Going All-Out With A Classic Balsa B-17-F – Part 10

Going All-Out With A Classic Balsa B-17-F – Part 10

I finished off the last session (Part 9) by promising you a “prodigious mess of balsa shavings” that would result from completing the job of shaping all four engine nacelles. Doing that turned out to be an exercises in using a sequence of traditional model airplane building tools…a modeler’s block plane, a model knife, and good old sandpaper used “loose” (hand held) and on a block…to create four iterations of a smoothly faired radial engine nacelle. This is one of the most evocative shapes of vintage aviation, and so of classic aeromodeling as well.These days all the decision making that comes with creating something like a B-17 nacelle falls to the mold maker at the foam ARF factory…or if you’re building just about any of the kits on the market, to that person’s opposite number making the molds for plastic or fiberglass parts. Only when we get to the level of shaping our own complex parts for custom kit modification or even real “scratch building do we get to be the one who makes the decision that “the shape is right. I have cut far enough and sanded smooth enough…” That’s where we are right now with this B-17. Let’s go shape some more balsa.

 

B-17-10-1 We’ve been here before. This is the aft upper fairing block on the right outboard (No. 4) nacelle that I shaped in cross section and outline as a “blank” and glued in place on the wing. The rest of the job of shaping it…contouring the outer surface…is one that’s best done in place, which is where we are right now. If we were using a 3-D CAD process we could define ALL the curves in advance, but that’s not what’s going on here. We have to make those 3-D decisions “by hand and eye”. Aside from being fun to do, going through that drill is a good way to understand what happens in the CAD process. That learning doesn’t necessarily work the other way around. At this point I’ll ask you to trust me about the best way to do this particular shaping job…this miniature modeler’s block plane is a lot faster and WAY more accurate than “free cutting” with a knife blade. You can the as-yet-untrimmed aft block on the No. 3 nacelle in the background and compare the work I’ve done on No. 1 with it.

 

B-17-10-2 More of the same … almost. This is another look at the same process on the bottom of the No. 3 nacelle. If you look carefully you can see where the two inboard bottom aft fairings extend further back (toward the trailing edge) than the others.

 

B-17-10-3 Here’s a better look at No. 3 lower. I’ve cut just about as far as I can with the block plane without risking digging in at the edges (even with that masking tape in place to protect the surrounding wing skin surface) or cutting too deep and creating a low spot that would require fixing.

 

B-17-10-4 The next step is to switch over to the sanding block. This one is made up with 80-grit production paper so I can use it to cut balsa decisively and get the fairing blocks shaped almost to the finished contour. Part of the trick here is to let the sanding block cut away balsa without having to use a lot of pressure…a light touch makes it easier to stop before you go too far. The “other half” of the trick is to WATCH the progress of the sanding block carefully as you work. What you see here is the point at which I’m going to stop and switch over to finer grit sandpaper.

 

B-17-10-5 Before I do that I want to define what’s going to be the front face of the nacelle by facing/squaring off the 1/8” balsa sheet extension I added last time. Doing the final shaping of the nacelle surface accurately will be easier with a defined surface here to serve as a reference.

 

B-17-10-6 With that done (you can see the smoothly cut-off faced I’ve just created) I can go back to finish shaping and initial smoothing of the nacelle. This 100-grit sanding block gives me a much finer touch in cutting the balsa sheet surface to exactly the finished dimension I want.

 

B-17-10-7 More of the same…here’s a look from a different angle at that finish shaping operation.

 

B-17-10-8 When the entire nacelle surface is shaped/dimensioned the way I want it, I can begin to round off the leading edge of the “extension ring” I made. When this is finished we want it to represent a neatly radiused sub-cowl leading edge that fits almost entirely inside the rear of the actual engine cowl.

 

B-17-10-9 This is what the No. 4 nacelle looks like “all shaped”. You can see the as-yet-unfinished No. 3 nacelle behind it for comparison.

 

B-17-10-10 Here’s another advance glance at what’s in our B-17’s future. This is one of the four Innov8tive Designs Cobra 2217/20 outrunner motors I’m using. The motor is trial-mounted (with just those two screws) to the motor mounting plate. In the background is one of the kit-standard clear molded plastic cowls rough-trimmed to check that it slip-fits the right distance back over the motor to create just the right clearance between the cowl trailing edge and the rounded sub-cowl lip/leading edge I just finished in B-17-10-8.

 

B-17-10-11 Before I remove all that protective blue masking tape around the nacelles I want to do some more sanding. Here I’m using a loose/hand-held sheet of 100-grit paper to fine tune the transition of the aft upper fairing into the surrounding wing surface.

 

B-17-10-12 I did the same thing on the other nacelles, too…this is the bottom of No. 2. As I explained earlier, the rear portion of this fairing must be removable along with the flap servo well cover that I will make to match the pre-existing opening. I’m using a razor saw to cut it off flush with the edge. I’ll set the cut-off piece aside to assemble it with the cover plate later.

 

B-17-10-13 This is the underside of the right wing, showing nacelles No. 1 and 2. It would have been nice to get a perfect fit between the nacelle skin sheet and the wing surface … but I’m not that good. This is a perfect place to use Deluxe Materials Model Lite balsa putty to correct every surface imperfection I can find. I’m using my well-tested artist’s palette knife to force the Model Lite into every opening as I apply and spread it.

 

B-17-10-14 That stuff dries fast! You can always tell when an application of Model Lite (or any other similar product) is dry enough to sand by feeling it…if it’s still even a bit cool as a result of solvent evaporation it’s not dry yet. This application IS and I’m using a handy Great Planes bar sander to reach accurately into that sharp corner to shape the Model Lite without marring the surrounding balsa.

 

B-17-10-15 Here’s another trick. This is a flexible-grit sanding pad that I’m using to work the nacelle surface down to a finer finish (this is 320-grit material). The flexible pad contours to the existing curved surface as I can smooth it without cutting any flat spots. LOOK CLOSELY…can you see where I’ve used the corner/edge of the pad to define the EXACT transition between the curved nacelle and the relatively flat wing skin? The little “bead” of sanded-off Model Lite is the tip-off that I’ve done some final, delicate shaping.

 

B-17-10-16 Finally it’s time for free hand fine sanding of the fully shaped nacelle with 320-grit production paper. This is where you develop a practical “feel” for what a correctly prepared balsa surface is supposed to be.

 

B-17-10-17 AND NOW …it’s time for some fiberglass cloth finish base work. In a different situation I might choose to wait until the rest of the structure (the fuselage in this case) was finished up to this stage in order to do all the fiberglass work at one time, but I’ve discovered (the hard way) that with a wing this large, the soft/light balsa I’ve used for weight control is too vulnerable to “shop rash” whenever I move the wing. The easy solution to that problem is to “glass: the wing now to give it some protection. Here you can see one of the packs of Deluxe Materials 2 ounce fiberglass cloth that I’m going to use along with a bottle of Deluxe Materials EZE-Kote. Watch to see how easy this stuff is to work with.

 

B-17-10-18 There are many different ways to mark and cut the actual working pieces from the sheet of fiberglass cloth as it comes out of the bag. On this project I’ve chosen to spread the “whole cloth” out across the structure and mark the necessary cutting lines roughly in pencil. What’s happening here is that I’m marking to “square-off” the inboard edge of a piece that’s going to fit an outer wing panel surface.

 

B-17-10-19 The next step is to spread the glass cloth smoothly over a flat surface and use a straightedge and pencil to mark accurate cutting lines that define my “working piece”.

 

B-17-10-20 You can’t see all of it here, but this working piece of 2 oz./sq. yd. fiberglass cloth cut from the piece you saw in B-17-10-19 fits the entire right/bottom wing panel from root to tip, LE to TE with a working margin (extra cloth to pull on, make up for alignment errors, and perhaps wrap around edges) of a couple of inches. Right now it’s just lying there, lined up and smoothed out, waiting for the Deluxe Materials EZE-Kote in the background that’s going to transform it.

 

B-17-10-21 This stuff is great to work with! I’ve poured out enough EZE-Kote into an open container (out of sight) to work from and I’m using a soft, flat brush…the same as I’d use for clear-doping fabric…to “flow” the EZE-Kote onto and through the glass cloth. On applications like this I have learned that it’s best to “work out from the middle”, keeping the fiberglass smooth (no bubbles or creases) and centered on the structure you’re glassing. The various grades/weights of fiberglass cloth you’ll get from Deluxe Materials are all going to be limp (“floppy”) enough to lie smoothly and tightly against any of the various curved surfaces like this wing skin that you’ll be using it on.

 

B-17-10-22 That “floppiness” will get you only so far. Trying to pull the cloth into the multiple curves/contours of the bottom/rear nacelle AND expect it to lie flat over the surrounding wing surface is usually going to be too much to ask for. On this particular job I’ve chosen to devote the piece of glass cloth to the wing surface itself and deal with the more complex nacelle separately. Here I’m using a No. 10 scalpel blade…you can see the gently curved cutting surface that’s not going to catch an edge or dig into the fiberglass … to cut free the portion of the main sheet that’s likely to “fight” against lying neatly in place on the nacelle. Throw away an “offcut” like this…don’t practice false economy by trying to find a place where it might fit. That’s a good way to get lost in fussy details and spoil the overall job.

 

B-17-10-23 Here’s one of those “wraparound” opportunities I’m going to pass on. This 2 oz. cloth is limp, but not limp enough to wrap around that TIGHT compound curve formed by the edge of the wingtip. Brush the EZE-Kote-saturated cloth as far around the curve as it wants to go without springing back and leave it alone. Did you notice the little slits I’ve cut into the overhang to help it “go around”? These worked well enough to get the glass cloth “over center” of the curve, and that was enough to ensure a bit of overlap when I do the same thing on the opposite (top) surface.

 

B-17-10-24 Can you see how the entire bottom/right wing surface has been smoothly glassed with the nacelles themselves still left uncovered as per B-17-10-22? Now I can concentrate on those sections by themselves. I’ve made a paper pattern that fits the entire bottom of the No.1 (outboard) nacelle.

 

B-17-10-25 As before, I’m using an ordinary soft pencil to trace out this pattern onto my large sheet of glas cloth. As it turned out this pattern fits ALL the nacelle surfaces except the two lower inboards, so I cut out six pieces like this one.

 

B-17-10-26 First I do a “dry” test fit.

 

B-17-10-27 When the new nacelle-bottom working piece fits right, I can go right to the EZE-Kote and begin brushing the cloth into place. Can you see how I am again starting at the center of the working piece and brushing “outward”?

 

B-17-10-28 When I’m done, that looks like this. I have an overlap of ¼” to ½” of the “nacelle piece” over the previously applied “wing skin piece”. This is where you get to see the payoff for not trying to push the “compound curve capabilities” of fiberglass cloth too far. All of your “glassing” jobs with EZE-Kote should look like this when you leave them to dry.

 

B-17-10-29 More of the same…here you can see the wing after I’ve glassed both bottom panels, the center section, and the lower surfaces of all four nacelles. This is where you go away and do something else while the EZE-Kote dries/cures hard. As it happens in my warm, dry shop, I was able to handle the wing at this stage after several hours, but as a matter of habit I let it sit overnight before turning it over to work on the other side.

 

B-17-10-30 Now we get to do the same thing all over again on the top. You can’t see it here, but as usual I have the right-side-up wing supported on a couple of my favorite “el cheapo” paper towel roll stand-offs for protection from shop rash.

 

B-17-10-31 This is just another look at the trimming process I’ve worked out using a “non-pointy” blade to cut away a section of cloth that might threaten to get wrinkly.

 

B-17-10-32 This is the top of the left wing. I’m brushing the last of the EZE-Kote onto the wing surface surrounding the top of the No. 2 nacelle. In the background you can see where the loose cloth between the nacelles is ready to be “EZE-Koted” down and around the leading edge.

 

B-17-10-33 OK … the top/left wing surface is glassed, along with the upper center section skin. The working piece of glass cloth for the upper right surface is resting loosely in place. Note that I’ll wait until most of the glass cloth surrounding the No. 3 and 4 nacelles is sealed into place before I trim away the wrinkle-prone nacelle sections.

 

B-17-10-34 Here’s the good part … I’m EZE-Koting the top of the NO. 2 nacelle just as I did the nacelle underside you saw earlier.

 

B-17-10-35 And, I finished up all the other nacelle surfaces in the same way.

 

B-17-10-36 In some places on this airplane I simply the let the working pieces of glass cloth on adjacent sections of structure…like where the top and bottom nacelle side pieces meet…overlap and fair intone another. With a firm structure underneath them, these sections will permit me to do a good job of sanding the overlapping cloth seams into invisible joints. ON this wingtip that’s too much to ask of the glass cloth, so I let the extra width of cloth “hang loose” to sand/trim like this when the EZE-Kote has dried hard. In some places, like this wingtip, a loose/hand-held sheet of 100-grit paper is the best way to get a neat trim.

 

B-17-10-37 On a well defined straight edge/corner like this right aileron well cutout using a sanding block (again 100-grit) is a better way to ensure that I get an accurate finished edge.

 

B-17-10-38 There are some places where a hand held sheet of fine sandpaper is the best way to get rid of those odd bumps and specks of dirt.

 

B-17-10-39 Everything you’ve seen so far involved the first bonding/sealing coat of EZE-Kote as I used it to attach the glass cloth to the airplane. (Remember that I applied a previous seal coat of EZE-Kote to the entire balsa structure before breaking out the glass cloth.) Now, after all the overlaps and overhangs have been trimmed off and cleaned up, I’m giving the entire cured/dried glass cloth surface a second coat of EZE-Kote applied just heavily enough to flow out and provide thorough, wet coverage. Go away (again) and let this job dry right. Note: Deluxe Materials recommends that this second EZE-Kote application…to be wet-sanded thoroughly when dry and BEFORE adding any additional primer/surfacer you might …is the best and lightest route to a superior finish. I’ve tried it their way and I agree.

 

B-17-10-40 Here’s the top of the wing, all glassed, sealed, and second-coated. Next time I’ll detail building the remaining wing structure…the flaps and ailerons.

See the entire build series: Building the Balsa B-17-F