Going All-Out With A Classic Balsa B-17-F – Part 3

Going All-Out With A Classic Balsa B-17-F – Part 3

As I promised last time, during this session in the shop I’ll begin the actual assembly of the wing and you’ll get to see how the modifications for flaps (openings in the original structural design) that began as some newly cut-out places in several ribs and new structure that I designed and drew onto the plan became tangible working parts of a real model airplane. As you watch this stuff come together you’ll be able to appreciate how important it is to make all the decisions about what is going to change before actually cutting out parts and sticking them together. Yes, you could go back and build modifications like these split flaps into an already-built structure, but the results would almost certainly be less precise (sloppy, even), heavier, and perhaps weaker as well. There are going to be more places like this where good model building practice demands that I visualize and plana modification before I start cutting things, and I’ll point them out as we go along. And … if I mess up and have to go back and fix some oversight, I’ll show you that, too.

 

Right away it becomes clear that building this wing is going to demand that we pay attention to details and make decisions about how I put the pieces together as we go along. Just the job of laying out the initial assembly of the left wing panel, beginning with rib W-1, is full of traps that will get you if you aren’t paying attention. For instance…right here W-1 is already assembled to the 1/8” x 3/16” bottom main spar and I’m using Deluxe Materials Roket Hot to lock W-2 in place at the rear spar joint. Easy enough, right? BUT…did you notice that W-1 is NOT at the end of the spars I’ve pinned down over the plan? It turns out that the 1/16” balsa sheet W-1 is not the base rib of the main wing panel…the heavier (1/8” sheet balsa) CCR works as the base rib and it is designed to be assembled initially as part of the center section, which you build separately from the left and right main panels. The original design could have included an extra rib at the inboard end of each panel that would “double-up” against CCR during assembly of the panels to the center section, but one of the primary goals of Past Tritle’s design philosophy is to save weight, and doubling the weight of two large wing ribs to make assembly a little easier didn’t fit the plan. What we’ll do in effect will be to build in a precisely defined open space where that extra rib might have been and assemble CCR as part of the center section into it when the time comes. It works fine…you just have to pay attention to the details to be sure it will fit when you need it to. This approach to drawing plans and writing building instructions…where you are NOT led by the hand through each and every cut and glue joint in an assembly sequence…was very common in the “good old days” of classic stick and tissue aeromodeling through the ‘30’s. ‘40’s, and ‘50’s. It demands more of the builder than today’s comprehensive “check off the box” construction manuals, but it also demands that you LEARN about model aircraft structure and about making your own decisions. It’s not for everybody, but I’ll suggest that it’s an excellent preparation for making intelligent modifications to kits, to real scratch building, and to your own original design accomplishments.

 

But wait, there’s more! As I used more Roket Hot to stick the front/lower 1/8” x 3/16” balsa spar to W-2, I noticed that with rib W-2 aligned exactly with W-1 that front spar did not want to seat completely into the pre-cut slot in W-2 unless I rocked W-2 slightly forward OR lifted the spar just a bit. Something’s not right! I stopped work, grabbed the instruction sheets, read through the entire wing construction sequence…and then I discovered what had been left out. Read carefully…the instructions say that the front edge of each rib, as well as the rear-facing surface of the ¼” balsa leading edge, must be kept in alignment perpendicular to the building surface. What they don’t say is that when you do this, ONLY the single full-length 1/8” x 3/16” balsa bottom spar will remain flat and in contact with the building board…all the other spars must curve UP off the board to fit the pre-cut holes in all the ribs. The instructions leave some uncertainty as to just how you should get everything to fit that way. Here’s what I did. I lifted the front spar off the surface to fit W-2 just here without distorting it, then left the rest of that spar “loose” for the time being. If I’d seen this coming I would have left out the front spar entirely until it was time to work on the bottom surface of the wing. Ignoring it for the time being was easier than trying to cut it out and replace it later. The time to add that 1/8” x 3/16” front/lower spar as well as all the other lower surface spars is AFTER the wing structure has been completed through the stage of sheeting the upper surface.

 

All the ribs are glued to the lower rear (main) spar. I have marked a centerline reference at the front of each rib as well as along the rear face of the leading edge. I have blocked the LE up so it rests square to the ribs and the building board (out of the picture at either end) and I’m using more Roket Hot to fix each rib in turn to the leading edge. You might notice that in spite of how I just mentioned leaving the front spar “loose” for now, here I have lifted it up dry (without adhesive) to check alignment.

 

Here’s another look at the rib front/leading edge joints. In this shot you can see clearly how the front lower spar (I’ve left it dry-fitted) has to be lifted above the building surface to fit properly.

 

With all the ribs squared off and glued to the lower spar and the LE, I went ahead and test-fitted the corresponding 1/8” x 3/16” balsa upper front spar.

 

That metal ruler works fine as a very narrow square to ensure alignment. I glued each rib-to-spar joint individually, double-checking that each was square as I went along.

 

Now we can get serious about putting together a structure that can be relied on to stay square while we build the rest of the wing around it. These are the first of a series of 1/16” sheet balsa spar webs (ALWAYS assembled with the wood grain vertical) that go in exactly per the kit instructions. Don’t forget that alignment square!

 

With the entire main spar assembly completed (all the 1/16” balsa sheet spar webs in place) it’s time to start defining the trailing edge. Remember the cut-out I designed and drew out to accept the split flaps? Here I’m test-fitting that flap well sub-trailing-edge cut from 1/8” balsa sheet and trimmed (tapered) to match the cutouts in each successive rib.

 

If you back to B-17 Blog No. 2 you can find an image that shows how the flap well cutouts in the ribs form a very shallow acute angle (not quite 90 degrees) where the flap well TE will go. I’m sanding in the necessary slight bevel to the top edge of the TE where it’s going to match those cutouts.

 

Put together, that assembly looks like this. This is all new structure that is not included/shown in any way on the original plan.

 

The original design calls for an upper trailing edge of 1/16” x ¾” balsa sheet set into notches already cut into the ribs. I’m including it as-is. You’ll see how it will serve as a reinforcement for the 1/16” balsa sheet wing skin (something I’m adding) that goes on later.

 

Here’s the entire inboard sub-TE installed. Notice that it stops at W-7, where the aileron well cutout begins. This is as per the original design.

 

Back when, at the same time I built the laminated outlines for the tail surfaces, I also made a pair of wingtip bow laminates. Now they are ready to be trimmed to exact finished size and fitted to the waiting wingtip structure. You can see how the tip bow centers on the cut-off rear end of rib W-11 and the centerline already drawn on the leading edge.

 

Adding all those 3/32” sq. balsa spars to the upper surface of the wing, in preparation for adding the 1/16” sheet balsa top wing skin, is going to continue the job of locking in whatever “twist” we want (or don’t want) in the panel. There are no jigging tabs on the ribs …or any other aids to alignment… in this kit, so I’m using the leading and trailing edge centerlines that I drew in as leveling references. Here I’m measuring the distance of that LE centerline just past W-1 off the building board surface. The specific distance isn’t important…what comes next is.

 

It’s time to play a round of the “successive approximations” game. I measured the distance of the trailing edge off the board at the same location spanwise as on the LE and then shifted the wing up and down until both the LE and TE centerlines were equidistant from the board with the main spar also resting securely on it. (As it happened, they came out even at 1.25”, but that’s not important. What IS important is that is that we establish a chordwise centerline for the wing and then block, brace or otherwise lock the wing in place with that centerline LEVEL.

 

I did exactly the same thing at the tip. It doesn’t matter that the distance off the board will different; that the LE/TE centerline is level DOES. Once that’s established, this is a good time to get started building WASHOUT (TE tip raised for a reduced angle of incidence at the tip to control tip stalling). With everything level I went back and RAISED the TE tip of W-11 by 1/8”. What happens next will begin to lock that little offset/twist into the wing structure so we can depend on it to be there to help ensure good flight characteristics.

 

I’ve added the first 3/32” sq. spar behind the main spar and now I’m trimming that AND the overhang I left on the 1/8” x 3/16” upper main spar to an exact match with the tip laminate. The sanding block turns out to be the best way to do this accurately.

 

Following that I added all the remaining upper wing surface spars just as called out by the kit instructions.

 

With all the structural parts that will contact the 1/16” balsa sheet upper wing skin in place, it’s time to cut and sand them into an accurate, smooth surface to which I’ll be able to glue the skin. Here I’m starting the job by using my old Master Airscrew block plane to shave extra wood off the top of the ¼” balsa leading edge. The top surface of the LE must match the curvature of the top of each rib.

 

The next step is to use as long a sanding block/bar as possible to blend all the upper edges of the ribs along with the various leading and trailing edge structures to form a single, smooth continuous compound curvature. When we glue the top wing skin in place all this work will define the finished shape of the top of the wing…so we need to get it RIGHT.

 

Next comes the first of what will be a sequence of attaching wing skins; that is, covering the entire upper (or lower) surface of each wing panel with a single piece of 1/16” sheet balsa. It is possible to do this using multiple pieces of narrow, hobby-shop-standard (3” wide) balsa and cutting the various sections of sheeting so that all the edges come together where a spar provides a base for a glue joint. Joining edges of sheet covering “in the air” is a bad idea that will almost guarantee a surface that’s impossible to sand smooth. The BEST way is to edge-glue those individual pieces into a single, full-chord-width WING SKIN that you can block-sand perfectly flat while you still have it properly supported by the building board surface. OR…you can do what I did and use extra-wide (also extra-expensive and hard to find) sheets that don’t need joining, only trimming at the edges to fit the exact outline of the wing. It turns out that I’ve been saving a special lot of choice balsa that included some 12” wide pieces of 1/16” sheet, and it occurred to me that this B-17 project would be a most appropriate place to use some of them. Here I have pre-trimmed one sheet to extend just beyond the outline of the left upper wing panel surface and I’m spraying what will become the outer surface lightly with water. This will accomplish three things. Spraying ONE side will induce that balsa to bend gently away from the wet side, pre-fitting it to the curved wing surface as well as softening the wood just enough to make bending around gentle compound curvatures easier. At the same time some of that moisture will penetrate to the opposite side of the wood and contribute to an excellent adhesive bond with the aliphatic resin wood glue I’m using … and … wetting the balsa causes it to expand slightly, so that after the glue “grabs” and the woods gradually dries, it also shrinks just enough to put a slight tension, or “pre-load” on the newly closed-up structure. This is a classic way to add strength to a closed sheeted balsa structure with NO added weight.

 

I’m using Deluxe Materials Aliphatic Resin wood glue for this job. The best way I have found to spread a controlled , even bead of wet glue along every joining surface on a wing structure like this is to use a brush of appropriate size. This is much easier to control than trying to squeeze an even bead from the spout of the glue bottle. It’s better to have the glue bead a little too wet than to risk a poor bond by leaving it too dry.

 

I chose to use a selection of appropriate weights to hold the damp wing skin firmly against the portion of the structure away from the edges. Where possible I have used clothespin clamps (with scrap balsa pads to protect the wood surface) to clamp the assembled wing skin so it cannot move while everything dries overnight.

 

When that has happened I can take off all the weights and clamps and remove the panel from the building board. This is the bottom of the left wing panel. There’s still some sanding to be done to clean up the outer face of the main spar assembly and all the bottom 3/32” sq. balsa spars have yet to be installed.

 

Here’s another look at the inside surface of the upper wing skin I just assembled. What I want you to notice is that the skin is firmly in contact with all the ribs, spars and edges everywhere (no voids or open spaces) and that there is just enough squeeze-out of dried wood glue to guarantee that I didn’t leave any dry joints. Leaving a bead of glue any thicker/heavier than what you see here will NOT add extra strength, but WILL cost you extra weight.

 

Time to start on the opposite (right) wing panel. I’m going to leave the left panel as you just saw it and get the right wing caught up to that same point before I go any further. This is where we run into a quirk of some kit designs that provides us with only a single wing panel plan (half a wing) to save the cost of printing the much larger sheet that could be needed to show the entire wing. The traditional “fix” has always been to OIL the half-plan you have to render it semi-transparent so you can turn it over and see through to build off the reverse image on the back. I don’t like oily stuff on my work board, so I chose to give my half-plan a heavy coat of clear nitrate dope, which provided the same see-through effect.

 

Now I’m repeating the same initial steps I did in assembling the first panel…except…here I’m using Deluxe Materials Super Phatic glue, which is designed to penetrate assembled (closed) joints in balsa structure in the same way fast/thin cyanoacrylate does. The Super Phatic takes longer to dry, but it’s less expensive and any extra squeeze-out is easier to sand.

 

Same as before … I’m double-checking for squareness and alignment at every step.

 

Same as before … I’m double-checking for squareness and alignment at every step.

 

Here’s the right wing again at the same stage of construction with the left wing panel (still as I left it) for comparison. That’s all for now.

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