Restoring A Classic PT-19 (Model) Airplane – Part 2

Restoring A Classic PT-19 (Model) Airplane – Part 2

Last time I shared the story of how I got this PT-19 model and chose to do this restoration project, and we made a good start looking at things that were wrong with it (and needed to be fixed). This time we’ll get in to some serious fixing.

 

PT-19-24 I used a razor saw to ensure accurate edges on that cutout. I need to inset a block big enough to include the handgrip opening (and some “backing”) I’m going to make because the interior of the wingtip is not solid far enough back to contain the hole.

 

PT-19-25 I measured and cut out a block of balsa to match the dimensions of the cutout, with some overhang left “to sand on”. On a job like this it’s a good idea to sort through your available wood and pick a piece that’s a close match for hardness with the wingtip you’re going to attach it to. This will make it a lot easier to avoid lumpy transitions when you do the finish shaping. As the eventual cutout will have radiused (round) ends, I’m beginning the cutout using a 3/8” drill.

 

PT-19-26 The next step is to use my Dremel scrollsaw to make the two linear cuts that join those 3/8”holes.

 

PT-19-27 With that done I can glue the roughed-out handhold block into the wingtip.

 

PT-19-28 Now I get to shape the insert. I’m cutting the outside contour first using coarse (80-grit) paper and being very careful to retain the curvature of the arc that defines the full scale wingtip outline.

 

PT-19-29 Now all I have to do is match the surface of the inset block to the rest of the wingtip. A No. 11 blade is a good way to make a serious start at that.

 

PT-19-30 Now I’m back to using the 100-grit block to true-up the edges of the new assembly.

 

PT-19-31 Some careful pressure with another piece of 100-grit held loose in my hand smooths the outer radius of the tip.

 

PT-19-32 I saved the fussy part for last. The inside of the handhold is radiused all around, top and bottom. I started that finishing job using a new No. 11 blade and making lots of SHORT, shallow cuts.

 

PT-19-33 And the call goes out once more for “Sanding Block Man”! This time an ordinary hobby knife handle turns out to be exactly the right diameter to fit 1/5 scale PT-19 wingtip cutouts with some 320-grit production paper wrapped around it. Actually there are no secret tricks to doing a shaping job like this…it’s just a matter of knowing exactly what you want the result to look like before you start and then disciplining yourself to work carefully and deliberately until it gets that way.

 

PT-19-34 When I removed the various control surfaces while stripping the old plastic covering, I elected to cut out all the old pinned, tab-type hinges. To get them out cleanly I had to take some extra balsa along with each one, and that left open slots like this one in the right wing aileron cutout, that need to be dealt with before I can do anything about installing new hinges.

 

PT-19-35 The little narrow cutouts I’ve made aren’t big enough to seriously weaken the various control surface edges I cut them into, but they must be filled in so the new hinges won’t be flopping around in oversize holes when I want align them precisely. All I need to do is add tight fitting inserts, or plugs, of balsa of comparable hardness to the surrounding structure and make sure they stay inserted. A generous shot of ZAP does that job here.

 

PT-19-36 Now I can use an 80-grit sanding block to cut each hinge slot insert smooth and flush with the surrounding surface. With this done, I can later mark and cut new hinge slots as if the old ones had never been there.

 

PT-19-37 With all the structural repair and modification done to the stage that the outer surface of the wing is complete, the next step is to create a finish. In this case I’m going to use lightweight (3/4 oz./sq. yd.) fiberglass cloth bonded and sealed with coating epoxy. The shape of the PT-19 wing allows me to cover the entire top (or bottom) surface of each wing panel with a single piece of glass cloth. Here is my working piece for the top of the left wing. Notice that I have left a generous overhang to hold on to and to wrap around all those edges. I’ll cut and overlap the “extra” cloth in the aileron cutout to reinforce that part of the structure.

 

PT-19-38 Here’s another look at “dry fitting” the glass cloth before I begin to use the epoxy. This is the underside of the left wing. I’ll stretch and smooth that folded-over section of the material back across the exposed portion of the balsa surface, and then…

 

PT-19-39 …it’s epoxy time! I’m using ZAP surfacing/finishing epoxy mixed exactly per instructions and then thinned by adding 25% of the epoxy’s volume of denatured alcohol. The viscosity of the thinned epoxy is just right to wet the cloth thoroughly and penetrate into the underlying balsa, and is just thin enough that it is reluctant to “puddle” on the surface and cause excessive material build-up. You can see that I’m brushing “out” from a definite starting point (in this case the center of the wing), smoothing the epoxy mixture and working out ALL the bubbles and wrinkles as I go along.

 

PT-19-40 Remember that working sheet of fiberglass cloth you saw lying loose on the left wing in image PT-19 31? This is how it looks all brushed down snug against the balsa wing skin with my alcohol-thinned epoxy mix. You can see how easily the weave of the glass cloth lets you work it around the tight folds and compound curves of the wing panel and tip. I worked it down into the handhold cutout as far as it would go without forcing and saturated that area with plenty of epoxy. I’ll trim that cutout, along with the wrapped-over edges, after the epoxy has cured completely.

 

PT-19-41 This is the underside of the wingtip showing you how I wrapped the cloth around “past center” on every edge.(Yes, this happens to be the right wing). The idea is to set up for a generous overlap when I glass the remaining surface. Next, though, I get to trim it.

 

PT-19-42 When that happens it looks like this. A piece of 100-grit paper in my hand easily cuts away the unattached/unsaturated (extra) glass cloth right back to where it’s properly bonded to the balsa surface.

 

PT-19-43 Off camera I finished glassing the rest of the wing and trimmed all the edges just as in image PT-19 36. The next step in finishing the wing will be lots of primer and sanding, but before I get into that I’m going to re-install the flap and aileron servos. They’ll be going back into their original mounting locations, but on modified mounting plates that enclose the entire servo. (Remember image PT-19 4?) Here’s how my Airtronics servo fits onto a pair of spruce mounting blocks that I previously measured, cut and ZAP’d to the 1/16” plywood plate that works as both the servo mounting tray and the cover plate.

 

PT-19-44 This is how all that goes together. I mounted the servo to the blocks using the off-the-shelf mounting screws that came with it. Inside the wing a servo extension cable reaches all the way to the center section where it will be accessible for hook-up.

 

PT-19-45 A little diversion…I need the aileron horn in place to line up the pushrod exit hole in the wing skin. I have marked a line directly back from the servo output to locate both the pushrod exit opening and the control horn itself. Here I’m using a small chisel to open a hole in the 3/32” balsa sheet aileron skin for the rectangular base of the nylon control horn to seat into. (I prepared for this earlier by building in a wide basswood block INSIDE the aileron to provide a strong mounting base.) The next step is to attach the horn…

 

PT-19-46 I made punch mark pilot holes for the two screws that are furnished with the horn and seated the entire assembly into SLO-ZAP.

 

PT-19-47 With the servo mounted and the aileron horn in place I was able to determine the dimensions of the pushrod to go between them and locate the hole that permits it to pass through the wing skin. I started with a nice pointy No. 11 blade and now I’m using a round wood rasp to put proper radiused ends on the opening.

 

PT-19-48 All closed up it looks like this…not dead-on scale, but close.

 

PT-19-49 I tossed out those clunky plastic landing gear strut fairings, but retained the 7/32” steel wire main gear legs that came with the model. As you can see here, I recessed the surface of the ordinary torque-mounting hardwood landing gear blocks that were already in the wing, mounted the LG legs with the usual metal clips and screws and then covered the whole thing up with Stits Lite Fill epoxy. (This is nothing but a concession to the scale fanatic in me. It will screw up access if I have to do repairs, but hides the non-scale hardware. If I had designed this model from the start I would have come up with a more elegant solution to hide the torque rods. This is a good example of a place where you have to decide how far you want to go in modifying an existing airplane vs. starting over). …THEN… with all that done I cut the gear legs off about 1” outside the bottom wing skin. What you haven’t seen yet is a set of Robart Robo-Struts that will attach to the protruding legs through a pair of machined strut adaptor fittings made for me by my friend Jim Miller. (Again, remember that I’m playing catch-up. This is my solution to restoring an existing model to bring it up to the standards I wanted).

 

PT-19-50 Now you see ‘em. I’m using a long piece of steel wire that matches the axle shaft diameter to line up the axle mounting holes in the ends of the struts with each other while I use an Allen wrench to lock down the set screws inside the adaptors. This, along with the red “permanent” Loctite you saw me adding in image PT-19 43 will keep everything in place during normal flight operations.

 

PT-19-51 Here’s a sneak peek at where we’re going…the left wing of the finished airplane.

 

PT-19-52 We had a look at the empennage (tail surfaces) earlier. As I mentioned, the problems I have to deal with included a film covered horizontal stabilizer instead of sheet covering of some sort to represent the plywood on the full scale airplane, and the vertical-fin-to fuselage fairing. I’m not sure how well you can see that in the photos (it’s the blue part in image PT-19 5), but the fairing on the model as I got it was some balsa block sanded into a sort of lumpy, convex bulged-out shape. It’s SUPPOSED to be a concave curvature that flows cleanly from the vertical fin into the horizontal stabilizer and the rear fuselage deck. Using my Paul Matt reference drawings and some full scale photos, I made sure I knew exactly what shape I was trying to represent, then determined that all the material that needs to be there for it to be right was already in that original lump. In other words, all I had to do was to cut and sand away excess balsa until the shape became correct. Here I’m just at the end of that process, with only finish sanding left to do on the corrected fairing.

 

PT-19-53 Here’s a good look at the horizontal stabilizer all stripped clean of film covering. (We’ll forget about the elevator for now.) To get that right…to represent a plywood-skinned surface…I could cut it all off and start over, insert/recess the outer edges of every rib as well as the outer faces of the top and bottom spars and “inlay” a new sheet covering the way I did on the wing, or smooth off the existing surface and skin over it. If I did THAT with 1/64” plywood the extra thickness of the completed structure would be inconsequential, the “scale” appearance of the model would be acceptable, and that’s what I did.