Joe Collicutt describes his finely tuned method of building from plans using Depron rather than balsa.
Lots of people have become used to using Depron sheet as a building material in the past decade or so and some have designed whole models out of the stuff. A typical example being the Ultimate biplane by Dave Royds, as featured in the December 2009 edition of RCM&E. However, I am not aware of many builders who have employed it extensively to make a sizeable scale model from plans designed for balsa wood.
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Recently, I decided to experiment with the idea and have had some success with a Dennis Bryant Chipmunk and a Brian Taylor Mk 1X Spitfire, both of which you may have seen in the magazine’s Pilots’ Pictorial pages over the past year. As a consequence, your friendly Editor asked me to write about the experience.
At the outset I ought to point out, if it is not already obvious, that using Depron throughout a model precludes the use of IC engines and dictates the use of electric power. Although not a great fan of wiggly amps, I was prepared to go this route to achieve my aim.
One of the attractions of Depron is its cheapness relative to balsa, but the overriding advantage for me is its lightness. It also happens to be easy to shape, albeit not with as good end results as those achieved with wood. Since it is light, as with all foam, the resultant models fly more slowly than their wood equivalents and this is what I primarily seek for scale models. The sight of Chipmunk models flown like aerobatic Extras makes me wince. So, a couple of years ago I ventured to make a Dennis Bryant Chipmunk, essentially from Depron.
Having previously built the model as designed, I was already familiar with the build process, which was a great help. The method of constructing the fuselage, using two balsa half shells stuck together longitudinally, was easily translatable into Depron, with little need for any reinforcement. However, it was clear the tail feathers and wings would require some stiffening with balsa in certain areas.
It is useful when employing a totally different material from that in the design to have a reasonably good appreciation of the basic principles of aircraft structures.
This does not require a deep understanding of primary, secondary and tertiary structures but this does give confidence in your decision making. Fortunately, most model fliers will quickly gain a knowledge of where the major stresses occur when witnessing a wing folding in flight, an undercarriage collapsing on landing or hinges working loose. So, when studying the plans at the outset of the build you can locate those areas likely to benefit from some added strength.
Inevitably there will be some areas where you will decide the original material must remain. These include spars, inter spar webs, undercarriage support, motor mounts and wing joiners. Having said that, it is often possible to substitute lite ply and thinner ply for hardwood and thick ply in both the U/C and engine areas, bearing in mind that less weight needs to be supported. The main reinforcement used was 2mm hard balsa strips. These were added to leading and trailing edges of wings, aileron and flap bases, fin and tailplane trailing edges and rudder and elevator leading edges. In some places it made more sense to replace balsa with blue foam block rather than Depron e.g., wing tips, tail and wing fairings and the nose area.
As you can imagine, with the extensive changes made, when the time came to test fly the completed Chipmunk there was more than the usual level of apprehension. However, a number of successful flights proved that the concept of replacing balsa with Depron in most areas was feasible and the resultant saving in weight produced the sought-after scale-like speed in flight. This saving can be up to 40% of AUW.
Buoyed with this success and inspired by the Silver Spitfire’s around the world ‘Longest Flight’, I decided to tackle the more demanding Brian Taylor Mk 1X Spitfire. The collection of photographs depicts various parts of the model during the build process and illustrate how Depron was utilised for the different components and where the original material has been retained.
The fuselage build hinged around a 6mm Depron spine cut in the shape of the fuselage profile. This not only stiffened up the fuselage structure but acted as a convenient jig for setting the thrust line, wing and tail incidences, as well as defining the location of formers.
TYPES OF DEPRON
Here it is worth mentioning, for the benefit of complete novices to Depron, the different forms of the material and how it can be manipulated.
Depron sheets come in a variety of forms and thicknesses. Some are more suited to particular components than others. I use 2 or 3mm white or grey for skins, thick for wings and fuselage, the thinner for tail feathers and control surfaces. Both are flexible enough to cope with most curvatures, provided suitable panel sizes are chosen.
I use 4 or 5mm ‘board’ or 6mm plain sheets for ribs and formers. The board is simply Depron with a paper or card skin. This is pretty rigid stuff and seems to work well.
Where a component required thicker than 6mm and was not a tertiary structure and candidate for blue foam, I would laminate to the required size.
The shape of components can easily be transferred to Depron sheet utilising its soft nature. I either make pinholes through the plan onto the sheet below or trace outlines, and with the tracing over the sheet I follow the outline with a pencil, which produces a slight indentation in the surface. The components are then cut out using either a Stanley knife or a scalpel, replacing blades on a regular basis. It is easy to tear Depron with a blunt blade.
The two main glues I used to fix Depron to Depron were foam safe contact adhesive (gallons of it!) and 5 minute ‘Lumberjack’ wood glue. I found that pins were not as effective as masking tape for holding components while the glue was setting. Contact glue was used for skins and the wood glue for most of the other components. Epoxy resin was employed as you would expect for ply to ply or ply to Depron.
As Depron is much softer than balsa more care is needed to avoid permanent depressions in the surface when cutting or shaping components. In all other respects I find Depron quicker to work than balsa and I like the fact that you can achieve most things with just a sharp knife, a straight edge and abrasive paper.
UNDERCARRIAGE & MOTOR MOUNTS
I decided it would be prudent to modify the design of the U/C mounts so that loads could be spread to a wider area of the structure. The 1/2” square hardwood U/C mounts on the Spitfire were replaced by longer lite ply plates and 1/8” thick ply mounts. The forces are now spread over three ribs instead of two in the original design. Also, the extra width increases the contact area, improving the resistance to the twisting moment created as the wheels touch the ground.
As for the motor mount, there are several good examples in existence of how builders much more skilful than me have converted plans from engine to electric motor propulsion. I unashamedly copied their ideas, using a simple box structure of ply and lite ply for the Chipmunk motor and battery support, and an even simpler tray design for the Spitfire. The tray was held firm by an improvised network of Depron formers and longerons infilled with blue foam.
This system was considerably lighter than the heavy ply and hardwood mount on the original plan. However, not all this weight saving was achieved in practice since some lead was needed in the nose to arrive at the correct balance.
A Depron model, like all foam aircraft, is unlikely to match the detail and finish or overall beauty of one based on wood. This is especially so in my case since I lack the dedication and patience required for that competition winner! My efforts to achieve a passable finish consist of applying a mixture comprising equal amounts of water-based varnish, filler and talcum powder. On occasion this will be brushed onto a covering of tissue paper or nylon. Halford’s acrylic paint and decorator’s emulsion provides the colour.
Always conscious of my main aim, which is to achieve the lightest airworthy model, I finish with less rather than more. The net result is a semi-scale model of indifferent appearance but which, in my opinion, looks great in the air.
The recent revelation that there may be a shortage of balsa available to model makers in the future owing to the demand for wind turbine wood could make Depron attractive to a wider clientele than hitherto. In which case my experiences may be of some help.
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