Going All-Out With A Classic Balsa B-17-F – Part 4

Going All-Out With A Classic Balsa B-17-F – Part 4

During our last B-17 session in the shop, I finished closing in the top surface of the wing using my favorite “wet balsa sheet” method. You may have noticed then that I made no provision for the cutouts in each leading edge that are going to be necessary when it’s time to add the basic structure of the four engine nacelles, nor did I explain my reasons for doing that. I saved that part for this session with you.

How best to arrange the sequence of doing the basic wing structure to balsa sheet skin to engine nacelle assembly is a problem that just doesn’t exist if you are building the airplane as originally designed. Except for a few 1/16” balsa sheet insets, there IS no sheet covering to be concerned with. However, we ARE going to sheet this wing and because each of those four nacelles constitutes an interruption of the long, smooth curved surfaces that make up the wing leading edges, I decided that it would be a good idea to invest some serious thought about how and when to complete each of what have become new steps in the process of building this wing. Because the wing structure as originally designed does not provide any built-in, reliable self-jigging or alignment features, I felt it would be a good idea to devote plenty of attention to keeping the various sections into which the nacelles divide each wing panel ALL IN LINE. Covering the structure with balsa sheet to “close up” each of those sections… inboard, between-the-nacelles , and outboard will lock the entire structure into whatever shape it happens to be when we go away and let the glued-down wet balsa skins dry. Three distinct sections separated by big cutouts in the leading edge are likely to end up in three different planes of reference unless we give them some help. Can you see where this is going? I will temporarily get rid of the separate sections by closing up the entire leading edge as if there were no cutouts involved and then bring them back after the entire “D-section” structure is locked into alignment … that’s why we left off last time with the top leading edge of each section sheeted into one piece, perhaps as if we were going to build a big single engine plane. The only reference to the nacelle cutouts so far is in the drawings.

 

The first thing I’m going to do this time will be to join both wing panels at the center. In fact, on this B-17 the wing center section is not built first as a distinct piece of structure; it is defined by the structure as we join the panels. The first thing that’s left to be done is to add 1/8” x ¼” balsa caps to the upper and lower middle (“straight”) portion of each of the two reinforced center spars I worked on earlier. These must be fitted so they will match up accurately with the top and bottom outer panel spars of the same dimension when everything is assembled. This aspect of the wing design/construction is exactly per the kit instructions.

 

When I installed those 1/16” balsa sheet spar webs, I made sure that the outer ends (the angled part) of each center spar would fit accurately against its respective web and between the upper and lower 1/8” x ¼” balsa spars,. Having made certain (off camera) that I’d gotten it right, I can now use that brush to wet the entire joining surface of one of those spar web “sockets” with Deluxe Materials Aliphatic Resin wood glue.

 

All that goes together like this. There aren’t any clothespin clamps on the front spar joint yet because I’ve left them off just long enough to get this picture details of the assembly.

 

Off camera I clamped that front spar joint, double-checked that both center spars were exactly parallel to each other along the middle (flat) portion that ends up between the outer panels and let it all dry overnight. All that is now the near wing panel…clamps removed…which I have squared and blocked firmly to the workboard. Depending on the alignment I have built in so far I then glued and clamped the center spars into the spar sockets on the opposite panel and left it all to dry overnight…again.

 

Once those center spar glue joints were safely dry I checked the panel-to-panel alignment (again) and then went on to add the 1/8” balsa sheet CCR ribs just as called out in the instructions. With the center spars and inboard ends of both wing panels now fully assembled, the next step is to cut each of the two CCR ribs into three pieces that fit ahead of the front spar, between the spars, and behind the rear spar in turn. Here I’m checking to be sure the rear portion of the right CCR is going to fit right when I rotate it that last little bit into line with the top wing sheet.

 

With the right panel CCR parts in place, I’ve squared the panel off with the center spar faces perpendicular to the work board and blocked/pinned the right panel inner trailing edge corner securely in that position. My next step was to cut another space block to the same size to hold the left panel in exactly the same relationship to the surface.

 

Here’s what that alignment checkout looked like from the other side before I installed the right rear CCR section.

 

With that done I could fit and glue all three sections of the left panel CCR and be sure the trailing edges would line up. (I put the spacer block back in while the glue was drying).

 

The next step was to stabilize the center section trailing edge by adding a 1/16” x ¾” balsa sheet insert.

 

Following that it was time to fit and assemble the 1/16” balsa sheet spar webs to the front face of the front 1/8” x ¼” balsa top and bottom spars…again, per the original instructions.

 

The sanding block is the best way to be sure the ends of each spar web fit precisely against each of the ribs they are supposed to connect and reinforce.

 

As that part of the assembly goes together it should look like this.

 

OK … I was busy off-camera again. With all the front spar webs in place I flipped the assembled wing right-side-up, gave all the exposed upper rib and spar surfaces another “square-off” pass with the sanding block and repeated the wet balsa sheet process to close up the top surface of the wing. Note that I’m using anti-crush strips of scrap balsa at the trailing edge so the clothespin clamps won’t damage the balsa sheet surface AND that I’m using heavy building weight to stabilize the entire assembly using the bottom center section surface as defined by the lower edges of both plywood center spars as a reference against which I’ll be able to measure the rest of the assembly of the wing.

 

That was another of those assemblies I insist on leaving overnight to dry. With everything off the board, I’m able inspect the work I’ve done so far. It happens that after I finished fussing and pulling all those center section joints into alignment that one of the CCR rib front sections didn’t mate up fully with the adjacent spar and rib faces. A “quick fix” that I’m comfortable with was to glue a length of ¼” sq. balsa into that corner to serve as a gusset. Here you can see the outer/lower end of it after I trimmed it off flush. At this point I have also begun to assemble the 3/32’ sq. balsa surface spars into the pre-cut notches in the ribs…again per the instructions.

 

You can see a few of those 3/32” sq. balsa spars lying loose on the wing, waiting to be trimmed to length and glued into place.

 

This is the center section seen from the bottom with all those 3/32” sq. spars in place. I discovered that the 3/32” sq. stock was a bit too “squishy” when it spanned the space between the two CCR ribs, so I substituted some 3/32” x 3/16” balsa I had on hand.

 

Now I have to get every bit of the wing structure that will contact the 1/16” balsa sheet skin that’s coming smooth and true so the skin will lie against it without either high spots (bumps) or low spots (voids in the glue joint). I’m starting by using this well-worn miniature block plane to trim most of the extra depth off the 1/8” balsa sheet flap well trailing edge to lie flush with the surface of all the adjoining ribs.

 

Same game on the leading edge. This is a bigger/thicker piece of wood so I can press a little harder and make a satisfying pile of shavings without worrying about cutting too deep.

 

I can only go so far with the plane…here it’s time to switch over to the old reliable sanding block. I’m using 150-grit production paper to cut all the surfaces into alignment (this means that I can’t detect any discontinuous joints/bumps when I run a fingertip over the whole thing).

 

After that I’ll go to a 320-grit block to work those surfaces to the degree of smoothness I want before I’m ready to do any gluing.

 

More catch-up. Here’s another place where I got caught by all the squeezing and pulling to make all the outer edges come out square. The bottom edges/surfaces of the CCR ribs are supposed to form a pair of straight lines from the leading to the trailing edge…but they don’t. I scabbed in several pieces of 1/8” x ¼” balsa scrap to form a surface wide enough that the 1/16” balsa sheet skins I’m about to add have a continuous surface to join over.

 

I’m starting with the lower center section skin…this wide piece of 1/16” balsa sheet is an offcut from those extra-wide wing skins I told you about. I’ve trimmed it to fit so that each edge runs exactly along the middle of the structural bases I just created by “filling out” the edges of the CCR ribs.

 

As usual I have sprayed what is to become the outer surface of that sheet with water and set it aside while I use the brush-and-glue technique to spread Deluxe Materials Aliphatic Resin everywhere the new skin will contact the structure.

 

All lined up and glued in place… I used those building weights again to provide firm pressure to the assembly. There are padded clothespin clamps on the trailing edge and I have used several pins at the leading edge where nothing else wanted to work.

 

As we have seen before, that glued-up assembly got to dry overnight before I handled it. NOW we catch up with the issue of those engine nacelle cutouts. The only practical reference to them is the junction inside the top skin where the various ribs and spars attach to the skin as defined by the plan. I can mark out the specific rib bay sections ahead of the front spar that must be cut out of the top sheet to define the nacelle positions. If I were to wait to do this until the bottom sheeting is attached I’d no longer be able to see those joints. I’m marking them now from the inside using a No. 11 blade to make punch marks that will show up clearly on the outer surface in exactly the right places.

 

A sharp blade and a straightedge make it easy to cut the necessary openings accurately from the top of the wing.

 

Not all of those cuts lined up exactly with the various rib faces, so I’m using that 150-grit block to true-up the edges I’m not satisfied with.

 

Here’s the 1/16” balsa sheet skin for the lower surface of the right wing FROM THE MAIN SPAR TO THE LEADING EDGE ONLY cut to size. Why am I closing up the lower surface only back to the spar? This is another of those judgment calls. As you’ll see soon I need to have both the upper and lower surfaces of the leading edge skinned to be able to finish the shaping/sanding that’s coming…but…there’d a lot of work INSIDE the wing I want to be able to do AFTER the leading edge is closed and finished and the basic nacelle structures are in place. By leaving the surface behind the spar open for now I retain the access I’ll need to install servo mounting plates, wiring access and so on. I’ll finish closing up the lower skin when all that stuff is done.

 

The joint where the lower leading edge skin meets the center section skin (this is also the “dihedral break”) is a good place to demonstrate making an accurate joint. This is a classic “cut-and-fit” operation and the sanding block is a good tool to use to get those final trims just right.

 

It should fit like this.

 

Just as before I’m using a handy-sized brush to get Deluxe Material Aliphatic Resin spread thoroughly and evenly on all the surfaces that are going to join the balsa skin.

 

I’ve used masking tape before, wrapped all the way around a structure that I’m “wet-balsa-skinning” to avoid having to try to make the tape adhere to wet balsa, to hold the sheet covering in place without pins or weights while it dries. This is a good example of the way it’s supposed to work.

 

The right wing leading edge lower surface sheeting is assembled/glued in place. I’m using the outer surface where the top 1/16”balsa skin joins the main spar as an alignment reference, with those building weights to hold everything FLAT against the work board. Note that because of the now built-in dihedral I’ve had to allow the center section and the left panel to extend out into space” off the end of the work board.

That’s enough for this time … I need a fresh pot of coffee. Next time around we’ll get to work on those nacelles.

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