Model resurrection

My beloved, ageing 72” span Harvard, built more than fifteen years ago from a Flair kit, hit the ground like a dropped jigsaw. We’d been through a great deal together over the years…balmy summer evenings, winter snow… just watching her parting the early morning mists of autumn as she left the runway was pure aeromodelling magic. The sad thing is that I wasn’t the pilot on the fateful day, for that particular privilege went to fellow clubmate, Robert. The fuselage and tailplane stood vertically out of the grass like a newly erected churchyard monument; as we slowly walked in its direction I consoled Robert, who felt he was to blame, but closer inspection revealed that I was the culprit. My son is an aircraft engineer, and he’s instilled in me the need to check airframes regularly and thoroughly. These little modelling air worthiness checks are very therapeutic and had kept the Harvard in the air all those years, but unfortunately I missed the worn elevator quick-link that parted company with the control horn whilst poor Robert was at the controls, and gravity did the rest.

Remember that old TV series, The Six Million Dollar Man? Well, as with the programme’s main character, astronaut Steve Austin, we can rebuild the Harvard! So often I hear model flyers complain that they’re either unable to obtain spares for their latest ARTF or that the cost of said parts exceeds the price of a complete new airframe; well, for us balsa-encrusted trad’ building types there’s rarely a spares shortage, since in most cases all that’s required is a plan or template and a fresh sheet of wood.

There is a small problem with Flair kits, though, in that the plans don’t contain full-size drawings of the various components. With other models it could be a case that although the original plans had all the detail required, the passing years have seen them become lost, or recycled as nesting material for a family of workshop-friendly mice. Salvation is to be found, however, in the crumpled remains of your broken model.

Having stemmed the flow of tears from dumping your pride and joy into terra firma, collect up every piece of wood from the crash site, however small. Not only is this the responsible thing to do, it also provides all the parts of the jigsaw with which to make a set of templates for cutting new parts.

Once back in the cosy surroundings of the workshop, remove the areas of damage and see exactly what there is left to work with. In the Harvard’s case the O.S. .52 four-stroke looked okay, but considering the engine’s age, plus the crash, I felt that a complete strip-down and check-over would be prudent. The alternative would be to package up the unit and post it off to those really helpful and knowledgeable folk at Just Engines, but I can’t resist a good tinker with a ‘real’ engine. Next on the list was the fuel tank; damaged but not split, I felt it was simpler and cleaner to go for a new one. During the impact the servos had ripped out their hardwood mounts, too, taking large sections of the fuselage’s framework with them as they made their escape. Fortunately standard servos aren’t expensive these days, so I bought a new set and marked the Harvard’s old (and now suspect) units accordingly, for use only as dummies during a building project.

Before taking a closer look at the fuselage my initial concern was the two main ARTF-style components, both of which may prove hard to replicate or replace: the canopy – a large feature of any model Harvard – and the aluminium radial cowl. Close inspection revealed the canopy to be undamaged, but the cowl had taken the full impact of the crash, ripping the aluminium. My first thought was to start a whole new career and enter the distorted metalwork as an item of modern art with the underlying label ‘A creation born from the forces of nature coming together on a man-made object’. But before I could inform the Tate Modern of my historic achievement it was pointed out that many people create similar masterpieces on a daily basis only they, being of more stable mind, call the creation ‘an accident’ and the result ‘junk’. My future blushes were spared by Robert, gallant fellow that he is, who sourced a replacement cowl from Flair and thereby brought thoughts of my début into the world of art to a conclusion as the old cowl made its final flight across the workshop and into the bin.

With all the covering removed, what was now left of the model’s fuselage appeared to be in good condition. There were a couple of damaged longerons, but considering the age of the airframe and the amount of glow fuel that must have passed over its surface, there appeared to be no ingress into the structure other than a slight trace around the lower tank area; this was a section designated for new wood anyway, so I felt pleasantly encouraged.

What about the wings, I hear you ask? Well, I’ve made a brief inspection at the damage, and although this is fairly extensive, the whole structure is basically balsa and liteply so I don’t feel intimidated by the repair prospects here. But I decided to put this to one side for now, to free up some bench space and make a start on my Airsail Chipmunk kit.

  

My building preference with most new projects is to start with the fuselage, but to create some workshop variety I decided to follow the Chipmunk’s instruction booklet and commence with the tail section, but not before I’d set some personal parameters, to have some idea of what I wanted out of the model. For instance it’s very tempting, with any scale project, to steam in and go over-the-top on detail and end up with an overweight airframe that’s blistering with vulnerable, intricate detail when all you really wanted at the start of the project was an everyday model that was personal to you. My first thoughts and initial parameters went along the following lines:

One area I still haven’t fully made a decision on is the addition of flaps, for although these aren’t included in the kit, the details and drawing are there should you fancy this extra piece of very useful scale detail. For my part I enjoy using flaps, and think they add a real sense of full-size to a model. On the other hand the extra weight they incur is, unfortunately, behind the C of G. Not that far behind, I grant you, but every little helps, and my one priority at this stage is lightness.

One thought was to dispense with the flaps, associated linkages and servo, the collective weight of which would theoretically provide a weight credit. I can then either save the full amount, with the profits going towards a lighter overall airframe, or spend some (or indeed all) of the saving on a different aspect of the model’s construction. One area where I’ve considered investing these extra ounces is in glass cloth. It’s not as if I’ve purposely avoided the technique in the past; on the contrary, I’ve always admired such a finish, and maybe the Chipmunk is the perfect opportunity to learn something new and become involved in the world of glass cloth.

With the basic ideas and parameters set out for the Chipmunk’s future, it was time to go through the kit’s contents and compare them with both the written instructions and the plans, since I’ve discovered over the years that a traditional kit may have some anomalies in the relationship between components and paperwork. I can only speculate as to how these little discrepancies occur, and assume that in many cases it would be difficult and costly with small production runs to continually update the plans, instructions and contents. A perfect example of this situation arises when looking at the ribs fitted to the tail section of the Chipmunk; the instructions refer to cutting the ribs from two different thicknesses of stock strip balsa and sanding them to shape once glued in place, however the kit is now supplied with a laser-cut sheet (of one thickness) that contains all the ribs for the tail section. There is, though, a separate piece of paper supplied by Airsail that indicates not only this anomaly but also a series of upgrades and amendments.

Bear this in mind and don’t be too eager, as it’s easy to end up confused and frustrated. Read all the literature in the box, and compare the various components with the cutting list, plans, instructions and any amendment sheets. If you still end up bewildered, then Internet forums can be a source of help. Alternatively ‘phone a friend for, unlike the inter-tube, friends bring biscuits with them.

One aspect of the Chipmunk’s packaging that I found very useful was that the stock strip wood is bundled together into packs applicable to specific areas of the airframe. Should you feel the need to open these packs before their time to check their contents, make sure you keep them within their groups otherwise life may become complicated later on when trying to identify where a random piece of unlabelled wood fits into the game.

With plan laid out on the workbench and duly covered with a protective layer of transparent film it was time to fit a new scalpel blade to the handle, crack open a fresh tube of glue, turn on the radio and start building the Chipmunk’s tailplane, elevators, fin and rudder. All are constructed in similar fashion, whereby the basic frame is glued together over the plan, leading and trailing edges of one side are then added, followed by the laser-cut ribs. Once dry, the frames are removed from the building board, turned over and the same process is repeated on the side that had previously been face down on the plan. I did find that there wasn’t quite sufficient wood at one point, due to either a shortage in the kit, perhaps, or poor cutting management on my part, but in all honesty this was a minor problem easily solved from my standard workshop supplies (posh terminology for ‘scrap box’). The moral here is to never throw wood away. For we traditional types, there could be a free airframe in the bin!

Once the basic open structure of the tail components was complete, the next steps were to add the control horn mountings and fix balsa blocks inside the elevators to take the wire joiner. Before rushing on and adding the balsa sheeting to the tailplane and fin I had to make a decision regarding the hinges, the choice of which is a very personal thing; not only as regards type and manufacturer but also the method of fixing, which can stir some heated emotions amongst builders. Those supplied in the kit are plastic, with a flexible joint that’s fixed into a slot cut into the basic frame of the control surfaces. I feel confident that this arrangement is more than sufficient for the job, but on the other hand the mechanical hinges fitted to my Harvard have survived not only hundreds of flying hours but also an impact that wiped out half the fuselage. Fitting the same type of hinge to the Chipmunk will mean adding some soft balsa mounting blocks before any sheeting is fitted, though.

  

My email inbox  received a number of enquiries asking for guidance on fixing and holding balsa sheeting in place, so it’s very timely that I can use the Chipmunk’s tail surfaces as an example of my own approach to the subject, which I also applied to the Harvard; none of the sheeting has failed, neither during the Harvard’s long life nor following its headlong plunge into Planet Earth. Let’s first take a look at the three different glues that I prefer, and some of their properties:

PVA a.k.a. ‘white glue’ – cures to a rubbery finish that can be difficult to sand.

Aliphatic – Usually has a yellow appearance and dries harder than PVA, making sanding easier

Cyano – Near-instant setting time, although some products claim to give 20 seconds. This lack of working time can make the therapeutic and tactile world of sheeting unnecessarily stressful 

One aspect of sheeting that’s often overlooked is the pins used to hold the balsa in place. We have two grades of pins in our workshop: thick and thin, with coloured plastic ends. There are no fixed rules as to which pin is best for the job – sometimes the thick ones just want to split the wood, thereby reducing their holding power, whilst in a different situation the heavier gauge shaft becomes an advantage. Conversely, the thin variety may not split the sheet but can lack authority and keep falling out. I’ve noticed that both hurt equally when inadvertently pushed into any of your ten digits, though!

One of the main problems I encounter when sheeting is negotiating the curves that crop up during the process. With a fairly resistant sheet of wood and a pronounced curve there’s a good chance that the pins may lose their grip during overnight glue curing, so selecting a softer and more pliable sheet is the first thing to consider here; alternatively you can steam the wood to shape, thus helping to reduce the load on the pins.

When sheeting the curve of a wing l.e. the balsa can tend to lift off the ribs between the main spar and l.e., and this is where the thin pins seem to win (nine times out of ten) for keeping the sheet in touch with the rib.

I mentioned above that the near-instant setting qualities of cyano can make sheeting a stressful pastime, but it does have its uses in the process. For instance, when the curve of the aforementioned l.e. sheet is very pronounced (as with some vintage models), I use cyano to fix the sheet to the l.e. then apply PVA or aliphatic glue to the ribs and main spar before bending the sheet over the structure; no pins are therefore required to hold the l.e. down. Another cyano trick that’s worked for me is to create a line of PVA or aliphatic along the spar or rib but leave a small space where you’d normally put a pin. In these spaces add a spot of cyano, place the sheet over the airframe, and the dotted cyano line effectively become permanent pins. The cyano makes for a one-shot go at placing the sheet correctly, so be wary. 

One fun aspect of traditional building is that of making decisions regarding various aspects of the model’s construction and components. With 99% of the woodwork completed on Janet’s Flair Puppeteer it’s time, once again, to play the decision game.

Her first choice is in respect of the ailerons; whether to go with the suggestion on the plan and use the fittings as supplied, driving the surfaces via a single servo in the lower wing plus pushrods and bell cranks, or the alternative method of installing a mini servo in each of the lower wing panels. Two separate servos will dispense with lengthy control rods, making the connection between the servo output arm and aileron control horn shorter; theory says this will make the control more positive, and there’s the added advantage that aileron differential can be set independently through a programmable transmitter. My own Puppeteer flew perfectly well with the single servo set-up, although this was back in the distant past, a time before computer-orientated transmitters and when servos had value and weight similar to that of a gold ingot. Personally I think she’ll go for the two mini servos, but I’m not placing any bets.

  Janet’s second area of debate is the wire u/c. Although it’s possible to incorporate a bungee-style suspension into the set-up, this does complicate the soldering process somewhat, and as she’s already decided to light up the blow torch she felt the standard wire u/c without any spring in its step would be a good starting point in learning the mysterious art of soldering.

Since news has got around about my Harvard’s mishap, a number of folk have asked, “Why spend so much time and effort on the rebuild? After all, it’s an old model, and badly damaged.” Firstly, for me, the rebuild represents hours of workshop entertainment, and secondly, in aeromodelling terms the Harvard is an old friend, and it seems unfair to simply abandon our relationship just because of one bad day. When all’s said and done, all successful relationships need a little work now and again, although I admit that in the Harvard’s case, for ‘little’ read a ‘lot!’