Going All-Out With A Classic Balsa B-17-F – Part 5

Going All-Out With A Classic Balsa B-17-F – Part 5

Last time I finished cleaning up the leading edge of the bottom of the wing and explained why I chose to leave the rear portion of the underside open for now. I also mentioned my reasons for getting the entire leading edge “closed up” before beginning any work on the four engine nacelles. Now I can show you why I wanted to do it that way.

 

Last time around I explained that before fitting and gluing the front/bottom 1/16” sheet balsa skin I had to “open up” (cut out) sections of the front/upper skin that correspond to the positions the nacelles will soon occupy. At that point it looked like this.

 

The reason I left the entire leading edge “squared off” was that I want to be able to carve/plane/sand the entire leading edge at one time to facilitate blending all of it into continuous series of curves. Although I have to leave the lower rear wing surface open for a while yet, it became obvious that to do that job right I would have to blend the long, straight portion of each leading edge into the corresponding wingtip structure. The wingtip needs to be complete for this to happen so it’s necessary to add the 1/16” sheet balsa bottom tip skin NOW. Here you can see the sheet balsa skin section I’ve cut and fitted to the right tip. At this point I have already water-sprayed it as I explained earlier and I’m brushing Deluxe Materials Aliphatic Resin onto every joining surface of the inside tip structure.

 

Set into place and “closed up”, the wingtip skin looks like this. Once again I have made liberal use of clothespin clamps to make sure the entire edge of the skin stays in TIGHT contact with the laminated tip bow. There’s no practical way to use masking tape to hold the inner edge of the skin firmly against the edge of the tip rib, so I’m relying on building weights to keep everything in place while the glue dries.

 

Once again I’ve allowed the whole assembly to dry completely before setting it loose from all those restraints. I’m using a razor blade to trim off most of the overhang of 1/16” sheet balsa from the bottom of the tip. As I have done before, I’ll leave the final trimming flush with the tip bow (and all the rounding and shaping) for a sanding block that I can control more precisely.

 

Back on the leading edge (you can just see the beginning of the wingtip curve at the right) I’m doing just that. The first step here is to square off the edges of the top and bottom skins flush with the still-flat face of the balsa leading edge.

 

You’ve seen me using this well-worn block plane before. Here I’m cutting generous (but still well-controlled) balsa shavings off the top edge where the balsa skin overlaps the leading edge. Keep in mind the what we want the finished appearance/shape of the leading edge cross section to be…a continuous smooth curve. What’s happening here is that I’m “sneaking up
on that shape I want by cutting away excess balsa on those blunt, squared-off edges without risking the job by cutting too far at once.

 

When I have cut away as much wood as I feel comfortable removing with the block plane the leading-edge-in-process looks like this.

 

Back to the sanding block, this time with medium-coarse 150-grit paper. In this step I’ll continue cutting away balsa until I’ve until I’ve reached/created exactly the cross section/curvature I want. This part of the sanding process is properly called “shaping”.

 

From a different angle that looks like this.

 

Now I’m using a longer sanding block (one of those aluminum bar jobs from Great Planes) with finer (220-grit) paper to begin the process of “smoothing” the wood that has now been shaped the way I want it to be.

 

When THAT part is done…as smooth as I need it to be for now…it looks like this.

 

To mark where I need to cut the lower leading edge skin I’m making punch marks through to the opposite side with the point of a No. 11 blade. This way I can then turn the wing over, use the marks to locate penciled cut lines, and cut the nacelle openings from the outside in with a straightedge to give me better control of the blade and minimize cracking/splitting of the balsa.

 

Marking those lines looks like this…

 

…and making the cuts looks like this.

 

With all that done I can use a razor saw to remove those sections of leading edge that would otherwise interfere with putting the nacelle structure in place.

 

All wood cut, THAT looks like this.

 

Now I can start working on the basic engine nacelle structure/framework. Pat Tritle’s plans and instructions tell you how to build up the various nacelle side frames (just like miniature fuselages) using 3/32” sq. balsa. I’m following his directions exactly on this part.

 

The plans call out that you make two each inside and outside frames of both the inboard and outboard nacelles (they’re all different, owing to the way the nacelles fit the tapered wing).Here I have most of them done and I’m working on an outboard/outer frame.

 

Just as if I were building a fuselage, I need to block-sand each completed frame smooth…each strip even with its neighbors and all the glue lumps or other bumps GONE…so the formers that are coming will fit accurately. It is WAY easier to sand each built-up frame separately, flat on the work board, than to risk breaking the assembled nacelles later.

 

Now I can join the respective pairs of side frames to assemble the basic structure of two inner and two outer nacelles.

 

If your fingers won’t fit safely “down inside” a nacelle to fit a part, use a tool to help you get it right.

 

Here’s the left outboard nacelle framework assembled and fitted part-way into the wing leading edge cutout we made for it.

 

Fitted in place, squared off, and glued securely (I used Roket Hot CA) both left nacelle frames look like this.

 

All four of them are in place now. If you have built the primary wing structure accurately you can depend on equally accurate nacelles to line up pretty well … but … ALWAYS double check by measuring and aligning directly over the plan to make sure they really are on the required centerlines and squared-off in relation to each other.

 

Here’s another of those places where things that are supposed to fit, don’t. In this case I’m going to attribute the problem to kit engineering, but no matter WHY there’s a problem in the end we still have to fix it. This is the right inboard nacelle. Look carefully at the N-1 former I’m holding up to the 3/32” sq. balsa cross member to which it is intended to be attached…the width of the former and the outside width of the nacelle at this point are supposed to be the same, and they are not. The N-1 former is on the order of 1/16” too wide on each side. I did some very thorough cross-checking. The N-1 formers should meet at their corners to form a circle, but they do that only if I lay them out flat on the work board WITHOUT the 3/32” sq. balsa structure to control the spacing. What’s wrong? The easiest explanation is that the top elevations on the plan over which each set of nacelle side frames is joined are about 1/8” too wide. There are a lot of things we might consider doing about that, but what’s important right now is FIXING the model we’re building. The formers (which all fail to fit the same way) are intended to fill out each nacelle to a perfectly round/circular cross section, and if we fiddle with that it’s going to show, big time. What should I do?

 

It turns out that if I “pack” or shim each N-1 former OUT about 1/16” away from its respective 3/32” sq. balsa cross member, they will all line up so closely that a light touch with the sanding block will correct the completed shape so closely that whatever tiny error in outside diameter remains, won’t matter. (BTW: We would NEVER do this kind of fix on a full scale airplane; even on a model intended to meet TOP GUN standards the best choice would be to cut out all the mistakes, draw up and document corrections, and start over on the parts that were wrong. For what we expect from this model airplane, “close enough” will work just fine.) So…what am I doing here? The balsa pieces I’m cutting are 1/16” x 3/16” strips from which I’ll cut and fit individual lengths to fit along the outer faces of each 3/32 sq. balsa cross member to move each former in turn OUT just enough to fit right.

 

I’m marking off the correct length to cut off the 1/16” x 3/16” strip…

 

…and cutting a batch of them to size.

 

When that’s done I can uses Roket Rapid CA to glue each of them in place. I started with the upper and lower faces of the basic nacelle frames…

 

…and then added the spacers to each of the sides. Whenever there was a little extra overhang I used the sanding block to clean up the corners of the nacelle frame assembly.

 

That permitted the N-1 formers to fit like this. See the circle formed by the combined outer edges of the formers?

 

This is what’s coming next. After a lot of searching and comparison of specs, I chose to use standard-issue E-Flite 10-25 electric main gear RLG units. The problem I’m dealing with is that NO off-the-shelf retract unit I’m aware of will fit this model exactly. “Working with what I’ve got”, I figured out how to make these E-Flite RLG’s do the job. Next time I’ll show you what I did.

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