Going All-Out With A Classic Balsa B-17-F – Part 19

Going All-Out With A Classic Balsa B-17-F – Part 19
I’m going to pick up right where we left off the last time we got to work on the B-17. You may remember that I explained why I felt that those really lightweight plastic cowls needed some stiffening so they’d stand up well over time as the airplane gets flown. I poured some reinforcing resin mixture into the front of each molding…now that is thoroughly cured and we can get on with the additional details that will help them look like B-17 engine cowls instead of molded plastic model airplane parts.

 

B-17-19-1 When I cut the excess plastic out of the front opening of each cowl I left a generous margin “to cut on”. I traced those red circles onto each cowl to mark the outside limit of each cowl’s front opening as defined by my scale reference drawings. This is another of those places where I did not want to trust the accuracy of the cut lines and opening diameters suggested on the original kit plan. Take a close look at the rear edge of the cowl at the bottom in this photo…I chose to make a significant deviation from the original design here. The kit plan suggests that you mark the outlines of the cowl flaps onto the finished surface of each cowl. You can see where I’m going here, right? Details like cowl flaps are attention grabbers, and this airplane needs something better. I cut off each plastic molding where the leading edge of the cowl flap ring is supposed to go, then measured and glued in a ring of 1/64” plywood that fits INSIDE the remaining plastic cowl body and extends to the rear to where the cowl flap trailing edges are going to be.

 

B-17-19-2 With the front opening circumference defined/marked I used a coarse rotary bit in my drill press to cut away MOST of what turned out to be excess material in the rounded front portion, or lip, of each cowl. Here you can also get another look at my new cowl flap base rings.

 

B-17-19-3 In the past you’ve heard me use the term “leave something to sand on”. Here’s how that works. I made another custom sanding tool with 100-grit production paper wrapped around an old 4 ounce dope bottle. The three cowls “behind” have been finish sanded to exactly the cowl lip profile I want, to match the scale drawing.

 

B-17-19-4 That last sanding job left a sort-of-sharp edge on each cowl lip. On the full scale B-17 that edge of made of formed sheet aluminum and it has “rolled” back on itself just far enough to enclose the sharp edge. I’ll replicate this on my model by hand sanding a corresponding “rolled edge” with some 100-grit production paper. (I’ll do some finer finish sanding here later.)

 

B-17-19-5 I’ve been busy off-camera again. While you weren’t watching I gave each of the cowl assemblies a generous spray coat of Stits PolySpray aluminum primer and set them aside for a while.

 

B-17-19-6 OK … here comes one of those deals where I am making this stuff up as I go along. Those squared-off motor mounts do a finer job of holding the motors but they won’t even come close to serving as anchor points for mounting the cowls. Here’s what I did about that. What you see here is a stack of four 1/16” plywood blanks that are big enough to contain a circle that’s the diameter of the inside of the cowl flap rings. I have all four taped into a single stack and I’m using the compass to mark that diameter onto the top surface.

 

B-17-19-7 I cut all four stacked blanks on my scroll saw and then tightened a machine screw through the center holes to lock them together, then used this simple lash-up to finish all four outer edges smooth and match them exactly to the inner cowl flap rings.

 

B-17-19-8 Again off-camera I measured and marked a series of lightening holes that also serve to increase cooling airflow. I gang-drilled the blanks and then used the scrollsaw again to cut out those center openings. Here I’m sanding in the finished inner diameter of each cowl mount. If you look closely you will see a 1/8” wide rim or flange on each mount. I messed up and cut the outside diameters about 1/16” short…adding these rings of 1/8” x 1/16” basswood is a neat way to make it look as if they were always supposed to be that way.

 

B-17-19-9 This is a finished cowl mount. See what happens to it next.

 

B-17-19-10 I centered each of the four cowl mounts on the 1/8” plywood face of each motor mounting box and added screw anchor blocks of ¼” x 3/8” spruce. (Sorry about that foreground focus.)

 

B-17-19-11 And … this is how it all fits together. You’ll get to see how the three radial mounting screws fit on the outside of each nacelle in a bit.

 

B-17-19-12 This is another view of one of the engine cowls (No. 3, right inboard) assembled into the exact location I want it to be and secured by those strips of masking tape.

 

B-17-19-13 Earlier I mentioned an extension of the centerline of each of those 1/4” x 3/8” spruce cowl mounting blocks back onto each boot cowl, but I think I forgot to show you this. By extending each of those lines forward…and back onto the outer surface of each cowl…and then measuring forward ¼” from each cowl trailing edge, I can do a precise job of locating the holes for those mounting screws. Here I’m using a 1/32” bit in my Dremel tool to drill the first of those holes right through the cowl and into the mounting block just inside it.

 

B-17-19-14 I will use flathead wood screws to hold the cowls in place. By gently using a countersinking bit …finger pressure only…I can open up each hole so the screw head will lie flush with the surface.

 

B-17-19-15 Starting one of those screws looks like this…

 

B-17-19-16 …and set all the way into place they look like this. (Sorry about that foreground focus).

 

B-17-19-17 Representing the cooling flaps on each cowl comes next. These cowl flaps are reproduced in the original kit as flared-out extensions around the trailing edge of each plastic cowl molding. “Doing cowl flaps” this way is a technique that shows up in a lot of kits…but I can’t get over the idea that done that way the finished job still looks like flared-out plastic flanges. We can do better. Remember how I cut those flanges off each cowl molding several sessions back and replaced them with 1/64” plywood inserts wrapped to match the open rear end of each cowl? Here’s why. Full scale cowl flaps are rectangular pieces of sheet metal curved to match the outer circumference of the main cowl, so that’s how I’m going to make them on this model. I’m going to use some .007” aluminum sheet for the flaps, and to make the surface of each one lie flush with the corresponding cowl surface I’m adding a spacer of 1/8” x 1/16” basswood that I have wetted and wrapped tightly around the front edge of each plywood insert like this.

 

B-17-19-18 I added a similar rear spacer to each cowl, then did some careful fine sanding to ensure that the collective outer surface of each ring lies .007” below the front cowl surface to match the thickness of the aluminum pieces I’m going to add next. At this point I’m using an ordinary emery board as a precision fine sanding tool to get that clearance just right. Note that the 1/16” thickness of the basswood spacers was intentionally too thick so I would have “something to sand on” for this final adjustment.

 

B-17-19-19 Here’s the No. 3 (right inboard) cowl with both spacer strips glued in place and sanded to fit. In front of the wing is a piece of the sheet aluminum that I’m using for the cowl flaps. Can you see where I have already scored it and begun to bend the strip at the rear to break it free? The width of that strip matches the fore-and-aft width of the cowl flaps I’m going to cut from it.

 

B-17-19-20 They go together like this. I used Deluxe Materials Roket Max (thick) to do the actual glue job, one cowl flap segment at a time.

 

B-17-19-21 It looks like this from the other side. Note that the top/center “flap” is actually a longer/deeper fixed panel that extends back beyond the moveable flaps.

 

B-17-19-22 No excuses…making and assembling four complete sets of cowl flaps takes a while. Because I knew my work wasn’t going to be perfect I left some extra material along the collective cowl flap trailing edges so I could use my 100-grit sanding block to cut them off even. (I smoothed off those coarse edges later with finer abrasive paper.)

 

B-17-19-23 All cleaned up and mounted temporarily, the No. 3 and 4 cowls look like this.

 

B-17-19-24 Now it’s time for an entirely different “scale detail” that will use more of that .007” aluminum sheet. There are two landing lights on a B-17…one set into the leading edge of each wing beyond each of the two outboard nacelles. I have already located and cut out the rectangular openings for both of them. Here’s what comes next. As you can see, this latest “aluminum bit” is cut to match the width of the landing light opening. None of this detail is shown on the original plan…I derived all the specifications from my scale drawings and photos.

 

B-17-19-25 This one is easy to figure out…I cut this blank over-length and then hand-bent it to match the appearance of the lights in my references.

 

B-17-19-26 Next I dug around to find some little light bulbs that looked as though they ought to be here…you’re on your own on this one…and drilled appropriate holes to insert them in what became the reflector/housings of each light assembly.

 

B-17-19-27 I assembled everything to this point using more Deluxe Materials Roket Max and lots of well-calculated finger pressure.

 

B-17-19-28 I think you could see this part coming…we need side panels for each landing light housing.

 

B-17-19-29 Those go in place like this.

 

B-17-19-30 When your adhesive has cured hard it all looks like this…oversize and in need of trimming. I found that making all the parts oversize this way and trimming everything to size later was the best way to get properly fitted details.

 

B-17-19-31 For that I did this. Little scissors/metal snips are my favorite way to trim excess sheet aluminum (and other similar stuff) as close as possible to the main surface before I subject the assembly to the stresses imposed by a sanding block…

 

B-17-19-32 …like this. The emery board is my tool of choice again here.

 

B-17-19-33 If you look carefully you can see the sheet of clear plastic I have wrapped around the leading edge and glued in place with more Roket Max. I held the clear panel in place with a wide, tight wrap of masking tape until everything had cured hard. With that done I removed the “holding tape” and cut a smaller piece to the exact size necessary to represent the actual landing light openings on the full scale B-17. The narrow “rim” of plastic not covered is going to be puttied, faired and blended to flow into and match the rest of the leading edge … later.

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