RC Foamie Vulcan Bomber
It is reasonably easy and low cost to make a RC plane from Styrofoam. Foam costs almost nothing - about Euro 2.50 for a block of 5x50x150 cm - and it is available in various thicknesses, 5, 8, 10 and 20 cm being common.
I am building a delta wing plane that resembles a Vulcan bomber, with a 1.2 m wingspan, and a Ducted Fan Engine inside the fuselage.
There are two
kinds of styrofoam, Expanded Polystyrene (the stuff used for exterior insulation of a
home) and Extruded Polystyrene (More smooth and easier to sand, but
heavier and used for roof insulation). Both kinds will cut super
smooth with a hot wire and it is the same plastic, so you can just as
well use the cheaper and lighter white beady stuff, since if you tune
your cutter temperature properly, then you don't need to use sand paper.
For reference, Polystyrene melts somewhere between 170 and 280 Celsius - it depends on the foam and the air pressure where you live. Therefore, you need an adjustable power supply and twiddle it to find the optimal current for your wire. I found that a steel guitar B-String starts to cut very slowly at 2 Ampere and cuts slowly and effortlessly at 2.5 Ampere. If you set it too hot, then it will make a too wide cut and if you set it too cold, then it will stick and the wire will flex and may even break if you force it. If it glows red hot, then it will just make a mess and break - the right working temperature does not glow.
A hot wire cutter is quite easy to make. Buy a packet of cheap strings at a music store, screw some wood together and put a rubber bungee cord, or a spring from a chest expander on the back to keep the wire taught. Musicians are artists after all and they also like to make things - if you tell the guy at the store that you need guitar B-string steel wire for a hot wire styrofoam cutter to build a radio control model plane, then he will understand and give you a packet of the cheapest strings. Make the bow at least wide enough to cut the width of a block of foam - 60 cm works for me. I glued little bolts into the ends of the wood to attach the string and pieces of copper wire, to solder the power wires to.
The trick is to cut very, very slooowly, while sliding the wire over templates (Melamine, Bakelite, Masonite... something thin and stiff), stuck to the foam with three or four sewing pins. With my bench power supply, a 60 cm guitar B-string works well at 5 V and 2.5 A. Therefore a big 5V USB wall wart may work for you, but it may not really be up to snuff and the magic smoke inside may escape. If you are like me and has a big bin full of old wall warts, then maybe you can just try them all until you find one that works well enough.
Toothpick trick: Stick three or four toothpicks into the leading and trailing edges, to guide the hot wire in and out of the foam block in a straight line.
You can smooth ridges in the foam with a fine sanding block. Do it outside, or the missus may want to divorce you. Holes and rough spots can be filled with light weight wall board spackle such as Tmel - when you pick it up, the container feels empty!
The best glue for foam is Bostic Mamut high hold construction glue. UHU makes a cyanoacrylate superglue that works. Any white wood glue also works. Test your glue on scrap foam as the wrong glue will dissolve the foam.
The best glue for foam is Bostic Mamut high hold construction glue. UHU makes a cyanoacrylate superglue that works. Any white wood glue also works. Test your glue on scrap foam as the wrong glue will dissolve the foam.
For wing spars, I prefer carbon arrow shafts, since they are very light and stiff. A pack of twelve costs about 12 Euro. Simply cut teeth in one end with a triangle file and stick it in a drill to slowly make a perfect fit hole. Then glue a shorter one in place with PVA wood glue.
You could use 8 or 10 mm wooden dowels as lower cost spars. The process is the same - sharpen a longer one and slowly drill a hole with it, then glue a shorter one in. Wood adds a little weight though. Note that a spar does not have to run the full length of the wing. The bending force is mostly at the root.
Only put glue on the last 10 cm or so, not the whole spar, or it may get stuck before you can push it in all the way!
To fit the wings to the fuselage, I put four pieces of aluminum tube in the fuselage and two short spars in each of the wings - keep them staggered. The spars slide into the tubes and then I drill little holes through for retaining wires - simple. First dry fit everything together and align it - only then glue the tubes in place to make up for any skew, since drilling holes is never perfect. When drilling a spar hole, you could start drilling at the wing tip, since the root is much thicker, there is more room for error.
In retrospect, next time, I would make the wings from two sheets of foam and then sandwich the spars in grooves, in between. That will be much easier and will ensure perfect spar placement. When I started this build, I did not know how well Mamut glue works and thought a single block of foam would be best.
Small pieces - wing tips, fins, elevons - can be strengthened and mounted with bamboo skewers and toothpicks. Put a skewer in a drill to make a hole, then wipe it with wood glue and stick it back in. Toothpicks, you can just glue and press in.
The elevons should be about 12% of the wing area, which brings us to the problem of hinges. For small planes, I glue small PVC tabs into the split ends of small pieces of skewers, so that I can push three hinges into the wing and flap with some white glue. The simplest method is to stiffen pieces of nylon rope with super glue and then stick them into the foam with more glue. There are endless ways to make light weight plastic hinges.
Another way is to simply glue the flaps on with Mamut or silicone grout - use masking tape on the top surface to hold them together and run a 2 mm bead down the groove to form a continuous hinge. Mamut works as a hinge, but silicone RTV is more flexible.
This is much easier to do than it sounds: I cut a V, stick tape on the top side, then open it up and apply glue on both faces, then put it down flat and work the glue down into the V with the back end of a skewer. The resulting bead is the size of the skewer.
For this bigger plane, I ended up going back to the Bronze Age and made copper wire hinges by wrapping 1 mm wire around a suitably sized jewel screw driver. The advantage is no friction and no play.
The vertical fin should be about 10% of the wing area and the rudder about 25% of that. The rudder is mainly used for take-off and landing. During normal flight I only use the elevons.
I make some bends in the wire hinge ends to give the glue a better grip, cut slots in the foam and work Mamut glue in there with a Starbucks stir stick, wiggle the hinge in and wait a day for the glue to cure. Do put some masking tape where the glue should not go, so you don’t get a stuck hinge.
Of course, there are cheap nylon hinges available at the hobby shops, but what is the phun in that?
You can make a wire control horn the same way, but do use stiff music wire for this!
Speaking of smoke, polystyrene smoke has the same toxicity as wood smoke. So, it won't kill you, but rather do your styrofoam cutting on the patio/garage/shed, not in the kitchen. Polyurethane smoke however, can kill you - so absolutely don't attempt to cut upholstery/mattress foam with a hot wire. For mattress foam, you need an electric steak/turkey carving knife.
Finishing a foamie can be a bit difficult. Some people cover the foam with Monokote or Oralite low temperature iron on covering film available from www.rcworld.co.uk but it costs an arm and a leg. You could also use wrapping paper, glued with diluted white glue. Packaging tape is much cheaper and available in multiple colours if you search around on the wild wild web, but will likely be a bit wrinkled.
Otherwise, you could give your foamy a light coat of exterior grade PVA (latex) house paint, or acrylic arts and crafts paint if you want more fancy colours.
European Red Tape
This size toy plane will weigh about 1 kg, so there are some crazy regulations to comply with.
Legal issues:
You can get a hobby pilot license here:
You can register as an operator here:
La voila!
Herman
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