When do we get to the good part? For me that’s a hard call, because they’re all the good parts. Covering, however, is kind of special because that’s the step that really transforms a bunch of subassemblies and frameworks into what is clearly going to be a flying machine. If you’ve been reading my stuff for long, you already know that I’m not going to use a film covering for this airplane. That stuff works as advertised, and your TigerKitten will fly fine with it, but I prefer the esthetic finesse and improved structural integrity that come with using fabric-and-paint covering and finishing products. On this model I’m going to use Polyspan, finished with clear nitrate dope and a variety of Stits Lite paint products. We’ll get into that part later. I’m also going to pass on explaining a lot about what Polyspan is here…I did that during the presentation of my Great Lakes Trainer project and you can check back there whenever you like for details. http://www.rcmodel.com/2011/07/building-the-oldflyline- great-lakes-2t-1a-kit-20/ This link will take you to Great Lakes blog 20, which introduces Polyspan. Blogs 21 through 23 offer more information on covering and doping that may be of interest as a supplement to what I’m offering you here in the TigerKitten story.
It’s time to start covering. Prior to getting to this step I’ve given the entire structure a heavy, wet coat of non-tautening clear nitrate dope. When that was thoroughly dry I sanded every surface that will come into contact with the covering with 320-grit production paper to get rid of all the little bumps and fuzzies that could cause snags. The other reason for this pre-doping is to seal the balsa so that the covering adhesive won’t soak in, but rather stay on the surface where it’s going to be needed as the heat activated stuff that grabs and holds the covering. Here I’m using a soft, round brush to put a generous bead around the entire outline of the wing…everywhere that the Polyspan must be adhered. I’m using fabric formula Balsarite, but there are other brush-on, heat activated fabric adhesive products that will work just as well.
I’m starting with a single piece of Polyspan cut to cover the entire bottom surface of the right wing. Since there is a severe concave compound curve formed by the lower leading edge fairing, I cut out a relief section to permit the main sheet of covering to lie flat around it. The Polyspan is cut exactly to the dimension I want it at the centerline of the wing but left with plenty of overhang (for now) everywhere else.
Stick it! I’m using an old reliable covering iron from Hangar 9, set to about 250 degrees, to activate the adhesive without causing any significant shrinkage of the Polyspan. I’ll work back and forth along this line, varying the pressure of the iron as necessary. The goal is to get the covering stuck firmly in place without any wrinkles, folds, or loose spots.
With the Polyspan stuck down along the wing centerline we have something to pull against to begin stretching the sheet smooth across the entire panel. Here I’m using the iron to adhere the covering at the inboard end of the leading edge securely down and around the front of the structure.
Next I pull the sheet of covering taut spanwise and adhere it at the outer edge of the tip assembly.
With the covering held in place at the root and at the tip, I can work along the leading and trailing edges in between and pull out on the covering just hard enough to make it lie smooth.
This is the center section with the edges of the covering adhered all around the perimeter. You can just see where the activated adhesive has turned the covering nearly clear in appearance where it’s wrapped around the leading edge.
I left the lower leading edge fairing uncovered until now. I cut a separate piece of Polyspan that can be worked over and down around the edges of the fairing.
Activating the adhesive on the small piece of Polyspan over this deeply curved fairing demands care. In a place like this you have to make the necessary extra effort to press and pull and push with the tip of the iron to avoid leaving any small patches of covering unadhered to bridge across tight curves and corners. Any such bridges will come back to haunt you as persistent flaws in your covering job if you ignore them now.
Now we’re working on the top surface of the wing. I’ve repeated all the steps I did on the bottom and now I have to deal with something new…the aileron horns. The trick to this part of the job is to make a small cutout that allows the covering to lie flat around whatever protruding part you’re dealing with and iron the material down tight around it.
This is the top surface of the right wing tip. I’m pulling and stretching the covering out, down and around the outer radius of the wingtip. Be sure to go over-center in places like this to guarantee that you’ll get a double overlap of the covering.
Here the covering has been pulled tight around most of the tip and lies flat across the trailing edge…actually the aileron well on this airplane.
I’ve made a diagonal cut in the covering and ironed the small triangular tab down and around the inner surface of the tip structure.
Now that last little end of loose covering around the rear end of the tip is stuck in place…and…I’ve wrapped the main expanse of Polyspan around the front of the aileron well. There’s a good overlap created here since the bottom surface covering already extended around that same edge.
I treated all the free edges of the covering around the trailing edge/aileron well the same way. With the entire wing panel…top and bottom… covered, I can begin shrinking the Polyspan tight. NEVER shrink the covering before both the top and bottom are covered. That’s a good way to introduce uneven stresses and create warps. Just to the right of the iron you can see a few shallow sags that have yet to be shrunk tight.
This is what freshly applied Polyspan should look like after being ironed at 275 – 300 degrees. There should be no bubbles around the edges and no puckers or wrinkles anywhere.
I covered the control surfaces and the fixed horizontal stabilizer just the same as I did the wing. Now it’s time for my favorite part…covering the fuselage. On this airplane we’re going to incorporate a classic feature, the fabric fairing between the vertical tail and the rear portion of the fuselage. This is a characteristic of many of the old time fabric covered airplanes you might want to model. You can’t fake it…but it’s not that hard to do right if you know how. Here I’m starting by covering the bottom of the fuselage from the wing cut-out to the tailpost. I started by adhering the covering along the front of the area…just as with the wing, I want something to pull the rest of the job tight against.
Next I pull the covering taut along the length of the fuselage and iron it down over the tail wheel mounting plate.
The next step is to pull the covering smooth across the fuselage and iron it down around each of the lower longerons.
With the aft bottom covered, I use a fresh (sharp) razor blade to trim off the overhang along the upper (inside) edge of each longeron.
Here comes the fun part! I have cut a sheet of Polyspan that reaches from the tailpost (the vertical fin trailing edge) all the way past F-1, and far enough up and down to provide a generous margin around the bottom longeron and the top centerline of the fuselage…including the entire vertical fin. I have cut a slot as precisely as possible to permit that sheet of covering to slip around the base of the fixed horizontal stabilizer (which I have already covered.) Watch what happens next.
I pulled the sheet back so that the front of the cutout fits snugly around the leading edge of the stabilizer and then adhered the covering all around the base…top and bottom.
Just to be sure I have everything under control, the next step is to adhere the covering along the front of F-1 (at the nose). I don’t want to get through the next few steps and then discover that the covering for the front part of the fuselage is out of line.
Now it gets interesting. I have adhered the Polyspan all along the tailpost/fin trailing edge. You can see where I have pulled it taut vertically…from the stabilizer toward the fin tip.
The next step is to adhere the Polyspan all along the lower edge of the fuselage including the rear longeron, the outline of the wing cutout, and the bottom edge of the nose. Again, the covering must be taut and smooth at this point, with no folds or deep wrinkles. We’ll get it TIGHT later.
Now we need to get the covering to fit into the complex curvature created by the fin leading edge and top rear of the fuselage. I start by slitting the covering down as close as possible to where it’s going to lie against the top center stringer and the fin leading edge AFTER I’ve pulled it tight. This requires good judgment..take your time. If you cut past the top stringer you’ll have a hole to deal with.
Now let’s go to the cockpit area, slit the covering there, and work it around the rear fuselage top and back along the dorsal (center) stringer.
Here comes the big secret to advanced fabric covering technique…pull, stretch and pull some more. The next thing I’m going to do is iron down the covering along the several inches right next to my thumb.
Now I do the same thing all the way around the fin leading edge. If you like, you can do some testing on scrap to determine how much you can pull on the covering material before it will tear. Usually that’s more than you expect.
All in place. Every edge is stuck down with plenty of structural overlap. The covering won’t be smooth at this point, but it must be free of puckers and creases.
This happens to be the opposite side of the fuselage. I covered it just the same as the first.
Here it is again from the other side, with the first coat of clear nitrate dope applied. SUMMARY
The fun doesn’t end here. Go to www.rcmodel.com to see what I did next. This installment of A Master’s Workshop is based on TigerKitten blog No. 11 …here’s the direct link: http://www.rcmodel.com/2012/05/the-return-of-the-tigerkitten-11/
As for next time, I’m going to speed up the action on the TigerKitten story. Last time I talked about building a BIG (100” span) Stinson Reliant from the Top Flite Gold Edition kit as an electric power conversion, and explained that I had stopped working on it to finish other projects. We were going to get back to that next time around. With all the good stuff going on in my shop, my builds got ahead of my blogs. In fact, the TigerKitten you see here is already finished and flying. Next month I’ll give you a look at the finished airplane…you can catch up on all the rest of my building blogs on-line…and after that we really will clear the bench for the big Stinson.