the big one. You already know that I won’t be using one step plastic coverings here. You may have seen the various articles I have written on using the Stits Lite covering and finishing system, as well as the material I have published about using silkspan. This time I have chosen something different.

Polyspan is not a new product. I first used it over ten years ago, and was impressed, but there has been some uncertainty regarding where to buy it. That issue has been resolved, and I’m going to recommend that you get some and try it yourself. From a model airplane builder’s point of view, Polyspan is best described as synthetic silkspan. It’s a polyester material that looks almost exactly like the heavy laid fiber teabag paper, better known to us as silkspan. It is of comparable weight but far stronger in terms of both tear and puncture resistance. (No, I don’t have numbers) Like silkspan, it accepts nitrate dope and any other compatible finishing materials with excellent results, but unlike silkspan it will NOT stretch or shrink with water. You shrink it with heat, just as if it were a prefinished plastic film of fabric, but after that it is a totally different material.

Most of the information that has been published to date regarding polyspan suggests that you adhere it to model airplane structure using clear nitrate dope, just as if it were silkspan or tissue, then heat-shrink the inevitable wrinkles out of the corners and curves. There’s a better way. I use a heat-sensitive adhesive like Balsarite Coverite Fabric Formula (I’ve also used Sig’s Stix-It with similar results.) IRON the polyspan into place, pulling and stretching as you go, and the puckers never happen. I’ll show you how this works, step-by-step.

My source of polyspan is Fly2Build .

I don’t think you are likely to come across their ads in the print media, but their website is well worth a visit.

http://www.thefinishcrew.com/FLY2BUILD/coveringmaterials&supplies.html

My source of nitrate dope, among other things, is Aerodyne. They are essentially a free flight specialty mail order outfit. Check out their website at aerodyneal@aol.com. Aerodyne is the most reliable source of nitrate dope for model building that I found have to date.

Let’s cut some material !

Ealier in the project I explained how I primed and sealed the entire balsa structure of this airplane with several coats of clear nitrate dope, and then sanded any and all surfaces that would later come in contact with the covering to get rid of the sllight roughness that comes up when the dope dries. That pre-doping also serves as a sealed base for the covering adhesive, which in this case is Coverite BalsaRite (Fabric Formula). I brushed a generous coat along every edge that will serve as an anchor for attaching the covering, but NOT to stringers, the top edges of ribs, and any other places where the covering should NOT adhere until after it is shrunk in place. The subtle shine of this wingtip indicates that it is properly primed, sanded, and coated with adhesive, ready to cover.

I'm going to start the actual covering process with the center section underside of the lower wing. The covering is going to adhere along the two fairing/edge strips and to the shallow formers at the leading and trailing edges of the structure that will serve as a fuselage belly fairing

I cut polyspan with the sharpest scissors I can get my hands on, just as with any other covering material I use, and leave enough of an overhang on all sides to permit grabbing and pulling the covering wherever it might be necessary.

We have to start somewhere...I like to use an end of the section being covered that offers a straight egde, to help keep everything i alignment as I go along. Here I have begun attaching the polyspan to the structure that forms the leading edge of thte fairing. What's important here is to stretch the covering smooth (not necessarily tight) and avoid folds and puckers. I'm bonding the polyspan only to the top/outside surface and not folding it around.

With one end of the covering panel attached, I can now pull against that bond and begin to stretch the covering smooth.

This is where the heat activated adhesive (BalsaRite in this case) does its thing. I have the covering iron set betwwen 225 and 250 degrees (F) for this job, so that the adhesive activates almost instantly when I press the iron down. A higher temp would cause the polyspan adjacent to the bonding area to begin shrinking, and I don't want that to happen yet.

The next step is to seal down the open sides of the panel. This part of the job involves careful pulling and pressing and fitting to get the covering exactly where you want it, then bonding it. At this point the covering needs only to be smooth. The trick is not to accept ANY overlaps…these will not shrink out later.

If I had attached the covering only to that narrow center fairing side strip, it’s likely that the covering might have slipped or broken loose later during the covering process. In order to get the widest possible attachment area, I let the polyspan extend out onto the main center section sheet and ironed down an additional half inch or so. The main lower surface covering will overlap this extra material and fit tight against the edge of the center section fairing.

Where the polyspan extends past the end of the fairing I want to be able to fold it down and stick it tight against the vertical portion of the structure. In order to do this neatly I have to trim the covering so it will fold without crinkling and bunching.

This is the trimmed foldover edge at the front of the fairing. You can see that I am pulling just hard enough to get the covering tight.

In the last image I was using my fingers to pull the extra covering tight; here I am ironing it down into position. In practice I woould have combined these two steps, but as I am doing the photo work on this blog by myself, I don't have enough hands to hold and pull and iron and shoot the image all at once.

This is the bottom surface of the right lower wing panel. I have cut a pipece of polyspan to the correct size and laid it loosely into position ready for heat bonding with the iron.

I used the iron to bond the sheet of covering all along the inboard edge of the panel, tight up against the center section fairing.

With the covering attached all the way along the wing root (the other end of the panel) I have something to pull against to begin working the covering tight. I begin with the outside center of the wingtip radius and PULL enough that you can see the covering beginning to get tight at that spot. I begin at the center and work out toward the leading and trailing edges to make it ewasier to avoid trapping puckers or wrinkles as I seal around the edges of the panel.

I am sticking the covering at the tip, using a fair amount of pressure on the iron and rotating it around the edge and down to help pull the polyspan tight.

Here I am working aroound the wingtip, toward the trailing edge, pulling and sealing with the iron as I go.

In the last sep I attached the covering all the way around the tip. Now I'm bonding it at the rear corner of the aileron cutout and along the inner portion of the trailing edge.

This is the inner trailing edge section you saw in the last image. I am sealing the edge of the polyspan down tight by rolling around the edge of the structure and pressing down with the iron.

I am bonding the covering at the inner front corner of the aileron cutout.

the next step is to attach the polyspan all along the front edge of the aileron cutout, which is in effect the trailing edge of the wing here.

With toe covering attached all th e wy qround the ailerin well cutout, I used a razor blade to slit it at the corners so I can fold it around the edges of the structure as a series of short, flat sections.

This is the inboard end of the aileron cutout with the foled over tab of covering stuck firmly in place.

The leading edge is the last part of the panel outline to be stuck down. It's important that the piece of polyspan be stuck smooth and tight all around the perimeter without any overlaps, puckers or creases. The covering across the open portion of the structure needs to be smooth, but not really tight,at this point.

I am working around the wingtip to be sure the polyspan is stuck and sealed all the way past the outer radius, i.e., over-center.

There are several ways to trim off the extra fabric, or overhang. Slicing under the loose edge with a REALLY SHARP blade is perhaps the safest. Trimmimg with the point of the blade against the structure would demand extra care to avoid cutting into the balsa.

There are plenty of places on any model where bits of hardware protrude from an otherwise smoth, flat structure. This is the end of the aileron pushrod guide tube. As per full scale aircraft practice, my technique here is to attach the entire panel of covering and then cut short slits as necessary to permit the polyspan to lie flat against the surrounding balsa sheet.

After that I stick down the new loose edges all around the protruding part so I can shrink the covering tight without having it pull away at this opening.

The entire lower surface of the wing panel has been covered and al the edge stuck down tight. The next step is to cover the upper/opposite surface, and the fisrt part of that job is to brush a generous coat of adhesive over all the folded over edges of the bottom covering so the top will have something to stick to.

This is the upper surface of the lower wing ready for covering. You can see where covering edges have been folded over from the bottom.

Here's the same panel with a sheet of polyspan cut, laid in place, and stuck down along the inboard end, or root, of the wing panel.

We are doing the pper surface of teh wing just the same way as the bottom. Here I am pulling the covering out and down and TIGHT around the radius of the wingtip.

I have ttached the upper surface covering al the way around the wing just as I did with the bottom surface. Here I am trimming the overhang at the tip. As you can imagine, it's super important at this point NOT to cut the covering on the opposite surface as you trim.

The covering is attached al the way around the perimeter of the top surface of the wing. there are no puckers or overlaps and the covering is smooth, but it is not yet tight.

This is the covering on the center portion of the panel, shrunk fully tight with the covering iron at about 300 degrees.

Here's the wingtip with the polyspan heat shrunk smooth and tight. Compare this image to the first one in this blog entry.

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