Chapter 5

 

Chapter 5

Building your model

Research shows that, in order to be a model builder, one thing is essential; you must build models! If you're starting off with a kit or magazine plan, you'll have an easier time following the drawings if you already know the basic methods for converting raw materials into airplanes; and a kit model can often be improved by adding a trick or two of your own. When you design your own ships, you'll want to have a variety of techniques at your command; so let's look over the standard building methods used for various types of model.

Simple gliders and ROG models are usually assembled from sheet balsa. If you are not building from a kit in which parts are die-cut, transfer your patterns for wing, tail assembly, and other parts to sheet balsa of the correct thickness. Select the proper grade of wood for each part, and fit the patterns to the wood using the straight edge of the sheet where convenient; cutting a piece from the center of a sheet is wasteful. To transfer patterns, trace them on plain paper, fit the paper to the wood, then lift the edge and slip carbon paper underneath.

Always lay out parts with the grain running the long way of the pattern, unless it happens to be a piece you intend to bend; balsa bends parallel to the grain (Fig. 15).


fig.15
fig.15
 

Cut out the parts with a razor blade, being careful to hold the blade perpendicular to the wood, not at a slant. If the wood is tough, make two or three light passes rather than trying to dig through with one; it's

both easier and neater. Cut outside the lines and sand away the excess to insure accuracy. Do your cutting on your workboard - the hardwood surface of the dining room table is tough on razor blades!

With the parts cut out, sand all leading edges to a rounded shape for streamlining; be careful not to round an edge which is to be cemented against another piece.

When assembling parts, use only a little cement, smoothly applied. For extra strength in important joints, double-cement by coating the parts and allowing the cement to dry, then cement again and join.

Some larger models employ a solid balsa profile fuselage, cut from thick sheet balsa; or a hollowed-block body, made by lightly cementing two blocks, carving to shape, separating and hollowing, then re-cementing. These are specialized types; more advanced models usually employ built-up construction, in which a framework is assembled and covered with tissue or other material. This produces a lighter and stronger structure, and makes possible interesting and realistic contours and efficient airfoils.

The box fuselage is versatile and simple to build accurately. The two sides of the fuselage are built over the plan (or sometimes cut from sheet balsa) and joined together by spacers, formers, or bulkheads. Cover the plan with waxed paper before starting assembly of the first side. Select the toughest balsa for the longerons and stringers and lay them out on the plan, holding them in place with straight pins (Fig. 16a); then cut and add the spacers, using cut-offs and medium wood unsuited for longerons. Careful cutting of the ends of the spacers for a good fit will increase the strength of the model. Make two of each piece as you go along, using the same strip (or strips of equal hardness) to insure perfectly matched sides. Place the extra pieces near their proper positions to save confusion. Use just a bead of cement on each end of the spacer, settle it firmly in place level with the longerons, and hold it there with pins if necessary. Don't stick the pins through the wood; put them alongside (Fig. 16b). Wipe off excess cement as you go.

When the first side is complete, leave it in place and build the second one directly above it. Be careful not to make your joints too juicy, especially on the underside, or you'll encounter difficulty in separating the sides afterward.

Allow half an hour for the cement to set hard, then pull the pins and remove the two sides from the plan very carefully. They will stick together, so leave them joined until you've sanded them carefully along the outline and on both faces, using a sanding block and fine sandpaper. Then use a razor blade to separate them.

If the two sides are to be joined by spacers of strip balsa, cement a pair of spacers to one of the sides, on the surface which was against the other when built (this leaves the sanded side out). Usually it's best to start at the widest point of the fuselage. When the two spacers have set (be sure they're straight), lay the side flat on the board and attach the other side; block up the rear so that the sides are parallel, or at an angle, as the plan top view indicates. Allow a few minutes to set up, then carefully draw the ends of the sides in to the proper distance (usually touching), and join (Fig. 16c). Don't use too much cement; it may soften the glue holding the side frames together and let things get out of control. The rest of the spacers can be added next. Hold the sides together with rubber bands wherever needed while drying.


fig.16
fig.16
 

If bulkheads are used, join the sides on the widest one, then proceed as for spacers. Before adding too much structure, be sure to install any necessary items inside the fuselage, such as L.G. mountings, engine mounting nuts, controls, etc. It's pretty difficult to lace landing gear struts to a bulkhead or drill mounting holes in a firewall after it is built into the model (Fig. 16d).

Add any necessary formers to the box only after it has been completed and squared up. Pieces of balsa strip, set diagonally, can be used to straighten out sections that are out of line. When placing stringers, check the notches for alignment, and whittle them a bit if necessary to straighten things out - but don't mistake a crooked fuselage for misaligned notches. Place stringers symmetrically, one on each side to avoid uneven stresses. To help bend a stringer around a deep curve, you can split it parallel to the curve, and cement along the split. This procedure can also be used for longerons; it makes them stronger, and prevents them from pulling the frame out of line.

A variation of the box fuselage is the diamond, where the sides are symmetrical and, after being joined, are turned on one corner so that the longerons become top and bottom keels and side stringers.

Fuselages having a strongly curved cross-section can be built in a number of ways. Crutch construction (Fig. 16e) is commonly used on contest-type, free-flight jobs, as it is strong, lends itself to pylon configuration, and gives a very solid engine mounting. The "crutch" consists of two heavy balsa members laid out on the top view of the fuselage and joined by spacers. This assembly represents a horizontal cross-section of the fuselage. Formers are added to it to build up the bottom portion of the body; then it is removed from the plan and the top half is added, plus any full-section bulkheads.

A similar method is the half-shell technique (Fig. 16f). Here the top and bottom outlines of the finished fuselage are represented by keels which are pinned to the plan. Bulkheads are split along a vertical center line and one-half of each is cemented in position. Then the half fuselage is removed from the plane and the other half is added. It is very important with this method to place stringers carefully to retain fuselage alignment. This is a good way to build a complicated scale fuselage, since it enables the designer to use outlines and cross-sections directly as patterns. To avoid cutting bulkheads, a fuselage can be built using a jig, which holds the bulkheads in place while keels, stringers, etc., are cemented.

If the fuselage is to be planked (see Chapter 6), provision for this covering is made by building the framework undersized by the thickness of the finished planking. For tissue covering, be sure the structure provides a smooth and continuous support for the paper. Stringers should project slightly from their notches, and bulkheads and formers should be sanded down between stringers to avoid ridges pushing up through the finished covering. Sand all joints carefully, after they are thoroughly dry.

No matter what kind of fuselage you use, make provision early for installing the power plant. The firewall or motor bearers for an engine have to be tough and solidly anchored - don't try to stick them on as an afterthought, or they're likely to come off the same way. Use fuelproof cement and plenty of it - but in thin layers, not all at once. Drill all necessary holes for mounting bolts and fuel line before installing the mounts, and secure the nuts firmly on the underside of the bearers or the back of the firewall by cementing a piece of hard balsa over them - or better yet, attach a metal plate to the back of the mount and solder the nuts to it (Fig. 17). Always attach the nuts with the engine in place, to be absolutely sure they're going to fit.


fig.17
fig.17
 

Install the fuel tank early in the game, too, not overlooking the filler and overflow lines. Spend a little time securing the tank in position for proper feeding; a tank that flops around inside the model can cause the engine to cut prematurely and is likely to fool the modeler as to how much fuel is in the tank; after stopping early a couple of times, the tank can bounce into a more favorable position, and keep feeding fuel to your poor old F/F while it churns right on up into the clouds.

If you're using rubber power, get the prop assembly ready first, starting with the prop itself. Machine-cut and plastic props are available, or you can make one yourself. To carve a prop, lay the outlines out on a block (Fig. 18) and cut out the blank. Start whittling by shaving off the right-hand corner of the uppermost blade until the top left and lower right edges are roughly joined. Then do the same for the other blade. Now turn it over and repeat. Take care to give the front surfaces of the blades a convex shape, and the backs a flat or a concave form. You can use the same technique to carve gas props in an emergency.


fig.18
fig.18
 

Another method is to cut a stack of balsa strips of the desired length and pin them together with a long pin through the center point. Use eight or ten strips about four times as wide as they are thick to build up the proper thickness. Fan them out widely and coat them with cement; then arrange them so that the tips retain enough overlap to allow shaping.

Sand your prop out carefully, and balance it by resting the two ends of a pin projecting from the center on two narrow strips set on edge; sand the tip which dips. Make the prop shaft from piano wire, and bend the end of the hook back so that it can latdi over the shaft to prevent opening under full turns. Slip the nose block and thrust button over the shaft, add a washer, bead and washer (or a ball-bearing washer) and then the prop. If the prop is to be free-wheeling, install the spring now, then bend the end of the wire at right angles. A pin projecting from the front of the prop stops the shaft from turning freely while pressure is on it; the spring disengages the prop when the rubber has unwound, allowing the prop to free-wheel.

By bending a loop in conjunction with the final bend, you can engage the prop assembly in a winder for fast winding (Fig. 19).


fig.19
fig.19
 

When winding the motor, stretch it out to double length, and start turning, drawing it out to three or four times its relaxed length by the time you have a couple of hundred turns; then start coming in slowly, and finish up with the nose block back in place. After the motor is wound, let it go without delay; don't stand around talking it over - that's tough on the rubber.

For small jobs, make up and install the motor while the fuselage is still accessible; it's pretty tricky sometimes to feed it in through the front after everything is covered. Of course, on large endurance models you'll be taking the motor out to service or replace it from time to time, but since the model is larger, it's not so difficult. Small back-yard flyers often get through their entire flying life on the original four strands, so prepare them properly, and get them in as soon as you have something to anchor to.

To measure out a rubber motor, double the front-hook-to-rear-hook distance, and add one-third of the same distance; this will give you the length of two strands. Double back the same length for the next two strands, and so on until you reach the desired number. Tie the ends together securely, and bind them just above the knot with thread. Now, unfold and lubricate the rubber thoroughly.

Next, double the rubber up again to make half the final number of strands of a little more than twice the final length. Hook one end of this long loop over something (a doorknob will do) and twist the other end in the direction opposite to the normal winding twist. Give it about 50 to 100 turns, depending on how long the motor is; then carefully, to avoid kinking, bring the two ends together, and release the middle. The rubber will twist around itself, shortening up to the desired length. If it's still too long, undo that last maneuver, and twist it a little more. When it's just right, so that it will hang snugly between supports, bind the loose end with a bit of soft string to keep it from unwinding when released, and install.

Wings are generally easier to build than fuselages but in their construction precision is even more important. Straight material must be selected, and there must be no stresses set up within the structure which may later result in warps. Most wings consist of a leading edge, trailing edge, spar, tips, and ribs. These components can be assembled to make wings that are flat, or with dihedral or polyhedral, tapered wings or straight wings, knock-off or integral wings, etc.

Flat wings are widely used on C/L models, which require no dihedral for stability. These wings are extremely strong, and easily built. Sometimes the tips and leading and trailing edges are laid out on the plan and the ribs added, but usually, since symmetrical or deeply undercurved ribs are used, precluding a flat layout, the ribs are placed in position on the one-piece spar, and the leading and trailing edges added (Fig. 20a).


fig.20, fig.21
fig.20, fig.21
 

A wing for a small rubber-powered model can sometimes be built flat and the leading and trailing edges cracked, bent, and cemented to the proper dihedral angle. Polyhedral or dihedral wings for larger rubber jobs and engine-powered models must be built with the angles as an integral part of the structure, for strength. One method is to build the wing in sections, each being laid out flat on the plan. The parts are then set up in the proper relation, and ribs and dihedral braces added at the joints to form a unit. More often, the spar and edges are first built over a pattern with the correct angles made by splicing strip balsa. The prefabricated members are then pinned to the plan, one panel at a time being in contact, and ribs added (Fig. 20b).

Most wings are so designed that the main load is carried by the spar or spars; accordingly, tough, straight material should be selected for this use. Any necessary joints must be double-cemented, and reinforced, preferably with plywood or celluloid (Fig. 21).

Since the spar must resist vertical bending stresses, it is always higher than it is wide; the deeper the spar, the better. Spars can be made of a single piece of balsa or lightweight hardwood, or they can be spliced, laminated, or built up. A box spar, made from four strips assembled to form a rectangular cross-section, is strong and serves as a jig for assembling a wing. By laminating a layer of celluloid between two layers of tough balsa, an exceedingly strong spar can be made. It is important here to cement the layers thoroughly without gaps. When splicing spars diagonally, cut carefully for an accurate fit.

Leading edges are usually formed from a single strip of medium balsa of proper size, notched into the front of the ribs, square or on edge. Sometimes the ribs are cut square, and the leading edge is cemented directly against them. A flat L.E. key strip can be used, fitted into a notch to help in aligning the ribs. This makes a good auxiliary member, but is not rigid enough to serve unsupported.

On larger models a wide strip of sheet balsa, moistened and formed around the ribs, makes a very strong leading edge. An alternate method is to cut the ribs to a V at the front and cement a flat strip against each angle. Fig. 22 shows several popular L.E. treatments.

When splicing a leading edge, match the dihedral angles to the spar to avoid built-in warps. It's easier to shape a leading edge after it is installed, and after leading-edge planking and/or cap strips have been added, since the rib stations serve as guides. Notches can be cut in the L.E. for the ribs, for extra rigidity.


fig.22
fig.22
 

Trailing edges are often of constant cross-section, and you can precut them to exact size using a power saw. A tapered trailing edge can be most accurately cut by first making a constant-section beveled strip of the proper length, then cutting the taper by removing a tapered strip along the thick edge. Another widely used T.E. is made by fitting a flat strip under the rear edge of the ribs, and notching another along the top surface, to form a hollow V section. Sometimes a thicker strip is used, notched into the bottom surface, with cap strips on top of the ribs to strengthen the joints. Since trailing edges are thin, they can often be scored and cracked to the proper dihedral angle, with a generous amount of cement rubbed into the break. Such angles can be strengthened by using gussets against the ribs at these points. The T.E. may also be notched to receive the ribs (Fig. 23).

While there are many different ways to make ribs, cutting them from sheet balsa is easiest and works fine. For a wing of constant chord, cut the ribs simultaneously by pinning together a stack of blanks, tracing a pattern on the outer blank, and cutting out with a saw or knife. The ribs are sanded as a unit, notches are cut as needed with a saw, and a matched set of ribs results.


fig.23, fig.24
fig.23, fig.24
 

For very large models, ribs are sometimes built up as individual sub-assemblies in order to save weight. It is doubtful if any net improvement results; anyway, it's too much work. Cut your ribs from thin, hard balsa, and use cap strips, for maximum utility with the least effort. Smoother covering results when caps are used, especially if the strips butt against the rear face of the leading edge and taper out at the trailing edge. These strips are usually used on the top surface of the wing, but can be added to the bottom too if desired. They should be of the same thickness as the ribs, usually, but not over 1/8 inch thick even on giant ships with foot-long ribs. Make them three to four times as wide as thick, and be sure when cementing them that they are in contact with the rib full length. Hold the caps in place while drying by sliding pins at an angle into the leading and trailing edges across the strips; in other words, don't stick pins through the wood near the end, or it's likely to split (Fig. 24). Use a sanding block to smooth out the joints after the cement is hard.

There is a lot of variety available in wing tips. Curved formers cut from sheet balsa are used to form round or oval tips. A block of soft balsa serves for short tips up to about two rib depths wide. For intermediate tips, a piece of sheet balsa cut to shape will serve, with the spar cut at an angle to support it, and the covering continued over the top. With one of the methods shown in Fig. 25 you can build a tip of nearly any conceivable shape.

While one-piece wings are generally stronger and easier to build than two-piece designs, the latter are practical for extra-large models, and for some special applications, such as low or shoulder wing layouts, or scale jobs where a one-piece wing would detract from the appearance.


fig.25, fig.26
fig.25, fig.26
 

Two-piece wings are fitted with projecting members such as spars, wood or metal dowel pins, or lengths of tubing, which slip into sockets designed to receive them, either in the fuselage or the opposite wing root. These keying members must, of course, be just as strong as the rest of the wing. The two halves can be held together by hooked rubber or simply by a tight fit.

Both for strength and efficient airfoil, sheet balsa planking along the leading edge of a wing is desirable on any model having a span of more than 24 inches (Fig. 26). Material of the same thickness as the ribs is cut in sections to fit each panel. For this purpose use straight-grained, medium-hard balsa; if it's too hard it won't bend easily. It should extend back no more than a third of the distance to the trailing edge, and should be supported along the rear edge by a spar; otherwise, it will dip between ribs when the covering tightens up. It's a good idea to dope the planking on both sides after installing to prevent swelling and buckling from absorbed moisture.

After the wing is completely assembled, use a sharp knife or razor to trim off excess balsa from L.E., tips, etc.; then settle down for a half hour or so with a sanding block to shape the final contours. The L.E. cross-section must be sanded out to complete the curve of the ribs accurately; otherwise you have a built-in head wind to start with. Check the shape with a template at each rib as you work to be sure its the same, full length. Fair the tips into the L.E. contour and thin them out to fit the T.E. taper. Don't let any projecting humps get by you, where the ribs meet the edges, or where the spar fits into the ribs. A good sanding job is the key to an efficient (and neat-looking) wing.

Finish up with a coat of fifty-fifty cement-dope mixture over the entire wing structure and daub it thoroughly into all joints. This welds the structure into a single unit and doubles its strength.

A great deal of time and effort has been expended by model designers on engineering elaborate tail assembly structures. This is unnecessary. Sheet balsa makes excellent rudder and elevator surfaces, and is light and strong enough for use on any kind of model, with the possible exception of very large models using an unusually thick section. The rudder and elevator need merely be cut from medium straight-grained balsa and sanded to a mildly streamlined cross-section. For tails made from 1/8" balsa or thinner (this covers up to about 18-inch elevator span) merely rounding the leading and trailing edges will do nicely. Of course, if you are building a scale model, you'll want to follow scale airfoil.

If a built-up tail is necessary (and frequently when it isn't, if you build from kits), the structural method is usually similar to that used in the wing, with an outline consisting of leading and trailing edges and tips, and ribs or spacers in the case of a flat surface.

On control-line models and R/C ships, control surfaces are hinged. A simple hinge can be made using strips of cloth in pairs; one tip under the stationary surface and the other over the moving one, the other member of the pair being reversed (Fig. 27a). Another method is to slip cotter pins over a length of wire, the ends of which are bent at right angles and inserted in the trailing edge of the fixed surface, while the cotter pins go into the moving portion. A notch should be cut in both surfaces at the position of the cotter pin for free movement (Fig. 27b). If the movable surface fits into the stationary one so that both ends are covered, a length of stiff wire projecting from each end ca'n be engaged in tubing set in the fixed portion (Fig. 27c). In addition to these homemade devices, manufactured types are available at the hobby shops.


fig.27
fig.27
 

When cementing and doping hinged surfaces, be careful not to clog the hinges. If your control linkages are to be enclosed, don't forget to connect things up before attaching the surfaces permanently to the fuselage.

For free-flight models, a knock-off tail is standard equipment. The rudder is cemented to the elevator, and the assembly is attached to the fuselage with rubber bands, for which dowels have been provided in the fuselage. The surfaces should be sanded and doped before being joined, simply because it's easier to do that way.

Grain should run lengthwise in an elevator, and vertically in the rudder. If the rudder is built up of several pieces, edge-cemented, the forward portion may have the grain running parallel to the leading edge. Sub-rudders may have grain running either way, depending on shape and thickness; vertically is usually best.

Movable tabs for adjusting trim may be cut from the outline of the rudder after it has been shaped, and held in position by soft wire set in rudder and tab; a tab can be cut from thin aluminum with small points projecting from its edge, which are pressed into the edge of the rudder. On large rudders the points can be spread slightly, so that as they penetrate the balsa, they emerge from opposite sides of the fin. The projecting tips can be bent back and cemented for a very secure mounting.

The landing gear is an aeronautical encumbrance on a model, but it serves the valuable function of coming between a relatively delicate structure and the unyielding earth, making take-offs possible, preserving propellers, and in general making everything a little more civilized. All sport and scale models have landing gears, even though they are usually hand-launched and land in grass (or treetops), and contest jobs often carry a wheel. This portion of the model must be rugged, since it takes landing and take-off shock repeatedly. This is where tough plywood and piano wire enter the picture. The main legs of the gear must be capable of taking the stress without bending permanently, and without tearing loose. They should be bent from wire of appropriate diameter and held firmly to a plywood bulkhead or mounting plate by thread or wire lacing, sandwiched between two pieces, or held down by a metal plate bolted in position. Joints should be wrapped with copper wire and soldered; lesser measures are inadequate. J bolts and similar fittings are all right for small models, but they're too flimsy for large ones, besides not being positive enough in their holding action. Don't rely on auxiliary L.G. struts to help much; they are usually of smaller-diameter wire, and merely bend in a hard landing. They are used chiefly for the sake of appearance on scale models.

Even a properly sized L.G. will flex amazingly when a model hits the ground hard. A nose dive can swing 1/16" struts, 2 inches long, in a 90° arc forward; a 3/32" nose-wheel leg can spring back far enough to punch a hole in the bottom of the fuselage, and rebound without taking a permanent bend. So don't look around for gremlins when holes appear in your model after an unsuccessful landing; just remember to beef up the gear in your next job.

You can take a lot of the jolt out of rough landings by installing a shock-absorbing L.G., or nosewheel. Bend the leg so that it passes horizontally through a length of brass tubing firmly attached to the mounting bulkhead. The upper extension of the leg is held by a rubber band or spring, so that it will swing back against tension as the model contacts the ground, absorbing the shock. Of course, rubber wheels and spring wire struts are in themselves shock-absorbing, but not adequately so for a real clobber-in. Fig. 28 shows a couple of simple arrangements.


fig.28
fig.28
 

Heavy R/C models are often equipped with knock-off gear, apparently on the theory that even if everything else is smashed, the wheels will bounce clear and survive. Of course, the same amount of effort and ingenuity could be used to build a sturdy shock-absorbing gear; still, with knock-off wings, tails, engines, etc., the wheels may as well go too. Probably some ghoulish types just like to see 'em scatter.

Another variation is the plug-in gear, which is removable for storage or repairs, or just to change the looks of the model. The mounting projection of the gear slips into a socket formed by a metal plate bolted or laced to a bulkhead, and is held by friction. By installing several sockets in the model, you can switch from two to three or four wheels in a moment.

The last word in landing gears for models is the retracting gear. Designing and building this type of installation constitutes a large subject in itself, and is dealt with in a separate chapter.

Whatever type of gear you use, the wheels (or floats) must be securely retained. A washer or bit of wire soldered to the end of the axle is effective and simple for this purpose. File a notch in the axle to help grip the solder. You can grind the end of the axle off flush after soldering, for a finished look. If you don't have soldering facilities, bind tough thread around the axle as a retainer and coat it with cement, or get a set of nylon or metal wheel collars. Use another retainer on the inside of the wheel to keep it from binding on the axle.

In building any type of model or any part of a model, keep in mind the fact that balsa is an extremely flexible material; it's not only bendable, it lends itself to alteration, correction, and replacement when necessary. Flawed construction can simply be cut out of a structure and replaced; if you whittle too deep, just glue that last shaving back on and try again; it will be as strong as ever. Tricky curves can be handled by cementing a balsa block, or even assorted scrap balsa in place, then sanding to shape. Joints can be cut free, if you're not pleased with them, and whittled down and reglued. A good sanding will do wonders for even a very rough job of assembling. To patch a hole or thin spot, just cut out a square around the flawed area, insert a chunk of balsa, and sand it smooth.

There is no reason for a modeler to be satisfied with an imperfect framework, as long as he has a sharp razor blade, balsa wood, and cement. And don't assume other designers have figured out all the possibilities in model structures. Keep experimenting - maybe you can do better!

 
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