Going All-Out With A Classic Balsa B-17-F – Part 13

Going All-Out With A Classic Balsa B-17-F – Part 13

Now that I have a fully framed -up fuselage assembled to the stage that I can actually set it in place against the wing, the logical next step might be to get to work “finishing” it by starting the process of balsa sheet planking/skinning. But… and you saw this coming, didn’t you… there are a bunch of “widgets” that go inside the fuselage that would be difficult (if not impossible) to work on with the completed outer surface/skin in place. What that means is that I have to give some careful thought to what will still be accessible after skinning and what has got to be installed (and working right) before we get to see what a closed-up fuselage is going to look like. The original design…per the kit…calls for mounting the wing to the fuselage using a pair of long ¼” x 24 nylon screws, one at the leading edge and the other at the trailing edge. A lower center section fairing gets built to fill out the bottom fuselage/belly contour beneath the wing. There is no provision, by the way, for a “knock-off” wing mounting to lessen damage in the event of a crash. I’m not sure I could design one myself that would actually work and not look ridiculous on what’s supposed to be a scale warbird. Let’s assume we all agree that this is OK because very few people are likely to be using a B-17 as a primary trainer. With the exception of the balsa sheet covering/skin I’m adding, this part of the airplane follows the kit instructions.

 

B-17-13-1 This is the bare, framed-up fuselage with the fully sheeted wing dry-fitted in place as viewed from the bottom/front. The lower section of the F-4 former fits into place like this…the only change I have made was to bevel the joining edge of the former a bit deeper to compensate for the thickness of the 1/16” balsa sheet that I already added to the wing.

 

B-17-13-2 When that’s done right the laser cut stringer notches in the former section I’m adding will match perfectly with the notches in the already-assembled section. When that’s done it looks like this.

 

B-17-13-3 I fitted the bottom F-7 section at the trailing edge the same way. For this assembly I chose to use Deluxe Materials Roket Rapid (for open assembly) to avoid the possibility of any excess application of adhesive penetrating through the joint and “grabbing” the fixed structure at F-7. (Yes, I could have dealt with that issue by putting some plastic wrap “in between”, but I chose not to do that in the interest of an unobstructed view of everything involved.)

 

B-17-13-4 I made a little change to the wing center section structure at F-4 and F-7. Those wing attachment screws will need something solid to “hold against”, and the 1/16” balsa sheet wing skin would not do that job. Here I’m using a drafting compass to draw out a circle of ½” balsa sheet that I’ll split in half and then trim to fit in place as wing screw reinforcements.

 

B-17-13-5 I cut out the circle and then split it into two equal pieces…one for the front and the other for the back of the center section. This is the front half before I beveled/tapered it to fit neatly between the F-4 former and the wing skin.

 

B-17-13-6 I have done that on both reinforcement inserts. Off-camera I also drilled the necessary holes for the attachment screws through each reinforcement and the entire wing structure and then on through the plywood mounting plates already built into the fuselage. After tapping those mounting plates for the ¼” x 24 screws I was able to assemble the wing to the fuselage mechanically for the first time. (NOTE: This is a really good time to re-check that everything is aligned correctly before adding any more structure.)

 

B-17-13-7 The lower sections of formers F-5 and F-6 are necessary to fill out the belly fairing contour. I transferred their locations to the bottom wing skin from the plan, relieved them to match the added 1/16” of wing thickness, and glued them in place.

 

B-17-13-8 This part demands care and judgment. The balsa sheet skin that is going to cover/close in these bottom wing center section formers (F-4 through F-7) forms an arc (part of a circle) in cross section to match the shape of rest of the fuselage. As viewed from the side, the upward-facing part of it where each edge of the balsa sheet skin meets the bottom surface of the wing must also follow a curve that matches the bottom skin contour. If I allow these edges to remain where they are marked on the original plan the skin will bulge (extend too far to either side) and spoil the flowing circular fuselage cross section. To fix that I have to draw in new lines to define those edges. In 3-D they would be exactly where the fuselage skin surface intersects the lower wing skin surface…a unique curve. If we were working in CAD we could define it mathematically, but that’s way beyond what I’ve chosen to use for this model. What I did (old tech!) was to replicate the curvature of that intersection from my scale three-view drawing on the actual bottom wing skin using a drafting curve and a pencil and then use those marks to create a paper cutting pattern to transfer that outline onto balsa.

 

B-17-13-9 It turns out that the outer ends of formers F-5 and F-6 extend too far out to match that newly defined edge. Here I’m cutting them off to match it (and I’ll taper them from each stringer to the wing skin as well) so that the resulting triangular shape will align each skin edge right on the new pencil line I just drew.

 

B-17-13-10 Off-camera I made that paper pattern to transfer my cutting marks to some of the 1/8”balsa sheet I’m using as fuselage skin. Here I’m getting ready to cut along one of those lines.

 

B-17-13-11 That intersection of the new balsa sheet and the existing wing skin is going to form a very sharp acute angle, so that the full thickness of the new sheet will be lying almost flat on its side. This is going to create a big, ugly bump in the finished surface unless we bevel the edge of the sheet like this. The cut-to-shape sheet of balsa I’m holding is going to get turned over so the beveled edge you see fits snugly against the wing skin and that bump turns into a very thin tapered joint.

 

B-17-13-12 I’m going to split that bottom sheet into two identical halves (left and right, along the centerline stringer) to make all the necessary bending and squeezing and gluing easier to keep under control. You can see both half-skins upside down, ready to join. At this point I’m using my old technique of applying Deluxe Materials Aliphatic Resin with a brush to get an even coating everywhere the new skin is going to touch the existing structure, EXCEPT…

 

B-17-13-13 …right along the centerline stringer. Watch. I’m adding a generous bead of Deluxe Materials Roket Rapid (for open joints) along the stringer as well as to what will become the joining edges of the half-skins.

 

B-17-13-14 See what happens when I set both half-skins precisely in place along the stringer? The Roket Rapid GRABBED those joining edges along the stringer without interfering with all that still-wet aliphatic glue waiting everywhere else beneath the skin halves. Now I can spray them with water for easy bending without worrying that the aliphatic resin will dry too quickly and then carefully form them into place with the securely fastened centerline joint as fixed base to push and pull against.

 

B-17-13-15 That looks like this…under what kids used to call a “playground pigpile” of building weights which is holding the entire assembly in place (without pins, which would tear up the wet wood without accomplishing much) while the wet balsa and glue bond it all into a nicely curved fairing. This is another of those overnight drying jobs. Later, we’ll have a look at how it turned out.

 

B-17-13-16 Some time ago I built the horizontal and vertical tail surfaces and set them aside while I built the wing and worked on the fuselage up to this point, where the next step is to install/assemble them. The horizontal stabilizer gets fitted into the fuselage structure pretty much as described by the kit instructions…a few pieces of fuselage stringers and formers have to be cut away so it will slide into place from one side. With that done I went through the old drill of ALIGNING EVERYTHING, checked my work with this big drafting square, and then used Deluxe Materials Roket Hot to grab and lock all those joining edges and surfaces together quickly, before any of it had a chance to move.

 

B-17-13-17 With all that done the tail section looked like this.

 

B-17-13-18 Time for more changes. As the model was designed the vertical tail was intended to be glued against the single 3/32” sq. balsa top fuselage stringer. I wanted a more substantial joint, so I began by cutting “flats” centered on the top of each of the formers that will contact the fin. Each “flat” opening matches the width of the completed vertical fin at that station, and they are cut deep enough that a strip of 1/8” balsa sheet cut to match the shape of the vertical tail will rest evenly across the formers. Here I’m using my 100-grit sanding block to finish off the cutout in the horizontal tail leading edge former.

 

B-17-13-19 Moving ahead along the top of the fuselage the modified formers look like this.

 

B-17-13-20 Here’s the 1/8” balsa sheet fin base insert ready to be fitted and glued into place. (I finished that job off-camera.)

 

B-17-13-21 Again I’m following the original instructions pretty closely with the installation of the rudder control horn. What you see here is a length of 3/32” hobby shop steel wire bent and cut per the plan, along with several odd bits of standard-issue model hardware. I found a servo arm in my “surplus stores” (scrap pile) that fit the dimensions of the tail structure. Watch to see what I did with it.

 

B-17-13-22 You are looking down at the top of the tail…the horizontal stabilizer has already been assembled, along with the new 1/8” balsa sheet vertical tail base I described in B-17-13-20. I have cut a small access hole in the fin base directly in line with the circular 1/8” plywood rudder horn doublers that were already added as part of the standard horizontal tail structure. Before getting to the point I’ve shown here, I drilled through both of those doublers with a 1/8” bit, which fits the outside diameter of those brass servo grommet inserts. The inserts fit snugly on the 3/32’ rudder horn shaft and will fit neatly into place in those holes in the plywood.

 

B-17-13-23 Seen from above this is how it comes together. The top brass grommet insert has been fixed in place in the upper plywood doubler, as has the bottom one you can’t see from here. These form a reliable bearing that permits the rudder horn to rotate freely without either binding or wobbling. The 3/32” wheel collar is already locked in place on the straight section of the horn wire at the correct location to permit the 90 degree-bent end to fit the pre-existing hole in the rudder while the wheel collar “bottoms” against the upper brass bearing.

 

B-17-13-24 With the top portion of the assembly in place, the bottom part looks like this. Remember that by bottoming the wheel collar against the bearing we ensure that the “working” part of the horn will match the hole in the rudder that allows it to do its job. What’s happening here is that I have installed the bottom 3/32” wheel collar snugly against the bottom bearing and I’m holding it in place while using a long Allen wrench to tighten the set screw that will keep it there. (Again off-camera, I added a drop of Loctite to each wheel collar and set screw to keep them from getting loose inside the airplane when I’m not looking.)

 

B-17-13-25 OK…how do I get the control input horn (that servo arm) to stay attached to that 3/32” steel horn wire? If I had pre-bent another 90 degree angle onto the lower end I wouldn’t have been able to assemble it all the way I just did. Trying to solder a connection up there inside the balsa structure would be a good idea. What I did was to grind a small flat on the lower end of the horn wire before assembling it to this point, press-fit this extra wheel collar into my spare servo arm, and lock it into place inside the fuselage with the standard-issue set screw. (You’ll get a better look at the whole assembly in a moment.)

 

B-17-13-26 Now we’re working on the tailwheel assembly. To put this in the correct scale location for a B-17 it is necessary that it be located separately from the rudder horn hardware. Here you see the tailwheel assembly mounting plate that I’ve made from a piece of 1/8” plywood cut to fit inside the fuselage along with a doubler of more 1/8” plywood that supports the tailwheel shaft bearing. NOTE: I made that stuff exactly the same way as I did the rudder horn assembly.)

 

B-17-13-27 Here’s another look at the tailwheel strut all bent to shape along with the tailwheel itself and the modified servo arm top end that’s about to go into place.

 

B-17-13-28 The mechanics of this widget work just the same as those of the rudder horn assembly.

 

B-17-13-29 Here’s the modified servo output arm for the tailwheel with the wheel collar already pressed into place.

 

B-17-13-30 Off-camera I glued the plywood tailwheel mount into place inside the fuselage, then slipped the servo arm down into place on the top side of the mount. (It would not fit through all those stringers into the fuselage with the arm already attached.) As with the rudder horn I’m tightening that wheel collar set screw to keep it there.

 

B-17-13-31 All in place the tailwheel assembly looks like this.

 

B-17-13-32 This is the finished rudder horn/drive assembly seen from underneath, looking toward the nose. On the picture’s left you can see the clevis-control rod assembly that links all this to the tailwheel steering mechanism we just worked on. The separate connection in the middle is the elevator horn, and on the right is the carbon fiber tube and clevis fitting that make up the push-pull connection to the rudder servo.

 

B-17-13-33 When I installed the tailwheel assembly there were a couple of squared-off corners of plywood sticking out through where the balsa sheet fuselage skin is going to be, and they needed to be trimmed off. In this case a powered rotary sanding drum is the best way to cut off that tough plywood without putting too much stress/pressure on the supporting balsa fuselage structure.

 

B-17-13-34 Here’s the whole deal in place. The carbon fiber tubes (there are two of them) are the rudder and elevator push-pull rods.

 

B-17-13-35 NOW I get to play with that classic, characteristically shaped B-17 vertical tail! To reinforce what would otherwise be a relatively weak sheet-to-sheet glue joint I located three short pieces of 3/16” dowel in the fin base and drilled holes to accept them in the 1/8” balsa mounting plate.

 

B-17-13-36 With plenty of Deluxe Materials Aliphatic Resin brushed in place, I lined up the fin installation and squared it off just as I did with the horizontal tail. The blue tape is doing the job of holding everything in place; the light diagonal “brace” is just to ensure that the alignment is maintained while the glue dries.

 

B-17-13-37 I spent A LOT of time trying to figure out the best way to sheet/skin this particular fuselage. Here’s how it all came out. I’m starting with the nearly cylindrical “tailcone” behind the vertical fin trailing edge (this will become the structural base upon which I’ll build the tail gunner’s enclosure). I’m going to skin it in two halves just as I did the wing center section bottom fairing, but in this case each half-skin will wrap all the way around to the opposite side. This is the first sheet of 1/8” balsa already cut and fitted and trimmed to attach to the top center stringer and wrap around to the bottom center.

 

B-17-13-38 Here I have locked down what becomes the defining edge of the assembly using Deluxe Materials Roket Rapid. What’s important here is that I can now push, pull and twist the panel as necessary without having that “primary” joint slip.

 

B-17-13-39 I did the same thing with the “equal-but-opposite” right panel, then water sprayed the entire outer surface and…

 

B-17-13-40 …with plenty of Deluxe Materials Aliphatic Resin brushed onto all the joining surfaces, I did a really tight masking tape wrap of the entire assembly, then performed my usual ritual of letting it dry overnight.

 

B-17-13-41 Remember that I promised you at the beginning that I would confess if I messed up? I did here…those two tailcone half-skins got cut off too short to meet on the opposite side like they were supposed to. That’s no big deal to fix, though. I cut an insert/patch from more of the same 1/8” balsa sheet, sprayed the entire joining area with water, brushed on more Aliphatic Resin…

 

B-17-13-42 …and then taped it all back together as tight as I could get it.

 

B-17-13-43 At the far right you can see where I’ve left that last “tape job” in place for now. Here’s where the real fun starts. I have decided to use one long piece of 1/8” balsa sheet running all the way from the nose (F-1) to the tail cone to get the strength/stiffness/stability that comes with having that uniform structural component extend the entire length of the fuselage. (Yes, I am going to install BOTH the left and right sides together to avoid twisting everything out of line with uneven stresses). What’s happening here is that I’m dry fitting the tail end of the left side section in place.

 

B-17-13-44 Trust me…there was too much going on while I worked on the assembly to this point to take any pictures! This is the fuselage with both the right and left main side sheets I talked about in place…wetted, glued, and clamped/taped very securely into place. Would you believe that this is another of my “let-it-dry-overnight” deals?

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