You’d be surprised just how much damage a model can sustain and still be repairable. Take the club member who flew his new ARTF CAP 232 into the concrete, for example. The fuselage was reduced to a mass of splinters, but after a spell in the workshop his model was back in the air looking as good as new. However, there was nothing miraculous about this resurrection, which came about because the owner not only flies ARTFs but also builds aeroplanes from scratch, and so understands both how models are put together, and how they can be repaired when they come apart.
Unfortunately, this understanding isn’t something that you’re likely to have gained by assembling ARTFs. So, in order to demonstrate some of the methods that’ll help your model bounce back from a mishap, I’m going to put you on a crash course, quite literally, by rebuilding an ARTF that has been badly damaged. And I hope you’re paying attention because the model in question – a Seagull Spacewalker – belongs to me. With a span of 50”, the Spacewalker was designed for electric power, but I’ve converted it to i.c. using an SC 15. Otherwise, however, its construction is typical of ARTFs, and it’s a delight to fly – or at least it was, right up until the moment that I handed over the controls to a friend and asked him to make a low-power dive straight into a concrete runway!
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The impact ripped the thin fibreglass cowling, and made matchsticks of the fuselage forward of the leading edge. It broke one wing in two, damaging a rib in the process, and snapped the fuselage behind the trailing edge. The engine, on the other hand, survived remarkably well, though the carburettor was bent and had chunks broken off the venturi. The advantage of SC carbs, however, is that they’re not only very good, but inexpensive to replace.
From the point of view of this article, then, the wreckage of the airframe was everything you could hope for, providing a perfect proving ground for a variety of reconstruction techniques. With this, the fuel tank was drained, the oil residue wiped off the larger bits of debris, and the model’s mortal remains swept into a bin bag.Article continues below…
This is how the internal structure of the fuselage looked after removing the localised damage.
…AND THE ANALYSISArticle continues below…
One of the problems with ARTFs is that the methods used to mass-produce them involve paring the materials down to the minimum. This is fine for weight saving, of course, but it does lead to some components having very thin sections, or being cut from the sheet in such a way that leaves the grain running across a narrow dimension. The end result is that, when the crunch comes, the structure breaks up into smaller pieces than home-built models, whose hand-cut parts arguably have a little more meat on the bones and therefore tend to be that bit more robust.
The shattered nose section looked like this. Bad, but not irreparable. Jigsaw puzzle anyone?Article continues below…
The new wing parts ready for fitting.
Despite this, the post prang examination revealed there was more than enough left of the Spacewalker to allow her to be rebuilt. The snapped wing hadn’t lost any wood at the trailing edge, so I’d be able to use that to get the spacing correct when I rebuilt the leading edge. Similarly, the break in the fuselage had enough splinters sticking out to help me align the two halves. At just £10 a time, it was tempting to simply replace the cowling, but it only takes a little fibreglass wing joining tape and polyester resin to repair damage of this sort. The real work, however, was clearly going to lie in the front end, which needed to be rebuilt from scratch. Mind you, even this isn’t as hard as it sounds. In fact, despite the model’s appearance, she’d suffered nothing that couldn’t be put right – which means that damage of this sort needn’t write off your ARTF, either. So let’s look at the different ways of repairing it.
Coming together. Note the clamps holding the bridging straps to the main spar.
Start by removing anything that projects beneath the wing, such as servos and horns, so that you can lay the panel flat on your bench. If the control horn is glued into place, as it was here, simply let the aileron overhang the edge of the bench.
These cross-strips are glued to the formers to stop them buckling.
The spar is the wing’s main structural member, of course, so to join the broken ends securely, I made up a new section that accurately replicates the construction of the original, which happens to be a sheet spar that runs the full depth of the wing and has slots for the ribs. Most models, however, use a spar made from strip wood on the top and bottom of the wing, with a web of thin sheet between the two. The approach to repairing both types of spar is the same, though: Make the new section from the correct grade and size of wood. If your ARTF’s original spar was made of a hard wood, then it would be worth using spruce rather than balsa to make the repair section.
The new piece (coloured) after being grafted into place; the bridging straps are in support from behind.
While I was making the piece for the spar, I also made a new length of leading edge, staggering the joints where it would be grafted onto the existing leading edge so as to avoid concentrating the weak spots in one area. Incidentally, a sheeted leading edge (repairs to which were covered last month) doesn’t require bridging straps at the spar or leading edge. It only needs them on the sheet at the ends of the repair.
The most important thing to remember about making these repair sections is that the joints between the old material and the new must be a perfect fit (as near as possible) and tapered (scarfed) over at least four times the thickness of the material.
The front of the fuselage was reconstructed using new sides and formers.
When I came to the damaged rib, I found that some of its sections were missing and decided that the easiest solution would be to make a new one by removing a complete rib and using it as a pattern. I didn’t bother duplicating the lightening holes, though: they don’t really save enough weight to be worth the reduction in strength that they cause.
Once satisfied that all the new parts would fit properly, the broken halves of the wing were pinned to the building board and rejoined, using bridging straps made of 1/16 sheet to reinforce both sides of the joins.
The reconstructed fuselage sides.
As you can imagine, this grafting technique – which you can also use to repair trailing edges and ailerons – will help you to save airframe parts that are in a worse state than the Spacewalker’s wing. If, for example, the damage had been such that I hadn’t been able to use the trailing edge to match up the two halves, I could have either used splinters as temporary guides to position the pieces, or even measurements taken from the opposite wing, to lay out the halves while I made replacement sections to join them.
Again, the first thing is to remove as much equipment as possible – tank, radio gear, switch and undercarriage – to give yourself room to work. Then trim away the covering from the damaged area.
On inspection, the Spacewalker’s formers proved rather thin and weak, so I reinforced the narrow parts (where the grain lies in the wrong direction) by adding strips of balsa.
The completed nose repair, ready for covering and a refit of the radio equipment.
Using the ragged edges of the break, I was able to align and space the two halves of the fuselage as I pinned them onto a board. From here, it was a matter of cutting pieces of wood to replace the broken sections. The trick is to make these replacement parts longer than the break and then cut them to create a tapered scarfing joint. Then, using the new sections as patterns, you can cut away the broken area to match their taper, glue the replacements into place, and fit the bridging straps behind the joins. I’ve coloured the model’s replacement sections in the photo (above) so you can see how they’ve been fitted into the fabric of the original airframe.
If your ARTF’s fuselage is built up from square strips, you can use the same technique to repair it, making the bridging straps or backing pieces from the same material as the sides. Remember, too, that these backing pieces should have a taper on both ends in order to create a progressive change of section, since sudden changes in thickness serve only to create stress points.
You may find that the final repair pieces have to be fitted after the fuselage has been unpinned from the board. If so, make sure that the assembly can’t flex, or you might end up with a bent fuselage, which will produce some very funny flying characteristics, possibly leading to even more repair work after the test flight! “What about the rolled sheeting on the top of a fuselage?” you ask. Don’t worry, I’ll come to this shortly.
Getting there! The assembled forward fuselage complete with formers and stringers.
ADAPT AND OVERCOME
Just as every crash is different, every repair will be different too, but the basic approach to repairing a model remains the same (piecing parts together and making new ones) as do the common-sense guidelines:
- When a part such as a spar is broken in several places, don’t try gluing it back together. Every joint creates a weak spot, and the fewer joins you have the stronger the repair will be. Instead, make yourself a new section that will replace the damaged area in one go.
- Always taper joints to avoid sudden changes in section. The latter only serve to concentrate stresses.
- Joins in a spar or main fuselage member should have backing pieces over the joint or, in the case of a join in sheet, thin sheet edge joiners.
- Don’t use glue to fill a joint. The strongest joints are those without gaps in the wood.
- Always double check alignments as you glue parts back together – bent fuselages and twisted wings are very bad news indeed.
THE TRICKY BIT
The hardest part of the Spacewalker to repair was the bit closest to the accident – the front of the fuselage. Here, the wood was broken and splintered back almost to the middle of the wing, taking with it the locations of the various holes for locating the wing bolts (though on models with a one-piece wing, of course, this won’t be a problem). The formers were also split and broken, and piecing the whole lot back together was quite a job.
However, with a little forensic work involving careful positioning of the broken parts and accurate measurements, I was able to cut two front fuselage sides from sheet. These pieces then served as guides as I trimmed the broken fuselage sides to match, before being fitted and braced with bridging straps.
The old formers were used to cut new ones, though the eagle-eyed among you will notice an anomaly. In one illustration (all, incidentally, taken from actual photos of the wreckage and repair), you can see two small formers and a front former, while a later drawing shows a different former for the rear part of the repair. The moral? Always be prepared to change your approach if you think there’s a better way of doing things.
With the two sides glued into place, the formers were added, together with the top stringers and, finally – as promised – the rolled sheet turtle deck.
Don’t be disheartened by a crash of this nature, practically anything can be repaired you know.
When only a short section of rolled sheet is needed, the best thing is to trim back the turtle deck until you reach undamaged material, and then glue bridging straps to the edges ready to receive the new section. To make fitting the replacement sheeting easier, the old deck can be trimmed in a straight line across the fuselage; this won’t compromise the fuselage’s strength, which should really lie in the sides.
The rolled sheet itself is quite simple to make. First, select a wood that will bend easily across the width. Be careful how you test this in a model shop; if you crack the material, the proprietor won’t be happy! Then, cut your replacement pieces to size, glue one edge of each to the fuselage sides, and leave them standing vertically until the glue has dried.
Once cured, wet the outside of one sheet and apply heat (if you don’t have access to a heat gun, a travel iron will do) so that you can gently bend it over until it touches the formers. Repeat this for the other side, and then trim the two pieces so that they meet neatly over the central stringer. If there is no central stringer, it’s worth adding one just to help support the join in the sheeting.
Finally, most ARTFs have assorted small pieces of ply to take screws that hold down cowlings, etc. These can either be salvaged from the wreckage and glued into place or made afresh.
If the damage to the fuselage had been even more extensive, it would probably have been possible to piece together enough patterns from the wreckage to scratch-build two complete fuselage sides and all the formers. That’s how the CAP 232 I mentioned earlier was rebuilt, and the model is actually much stronger for having solid sheet sides.
The principal tools needed for simple repairs were listed last month. The only additional requirement for more complex repairs is a fret saw, which you’ll need when you start working with plywood.
A simple hand fret saw will cost you from £7.00 upwards, while the spare blades cost about £2.50 a dozen. Otherwise, you can use a coping saw, though you won’t find it as handy when cutting tight radii.
If you have trouble finding a fret saw, call Squires (01243 842 525) for a copy of its 480-page catalogue. Every modeller should have this catalogue – it’s free, Squires’ minimum order is just £10, and orders are also sent post free.
In terms of materials, substantial repairs will call for balsa sections, 1/8” liteply, and 1/8” birch ply for formers and strengtheners, though it’s probably best to buy the wood once you have worked out just what you need for the repair in question.
THE FINISHING TOUCH…
Once all the woodwork’s done, you’ll be ready to hide all the joins by restoring the covering. As I mentioned last month, there are two coverings in common use on ARTFs: regular film, and a material resembling the old Fablon self-adhesive plastic sheet. If your model is film covered, you can make your repairs with the aid of nothing more than a small travel iron. Self-adhesive coverings, on the other hand, are best repaired with Solarfilm Solartrim, finding as close a match as possible for the original covering.
…AND THE FINAL WORD
I know that a lot of people who fly ARTFs don’t believe that they can repair them after a bump or crash. I also know of a model shop that has a back room full of crashed ARTFs because of this attitude. If that’s the way you feel, I won’t argue with you. However, I will just say that if there’d been ARTFs when I started modelling, at 14 years old I’d still have been able to keep a damaged model serviceable. Back then, of course, we had to use our brains to make things rather than simply open our wallets and buy new. One to remember before you decide to write off your ARTF rather than attempt a repair.
It’s always easier to make a part and then trim the damaged section to fit than try to make a part that fits the damage.