Restoring a Classic PT-19 (Model) Airplane – Part 3

Restoring a Classic PT-19 (Model) Airplane – Part 3

We left off last time with plenty of work to do on the tail surfaces…let’s get going on that.

 

PT-19-54 I made a pattern, then traced and cut out four identical 1/64” plywood skins.

 

PT-19-55 Here’s one of them all ready to attach.

 

PT-19-56 You can see just how it’s going to fit. As it happens, the top and bottom skins are nearly identical…the only changes I needed were some differences in trimming where the inboard end of each skin meet the fuselage of fin fairing.

 

PT-19-57 Time for Titebond… as before, I used a brush to get a wet, even coat of glue everywhere the new skin would contact the old structure.

 

PT-19-58 Here’s the first skin in place.

 

PT-19-59 This is a place for some tight, accurate clamping while the glue dries. I have the bottom skins in place as well and I’m beginning to add some plastic clamps (they were given to me; I don’t have a brand name) to lock everything in alignment.

 

PT-19-60 Finished, that clamping job looks like this. At times like this, I leave the whole assembly overnight to be sure it’s dried properly before I handle it. BTW: That fuselage covered with pink-tinted fabric in the background is part of the big Stinson SR-9 I was working on at the same time as the PT-19 restoration as the subject of my Master’s Workshop column in FLY RC Magazine

 

PT-19-61 The glue is dry, all the clamps are off, and I have squared-off the surface outline/edge consisting of the trimmed-back original balsa leading edge and the new plywood skins. I’m doing this to provide a flat, accurate base to which I will laminate those “extra” ¾” x 1/16” balsa strips lying on the board in order to fill out the correct leading edge cross section.

 

PT-19-62 I have already soaked these laminating strips in warm water to make them easier to bend and now I’m brushing a generous layer of aliphatic resin wood glue onto what will become the joining surfaces.

 

PT-19-63 Unlike the process of laminating a new structural outline, which requires a pattern and a template of some sort around which to form the part, laminating what is simply an extension of an existing structure is easy…the thing creates its own pattern and I just add on to what is already there. Masking tape works far better here than pins or even clamps and blocks. Don’t forget to pull every piece of tape TIGHT to eliminate every last trace of space (openings) between the laminated strips.

 

PT-19-64 All dried and tape off, the newly laminated leading edge looks like this.

 

PT-19-65 The next step is to shape the new laminate to match the surface of the existing plywood skins and then round it to the correct cross section. I’m starting by (carefully) slicing off extra wood using an ordinary No. 11 model knife.

 

PT-19-66 A coarse (60-grit) sanding block is the right tool to use to cut the laminated edge smooth and flush with the rest of the structure.

 

PT-19-67 I rough-shaped the leading edge radius with the 60-grit block and then switched to 100-grit…and took advantage of the straight line reference provided by that long sanding block…to ensure that the final shaping and smoothing of the leading edge results in an aerodynamic shape that is straight and smooth.

 

PT-19-68 With the entire new leading edge/tip sanded to finished shape, the horizontal stabilizer looks like this.

 

PT-19-69 Here it is again from the other side.

 

PT-19-70 This revised, plywood-skinned structure doesn’t NEED any reinforcement …but…I’m going to add covering material to it to help me build up a slick, smooth (scale) finish base without the added weight multiple coats of sanding primer would cost me. This is especially important on the tail (of ANY airplane) because every unit of extra weight you add here will demand multiple units to be added at the noses for balance. My choice of material here is traditional silkspan (medium or “GM” weight).

 

PT-19-71 My preferred method of attaching silkspan to a closed (sheeted) surface as an aid to getting a good finish base is to wet the material with water, smooth it out across the structure to be covered, then brush clear nitrate dope right through it and onto/into the underlying wood. Working OUT from the center is the best way to avoid/control wrinkles here.

 

PT-19-72 As you may remember seeing me do on earlier projects, I’m using the old time technique of slitting the wet silkspan into narrow “tabs” that will fold around the outer edges without wrinkling.

 

PT-19-73 Once that clear nitrate dope has “grabbed”, but before it dries hard, I used a new (sharp) razor blade to trim off the excess from those “tabs”, leaving a double overlap of silkspan around every edge.

 

PT-19-74 The elevator needs its own modification. On the full scale PT-19 the two elevator halves are fabric covered, so our basic structure is OK, but each section includes a big trim tab. Representing these tabs isn’t necessary to fly the model, but NOT building them would really lose points on scale appearance. I used my Paul Matt scale drawings again to determine the scale outline of the trim tabs and locate them on the existing balsa elevators. Here you can see how I have used a razor saw to create a “pocket” where the new trim tab will fit.

 

PT-19-75 I cut away enough “extra” elevator material to leave room for a new “tab pocket sub-trailing edge” insert cut from some scrap ¼” balsa sheet.

 

PT-19-76 I left extra depth (vertical dimension) on the sub-trailing edge so I could go back after the glue dried and trim it to a perfect fit.

 

PT-19-77 I began the process using an ordinary razor blade and then switched to a sanding block to finish the outside face of the new parts exactly even with the pre-existing elevator ribs.

 

PT-19-78 I’ll get back to the actual trim tab later. Here you can see that (while you weren’t looking) I used the Stits PolyFiber process you’ve seen me use before to cover and edge-tape the elevator sections. I’m going to use Robart Hinge Points (large) to mount the elevator…here I have marked their locations (using the scale reference drawing again) and I’m drilling pilot holes on the horizontal stabilizer trailing edge.

 

PT-19-79 There’s more than one way to make those holes fit the Robart hinges exactly. Here I’m using a reamer to enlarge the pilot hole I drilled earlier…just enough to allow the hinges a gentle press fit.

 

PT-19-80 That looks like this.

 

PT-19-81 Notice that the that the scale outline of the elevator leading edge includes recessed cutouts where the hinge assemblies fit on the full scale PT-19. One of the reasons I’m using these Robart Hinge Points is that they allow me to replicate that geometry. Here I’m re-marking the exact center of one cutout before drilling the hinge hole.

 

PT-19-82 The hinge goes in like this. It’s important to make sure that all the hinge pin centers of rotation align exactly along the elevator centerline and fore-and-aft as seen from above…if not, the hinges will work against each other and the elevator surface will bind and jerk instead of moving smoothly through its range of motion.

 

PT-19-83 The hinge is dry-assembled into the elevator and I’m checking for an exact fit into the trailing edge of the horizontal stabilizer.

 

PT-19-84 I’m going to seat the portion of each hinge assembly that fits inside the structure into some 30-minute epoxy. Before I do that I’ll OIL each hinge pivot assembly generously to keep any of that epoxy that happens to squeeze out from sticking where I don’t want it to.

 

PT-19-85 Once all the hinges are aligned in the elevator and the epoxy has cured hard, I can use the same oil-and-glue technique to set the free ends into the horizontal stabilizer. One of the reasons I use a relatively slow curing adhesive here is that I have plenty of time to align everything as exactly as I can get it, then clamp and tape it all so nothing can slip…and leave it overnight.

 

PT-19-86 All set up with those random oozes of excess epoxy cleaned off the hinges and all the tape removed, the stabilizer-elevator assembly looks like this. I’ll come back later to fit the elevator trim tab.

 

PT-19-87 I’m going to use the same hinging process on the rudder, but first I’ll need to make and install the two-sided control horn that works with a pull-pull cable control loop. I drew out this horn to match the size and shape indicated on the scale drawing, and then cut and drilled it from some 1/16” Micarta (“Paxolin” in Britain) . G-10 epoxy board would work just as well. Note: what you see is thte right half of the horn; the opposite side is identical.

 

PT-19-88 The elevator control hookup is a standard push-pull deal with a metal control horn silver-soldered to the elevator joiner rod. I used a heavy duty (4-40) rod-end fitting to attach the push-pull rod from the elevator servo. Note: the access opening you can see all this through matches the access panels on either side of the tail of the full scale PT-19, and it’s going to get finished off with scale panel covers.

 

PT-19-89 When it’s all done the tail assembly is going to look like this.