Restoring A Classic PT-19 (Model) Airplane – Part 4

Restoring A Classic PT-19 (Model) Airplane – Part 4
JULY 17, 2016… A NOTE TO ALL MY READERS: As you may have noticed I have not been posting material as often as in the past. Over the past year my wife and I have been involved 24/7 as the only caregivers for an elderly family member suffering from advancing Alzheimer’s disease with severe complications. As you might expect, building airplanes and writing about them has had to be set aside from time to time. Thank you for your patience. Indeed, I have several new projects (including some new stuff you’ll never see coming) ready to share and as soon as possible I expect to be able to get caught up and get them posted.

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PT-19-90 Back in image PT-19 2, while I was showing you what the model looked like when I got it, most of the old plastic film covering had already been stripped off the fuselage and we got a look at the balsa sheet structure that it had been concealing. Here’s a closer look from a different angle that allows a better assessment of what’s wrong with this model…what I’ll have to do to make it airworthy…AND some of what’s right, as well. What can YOU see? My attention goes to the notion that from what we can see here the basic fuselage structure appears to be complete and still properly squared off/aligned. Based on my study of my PT-19 scale drawings and photos it appears that the general shape of the cockpit opening cutouts is at least very close to being correct, and that the balsa sheet top deck structure of which they are a part is intact. It’s also easy to see that there are several places where the balsa surface appears “punky”…maybe even oily? There’s real damage to the side stringers and we can see that although there’s a wing root fairing base in place, the portion of the fairing/fillet surface we can see is very rough.

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PT-19-91 Let’s get a better look at that. Without any plastic covering to hide it, the wing faring/fillet turns out to be a half finished, poorly sanded batch of some sort of wood filler. What’s the best way to deal with that…should I cut it all away and start over, or can I save what’s there without compromising the quality of the restored model? There are also clearly problems with those side stringers, which measured out as 1/8” x ½” balsa and, as you may have guessed already, were REALLY SOFT.

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PT-19-92 Back at the tail…same side…we can see the extent of the stringer problem. This is where I made the decision to replace ALL of the exposed stringers (the ones not covered and reinforced by adjacent balsa sheet structure). My best guess for the cause of all that breakage is balsa stringer stock that was too light/soft for anything but unrealistically gentle handling. When I’m done getting it fixed the way I want it this airplane is obviously going to weigh at least ten pounds, probably more, and I expect to fly it often. So…I feel it’s worth the slight extra weight to substitute 1/8” x ½” spruce for the original balsa stringers. I don’t want to be afraid to pick it up.

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PT-19-93 It may take a moment to orient yourself to what’s going on in this shot…you are looking at the right fuselage side where the rear end of the pre-existing rudder/elevator servo tray makes a right angle joint with the wing trailing edge station fuselage former. The light colored, angled-off panel behind my finger is the original 1/8” balsa sheet inset along the entire length (chord) of the wing root that serves as a fabric anchor and the vertical surface against which the concave wing root fillet/fairing is formed. The blue-splotched tan area beneath that is the pre-existing fillet that I’m going to have to deal with. Got it? Let’s get to work. Replacing all the stringers means cutting out all the old stringer structure that has not yet fallen off the airplane, and that means decided EXACTLY where to cut. You can see where the midline 1/8” x ½” balsa stringer has already been broken off at the former. I’ll work at replacing all of that later…right now I’m using an ordinary modeler’s razor saw to cut a new 1/8” x ½” slot in the TE former where the old/missing balsa stringer used to butt-joint. I’ll add a new spruce stringer…pretty much along the line indicated by the edge of the razor saw blade…that will extend all the way forward to the rear edge of the boot cowl sheeting at the wing leading edge. Watch how all this comes together…

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PT-19-94 …right here as I use an ordinary razor blade to separate what’s left of that old 1/8” x ½” balsa stringer from the edge of the servo plate. I’m creating a clear channel for the new 1/8” x ½” spruce stringer…
 
 
 

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PT-19-95 …that you see being test-fitted here to slip into place in the pre-existing structure. That butt end of the spruce stringer is going to slide forward…
 
 
 
 
 

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PT-19-96 …like THIS. (Yes, this is the left side of the airplane, but it works just the same).
 
 
 
 
 
 

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PT-19-97 Still on the left side…the existing 1/8” balsa sheet fillet base on the right side was in good condition, but this one was chewed up, so I cut away the damaged area and spliced in some new balsa.
 
 
 

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PT-19-98 Back on the right side, you can see how the both the new center and lower 1/8” x ½” spruce stringers fit into place and blend in with the surrounding structure. It worked out that the lower edge of the “turtleback” (upper/rear fuselage deck) balsa sheet skin was sound enough that I saw no reason to replace any of it. Here you also get a good look at the angled 1/8” balsa sheet wing root fairing base AFTER I cleaned it up. As you can see, the darker colored balsa filler-conglomeration that’s supposed to work as the actual fairing itself still needs a lot of work.

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PT-19-99 SANDING is the solution to a lot of problems. Remember my concerns about the condition of that 1/8” balsa sheet turtledeck structure? It turns out that the balsa was in pretty good condition beneath a layer of dust and traces of film covering adhesive. I washed it down with lacquer thinner and once it was dry, sanded it just enough with 150-grit production paper on that long sanding block to get rid of any stubborn little bumps…and to prepare it for some serious reinforcement that’s coming along soon.

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PT-19-100 I discovered a real problem with the vertical-to-horizontal tail fillet. Check out image PT-19-5 as it appeared on the model when I received it, and see if you can spot the error…but don’t feel bad if you can’t. It’s very subtle unless you already know the PT-19 well or study your scale references very carefully. The curve of the existing fairing, especially around the vertical fin leading edge, is convex…it bulges OUT. At this point in the restoration I have stripped off all the old plastic covering. In fact the vertical tail fairing on a PT-19 is supposed to follow an “inside curve”…it forms a smooth CONCAVE surface that intersects all the surrounding surfaces on a tangent. By the way, when I pulled the film off this section of the fairing, most of the adhesive/color stayed stuck to the surface. What you see here is what was left after I washed it all down using lacquer thinner (acetone would also work OK) as a solvent.

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PT-19-101 It turns out that this is not going to be too difficult an error to fix. Although the shape is wrong, each side of the fairing was made from a single piece of balsa with only a bit of filler (the white stuff) around the edges. My job was to cut away enough material (balsa) to change the bulged-out convex curve to a smoothly sweeping concave one. I decided to use a coarse rotary sanding drum to cut away most of the excess balsa. The trap here would be to cut too deep and need filler to cover the mistake. Don’t do that…

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PT-19-102 …switch over to a contoured sanding tool and sneak up on just the right cross section/profile.
 
 
 
 
 

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PT-19-103 Here’s a close-up peek at what the finished job is going to look like…this is what you’re shooting for.
 
 
 
 
 
 

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PT-19-104 Let’s move on to finishing the rest of the fuselage, starting with the nose. This is the left side of the nose/boot cowl showing the original balsa sheet structure that I stripped of plastic covering and washed with solvent. It looks a little battle-weary, but essentially intact…worth saving. The plywood side of the engine (motor) mount box is pretty clean.
 

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PT-19-105 The firewall/motor mount base plate has a few too many holes drilled in it … did somebody keep changing his mind ?…but it, too is otherwise intact and probably flight-worthy.
 
 
 
 
 

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PT-19-106 I chose to add a layer of 2 oz./sq. ft. glass cloth over the entire balsa sheet cowl structure ahead of the wing leading edge. This location appears on the surface of the full scale airplane as a distinct structural transition, so it’s a good place for our actual transition from simulated sheet metal to the open-stringered, fabric-covered fuselage side behind it. I attached/filled/sealed the glass cloth with a single wet )penetrating) coat of ZAP Coating Epoxy thinned 25% with denatured alcohol. I’ll add some additional reinforcement next.

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PT-19-107 Leaving that epoxy/glass application alone overnight permitted it to cure thoroughly. With that done I trimmed away all the loose edges and overhangs and used a 150-grit sanding block to get the entire glassed surface bump-free and smooth enough for the next step…using the Stits PolyFiber covering system to do a “true-to-scale fabric covering job” on the entire fuselage. At this point you can see the single piece of Stits PolyFiber fabric I have cut to cover the fuselage sides and top deck with a single piece of material which will extend forward over the boot cowl surfaces I just fiberglassed and provide a seamless finish base. I’ll be using Stits PolyTak fabric adhesive (in that can) by pouring out a bit at a time into my tin can working pot.

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PT-19-108 As has been the case before, I did a lot of work (in this case, fabric covering) “off camera”. Check out my blog series on building the Top Flite 100” Stinson SR-9 for a thorough discussion of using the Stits Process system of covering and finishing. At this point “all that stuff” has been finished to the point that the fabric covering is attached, shrunk tight, and sealed ready for sanding base coats of Stits Poly Spray. Before I can do that I need to finish fixing those messed-up wing root fillets, and this is what’s going on here. I have used three layers of “fine line” automotive masking tape to define what will become the upper edge of the simulated formed sheet metal fairings. Three layers will build up enough depth that the fillet material I’m using will provide a convincing “sheet metal edge”.

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PT-19-109 That extra tape (the blue stuff) is just there to protect the fabric finish above/past where the simulated aluminum is going to be. We need it because that fine line stuff is only 1/8” wide.
 
 
 
 
 

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PT-19-110 More Stits products…the “blue stuff” is Stits Lite Fill 2-part light weight epoxy putty. I built up the rough shape of the fairing (just thick/deep enough oversize to permit sanding it to the correct finished shape without having to add any more) using my old artist’s palette knife and then smoothed the not-yet-cured surface to a nearly finished contour with my fingers generously wet with ordinary water. In a warm room Stits Lite Fill will have begun to set sufficiently in an hour or less that you can leave it without having to worry about you newly shaped fairing running or sagging.

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PT-19-111 Once again, curing overnight is the secret formula for making sure the job is ready for sanding. This is one part of scale aeromodeling that demands plenty of patience, attention to detail, and practice. You have to depend on your scale references…drawings and photos…to get an accurate idea of EXACTLY how the finished fairing is supposed to be shaped and then put your hands and your sanding tools to work to make it that way.
 

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PT-19-112 Here’s another look at the same shaping job from a different angle.
 
 
 
 
 
 

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PT-19-113 And again … you are looking at the left side of the fuselage from ahead and below. At this point I have finished defining the curvature/contour of the fairing with coarse (60-grit) production paper on a variety of curved blocks and I’m using a hand held piece of 150-grit to start smoothing away the sanding scratches left by the coarser abrasive.
 

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PT-19-114 Off camera I worked the shaping-and-smoothing job to the point of having a nice 320-grit finish overall…this is as fine as I’ll want to get it prior to starting to work with the Stits Poly Spray aluminum based sanding primer that will become the real base for my paint finish. This is the point where I can peel away those layers of masking tape that defined the shape of the fairing. I’ll leave the restored fuselage at this stage of finish for now and go on to bring the rest of the model up to this same level.

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PT-19-115 What’s left to fix is the cowl. This is how it looked when I got it. As far as I can determine this started out as a custom formed ABS plastic PT-19 cowl purchased “somewhere”, assembled/glued up from several separate pieces, and then cut out to clear a glow engine of some sort. As with most of the rest of this PT-19, I found it dirty and showing signs of rough handling and as before I cleaned it aggressively with lacquer thinner on the chance that there might be fuel/oil residue hiding in the rest of the grime.

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PT-19-116 Here’s the result of another big judgment call. I have the mostly-intact but significantly-messed-up molded plastic cowl for a large scale model that will never look right unless it’s fixed right. My choices for doing that include finding/buying a whole new cowl, creating a plug/mold and laying up an entirely new epoxy-glass cowl, or fixing this one well enough to look right and stand up to the stresses of regular flying. Since I Have no idea where the original came from, getting a replacement won’t work. I could make a new mold with all that entails, but this PT-19 does not have to be TOP GUN accurate and I’m building only this one airplane…so…I will fix the broken parts. There is certainly more than one way to do this…I’ll share with you what made sense to me. It happened that I had lots of 1/64” aircraft plywood on hand, including several offcut scraps that were big enough to cover the open spaces neatly. As you can see here, I trimmed them to fit each “hole” as neatly as I could while leaving a flange of at least ¼” of extra plywood to rest inside the original cowl where I could bed it into a generous bead of epoxy. Did you notice that each of these “patches” is convex (bulged out). I cut each piece to fit so that convexity would work with me in restoring the original curved cowl surface. To hold it all in exactly the position/shape I wanted I used medium-cure (15-minute) epoxy to guarantee enough working time, set each patch into position, and then stuffed the inside of the cowl with wadded-up paper towels until I had created enough outward pressure to hold the shape you see while everything cured properly.

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PT-19-117 I did the same thing on the other side…it looked like this.
 
 
 
 
 
 
 

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PT-19-118 I would not have used this patch-and-fill technique without a molding/surfacing product I could trust to hold its shape without shrinking or cracking, to stand up to what I hope will be years of everyday flying field bumps and stresses, AND to be light enough that the weight gain from using it will not become a liability. Stits Lite Fill two-part epoxy filler, which is certified for full scale aircraft use, is perfect for this job. This shot shows you three successive steps of the filling-and-finishing process. The dirty/dull blue areas where you previously saw plywood patches are the first application that I’ve finish sanded to blend with the surrounding cowl surface. The pink feathered edges are old (original build) primer showing through. The brick red spots are ordinary quick-drying cellulose based autobody primer “spotted” on to fill a few stubborn low spots and pinholes and then fine-sanded flush. The brighter blue band of filler around the rear edge of the nose bowl is freshly applied, just-cured Lite Fill. If you look closely you can see where I have used it to fill up to the edge defined by that blue masking tape. Just as I did on those wing root fairings, I’ll sand the cured Lite Fill back to cut of cleanly at the built-up tape edge and feather smoothly into the pre-existing cowl surface ahead of it.

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PT-19-119 Doing that sanding job looks like this.
 
 
 
 
 
 
 
 

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PT-19-120 Remember the masking tape? Peeling it away when the sanding is completed exactly the way you want it leaves this simulated sheet metal overlap edge. Doing that job looks like this.
 
 
 
 

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PT-19-121 I did some more of that off-camera stuff to get the cowl to this stage…here you are looking at the other side and you can see that the big opening that got patched with 1/64” plywood in image PT-19-117 has been filled in with more of the Stits Lite Fill that I used to make the wing root fairings. I’ll say more about using Lite Fill in a bit, but first I need to show you the OTHER process I’ve come to rely on for challenges like this one. What’s the best way to tie together, reinforce and finish this fixed-up cowl? The answer is lightweight fiberglass cloth and epoxy resin. Here I have cut a piece of 2 oz./sq. ft. glass cloth, which is my standard for a job like this where I’m “fixing” a pre-existing structure that just needs a little help. As you’ll see, the piece of cloth is going to be just big enough to cover the part of the cowl I’m working on…I won’t try to cover the whole thing with one piece. I’ll keep it there with more of that ZAP Finishing Resin thinned with 25% denatured alcohol as before so it will penetrate the cloth quickly, without my having to press and pull and otherwise fuss with it.

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PT-19-122 What you see now is the nose bowl (front of the cowl) with the glass cloth held in place by the cured resin (and saturated with it so the weave is thoroughly filled). Once it has cured long enough to be dry to the touch and stiff enough to handle, I’ll trim away that overhang of loose cloth even with what will become the finished rear edge of the nose bowl.
 

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PT-19-123 While I’m waiting I’ll do the top/removable section of the main cowl.
 
 
 
 
 
 

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PT-19-124 First I just drape the glass cloth loosely across the area I want it to cover to confirm that I’ve cut it to the right size, then brushing the “25% thinned” finishing epoxy over-and-into the glass cloth makes it look like this. I stopped partway through the application to show you the difference between the loose/dry cloth and the way it looks when it’s properly resin-saturated and bonded. (Yes, I’ll finish the job by attaching the cloth that’s still loose here all the way down to and over the structural edges. I’ll leave enough overhang to ensure good coverage/adhesion around those edges and trim it when it’s hardened.

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PT-19-125 All finished…sanded, primed, and painted…that cowl is going to look like this. I mentioned this before, but I’ll repeat it here. In this discussion of restoring my PT-19 I have pretty well skipped over talking about the process of finishing the model with Stits products. Go back to my online blog series at www.rmodel.com on “Building the StinsonSR-9” (or refer to your back issues of FLY RC for past “Master’s Workshop” columns) for all those details.

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PT-19-126 That top cowl comes off for access to the battery pack, which happens to be a single 6S 5000 mAh Venom LiPo that slips under and past the top deck at the front windshield.
 
 
 
 
 

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PT-19-127 Have you been waiting for THIS, or WHAT? Once again, I’ve been busy off-camera doing all the little tasks of final assembly that seem to take just about forever for all the details to come out right. With all the pieces we’ve been working on put together into the shape of an actual airplane, I did an exhaustive pre-test-flight checkout…first in the shop before I even got close to being ready to take the model out to the field, and then at the field itself, on the ground, before I was seriously considering pointing her down the strip and opening the throttle. When that moment finally came, I was ready for THIS. I can’t guarantee that master model photographer Gary Ritchie got this shot on the first flight, but it might as well have been…after all that work she flew off beautifully “by the book”.

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PT-19-128 On this low fly-past, it would not be difficult to convince yourself that you’re watching a full scale PT-19.
 
 
 
 

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PT-19-129 Would you like to see this part for real? We are working on a video of this model FLYING at our Puget Sound Silent Fliers field near Olympia, WA. Let FLY RC know if you would be interested in seeing it.
 
 
 

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PT-19-130 This turned out to be a great airplane. She looked like this…neat, clean and pretty…AFTER I taxied back and parked her following that first flight.