Going All-Out With A Classic Balsa B-17-F – Part 1

Going All-Out With A Classic Balsa B-17-F – Part 1

For almost as long as there have been model airplane builders there has been the B-17. The first of them appeared in the late 1930’s. We aeromodelers as a group were around in significant numbers for only a decade or so before that. When the early-model Boeing B-17 bombers first appeared they did indeed represent the ultimate in military aircraft development, and the later, more sophisticated models that went on to operate all over the world during World War II maintained that tradition of excellence. Today a B-17 is anything but high-tech, but it would be tough to find any other airplane that has earned more respect and become more recognized over all those intervening years.

We aeromodelers have always been right there at the front of the line to get up-close-and-personally involved with any B-17 we can get near, and generations of model builders have announced their intention to take on one of the classic challenges of the world of model airplanes by saying, “I’m gonna’ build a B-17 and fly it!” As I prepare this material it’s easy to recall LARGE RC B-17 models – both gas and electric powered – that have appeared in the magazines and on-line, and there are several mid-size (6-foot-plus wingspan) ARF versions on the market waiting to grab your attention. I’m not up for another ten-foot-or-so model in my shop right now, but there’s plenty of room there for a smaller one … SO … I’m going to begin with the 75 ½” span B-17-F designed by Pat Tritle and kitted by Dare Hobby (now part of Brodak Manufacturing). Pat designed his B-17 as a big classic stick-and-tissue airplane intended for electric powered RC flying, and as he presents it the completed model is a wonder of lightness and slow, scale-like flight. I have built, flown and reviewed several of Pat’s other designs (check out my write-up the Dumas kit of the DeHavilland Dragon Rapide in the October, 2010 Flying Models Magazine.) I will never be able to match Pat’s skill at designing and building VERY LIGHT models that fly reliably, but I can come close, and I have discovered how much I enjoy pushing the envelope of his design philosophy just a bit in the direction of better scale detail and improved durability out in real world flying conditions. And yes, I have discussed this personally with Pat Tritle and have his blessing to use his B-17 as the starting point of my project. Quite a few of you have made it clear to me that you appreciate my taking the time to explain the “how” and “why” of  what I do in detail so I’m going to do that with this project as well. In fact, I’m going to take this series as a fine opportunity  to open my shop door wide and not just let you watch as I modify and build this airplane, but also share my thoughts as I make the decisions about what I feel needs to be changed.

What am I going to change? Let’s look at what I have to start with. As Pat designed it, this B-17 exemplifies built-up, stick-and-tissue “open” structure…the wing panels and tail surfaces consist of leading and trailing edges and ribs that enclose lots of open areas that need to be closed in and defined (covered) with tissue, and the fuselage works the same way with lots of stringers … also to be tissue-covered … around a series of bulkheads/formers that define the shape. A few sections of the structure, in high-stress locations, are “filled in” with 1/16” balsa sheet but most of the surface of the model is unsupported doped tissue.  This works out to be very light, and it flies fine, but it leaves something to be desired in terms of scale appearance. The entire full scale B-17 was skinned with sheet aluminum with the exception of the rudder, elevator and ailerons, which were fabric covered. I’m going to address this issue by adding a sheet balsa skin where there “should be” aluminum and simulate the fabric with doped silkspan. With one or two exceptions,  which I’ll address as separate issues, the actual shape … cross section and outline…of the model is pretty close to scale and won’t need to be changed at all. How I modify the structure with sheet covering without adding enough weight to spoil the performance of the model is what a lot of this series is about. Where scale appearance is concerned, the only other important deviation of the model as it is designed is the lack of a retractable landing gear, but (at least for me) that’s impossible to ignore…so…I’m going to modify my airplane to use an existing (that means available to buy if you want one) electric retract gear system. I’ll explain all the details of these modifications as we go along. Let’s get started building a B-17. 

 

Where do we start? In this case that will be with an inventory of the Dare Designs kit that I started working on over ten years ago and put aside to wait for the availability of better power systems (which turned out to be brushless motors and LiPo’s) to get my head around all the accessory goodies I’ve accumulated since I decided to revive (dust off) the project and use it as the subject of these articles. Here’s a look at the original (old) box label showing the model completed using the original doped tissue covering. That’s going to change.

 

This is where I stopped … ‘way back when. Built as per the original plan and instructions, the vertical tail is an open structure consisting of a laminated balsa outline (2   1/16″ x 1/8″ strips) filled in by those vertical “spars” and those two-part balsa strip ribs. Imagined that the “open” structure you can see continues all the way down to the base rib … as if the partial balsa sheet covering had never been started … and you understand what the vertical fin was originally supposed to look like Built that way it would be exquisitely light, but as I have explained, I chose to sacrifice a bit of that lightness for what I consider a big improvement in scale fidelity/appearance.

 

After cleaning off all that dust, my first task was to decide on the best way to closes that big gap in time by finishing the job of adding a sheet balsa skin to the rest of the basic fin structure. If you examine this image carefully you may be able to see that the section of skin already attached is/was 1/32″ balsa sheet. After much thought I decided to switch over to 1/16″ sheet for the rest of the job. At the expense of a very small increase in weight this is going to give me a much more substantial, “forgiving” surface to work on. I expect to sand away most of the difference between the old 1/32″ and the new 1/16″ sheet over the entire airplane. Here I’m marking the cut line on a piece of 1/16″ sheet I’ve chosen to “close up” the tail.

 

I’ve cut the new 1/16″ balsa sheet skins for both sides of the portion of the vertical tail that remains to be closed in from the same piece of balsa. This ensures that both will have the same resistance to bending and so minimize the chance of warping the structure. Here I have dry-fitted the left skin into place.

 

For that same reason I’m going to assemble both side sheets simultaneously. The first step is to use my favorite “skinny brush” technique to “paint” aliphatic resin glue onto every surface that will contact the new sheet balsa skin. Today I’m using materials from a new product line, Deluxe Materials.

 

Off-camera, I sprayed the outer surface of each sheet with water…this will increase its flexibility as regards bending to conform to the mild compound curve formed by the fin surface. By wetting just the “outside” face I get the balsa to pre-curve itself gently in the direction I want it to bend. Here I have placed the sheets on both sides of the fin, lined them up exactly with the pre-existing covering which I want them to match, and used a clothespin clamp to fix BOTH sides in place at the trailing edge. Notice that I’m using blocks, or pads, of scrap balsa to distribute the pressure of the clamp across the surface of the new skin to avoid denting/crushing of the wet balsa.

 

Here’s the same game at the leading edge.

 

For a job like this, use as many clamps as necessary to ensure that there are NO gaps between the new skin and the leading/trailing edges. You can see that I used clamping pads all the way around. Don’t leave any voids…any you miss will become structural defects and will be very difficult to conceal later on as well.

 

While that assembly dries, it’s time to work on the rudder. Under all the dust I found the original outer surface built up of a laminated outline, a substantial balsa leading edge, and 1/16″ balsa sheet ribs. In order to improve scale appearance I had already doubled the number of ribs called for in the kit. This meant that the spacing between ribs got cut in half…not an exact match to the scale spacing, but close and OK for the level of accuracy I’m shooting for on this airplane. One thing the original design overlooks is the rudder trim tab. I couldn’t get comfortable with that, so I used my scale references to draw in the tab location on the original plan with red ink.

 

I cut off the three lower ribs to create an opening the size of the trim tab PLUS a 1/8″ thick balsa insert “faceplate” for what becomes part of the fin trailing edge. Here I’m marking a piece of 1/8″ balsa to fit the new cutout.

 

Along with that new aliphatic glue, I’m trying out Deluxe Materials’ “Roket” cyanoacrylate adhesive line. Here I’m using Roket Hot (fast/thin)to finish up the assembly of the trim tab cutout.

 

All assembled, it looks like this. The cut-off portion of the trailing edge gets tossed. I’ll use doped silkspan on the rudder to replicate the fabric covering on the full scale B-17, but the trim tab was sheet metal covered. I’ll get to that in a bit.

 

Time for some clean-up with the sanding block. Here I’m beginning by trueing-up the vertical fin trailing edge with 100-grit paper on a block long enough to span the length of the working surface.

 

The next step was to sand back the outer skins to match the contour of the laminated vertical tail leading edge.

 

When that’s done right, the outline of the tail matches the shape shown on the plan. Notice that there’s a subtle curve at the top of the trailing edge that’s matched by the rudder leading edge shape.

 

The contour of the vertical tail tip I just showed you is called an “outside curve”, and I worked on it using the sanding block you’ve already seen. The leading edge, where it transitions into the dorsal fin, forms an ‘inside curve” and we need a curved sanding block to work on it without gouging/digging in at the edges and spoiling the job. You’ve seen me do this sort of thing before. Here I’ve chosen to use an old can of a convenient diameter as the new tool.

 

The first step in forming the leading edge was to trim the sheet surfaces even with the edge laminate to match the scale outline. Only when that part of the job looks like THIS is it time to go on to rounding and smoothing the leading edge.

 

The next step was to use the long flat sanding block to true up the outer surface of the sheet balsa skins. Doing that right became especially important on this job because I had to make “new” sheet balsa match with pre-existing old covering so that the transition will not show on the finished model. This joint passes the “fingertips test” when I can NOT detect the transition from one piece of wood to the other just by feel.

 

Now I have to play catch-up. The original balsa framework was designed to be tissue covered, and as such the thickness of the vertical tail and the rudder matched. Since I have added 1/16″ sheet skins to both faces of the vertical tail, I have to do the same with the rudder to avoid a “step” where the finished surfaces will meet. Here I’m using my well worn Master Airscrew balsa stripper to cut some scrap 1/16″ balsa sheet to 1/16″ and 1/4″ widths.

 

Before I get to use those strips, I have to trim and sand the existing rudder structure to a smooth surface. I’m using a No. 11 blade to rough cut those oversized “extra” ribs close to their finished shape.

 

Next it’s the turn of the sanding block to blend the original leading and trailing edge and ribs into a flat surface.

 

I used Roket Rapid (medium viscosity) in an open joint assembly to add caps made from those 1/4″ strips I just cut to the rudder leading edge.

 

Same game for the 1/16″ sq. capstrips that bring the surfaces of each of the ribs up to the required height.

 

I didn’t add any build-up to the trailing edge…it’s OK as originally constructed. What I DO need to do is to use the sanding block to cut a smooth transition from the new leading edge thickness to the original trailing edge.

 

Now it’s time to radius (round off) the rudder leading edge. I’m using a miniature block plane to start the job of shaping the original squared off structure.

Sanded to shape, the leading edge looks like this.

 

Back to the vertical fin. Now I’m sanding the finished radius into the leading edge. I’m starting with my straight sanding block on the outside curve formed by the tip.

 

For the inside curve I’ll go back to that custom round sanding block.

 

With the curvature/radius of the edge established, I’ll go back with a hand-held sheet of 100-grit paper and smooth out the entire surface.

 

Something I overlooked…the plan specifies inserts of 1/4″ balsa to serve as hinge point reinforcements on the inside face of the trailing edge. These have to go into place before I go any further.

 

That’s not all I overlooked. On the full scale B-17 there are two cutouts in the rudder leading edge to create clearance for the two hinge assemblies and I’m going to modify the model to include them because they are such an attention-grabber of a scale detail. The problem is that the existing rudder leading edge doesn’t go far enough back to accommodate doing that. The fix is for me to add tapered 1/16″ balsa sheet inserts at the rear edge of the T.E. to extend the simulated sheet metal structure. Here I’m marking where that insert will meet the 1/16″ capstrips I just finished adding to the ribs.

 

Now I’m cutting out a short section of each of those capstrips so the 1/16″ sheet insert will slip into place and match the height of the existing surface.

 

Here’s the insert, cut to shape and ready for assembly using more Roket Hot.

 

With one side assembled, the rudder leading edge extension looks like this from the inside.

 

I’m going to use medium size Robart Hinge Points here. You can see where this one is going to go. The circle drawn on the base rib locates the center of rotation of the rudder on the hinge line, and allows me to line up the hinges along it.

 

I’m using my old Dremel saw to cut the 3/16″ wide slots into the leading edge for those two Hinge Points.

 

I drilled a 1/8″ hole at the center of each hinge cutout and now I’m using a tapered wood rasp (file) to open the hole just enough to permit the hinge to slip into place.

 

That looks like this.

 

Now it’s time to make the sheet metal trim tab. I’m going to cut a “blank” from this sheet of soft/light 3/8″ sheet balsa to fit the plan.

 

With the blank held in place I’m using a pencil to mark where to cut it to match the thickness of the faceplate.

 

Now I have some carving and sanding to do. Here a nice long blade is just right for cutting back the surface of the entire blank to the correct thickness.

 

As usual I’m using a sanding block to finish that shaping job.

 

This is what the cross section/end of the shaped trim tab should look like.

 

Here’s the entire vertical tail assembly so far, dry-fitted. I’ll cut off those protruding ends of the Robart Hinge Points before final assembly. For now the trim tab is just pinned in place.

Did you notice some other parts in the background on the bench as we went along? Next time I’ll bring the horizontal tail assembly up to this same stage of completion, then get to work on the wing.

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