Going All-Out With A Classic Balsa B-17-F – Part 2

Going All-Out With A Classic Balsa B-17-F – Part 2

Last time I finished “picking up where I left off” on this airplane ten years ago, dusting off the partially-built vertical and horizontal tail surfaces, and bringing the vertical tail and rudder up to the stage at which they are ready to be assembled to the rest of the airplane when the time for that comes. In this session I’m going to do the same with the horizontal stabilizer and the elevator, and then move on to working out the first of what are going to be many structural modifications to the wing to bring my finished B-17 up to the standards I’ve described. With some of those decisions made, I’ll start the actual cutting and gluing that will leave me with a set of parts ready to go into the actual preliminary assembly of the wing panels.

 

This is the horizontal tail assembly as I left it back when. I had already finished sheeting the horizontal stabilizer, but the elevators (left and right halves) are waiting for me the way the rudder was, with a bunch of extra ribs “roughed in” as rectangular 1/16” balsa sheet inserts that still need a lot of shaping.

 

Here’s a closer look at the horizontal stabilizer…you can see where I left off with the top and bottom sheeting “squared off” with the leading edge, but with no further shaping of the airfoil.

 

The original designs calls for a pair of circular plywood reinforcements (where the rudder control horn wire is going to pass through) top and bottom on the fixed rear center section of the stabilizer. I’m marking their locations exactly per the plan.

 

I’m using one of several products here that are new to my shop…this is Deluxe Materials Aliphatic Resin (wood glue). You can see how I have located a center point for the reinforcing disc and marked its location for easy alignment. (I did the other side the same way).

 

A clothespin clip is just what I need here to clamp both the top and bottom discs in place while the glue dries.

 

While that’s happening I’ll work on the elevator. Remember that I added half of the ribs you see as rectangular inserts? That left a lot of squared-off corners sticking out, which I trimmed roughly to shape with a No. 1 blade. Now I’m using my 80-grit sanding block carefully/”by feel” to cut all those edges even with each other.

 

With that done I can go to a more traditional block sanding technique (against the building board) to cut the surface of all the ribs even AND bevel the edges of the balsa leading and trailing edges to match the angle defined by the taper of the ribs.

 

With the heavy cutting/shaping done I went to finer (100-grit) paper to work the entire surface of the elevator super-even and smooth. You can see from the sanding dust that I had to take off quite a bit of balsa to get it that way. Assembling a structure like this elevator a bit “oversize”, with extra thickness designed into the various structural parts so they can be aggressively sanded to a precise finished shape is an old model building technique.

 

With all that done and the upper and lower surfaces of both elevator halves shaped and finished exactly the way I want them, I’m using a model builder’s block plane (this one is from Master Airscrew) to rough-trim the beginning of the finished half-round cross section of the leading edge.

 

When I have cut away as much material (balsa) as I comfortable taking off with the plane, it looks like this. Finishing this shaping process is a sanding block job (which I finished off-camera).

 

Same sanding block, different part of the airplane. I’m using coarse (60-grit) paper to start the process of “rounding off” (cutting a neatly radiused curve) the leading edge of the horizontal stabilizer. Some kits may provide a pre-cut gauge/pattern for you to use to check the accuracy of a job like this. This Dare kit does not, but I’m comfortable using the old time sand-and-check, sand-some-more and check-some-more method.

 

When that leading edge curvature was just right, I switched to a finer (100-grit) block, blended the new contour back into the existing surface, and then went on to bring the entire top/bottom skin surface up to a better degree of finish.

 

Now I’m back to the elevators, which have already been shaped/sanded to the correct cross section and requisite smoothness. As with the rudder, I have drawn the elevator trim tab dimensions onto the original plan (that’s what the red ink is) and I have just cut free all of the original elevator structure that must “go away” to make room for the new tab.

 

When I did that I included an extra 1/8” in the cutout to allow for a 1/8” to allow room for new 1/8” balsa tab cutout faces (elevator sub-trailing edges). These are necessary to define the trim tab cutouts and give the elevator covering (coming later) something to be attached to. I am going to assemble them using Deluxe Materials Roket Hot CyA.

 

With those new sub-T.E’s in place and sanded to blend into the rest of each elevator, I can cut and fit the balsa sheet trim tabs. These will not be installed until after most of the covering and finishing work is done.

 

Now it’s time to work on the elevator hinges. On the full scale B-17, there are metal pin-and-bearing assemblies located on the lateral (side-to-side) centerline of a circle that defines the cross section of the elevator leading edge. This means that the hinge points must align somewhere behind the structural leading edge. In my opinion, Robart Hinge points (medium size) are the best way to achieve the appearance (and motion) I want to represent. (If this were a TOP GUN caliber model I’d be making custom hinges to keep the scale judges happy, but that’s not the case here). Replicating the scale shape of those LE cutouts means cutting far enough back into the leading edge to weaken it, so I’ve added some reinforcing blocks to distribute stress loads better. NOTE: The existing leading edge shape is just about right for “scale”. I don’t want these reinforcements to show through the covering, so I have recessed them by about 1/16”. You’ll see later how that comes together. Right now I’m using my old Dremel scrollsaw to make the fore-and-aft cuts that define those new cutouts.

 

With that done, it’s easy to use a No. 11 blade, cutting along the grain, to “pop” the cut-out bits free. The inboard hinge slot at the right is already done.

 

I have finished making those cutouts and drilled both the elevator LE and the horizontal stabilizer trailing edge for the Robart Hinge Points. Note that the working section of each hinge will lie behind the face of the stab TE so the radiused elevator leading edge will clear it just enough to rotate freely. Right now the hinge assemblies are “bottomed” in place in the elevator, but remain about ½” out from what will become their finished location in the stab TE.

 

Here I have the elevator-stabilizer assembly dry fitted to the finished position and I’m holding a finish-shaped trim tab next to its cutout.

 

Here’s the entire tail assembly…vertical and horizontal…dry-fitted as it will go on to become part of the finished airplane. I’m going to put all this aside for now…

 

…and it’s time to start working on the wing. The first part of this process is to modify the 1/8: balsa sheet wing spar joiner/reinforcements. The kit instructions call for reinforcing these with fiberglass cloth and epoxy. Although I don’t intend to stress my B-17 beyond what the designer had in mind, it IS going to be heavier, and that leaves me with the same problem…the wing center section is going to have to carry increased loads. I’m going to turn each of the 1/8” balsa sheet joiners (front and rear) into a composite structure using each original 1/8” balsa piece as a core, laminated to 1/64” aircraft plywood doublers on both sides with a piece of Deluxe Materials Carbon Fiber Tissue included in each joint. Here I’m getting started by tracing one of the four plywood doublers I’ll need onto a sheet of 1/64” ply.

 

While you weren’t looking I cut out four identical 1/64” plywood doublers (two each for the original balsa spar joiners, which are about to become spar joiner cores. I’m using one of them as a pattern to trace out a matching laminate of that Carbon Fiber Tissue. (I made four of those, too.)

 

It takes a firm touch with a sharp blade to cut the carbon fiber tissue cleanly. Here’s one of the pieces that will be laminated between one side of the 1/8” balsa core and one of those 1/64” plywood doublers.

 

There are a lot of adhesives that would work OK for bonding the plywood-carbon-balsa laminate together. On this project I’m trying out many of the innovative products from Deluxe Materials. EzeKote is advertised as an excellent medium for bonding fiberglass to balsa surfaces in place of the more traditional two-part epoxy. EzeKote is water based and for practical purposes it’s odorless and non-toxic. I already know from a previous project (http://www.rcmodel.com/2014/07/finishing-those-foam-arf-rc-scale-airplanes/) that it does that job exactly as advertised, so I’m trying it out here as well. Right now I’m brushing a wet coat of EzeKote onto one side of the 1/8” balsa core.

 

I laid the carbon fiber sheet squarely in place against the wet EzeKote, set the sub-assembly off to the side, and repeated the wet-EzeKote treatment on one side of one of the 1/64” ply doublers. I made no attempt to brush EzeKote directly onto the loose carbon tissue.

 

Off camera I assembled the balsa-carbon fiber-plywood laminate (making sure I used enough EzeKote to get an even squeeze-out all around the laminate when I pressed it together), then turned the assembly over to repeat the EzeKote-carbon tissue-ply sequence on the opposite side. You can see the carbon tissue overhang on the underside of the assembly and the carbon tissue and ply pieces that will complete the laminate on this side waiting for me to assemble them.

 

“Squeeze it and go away!” I have used lead blocks (building weights) to apply even clamping pressure across the freshly assembled laminate. This is the front spar joiner. I made the rear joiner in the same way and left both of them to dry hard overnight.

 

Now it’s time for some design and engineering work. In the interest of simplicity and lightness, Pat Tritle did not include flaps in his original B-17 design. As I explained earlier, I have my own reasons for including them, and that means that I must do some structural modification engineering to make a working flap assembly that will integrate smoothly with the rest of the existing wing structure. (I could design an entire new wing, but that would subvert the whole intent of the project.) Using my scale reference drawings as a source of data and dimensions, I determined that the B-17-F used split flaps (simple single surface flaps that hinge down from the undersurface of the wing and retract flat when not deployed). My first job was to scale the flap outline dimensions off the reference drawing and transfer them onto the full size working model plan. Because Pat Tritle used the correct scale aileron outline, I could simply draw in the flaps in their matching scale location without having to “fix” anything. The red line I’m pointing to denotes the leading edge (front) of the actual opening that I’m going to make in the undersurface of the wing…it includes space for the fixed supporting member that will go in along with the moveable flap. The other red line denotes where the trailing edge of the moveable flap surface will lie in relation to the actual wing trailing edge.

 

This is a detailed look at the penciled-in notes I made for the modification necessary to the main wing ribs (W-1 through W-6) for the flap to fit. See if you can follow my logic in working it all out.

 

Now it’s time to transfer those dimensions to the actual ribs that will need to be modified. This will involve removing a carefully marked triangular area from the bottom/rear of each rib.

 

Here’s a better look at one rib with the exact cutting lines drawn in.

 

How does the shape of the modification/cutout change as the wing tapers in chord and thickness from W-1 to W-6? By comparing two respective ribs it’s easy to figure out that the chordwise centerline of each rib has to be the reference we need to deal with…it won’t change from one rib to the next although the outlines will.

 

Here’s one rib (W-2) with the modification cutout made. If you look carefully, you can just make out that the outer edge of the rib underneath (W-3) changes. The cutouts will all begin on the centerline and become progressively more shallow as I work toward the tip.

 

Here you should be able to see how the depth of the cutout actually changes from one rib to the next. Not clear? Don’t worry … next time I’ll get started with the actual assembly of the wing and you’ll be able to see how it all comes together.

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