Going All-Out With A Classic Balsa B-17-F – Part 6

Going All-Out With A Classic Balsa B-17-F – Part 6

We finished our last session in the shop with my showing you the E-Flite 10-25 electric RLG (retractable landing gear) system that’s going into the B-17. As I suggested then, there are a lot of things that have to happen…and a lot of decisions to be made… before I’ll be able to glue a pair of landing gear mounting plates into just the right place in the structure I’ve built so far, and then screw the RLG units to them. Remember that the model started out designed for a (non-scale) fixed main landing gear, and that design included a mounting plate, with its own dedicated structural reinforcements, that was to be built into the wing structure at an odd angle that reflects the pronounced forward sweep/angle of the full scale B-17 main struts. Right here is where it helps a lot to appreciate all the things that are happening at once in the decision making/design/structural modification I committed myself to. First big question: Why didn’t I design in all the necessary modifications at the “plans-altering” stage, before I had the primary structure of both inboard nacelles built? After investing a lot of time and effort, I decided that I was not going to find a pre-existing (on the market and available to buy) RLG system that would fit the airframe out-of-the-box. At the same time I’d had enough experience with building the structure up to that point to realize that there are a lot of places where structural inconsistencies can creep into this particular model airplane kit project, with the result that I want to have you take your working measurements from the structure you have built and not from my patterns if you decide to do this RLG modification on your model. So…here we are with the wing structure and primary nacelle frames completed to the point where we must decide what to change to make those new RLG units fit (and work) before going any further. That brings us to step one of this building session.

 

The N-2 formers are used only on the two inboard nacelles (the two with the landing gear in them). As the inboard nacelles are longer so the RLG will fit, the N-2’s are necessary to help the circular cross section of the forward portion transition into the correct contour when each nacelle blends into the wing leading edge. In other words, we need ‘em! This is where we discover that then bottom N-2 on each side is going to interfere with the RLG mounting plate I’m designing, so I have to cut each of them free and keep them to re-install later. (Yes, if I’d seen this coming I’d have left them off until then).

 

Did you notice in the last image that I’d marked off a rectangular area on the bottom wing skin behind each F-2? Now I get to cut an opening there. Remember that I mentioned leaving some decisions on the landing gear mounting “for later”? That’s now. It turns out that the most practical way to add reinforcement to the wing structure around each inboard nacelle will be to go back and install the diagonally slotted plywood landing gear braces as originally designed into the kit.

 

I’m not going to need those slots, but I am going to use them as an easy way to cut each plate “in half” so I can slip it where it belongs inside the existing wing structure. Here I’m using my old reliable metal rule/straightedge to ensure neat edges in the 1/16” balsa sheet skin where I make the cutout.

 

With a little trimming those plywood plates fit right into place. In order to make use of the original kit parts as far as possible (that’s the old-timer in me remembering how it felt when I couldn’t afford to waste anything) I’m going ahead on the educated guess that the discontinuity/gap between the halves of each plate isn’t going to hurt anything in this particular case.

 

I’ve chosen to accept that narrow gap in the plywood, but I can’t tolerate a poor fit between the plates and the balsa ribs they reinforce. I have already spread a generous coat of Deluxe Materials Aliphatic Resin glue on the inner (hidden) face of each plate and after fitting them into place I’m using some scrap wood inserts to brace the assembly while the glue dries. To make them work each brace must fit so tightly that it demands a firm push to “seat” and thus apply pressure to the plates.

 

This part makes sense when I explain what’s coming next…a landing gear mounting plate cut from 1/8” plywood is going into the opening defined by those two lower nacelle longerons, and I have to “relieve” (cut away) material from the bottom edge of the ¼” x 1/8” balsa front spar cap. Cutting holes in wing spars isn’t always a good idea. In this case I will depend on the 1/8” plywood that will replace this cut-out piece to replace its strength as well.

 

I considered providing a template/measurements for the RLG mount plates, but considering the possibility that you might choose different retract units AND that the actual dimensions of your nacelles may vary from mine, I’m choosing to assume that if you are building one of these things on you own you can measure dimensions as well as I can. As I have mentioned before, the critical part is that the main wheel axles must line up fore-and-aft in the location shown on the plan OR you’ll have ground handling difficulties with your airplane. Here I’m finishing the job of cutting out one plate on my old Dremel scroll saw.

 

Here are the two finished 1/8” plywood mounting plates. Note that (to match the full scale layout) the RLG unit centerlines are offset from the midline of each nacelle plate. The two finished plates are asymmetric and are going to be mirror images of each other.

 

Now you get to see how all this stuff is going to fit together.

 

Before I can assemble the landing gear plates I’m going to add reinforcing rails cut from ¼” x 1/8” spruce. These will also provide stout “attachment points” for the sheet metal screws I’ll use to mount the RLG units. I want super-reliable bonding in this assembly, so I will use Deluxe Materials Aliphatic Resin and more of those clothespin clamps to hold it all together overnight.

 

This is what the right inboard nacelle looks like with the corresponding mounting plate assembly ready to be fitted into place.

 

Before I do that assembly process I have to mark and cut out one more pair of pieces. I decided to cut out the remains of the front spar where I made that 1/8” recess earlier and replace the entire section (rib bay) with a stout, load bearing reinforcing former of 1/8” plywood. To make it fit the RLG mounting plate accurately I’m using the underside of the plate to mark the exact cut line on the initially oversize former.

 

Ready to install (with more Deluxe Materials Aliphatic Resin), the new former looks like this. There is another one (mirror-imaged) for the other nacelle.

 

I lined up the former and the RLG plate with everything else I could reach to measure from, added a generous application of Aliphatic Resin, and clamped it all firmly in place for another of those overnight drying sessions. One of the advantages of using a slow-drying adhesive on an assembly like this is that you get extra time to double check and re-adjust when necessary as part of the deal.

 

With the Aliphatic Resin glue dry and all those clothespin clamps back in the storage box, I was able to check the fit of that N-2 former that had to be removed. It turns out that the position of the bottom (outer) face of the 1/8” plywood plate matches the lower edge of the cross member structure I had to remove, with the result that I can simply glue N-2 back in place at the same fore-and-aft location it used to be. Note: By design, the rear end of the narrow part of the RLG unit cut-out defines the location of N-2, so I knew in advance that it would all fit together right.

 

The way this model was originally designed, the nacelle frames are “square” to the wing spars…this means that they align with the dihedral angle and are NOT parallel to the ground reference plane. A check with my scale drawings confirmed that the main landing gear struts/legs are perpendicular to the ground (and so NOT at right angles, or “square”, to the nacelle/wing spar reference, as the mounting plates ARE). We have to get rid of that small difference of a few degrees, and the most practical way to make this happen is to bevel a piece of 1/16” plywood to form a shim that holds the RLG unit mounting face at the desired angle to the plywood mounting plate. While I hold it up here you can see the taper I sanded in. The shim is going to be glued to the RLG plate where I have made cross-hatch pencil marks.

 

While that glue joint is drying I can deal with the void I created in the 1/16” balsa sheet wing skin when I made access openings for those spar reinforcing plates. The aft portion of the RLG plate fills the opening and restores the structural strength I took away, but it leaves a low spot in the skin surface. As it turns out, the extended portion of the bottom of the nacelle as it fairs into the wing will cover/fill PART of that opening, but not all of it. The best way around that problem is to fill the entire hole with light balsa which can be sanded flush. Here I’m brushing on more Deluxe Materials Aliphatic Resin with the filler block ready beside the cutout.

 

With that glue dry, it’s easy to block sand the balsa insert flush/smooth with the rest of the wing skin. In a place like this it helps to be very careful not to permit the sanding block to dig in and tear up the surrounding balsa sheet.

 

Now I can move on to the next part of the job of inventing/designing more of the changes that will be necessary to build in the aileron and flap servos and their connecting cables along with the ESC signal, power and RLG cables where they will work correctly, look “scale”, and be accessible for maintenance. Lots of experience has taught me that servos MUST be accessible in any model you care about. To me, mounting them on the wing surface like some kind of electromechanical warts is not acceptable in terms of appearance, so they have to be built in, or attached to suitable mounts within the wing structure with appropriate removable cover plates. BUT…there’s more. There will be an ESC signal cable and three heavy duty power wires coming out the back of each of the four nacelles and RLG cables as well on the inboard nacelles. I COULD just thread them into place through a series of properly placed holes in the respective spar and rib locations and sheet over everything, but I know better. If you seriously intend to FLY an airplane like this (as I do) you MUST be able to get at whatever “good stuff” is inside it from time to time. That means practical access hatches. I gave a lot of thought to what might be the best way to accomplish all that here. In the end, I decided to arrange the flap servo mounts and control linkages so their access openings would be directly in line with where the inboard power and RLG connections emerge through their openings in the spars…and… at the expense of having to include short lengths of cable-in-tube connector to the aileron servos I was able to locate THEM directly behind the outboard nacelles to facilitate the same “shared space” arrangement. Here I’m installing the first of several ¼” x 3/8” spruce servo mounting rails. Watch how this plan comes together…

 

I thought about making separate plywood servo mounting plates, but simply extending those original rails with ¼” x ½” spruce trimmed to fit the exact width of the servo turned out to be the most practical way to do the job. This is the right outboard nacelle (from the bottom) at the point where I’m rough-cutting the access hole through the 1/16” balsa spar web for the power and ESC cables.

 

You know that I wasn’t going to leave those openings “rough cut”. Another of my custom sanding blocks (this time a piece of PVC pipe left over from building a transport frame for another model) does the job of cleaning up with some 100-grit production paper. Many years of hard-earned experience around full scale airplanes as well as models have taught me not to be comfortable with ANY rough/unfinished edge anywhere inside an aircraft.

 

Here’s the aileron servo installation in the right wing, so far. If you look carefully you can see the yellow outer tubing of the tube-and-cable aileron linkage and the 1/16” balsa sheet exit guide/mount I added several rib spaces outboard where the aileron horn is going to end up.

 

It’s pretty much the same game with the flap servo and the inboard nacelle except that here I was able to line everything up so the servo is going to be able to drive a push-pull rod direct to the flap horn.

 

Several different things are going on in this image. If you compare it with the previous illustration you’ll see that I have “framed in” a rectangular opening (cover plate base) around the aileron servo using scrap bits of 1/8” x 3/8” spruce. Did you notice that the “side” pieces are on edge? I did that to get extra depth to accept some screws that are coming later. (I also framed in the flap servo access the same way.) That’s not all. Do you remember that I agreed to let you in on any “mess-ups” that I might allow to get by me? Here’s one. That piece of ¼” x ½” balsa just ahead of the steel ruler (the wing sub-trailing edge at the front of the flap well) is supposed to be straight. As you can see, it’s not. I’m not sure whether my error was in drawing modifications, measuring, or cutting, but the result is that the ends of ribs W-4, W-5 and W-6 don’t line up. The payoff is that curve, and the questions is, “What’s the best way to fix it?”

 

The hard core purist in me would have cut out the offending curved edge, fixed the mis-measured ribs, and then glued in a new piece of ¼” x ½” balsa to start over…but…that would have risked breaking the surrounding structure. The extra weight that would come with such a repair would probably be greater than the result of what I did, which was to glue an extra piece of ¼” x ½” balsa along the portion of the sub T.E. that’s flawed. Deluxe Materials Roket Hot along with a couple of clothespin clamps provides the most realistic fix here.

 

Now I can use that straightedge ruler to guide my No. 11 blade along a correct (straight) cut that defines the repaired sub T.E. (Did you notice the clamp helping me keep the ruler straight while I made the cut? There’s another one out of sight at the right.)

 

Now I can block sand the surface of that add-on repair perfectly flush with the pre-existing structure at the same time I even off the servo access framing I added earlier.

 

NOW all the good stuff that has to be done inside the wing is finished and I can go ahead and close up/finish sheeting the bottom/rear wing surface that I’ve left open until now. Just as I did with the upper surface I pre-cut a 1/16” balsa sheet wing skin to fit the open area and water sprayed it in the outside face.

 

Again as before I’m using a brush to add a wet bead of Deluxe Materials Aliphatic Resin along EVERY structural edge of the main wing structure that is going to touch the balsa sheet skin.

 

We’ve done this before, too. Plenty of masking tape to close up all the edges, then block the wing STRAIGHT on the work board and add enough building weight to be sure all of those pre-glued joints stay tight while it all dries.

 

Again just as before, with all the weight and tape removed it’s time to do some sanding. Here I’m starting by using a relatively wide sanding tool to ensure that the new bottom wing skin exactly matches the existing surface.

 

The outboard 3/32” balsa sheet rib in the flap well area (which also defines the end of the aileron cutout) has gotten pretty well beat up with all the work going on around it so I’m going to reinforce it. I cut a triangular patch from a scrap of 1/8” balsa sheet and glued it firmly against the outer edge of the compromised rib.

 

Sanded flush with all the surrounding structure that reinforcing patch looks like this.

 

There’s one more extra bit I want to add to the flap well area before moving on. At this location the 1/16” balsa sheet upper wing skin ends in an un-reinforced edge at the same place there should be a shallow recess as seen from the bottom where the trailing edge of the flap will fit. I’m going to address both issues by adding this 1/16” x 3/16” strip of basswood (which is harder and a bit stiffer than balsa) as a T.E. doubler. Here I’m brushing Aliphatic Resin glue along the edge where the reinforcing strip is going.

That long, open joint where the 1/16” x 3/16” basswood is getting glued to the 1/16” balsa sheet upper skin is the perfect place for warps, waves and bumps to appear as the glue dries. I am clamping odd lengths of ¼” x ½” spruce onto both sides of the joint to force everything to STAY STRAIGHT.

 

With all that new work dry I can again trust my sanding block to blend all the odd edges of the bottom wing skin and the T.E. reinforcement flush and smooth.
That’s enough for now…there’s plenty of “little detail stuff” left to do on my own before I can get started with closing up the nacelles, which is what we’ll work on next time.

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