Going All-Out With A Classic Balsa B-17-F – Part 7

Going All-Out With A Classic Balsa B-17-F – Part 7

I finished our last building session by closing up (adding the last of the 1/16”balsa sheet skin) the lower surface of both wing panels and doing some final reinforcement and preliminary sanding around the flap well access. With that done, the next step is to locate, mark and cut out the portions of the wing skin that are now covering (and hiding) those flap and aileron servos along with all the access to cables, connections and so on that I want to build into this airplane. That’s where we’ll start this time.

 

B-17-7-1 When I framed in the four flap and aileron well/access hatch locations and sanded them flush with the rest of the wing surface/ribs and spars it was with the intention of closing up the entire bottom surface with a single sheet of 1/16” balsa. On this model that makes more sense than trying to “piece together” a wing skin of multiple sheets arranged around the various openings…that would be a good way to introduce lots of “discontinuities” (bumps). The downside of closing the structure with a single sheet is that I have to locate those openings through it and cut them out without marring the balsa sheet around them. Pre-marking the exact openings on the sheet before assembly would have required exact registration (alignment) during assembly. I prefer to devote all my attention to getting the balsa sheet skin attached smoothly and consistently to the underlying structure and then deal with all these openings like this…by measuring from the plans I can locate and mark the approximate center of each opening and be “off” by quite a bit without causing trouble. Watch…

 

B-17-7-2 I have rough-cut a hole about an inch across as close to the middle of the hidden flap servo hatch opening as I can get. Now I’m slicing carefully through balsa that’s eventually going to be cut away while the blade locates the exact edge of the pre-framed opening.

 

B-17-7-3 With that done it’s no big deal to locate the rest of the edges by “feeling” with the No. 11 blade.

 

B-17-7-4 Off camera I drew in pencil lines to mark the edges of the cutout I wanted to make, keeping my cuts at least 1/16” inside where the finished edge will be. Then I could easily see what I was doing using this 100-grit sanding block to finish each skin-to-hatch transition neatly.

 

B-17-7-5 With that done, along with a few passes with 320-grit paper on the wider block to double-check that the whole deal is smooth and even, the new flap servo access opening looks like this. I’ll work on the actual cover plate (all four of them) later in the construction process.

 

B-17-7-6 All that work on designing, cutting out and fitting the landing gear mount assemblies leads up to this. This is the right main gear assembly seen from in front and below. Did you notice that there is balsa structure (an N-1 former) that is going to be in the way when I try to retract the strut? I have a good reason to leave that as-is for now…watch this space for an explanation later.

 

B-17-7-7 How do you cut out CLEAN stringer notches into formers like these N-1’s AND keep them all in line from one former to the next? As it turns out, although most of the stringer notches in this kit are pre-cut, there are a few places (like the corner longerons on each of the nacelles) where I need to re-define them. One of those old time tricks that I learned from Cleveland kits during the 1950’s is to glue a strip of medium-fine sandpaper cut toa precise width (here that’s 1/8”) to a straight piece of hard balsa of the same thickness. That custom cutting/sanding tool looks like this…

 

B-17-7-8 …and using it looks like this. The kit plans does not show clearly that we need a 3/32” sq. balsa stringer here, but with all the adjustments I’ve been making to the structural design, including them has become necessary to ensure a smooth, consistent outer contour when I skin the nacelles with 1/16” balsa sheet.

 

B-17-7-9 Here you get a good look at the “new” stringer notch I have cut into the assembled nacelle along the outer edge of the longeron. I’ll put the extra stringer in place in just a moment. Right now, though, I’m using Deluxe Materials Roket Hot to assemble the free rear end of this side stringer to the 1/16” balsa sheet wing skin. Look closely…the rear end of the stringer is tapered/beveled to match the odd angle of the joint exactly. You can also see that the front end of the stringer lines up with the curve that the stringer (and later the nacelle skin) will follow when I bend it to fit into the corresponding notch in the front N-1.

 

B-17-7-10 All that happens when I use “calibrated thumb pressure” to form the bend. I’ll use a generous drop of Deluxe Materials Roket Hot to lock up that joint.

 

B-17-7-11 More of the same. All the remaining 3/32” sq. balsa nacelle stringers get installed in the same way.

 

B-17-7-12 With all the stringers on the nacelle in place I can use my 100-grit sanding block to cut off all the overhanging excess balsa and true up the face formed by the front N-1’s.

 

B-17-7-13 While you weren’t watching I finished assembling the stringers to the remaining nacelles and cleaned up the front faces of each of them the same way. Now I need to make sure that the outer nacelle circumference/surface that is defined by each group of assembled stringers is accurate. The curves defined by the outer faces of the stringers dictate the shape of the 1/16” balsa sheet skin (the outer surfaces of the finished nacelles) that comes next. If you don’t get that right now, the error will always show. Important note: The assembly of N-1’s, stringers and glue joints that I have to cut/sand “perfectly round” is just about guaranteed to be harder (tougher to sand) than the 1/16” balsa sheet skin it is going to blend into. If I don’t protect that skin adjacent to the structure that I’m going to shape with the sanding block I’ll almost certainly scuff and gouge it seriously. That’s why those protective layers of masking tape are in place wherever the abrasive could slip and cut whatever happens to be in its way. Can you see that right here working “around” the nacelle in a spiral pattern?

 

B-17-7-14 This is the same nacelle seen from the other side. You can see some of the sanding dust that shows I’ve cut away a considerable amount of wood. This is one of those aeromodeling tasks that must NEVER be rushed…you have to sand and check and sand some more until the surface curvature you are defining is correct or the errors you leave in your work will always show.

 

B-17-7-15 When I have all four nacelles shaped and sanded the way I want them…top and bottom, front and rear…I can go on to the next step of making paper patterns for the various sections of 1/16” balsa sheet skin that will close up and define the outer nacelle surfaces everyone is going to see. This is one of the important changes I’ve chosen to make from the kit design…originally these nacelles were intended to consist of tissue covering over exposed stringers. As I have suggested earlier, I am so turned on by the look of a classic, round radial engine cowl and nacelle that I’m not willing to compromise. The small increase in weight is going to be part of paying my dues for improved scale appearance. Making the various paper patterns for the panels that form the nacelle skins is a cut-and-try process. I made the preliminary paper pattern you see for the panels that fit both the inboard and outboard halves of the rear portions of the right outboard nacelle by tracing roughly against the structure and then drawing a “clean” copy on fresh paper using a drafting curve.

 

B-17-7-16 Test-fitting the pattern to the actual structure revealed the need for a bit of extra width/depth at the rear outside edge. If you look closely you’ll see where I have marked the error with a pencil line.

 

B-17-7-17 In this image I’m again using a drafting curve to mark a smooth curve (that should match the contour of the wing skin accurately) onto a corrected pattern on yet another fresh piece of paper.

 

B-17-7-18 When I cut out THAT pattern and checked it against the nacelle structure it fit correctly, so I went on to use it to mark cutting lines onto a piece of the 1/16” balsa sheet that I had already selected for the nacelle skins. Here I’m using a No. 11 blade to cut the critical curvature/edge that MUST match the shape of the wing skin.

 

B-17-7-19 Remember what I said about “cut-and-fit”? Holding the newly-cut balsa panel in place I can see that it comes close to matching the wing leading edge curve…BUT…I can still make it fit better.

 

B-17-7-20 Some 100-grit paper on a round block is just what I need to “open up” that inside curve just a bit…

 

B-17-7-21 …with the result that it now fits with noticeably improved accuracy. Off-camera, I dampened the outer surface of the panel with water and glued it into place.

 

B-17-7-22 I finished closing up the rear portion of all four nacelles in pretty much the same way. Here I’m using a new pattern to begin marking more 1/16” balsa sheet for one of the inboard nacelles.

 

B-17-7-23 Just as before, I test-fitted each panel to its final location on the airplane and corrected wherever it was necessary.

 

B-17-7-24 Now I’m applying a bead of Deluxe Materials Roket Rapid to the critical edge of the panel…

 

B-17-7-25 …and pressing it (very carefully) into place, aligned /fitted to match the wing skin. I have left all the other edges of this panel slightly oversize so I can fit and trim them to an exact match where the panel contacts the nacelle structure…then with this first/most critical edge glued securely I can roll/wrap/press the remaining free edges into place and glue them in turn. Did you notice my other “freebie tool”? Paper towel rolls make excellent pads to rest potentially vulnerable structure on while you’re working on it.

 

B-17-7-26 Now I’m going to sheet/close the forward section of the nacelle. Because this portion of the structure…from the sub-firewall to the second former station…is cylindrical (not tapered) the sheet covering panels likewise don’t have to be tapered, so making them is a lot easier. Here I have aligned what will become the rear joining edge of the new panel with the structure it has to match and I’ve drawn a straight pencil line to mark a simple cut that will give me a panel section (grain along the axis of the nacelle for easy bending) with enough overhang in the front to give me room for adjustment if one part or another of the front former isn’t exactly square.

 

B-17-7-27 I have cut the new panel section along the line a marked and held it in place where it’s going to go on the nacelle. Notice that I have carefully aligned the “bottom” edge of the panel/sheet along the nacelle side stringer where it is going to be attached. With that alignment as a reference I then made those pencil marks along the top stringer…they become the alignment reference for the top of the panel.

 

B-17-7-28 As it turned out that top/front panel “blank” fit so well that I left it long enough to wrap around to the inboard nacelle side stringer, giving me one-piece coverage of the top 180 degrees of the nacelle circumference. Again off-camera, I water sprayed the outer face of that panel, then applied Deluxe Materials Roket Rapid to all the joining surfaces on the nacelle, wrapped the panel into place and used calibrated hand pressure to hold it firmly-but-gently exactly where I wanted it to be. With an “open joint” cyanoacrylate adhesive like Roket Rapid and consistent “clamping pressure” on a joint like this, a minute or so is all it takes for the CA to grab. This is an example of one of those places where “hand-holding” makes more sense than the time and effort of taping, clamping, etc., to get the same result.

NEXT TIME we’ll get to do some really neat stuff on the nacelles.

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