Going All-Out With A Classic Balsa B-17-F

Going All-Out With A Classic Balsa B-17-F – Part 8

I finished our last session of B-17 building by closing-up/sheeting-in the forward section of one of the one of the engine nacelles. I’ve been doing nacelles for quite a while now and I’m sure it has occurred to you (as it did to me when I was planning the project) that four nacelles …all of them slightly different…are going to demand not just some seriously skilled balsa work, but also a lot of patience.That last part is the key to getting it right. As I’m about to show you, as very often happens with building model airplanes, not everything connected with doing nacelles came out exactly right on the first try. One of the lessons I want to share this time around is how to go back and get it right when whatever it is that’s getting built at the moment falls short of your expectations. Being able to apply that lesson, along with a generous shot of that patience (as in, “No, I don’t have to finish closing all four nacelles today!”) is one of the secrets to doing the sort of model building people talk about. Once all the sheet balsa nacelle skins are in place, I’ll go on to fill in some more of the shape that will go a long way to defining the character of the B-17. When I close the shop door this time around there’s still going to be a healthy dose of nacelle building left…but…it’s going to get easier to squint a little into the shadows in the corners and imagine four R-1820 radials chuffing and rumbling into life one after the other. Let’s cut some balsa…

B-17-8-1 Last time I started with the front of the left/outboard (No. 1) nacelle…now I’m working on the right/inboard (No. 3). This is where the get-it-right part begins. In a perfect world, the ring-shaped joining surface created by the open edge of the 1/16” balsa sheet skin where it attached to the N-1 formers at the wing leading edge would be exactly circular and lie neatly in the single vertical plane defined by the N-1’s. If that were the case I’d be able to cut a single piece of 1/16” balsa sheet (with the grain running front-to-back for easy bending) that would wrap a full 360 degrees around the structure and close it. In this real-world project there are going to be imperfections…bumps and wiggles…that will require corrective adjustments (trimming) to the front skin material to allow me to make a neatly fitted, closed joint all the way around the nacelle circumference. By making this front skin in four quarter-circle (90 degree) segments I can make those adjustments easier to manage. Here I’m starting by marking a piece of 1/16” balsa sheet wide enough to cover ¼ of the nacelle circumference and long enough to provide some extra margin for trimming at the front and/or rear.

B-17-8-2 Cutting the first of the four front skin panels I’ll need for this nacelle from a stock 1/16” x 3” x 36” balsa sheet is just the usual straightedge-and-razor blade job.

B-17-8-3 Because I have planned for extra length/overhang it’s not necessary to measure each panel individually to get an exact measurement…I’m using the first one as a pattern to mark off the rest.

B-17-8-4 Here’s the front cowl skin ready for some careful cutting and fitting into an accurate assembly.

B-17-8-5 Here’s the first of those inaccuracies (errors) that are going to need fixing. Clearly one edge of the new skin panel must be centered on that 3/32” sq. stringer, so I’ll have a joining surface/gluing base for the one that will fit next to it. BUT…can you see where, for whatever reason, I have allowed the joining edges of the rear skin panels to end up out of line with the stringer? What’s more, the rear panel edges don’t line up. What’s the best way to correct all the misalignment to get an accurate finished nacelle surface?

B-17-8-6 Look very closely at the joining edges as I hold the new panel in place. I used the sharp corner of a sanding block to trim a shallow notch into the corner of the front sheet to achieve the fit you see here. This is easier than trying to cut and chisel away extra material on the old glue line over the N-1 former.

B-17-8-7 This is the same panel as seen from the other side…the trimmed joining edge we just looked at is at the top where I’m holding it firmly in place with my thumb. Before positioning this panel I made the pencil mark that’s on the left of the seam to reference the centerline of the side 3/32” sq. stringer I’ll use as a joining base. With the skin panel temporarily bent into place I then made the pencil mark on the right side to provide a reference for cutting off the excess panel width so the finished edge will line up with the stringer.

B-17-8-8 I marked the front edge at the stringer centerline, too. The result looks like this.

B-17-8-9 The narrow cut-off piece of balsa is the portion that would have extended beyond/over the stringer centerline.

B-17-8-10 When I test-fit the trimmed skin panel it looks like this.

B-17-8-11 As with all the other assemblies on this airplane that require panels of sheet balsa to bend to fit a curved structure, I’m spraying what will be the outer face of this one with water. If you look carefully you can see that the 1/16” balsa sheet has already started to assume a curve away from the moistened surface.

B-17-8-12 This is a classic example of “open joint assembly”. I have to put adhesive on ALL the structural surfaces (stringers, formers, etc.) that will contact the panel I’m attaching before I fit it into place. Traditional aliphatic resin (or even old-time model airplane cement) would do the job here by remaining wet long enough for me to fit the assembly together and hold (clamp) all of it in place long enough for the adhesive to harden…for an assembly like this I would normally use masking tape to avoid poking holes or otherwise marring the sheet panel with pins. This is where Deluxe Materials Roket Rapid is just what I need. Once the joining surfaces are closed (pressed together) under gentle pressure the adhesive will take hold (“grab”) within a minute or so. That’s a reasonable length of time for me to hold everything in place with my hand. When “handholding” is tthe better choice and when you should rely on tape, clamps, or whatever to do that job for you will always be a judgment call on your part as an aeromodeling craftsman. Watch how it works here…I’ll begin by laying a smooth, full bead of Roket Rapid along each of the surfaces to be joined, even the little narrow ones.

B-17-8-13 The Roket Rapid easily stays “wet” for the minute or two it takes me apply it to every part of this complex joint and then allows me time to accurately align the panel along the top stringer. Remember that since I’ve already double-checked the fit, lining up this part of the joint ensures that all the other edges will fit as well.

B-17-8-14 With that done I can wrap/roll the rest of the panel around and into contact with the pre-glued nacelle structure…

B-17-8-15 …and HOLD it there by hand for the minute or so it takes the Roket Rapid in the now-closed joint to grab and take over the job of holding. I could have used a masking tape wrap to do the job, but this is a good example of a case where using cyanoacrylate allows me to make a strong, accurate joint in less time.

B-17-8-16 This is the same panel/assembly seen from a different angle.

B-17-8-17 Remember what I said about opportunities for less-than-perfect joints that may jump out to surprise you? Look carefully…the upper edge of the new 1/16” balsa sheet panel is aligned exactly along the top stringer, ready to form a perfect joint with the panel that’s already in place. The square-cornered panel doesn’t fit. What’s going on? For whatever reason I’ve permitted an alignment error to creep into the N-1 formers as they align with the wing leading edge. What should I do? Having this particular skin panel joint cut at a corrective angle will not compromise structural strength, nor will it show after I’ve finished skinning the nacelle and done some careful sanding, so…

B-17-8-18 …I’m going to adjust (trim) the panel edge to cancel out the error. I’m measuring the width of the gap that’s defined by the misalignment…

B-17-8-19 …and transferring that measurement to the panel.

B-17-8-20 That allows me to mark a reference line that I’ll use to cut off the extra 1/16” balsa sheet that’s getting in then way of a proper fit.

B-17-8-21 This happens a lot. I trimmed off almost, but not quite, enough balsa. Off camera, I repeated the measure, mark and cut operation.

B-17-8-22 On the second try it fit the way it’s supposed to. If you check back over the last several steps you’ll see where I left plenty of extra length on these panels in case this kind of trimming became necessary.

B-17-8-23 All things considered, it’s a good idea to check again that everything fits. Remember that I’m allowing the overhang on the front to remain there for now.

B-17-8-24 You couldn’t see it, but I found a little “wide spot” along the top/joining edge of this panel, so I used my 100-grit sanding block to “feather off” a tiny adjustment that would have been too delicate to get right with a razor blade and straightedge.

B-17-8-25 Now I can get out the Deluxe Materials Roket Rapid again and start gluing-up this panel assembly just as I did the last one.

B-17-8-26 As before, I’m relying on calibrated finger pressure to clamp the multiple gluing surfaces of this complex joint long enough for the Roket Rapid to grab.

B-17-8-27 Now I’m moving on to the adjoining nacelle…the right inboard…and going through the same process of fitting the new panel piece to the existing structure.

B-17-8-28 As before, I measure-and-trim-and-check until I have a fitted joint I won’t need to hide.

B-17-8-29 Here I have moved on to yet the next nacelle…more of the same until I have them all the way I want them.

B-17-8-30 THAT looks like this. Here it’s easy to see all that extra 1/16” balsa sheet I left overhanging the sub-firewall (front) N-1’s in case I needed it for trimming and fitting.

B-17-8-31 Now I can trim away all that extra sheet balsa. I’m rough-cutting the assembly to shape using a SHARP razor blade and a slicing motion.

B-17-8-32 The 80-grit sanding block makes an easy job of blending all those protruding edges into the surface defined by the N-1’s so smoothly that I can’t feel a discontinuity.

B-17-8-33 Here’s a dry fit of one of the laser cut plywood firewalls directly out of the kit box. Before I can glue these in place I have to finalize any decisions regarding the motor mounts and mark/cut/drill any openings I might want to make in the firewalls while I still have them “loose” and fully accessible to work on.

B-17-8-34 Another sign of things to come…this is one of the aft-nacelle fairing blocks that I’ll be cutting from ¾” balsa sheet and attaching to the top and bottom of each nacelle rear-end in turn. There will be a LOT of trimming and fitting and sanding involved here, so I’m going to put off working on those details until next time to assure having plenty of space to explain it all.

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