Introduction: Foam Built Model Airplanes
Two articles on constructing large and small radio control model airplanes from cheap, easily sourced polystyrene foam sheets.
Step 1: Polystyrene Model Airplane Design
Why Polystyrene Foam?
Model aircraft construction in polystyrene offers several
advantages over conventional methods. Cost is far lower, structures are less complex and tend to absorb more damage in a severe crash, sparing vital engine and radio components. Time taken is often less, and there is a fair amount of “artistic” effort involved in shaping components, which some find very rewarding. The parts count on any particular airframe is usually far lower than from conventional building techniques. Different shapes and designs can be experimented with quickly and cheaply, and most important of all, the airframes have a high degree of “flyability”. From experience I construe this to be largely because they are light, and have on average thinner wing sections than conventionally built aircraft. Thin wing equals less drag.
Step 2: The Raw Materials
Sourcing polystyrene is not difficult. Local D.I.Y and
Building Merchants are the best suppliers. Sheets come in eight foot by four foot sizes, and a variety of thicknesses. 20- 30 mm is best. White Polystyrene foam is far cheaper, lighter, and more available than blue foams. It just requires a little more reinforcing in strategic areas to render it fit for purpose. Take a craft knife to the store, and measure and cut your sheets in half to transport them home in the family car. One sheet provides material for several models. Brown paper is the next ingredient. Rolls of brown wrapping paper are available in many stationary outlets. Other wrapping paper may be used. Avoid silver “metallised” papers, as these can cause problems with RC receivers. Balsa and spruce for wing spars, plywood for dihedral braces and engine mounts, PVA and epoxy glues are the other major requirements.
Tip: Avoid balsa cement, Ados type contact glues, and Cyanoacrylates, (Superglues) as these all attack polystyrene foam.
Step 3: Getting Started
Decide what you are going to build, and get any
drawings, photos, 3-views necessary. You need to decide on the basic construction you will undertake. 25mm polystyrene sheet can be carved to produce quite rounded contours. Let’s say you are going to build a 49” span model of the Mitsubishi Zero, a great design by a guy called Jiro Horikoshi. One of his main design parameters was lightness above all else - you should do the same. Decide on the dimensions for the plane, and then start cutting! Start with two equal fuselage sides, and a couple of rectangular formers to join them together at the correct (outside) dimension. (see Fig.1)
Tip: cut with a very sharp craft knife blade, fully extended. Draw the
blade firmly along the line you want cut at a very shallow angle. Repeat if necessary. The main point is; keep the blade sharp, and moving. Use an oilstone to whet the blade every few strokes. Replace the blade every month or so. (see Fig. 5)
Glue the two formers to the fuselage sides with PVA glue and pin them for
an hour or two.A 1/8 inch balsa doubler can be PVAed to the insides of the fuselage, from the nose to the rear of the wing position. Pull the two ends together and glue, clamping with a clothes peg. You now have the basic structure of the fuselage to start doing things to. At the front, insetting a ply engine mounting plate is best done with 5 minute epoxy. Engine bearers glued underneath, and a thin piece of ply bottom and back make a fuel proof “box” under the engine. At this stage you also have to decide on the various thrust and incidence lines, etc. (see Fig. 2)
Tip: Try going with a “zero –zero –zero“ setting for your Zero. No pun intended: this means the engine has no up or down thrust (maybe a little right-thrust) the wing is set at zero incidence, and the horizontal tail plane is also at zero incidence.
Laminations of Polystyrene sheet can be built up to
form the basic shape of the cowling, epoxied either side of the ply engine mounting plate, and carved to allow the engine to be dropped in. The top of the fuselage should be added now. For a small model like this Zero, the curves will allow this to come out of one piece of polystyrene glued to the top, then carved and sanded to the correct contour. The bottom of the fuselage is left open at this stage to allow for installation of the radio equipment. The fin and tail plane are next, carved out of polystyrene sheet, with a basic symmetrical aerofoil shape carved into them. The rudder will be hinged with conventional hinges, so it can curve in and taper towards the tip, but the horizontal stabilizer must have a flat, straight top near the trailing edge, as it will be hinged with the paper covering material. And if you curved the top, you would have a curved hinge line, which wouldn’t work effectively, if at all. Hard balsa spars are added at the trailing edge of the stabilizer and fin, and the leading edges of the elevators and rudder, which have been cut free. Remember to allow for the thickness of these spars in the overall shape of the surfaces. The edges of the spar and elevator are bevelled to allow full down deflection of the control surface. Thin strips of masking tape can be added to the elevators and fin to simulate scale rib spacing. They will show through the paper covering. (see Fig 3)
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