Going All-Out With A Classic Balsa B-17-F – Part 9

Going All-Out With A Classic Balsa B-17-F – Part 9

Part 8 of our “building the B-17” story ended with my giving you a sneak peek at the first of the aft nacelle fairing blocks stuck into place. In showing you that (as a hint of things to come) I got a little ahead of myself. Today I’m going to back up a step and explain how that first fairing block got to be that way.

 

B-17-9-1 As I have mentioned before, the original design of this model called for the engine nacelles to be fully “stringered” with a surface of tissue or film covering. To that end the kit plan includes patterns that denote where wingtip-like outlines of 3/32” balsa sheet define the outer edges of each aft nacelle fairing. I could have finished building each nacelle that way, then sanded each set of stringers, formers and outlines true and used more 1/16” balsa sheet to close in the remaining open structure just as I did with the forward parts of the nacelles. However…the penalty of fussiness and heavy glue joints that would come with doing it that way pretty much cancels out any “extra weight penalty” that might come with using shaped solid (light) balsa blocks. What’s going on here is that I have measured and cut out rectangular pieces (blanks) of a sheet of ¾” balsa that are just big enough to enclose/cover those nacelle fairing outlines on the plan. Doing this leaves me with cutting out curvatures in several dimensions to deal with. I began by making a rough paper pattern (off camera) to transfer the curve created by the wing surface to what will become the lower /mating surface of the fairing block.

 

B-17-9-2 Again while you weren’t looking I used my bandsaw to cut away the extra wood below that line. (That on-edge operation was a fussy job that demanded all my attention, so I didn’t get any photos.) What you see here is the still-flat upper surface of the block with the paper pattern I cut from the plan aligned over it. Tracing a reliable cutting line with a pencil along an unsupported edge of a paper pattern isn’t easy, so I used a drawing curve for support.

 

B-17-9-3 Now it’s clear how all that is supposed to work. It’s a simple job to cut out the partially shaped fairing block along the traced line.

 

B-17-9-4 A test-fit comes first. I’m going to need adhesive (Deluxe Materials Aliphatic Resin) only on the portion of the front face of the fairing that meets the rears former of the nacelle…so…I have traced a penciled outline to mark it off.

 

B-17-9-5 Aliphatic resin glue needs to be applied heavily enough to form a “wet”, filled-in joint to work properly. Here I have applied a generous coat to the fairing base and “fingertipped” it smooth. Note: with this much glue on one of the joining surfaces there’s no need to put any on the other.

 

B-17-9-6 And then I just line it up and press it into place.

 

B-17-9-7 According to the full scale aircraft woodworking manuals a clearly visible “squeezeout” guarantees that you have thoroughly wetted the joint. After pressing the fairing firmly into place that looks like this.

 

B-17-9-8 The next step is to wipe away as much as possible of the squeezed-out glue. A “fillet” of aliphatic resin adds no appreciable strength…just weight.

 

B-17-9-9 A building weight is the best way to keep “clamping pressure” on this particular joint. Although this glue product is relatively fast drying I always leave an assembly like this overnight before handling it.

 

B-17-9-10 While that assembly is minding its own business at the other end of the bench, it’s time to start working on the motor mount adaptors that fit between each of the Cobra 2217/20 outrunner motors (from Innov8tive Designs) and its respective firewall. Pat Tritle designed this model around other (brushed) motors and included all the necessary laser cut motor mount parts for them in the kit, but there would be unsuitable on several counts for the motors I’m going to use. I need a sturdy, flat plywood mounting face to accept the four-bolt mounting flange that comes with each Cobra motor AND a stand-off of exactly the right dimension to place the face of each prop driver just the right distance (about 1/8” in this case) ahead of its respective front cowl. If you study this image of my marked-up plans you can see the outlines and dimensions that I derived and drew in to do that job. You have all the info you need right here to make your own mounts if you are using the same motors. You can see two finished mounting faceplates and one pair of standoffs as well.

 

B-17-9-11 More motor mount parts…I’m holding one standoff next to the mounting faceplate I have to join it to.

 

B-17-9-12 That joint has to be at a right angle (square) to fit right. Here I’ve improvised a simple jig using one side of a miter box and the bench surface to define that angle. Carefully applied finger pressure holds the parts in place while I run a generous bead of Deluxe Materials Roket Hot along the joining edges…this will grab FAST and permit me to let go of the assembly right away.

 

B-17-9-13 I turned the assembly end-for-end with the securely glued joint on the “outside” and aligned the opposite standoff plate in my “jig”. ( I made the remaining three mounts in the same way.)

 

B-17-9-14 I’m using the 1/8” plywood firewall/formers exactly as they are supplied in the kit. The pre-cut cooling air/power wire access holes will work with my newly-developed motor mounts if I line everything up correctly. A moment of study should make it clear how I arranged those alignment reference marks.

 

B-17-9-15 Again I’m going to depend on Deluxe Materials Roket Hot for reliable assembly of the mount to the firewall. See how all my various reference marks line up.

 

B-17-9-16 Here’s an advance look at all four motor mount assemblies with one of them held loosely in place near the No. 4 firewall.

 

B-17-9-17 Roket Hot is strong stuff and it would probably keep those mount assemblies together on its own…but…a motor mount is a set of really critical working joints and I want to go one better. Here I’m using small pieces of Deluxe Materials 2 oz./sq. ft. fiberglass cloth as reinforcing patches to bond each mount assembly together AND keep it attached to its firewall. I’m using Deluxe Materials Speed Epoxy (60 minute, which is a slow-setting, tough-holding formulation) thinned with just enough denatured alcohol (about 15% by volume) to make it easy to brush on and fully penetrate/wet the cloth weave.

 

B-17-9-18 As a last touch on one of the firewall/mount assemblies I’m using this short, stiff brush to make sure the glass cloth is seated completely into the 90 degree corner joint and thoroughly wetted with epoxy. With that done, this becomes another of those jobs I go away and leave overnight before handling it again.

 

B-17-9-19 Next morning it was easy to clean up all the glass cloth overhang with a 100-grit sanding block. If you look closely you can see that I have not yet “opened up” the mounting bolt holes.

 

B-17-9-20 This assembly is a pretty straightforward job of applying Deluxe Materials Aliphatic Resin generously and using ordinary masking tape to hold the joint I alignment…with each firewall/mount assembly centered on its nacelle front…while I go away and leave it again.

 

B-17-9-21 Here’s what all four assemblies look like taped and set aside to dry. (Yes, that’s one of my several “shop helper” cats in the background, leaving me alone with the airplane for the moment because she has already had her fill of attention.)

 

B-17-9-22 The original kit design includes “cowl rings” that are laminated into pairs and attached to the front of each firewall to make the various relationships between model-sized engineering and scaled dimensions come out right. I’m going to use them exactly as intended.

 

B-17-9-23 These rings are cut from ¼” balsa sheet and are intended to be double-laminated to form ½” deep assemblies that will be attached to each firewall. Here I’m applying a generous bead of Deluxe Materials Roket Rapid (for open joints) to one of the cowl ring surfaces.

 

B-17-9-24 Holding a pair of rings firmly together (with the grains at right angles) for a minute or so is sufficient to assemble each laminated pair.

 

B-17-9-25 What happens next is that I used more Roket Rapid on the rear face of each cowl ring assembly to stick it firmly in place against its respective firewall. As with thee firewall-to-nacelle assembly, the fit may not be perfect. Center everything as closely as possible.

 

B-17-9-26 Remember that when I decided to skin/enclose the stringered nacelles with 1/16” balsa sheet I created some extra thickness/width of structure that would demand compensating measures later on? Here it is. I have to “make up” that 1/16”around each of the firewall/cowl ring assemblies that were designed to fit the open-stringered nacelle design. My first step is to clean up what will become another joining surface around the circumference of each nacelle front.

 

B-17-9-27 My next step (off camera again) was to cut a supply of ¾” wide strips of light/soft balsa sheet cross-grain to permit easy wrapping around each nacelle. Remember those wide sheets of 1/16” balsa sheet that I used for wing skins? I used an 8” wide offcut from them to cut strips that turned out to be almost long enough to enclose each nacelle front in one piece. Here I’m holding one end of the first strip in place “dry” to check the fit.

 

B-17-9-28 Remember all those water spray treatments we’ve done on balsa sheet to make it easy to bend? Same deal here. I wetted the strip, applied more Aliphatic Resin around the entire circumference of the narrow portion of the nacelle and the wrapped the wet balsa strip firmly against and around the wet-glue surface. Long strips of masking tape in tension around the assembly pull the wet balsa tightly against the nacelle base and hold it there for drying. Remember also that balsa we assemble “wet” will shrink appreciably as it (and the glue) dry…this results in everything pulling together into a really tight finished assembly.

 

B-17-9-29 This is how the No. 3 nacelle looks all taped. In this shot the glue has dried and I have already pulled the tape from No. 4.

 

B-17-9-30 Getting the fit perfect here would be great, but that doesn’t always happen. Can you see where my failure to pull the wet 1/16” balsa sheet firmly enough against the nacelle structure has left some voids? If you look carefully you can see that where I’m applying firm squeezing pressure with my thumb, the void has closed up? I went around the front of each of these assemblies, inch by inch, and ran a generous shot of Deluxe Materials Roket Hot (fast) into each void in turn as I held it closed.

 

B-17-9-31 With that done each of the circular joints should look like this.

 

B-17-9-32 The next step was to cut out and install all the rest of the aft nacelle fairing blocks. Each of these went in just like the one I’ve already shown you. Here are the upper fairings on No. 1 and 2 nacelles with building weights in place for clamping pressure.

 

B-17-9-33 The next step was to cut out and install all the rest of the aft nacelle fairing blocks. Each of these went in just like the one I’ve already shown you. Here are the upper fairings on No. 1 and 2 nacelles with building weights in place for clamping pressure.

 

B-17-9-34 The rear portion of each of these two fairings is going to be removable along with the flap servo cover plate. What’s the best way to shape them? I’m going to glue the front/upper surface (ahead of the flap servo opening) in place to secure the entire fairing block so I can work on it, shape the entire fairing, and then cut it off across the front of the opening, to be attached to the cover plate (and become part of it) later. You’ll get to see this later. Right now all the bottom fairings are in place and the glue is drying.

 

B-17-9-35 Top side again. These are nacelles No. 3 and 4 with the front circumference filled in with those 1/16” balsa sheet strips and the partially shaped aft fairing blocks in place. The next step (next time) will be to create a prodigious mess of balsa shavings and sanding dust to finish the job of shaping each nacelle.

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