Building Those Old Kits – Not Enough Information?

Building Those Old Kits – Not Enough Information?

On lots of the plans sheets that came with those old (stick-and-tissue, printwood, rubber powered and maybe sort-of-flyable) model airplane kits pretty much all you got was plans. Instructions that actually helped you figure out what came next were not always part of the deal. For a lot of the kids (myself included) who wanted to join the elite ranks of real model builders whose airplanes actually flew, it sometimes seemed as if we were up against a conspiracy of silence, almost as if there’d be one of those  experts who might jeer, “What’sa’matter, kid, you’re supposed to know that?” and leave you feeling like a scared freshman on the first day at High School. You had to really want to do it.

A while back, John Liacos, an rcmodel.com reader from Maine, emailed me with this question: “I have a couple of Cleveland Biplane kits to build. Any suggestions as to the best way to attach the second wing? The plans are no help. I did a terrible job on my last biplane.” I thought about this for a while and  then emailed John back to tell him that I suspected he was far from being alone with a problem like that I’d post an answer to him and anyone else who might be interested as part of an online blog, Here it is.

John sent me a photocopy of a section of the plan, and from what I can see it appears to be from a Cleveland kit ( printwood, cut-out-the-balsa-parts-yourself stick and tissue) of a 1930’s vintage Boeing P-12 USAAF fighter. My guess is that the kit is from of a series of ¾” = 1’ designs and dates from sometime in the 1940’s. It provides a characteristic example of the sort of rubber powered “flying scale” model airplanes that just about everybody learned to build (or tried to) from the 1930’s on into the mid ‘50’s. Never mind laser cutting…this was even before the days of die-cutting, formed plastic parts of any kind. What you got was a box of balsa sheets with part outlines printed on them, some strip wood, a few balsa blocks and bits of dowel and maybe some fine steel wire, some tissue and…oh, yeah…a paper plan sheet that might include a few “construction notes” but very seldom detailed instructions on how you were to go about turning  that stuff into a little airplane that you might actually take outdoors on a nice day and set free into the sky.

This is where we need to talk about the claims made on the boxes and in the magazine ads for models like this one. They were “scale”… often painstakingly accurate with correct cross sections and outlines, maybe even the right airfoil (wing section) and nearly always tantalizing details on the plans that suggested you’d be able to make cowling and dummy engine details, non-operating scale control surfaces, machine guns and bombs and flying wires and access panels and on and on…just how good a model builder were you? The deficiency came with the flying part. Nearly always they did not because they could not. Remember that we’re talking about the days when practical radio control of any sort for model airplanes was non-existent. Free flight meant (as it does now) no control of the model once you let go of it. Such models as were entrusted to flight nearly always flew once…very briefly…before destroying themselves. An accomplished aeromodeler could get one to fly by leaving off all those enticing details, controlling weight obsessively, and applying hard-won skills of balancing and trimming and subtle tweaking. That happened just often enough that as an average modeler you might hear about some guy who usually lived impossibly far away who had done it…just enough to keep you dreaming and going back for another kit every time you had twenty five cents, but usually not much more. The folks who designed the models and produced the kits knew this. Perhaps their reasoning was that the vast majority of guys who bought their kits would know better than to expect them to fly, build them as “shelf models”…for display only…and not worry about functional details like mounting the upper wing of a biplane so it wouldn’t fall off if you picked it up carelessly. If you wanted to go beyond the display thing, you did it the hard way, at the expense of lots of time, skill, and devotion. Mostly, that didn’t happen.

OK…fast forward to NOW!  Reliable RC is an accomplished fact, as are super-miniaturized equipment and highly sophisticated electric power systems. It doesn’t take much imagination to realize that those old designs were crying out for these high-tech goodies…they just had to wait a half-century or so. That brings us to here and now. It’s not so difficult to find examples of the old scale free flight kits, though many of them are considered collectible and may command some serious prices. But…many of the old plans are conveniently available and dozens of those designs are back on the market as modern re-kittings, with the result that there are lots of today’s model builders who have a good reason to learn those old stick and tissue skills that threatened to wither and die in the shadow of today’s high tech, ARF-dominated RC model aviation culture. Whether you are interested in building one or another of those kits as-it-was as an exercise in nostalgia or you want to continue the best of the “good old days” with current electric RC technology, it’s good to be able to figure out what to do if the old engineering came up short for practical flight.

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Let’s look at the plans John sent me. His concern is with the “second” (upper) wing. I’m going to assume that the lower wing attaches to the fuselage either as a one-piece structure that glues into an opening provided by cutouts in the fuselage belly OR as two separate panels glued against rib-shaped base plates included as part of the fuselage structure. Whether the lower wing design called for one or more wing spars to carry though the fuselage for structural integrity we don’t know. In any event, that’s a problem for another time. There’s no mystery about the intended attachment of the upper wing to the cabane struts other than how it might ever have been expected to work according to the plans! What they show is a flight of fantasy. It would appear we are expected to believe that we should bevel the upper ends of the cabanes to achieve a precise fit to the tissue-covered undersurface of the wing and then just glue ‘em together.

Have a look at Figure 1, which is based on one of the plan views John sent me. You can see at points (1) and (2) where I have added a couple of red circles to direct your attention that there is nothing there to suggest any kind of a structural joint. Point (3) shows us the same difficulty from the top.

 

The plan even suggests that you ought to be able to create neat little faired fillets around the each strut end and expect them not to fall apart the first time they get touched. And oh, by the way, what do you make those from?  Check out Figure 2, which also comes from John’s plans.  

 

 

Let’s get on with cutting some balsa. The cabane-to-wing attachment is primary structure.   It must work under any and all loads that might be imposed on the airplane…its job is to hold the whole thing together. If those little fillets fell off and blow away in flight nothing else bad would happen…not so with the upper wing. For this exercise we have to assume that the lower ends of those cabanes are attached to something substantial down inside the fuselage…probably either the upper longerons or fuselage formers three and four with which they appear to align.

What corresponding part of the wing structure are they supposed to hook up with? The correct answer is…the spar (or spars) which run spanwise and are supposed to be the primary load bearing part of the wing. In this model there is no main/forward spar, but the leading is unusually stout and expected to do that job. (On such a small, light model this is OK). The problem here is that as we view the airplane from the side, if the wing and fuselage structures are shaped correctly and in the right positions relative to each other, the upper ends of properly located cabane struts get to connect up with the inner edge of  wing rib number 3, which for those purposed might as well be empty space. To fix this we have to do what the model kit designer  did not…we have to add load bearing structure to accept those cabane ends. There is more than one way to do this. I’ll show you the method I’d use if this were my project and all I wanted to achieve was to build the model so that it would have a chance at flight as intended, as a rubber powered free flight job. If I were going for an electric RC conversion there’d be more to it. (and I’ll address that issue in another blog…soon.)

The cabane struts appear to be called out to be carved/sanded from 1/16” x 1/8” balsa strip, and the wing ribs are almost certainly cut from 1/16” balsa sheet. With those dimensions as a starting point, I’m going to cut out a couple of pieces of 1/8” x 1/2” balsa from my stash of leftover balsa stock to create the missing structure. There will be two separate pieces, one on each side of the wing. These will fit between the leading edge and the spar, one on either side of rib number three and flush with the lower surface of the wing (so the tissue covering will lie directly on them) and form little shelves ½” wide.

Have a look at the simple sketch I created as Figure 3 to see how this is going to work.

 

Don’t glue them into the wing just yet. Instead, make cutouts as necessary so each shelf will fit over the cabanes and maintain exact alignment with the fuselage centerline (and the rest of the airplane) per Figure 4. The trick is to be sure the front and rear distances D/f and D/r are in exactly the relationship to each other as on the plan (whether it’s this one or your own) so that the incidence angle of the wing relative to that centerline will remain as designed.

Check out figure 4 to see how this works.

 

 

 

Once those parts are joined in alignment you can cut off the tops of the cabanes flush the tops of the mounting shelves, trim out 1/8” from the bottom of each number 3 rib and let each wing panel slip down and rest on its mounting shelf.

 

All this is what I’d suggest to John for a simple, practical fix for his model. If you can get your head around what’s going on here you’ll be able to see similar problems in advance on other projects you might be working on and apply the same logic on your own.