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

Going All-Out With A Classic Balsa B-17-F – Part 17
It has been a while since we did any work on the B-17. Several months ago I hinted that there might be some changes in the works, and left it at that. In fact I have been busier than you want to know with some heavy-duty family responsibilities. FLY RC editor Matt Maziarz agreed that this would be a good time for him to make use of an “extra” project we had been waiting to find magazine page space for, and I decided that I would take advantage of the same opportunity for rcmodel.com. That’s how you got to check out my PT-19 project over the last four months. With that said, I can assure you that the B-17 is alive and well; indeed, I am heading into what you might call the home stretch of the project. With every cool “scale detail” I work on, I keep hearing a little voice calling out, “Finish me, finish me!” I’ve learned not to ignore such things, so it was high time to dust off all the B-17 components and get back to work filling in the last structural details of the wing as promised.

 

B-17-17-1 Let’s begin with a good look at the airplane as it exists so far. If you squint a bit it’s not so hard to imagine that with just a few more components stuck in place I could fly it. Adding four motors and cowls along with control surfaces would just about do the job, right? Actually, yes, but there’s a huge difference between a model airplane that will fly and a SCALE MODEL AIRCRAFT that will stop people in their tracks to get a better look at it. That’s the part I’m getting started with now. Try keeping track of all the “extra” stuff that still needs to be done and how much interest and appeal each “mini-project” adds to the completed airplane.

 

B-17-17-2 “Way back when” I built the main landing gear/retract mounting rails into the inboard engine nacelles, and explained that to achieve a smooth, consistent job of adding the outer surface skins it would be better to “sheet right over” each wheel well opening and close up nacelles No. 2 and 3 with uninterrupted pieces of balsa sheet. You can see here how well that worked. This is the No. 3 (right inboard) nacelle and base cowl, and it’s pretty close to being a “no excuses” job so far. The opening you see to the right/rear is the right flap servo access with the cover off. The little “x” a few inches toward the wing leading edge is the locating point for the centerline of the extended strut, which I located by transferring the relevant dimensions from the plan.

 

B-17-17-3 The purpose of that little mark is to permit me to cut a trial opening through the nacelle skin without risking making a hole outside of where I want the edges of the finished wheel well to be. With a small reference hole made to allow me to make accurate measurements from the actual retract gear unit it was easy to cut and trim until I had this slightly oversize opening in exactly the right place.

 

B-17-17-4 Now all that makes sense. You can see where I enlarged the wheel well opening here and there for the clear access I needed to install the RLG unit accurately. A few steps further on I’ll show you how I corrected the oversized hole to get the correct scale wheel well shape.

 

B-17-17-5 Here’s the right main retract unit with its wheel attached, all mounted with the strut and wheel in the fully retracted position. Remember that the lower portion of each main wheel is supposed to remain outside/below the cowl circumference on a B-17.

 

B-17-17-6 I’ll get back to completing the wheel well openings in a bit, but first I want to finish closing up and fairing in several other holes in the airplane that still need my attention. I’m going to close up all the servo access openings on the bottom of the wing along with completing the bottom aft fairings sections of the inboard nacelles. First, though, I’m taking advantage of this break in the action to put a fresh sheet of 220 grit production paper on the block and go over the entire outer surface of the wing…again. It’s difficult to sand too much in places like this and all too easy to overlook “ugly seed” rough spots you didn’t notice before.

 

B-17-17-7 This is the underside of the left wing behind the outboard (No. 1) nacelle. During a previous session working on the wing I built this inset servo cover plate frame from ¼” x 1/8” spruce, and now it’s time to make and install the cover plate. Can you see where the frame edges are recessed 1/32” below the surface of the wing skin? I did that so the cover plate I just cut from 1/32” plywood will fit flush when I’m done with it.

 

B-17-17-8 I found a stash of little (3/8”) flathead wood screws and decided to use them to attach all the cover plates. First I marked all four corners and drilled right through the frame with the plate in place, then I removed each cover plate and used a countersink bit to shape the outside of each mounting screw hole so those flathead screws will fit flush with the surface.

 

B-17-17-9 That looks like this. (This is the No. 1 nacelle again.)

 

B-17-17-10 Now we are looking at the wing lower surface behind the No. 2 (left inboard) nacelle. If you dig back through my earlier installments you’ll find that the lower/aft fairings of the inboard nacelles extend back and over the place where the flap servo cover plates have also got to be. Earlier I rough-cut each nacelle fairing full length and then cut off and saved the portion of each one that needs to be made removable as part of its respective cover plate. You can see just where this one is going to fit.

 

B-17-17-11 The first part of that fitting job was to cut the outline of the fairing block (using the scale reference drawings as a guide) and then make those part-circle cutouts at the front that were necessary to make everything fit. At this point the fairing block has been glued in place as you see it and becomes part of the servo cover plate.

 

B-17-17-12 That long, thin hobby knife blade makes those long, shallow accurate cuts I need to rough-shape the fairing easier, but I still have to rely on all that making tape to prevent nicks in what is supposed to remain smooth wing skin surface.

 

B-17-17-13 The next step is to use a relatively coarse (100-grit) sanding block to finish shaping the fairing.

 

B-17-17-14 All done, that looks like this.

 

B-17-17-15 Remember those main gear wheel wells that I left a bit oversize (and sloppy)? Here’s the fix. Again using my reference drawings to determine the correct shape, I cut out a paper pattern to match the scale inner contour of each wheel well opening and allow the outer edges to coincide with full scale sheet metal panel joint locations.

 

B-17-17-16 I traced and cut out two of those covers from more of that lithoplate (.007” aluminum), bent each to match the curvature of the nacelle, and used Deluxe Materials Roket Rapid (thick) to fix them in place. Due to the many small imperfections (mistakes) that I allowed to creep into the surface contour of each nacelle, I decided to use more of my Stits Lite Fill epoxy (the blue stuff) to build up a filler base into which I could sand a more accurate shape.

 

B-17-17-17 I let the Lite Fill extend over and around the nacelle front (the sub-cowl base) so I could use some more 100-grit paper freehand to correct the radius of the cowl lip.

 

B-17-17-18 All sanded out, the No. 3 (right inboard) nacelle looks like this. There is still some blue epoxy sanding dust visible (and a LOT MORE inside the wheel well where you can’t see it). My next step is to use my high pressure air nozzle to clear all of it away.

 

B-17-17-19 Now let’s play with some of the control surfaces…the flaps and ailerons need some more work. If you want to you can go back and check out how I got all those aileron hinges located and dry-fitted. The next step is to make and install the aileron control horn. On a relatively small, light model like this one I have had good success over the years making custom-fitted horns from cut-off servo outputs of various sizes. Watch…

 

B-17-17-20 I cut off one arm to the necessary size to permit about half of it to fit inside the mounting slot I cut for it (for secure mounting), and the rest to extend outside the surface of the aileron to serve as a control horn lever. As you can see it all lines up with aileron push-pull cable tube that’s already in place.

 

B-17-17-21 I used more Roket Rapid to lock the base of the control horn in place…note that I planned my cut on the servo output arm so that several of the little holes would line up inside the cutout so the adhesive would penetrate through the horn and help lock it in place. You can see how the cutout slot I made in the wing trailing edge is going to permit the horn to move forward as the aileron rotates on its hinges.

 

B-17-17-22 Once I checked the ailerons for free movement I left them loose for assembly later, and moved on to the flaps. Again you may remember that some time ago I delayed finishing the flap hinge job because I wasn’t satisfied that I’d made the best choice of hardware. Now I’ve got it figured out. I will use medium-sized tab-type nylon pinned hinges that will lie flat within the structure and permit a “clean” outer surface that will be close to the correct scale appearance. Because the flap well trailing edges and flap leading edges are made of spruce for durability, cutting hinge slots properly required that I drill 1/32” holes to define each end of each slot. With the holes drilled, I can use my No. 11 blade to open each slot like this.

 

B-17-17-23 This is the wing sub-trailing edge at the front of the left flap well. On order to seat the front tab of each hinge so that it will be supported by solid wood all around I am fitting them into the ¼” deep TE at a 45 degree angle. (Yes, I could have designed that structure thicker/deeper to seat those hinges properly, but at the expense of extra weight. The angled installation makes better use of the structure.)

 

B-17-17-24 All assembled, the flap looks like this. Note that you are looking at the bottom of the wing … these are “split flaps” and do not show at all from the top.

 

B-17-17-25 It was time to do something about all that balsa finish…while you weren’t looking I sprayed the entire wing with a generous coat of Stits Poly Spray aluminum primer. This stuff adheres very well to the Deluxe Materials EzeKote/fiberglass surface base that I built up earlier in the project. Here I am starting what is going to be an involved, demanding finishing process by dry-sanding aggressively with hand-held sheets of 220-grit production paper. This is one of those places where I chose to begin the sanding job “dry” to permit a better look at the surface as I cut material away from it.

 

B-17-17-26 More of the same…can you see here how I’m letting the sheet of 100-grit conform itself to the curvature of the leading edge? The appearance of the sanded-out surface of the No. 4 nacelle and the surrounding wing provide a realistic idea of how much of that first coat of Poly Spray primer has to be sanded off to get an acceptable finish without leaving WAY too much material/weight on the airplane.

 

B-17-17-27 There are more “scale detail” holes to cut before I do any more priming and sanding. Can you see the penciled-in locations for two of the many air intake holes that are going to be cut into the wing leading edge? As always I relied on my scale reference drawings and full scale photos to get them located right.

 

B-17-17-28 A 3/32” drill bit in my rotary tool is the best way to define the corners of each opening.

 

B-17-17-29 After that I can cut out each hole with my No. 11 blade.

 

B-17-17-30 An ordinary “fingernail grade” emery board makes an excellent sanding stick to finish defining and cleaning up the shape of each air intake opening.

 

B-17-17-31 After getting all those air intake openings right, right, I gave the entire wing a second (heavy) coat of Stits Poly Spray. What you see here is the primer surface completely dry, before any sanding. If you look carefully you can see that there are bumps, low spots and other imperfections EVERYWHERE. It happens that aluminum (silver) primer does the best job of showing where every surface imperfection is hiding…that’s why aluminum primer is the best choice if you want a really good finish on ANY airplane.

 

B-17-17-32 Now that I have a sufficiently thick primer base on the way, I can switch over to 220-grit wet-or-dry paper. The trick to making this stuff work best is to keep it REALLY WET. Here I’m just getting started.

 

B-17-17-33 Wet sanding makes a mess…here you can see the upper right wing surface before I start wiping off all that wet sanding sludge.

 

B-17-17-34 At this point my finish base is smooth enough for me to start working on some surface details. The scale drawing and photos show there is a sheet metal overlap joint around each of the curves where the engine nacelles meet the wing surface. There are a lot of ways to replicate such details. Here I’m trying something new…I’m using Deluxe Materials Micro Balloons to thicken (add body) to some Stits Poly Spray. Watch what happens.

 

B-17-17-35 I masked the location of each of those overlap joints around all four nacelles with several layers of 3/32” fine-line masking tape…the more layers of tape, the deeper/more prominent the defined edge will be, I used three layers here…

 

B-17-17-36 …and then painted on several quick coats of “silver stuff” to build up a layer at least as thick as the layered masking tape. Later I will sand this stuff even with the top of the tape on the inside of the overlap and show you how it works, but right now I’m waiting for it to get really dry and that’s enough for today.

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