Chapter 3


Chapter 3

Engines, accessories, and gimmicks

Model shops stock plenty of hardware for model building. Some of these items replace handmade components, improve flying, or make possible new and fancy gadgets, while others fill needs we didn't even know we had. By knowing what's available and making use thereof, you can save work, build better ships, and spice up your flying with a few extras.

The basic accessory is the internal-combustion model-plane engine, still called a "gas" engine as a hangover from the days of the gasoline-fueled ignition engine. The simpler glow and diesel engines have almost completely supplanted ignition today, except for occasional use in R/C or C/L jobs where a high degree of engine control is desirable. For ordinary flying, the advantage of a timer lever which can be advanced or retarded to control engine speed precisely is not enough to offset the heavy and unreliable ignition system of coil, condenser, batteries, etc., which must be carried by the plane.

An alcohol-base fuel is used with a glow plug, while diesels burn an ether mixture. The diesel has the possible disadvantage of requiring two adjustments rather than a needle valve adjustment alone, as on glow engines. Although the diesel engine is favored in most of the world, U.S. modelers seem to prefer glow; however, an increasing number of foreign-made diesels are now being imported.

Aside from its simplicity, the average glow engine is (maybe) a little easier to start than the average diesel; but all engines are so easy-starting now that this point is not too important. Diesels are generally more heavily built than glow engines, but they will swing a heavier prop - though in general at a lower speed. But rpm in excess of 10,000 is commonplace with any modern engine, and nobody really needs more, unless he plans on entering speed competition. Glow engines seem less subject to vibration, but diesels require no batteries (which run down) or glow plugs (which burn out). Some manufacturers turn out both a diesel and a glow version of their engines, so you can try both in the same plane. Mountings, tanks, and accessories are about the same for both varieties, but don't get the fuels mixed up.

Model engines are exceedingly simple and efficient machines with only three moving parts: a piston, a connecting rod, and a crankshaft. Most of them are two-cycle engines and draw fuel into the combustion chamber, fire, and expel exhaust solely by the up-and-down movement of the piston in the cylinder. Fuel flow is regulated by a spray bar and needle-valve assembly, which can be adjusted to mix more or less air with the fuel for a rich or lean mixture. Diesels also have an adjustable counter-piston opposing the power piston, enabling the flyer to vary compression to fit conditions, such as different props, temperatures, fuels, etc., and to control speed.

A pod-and-boom job with tricycle landing gear.
A pod-and-boom job with tricycle landing gear.

Glow plug engines are fired by a glowing platinum filament, which is heated for starting by a 1-1/2 volt battery, after which the heat of combustion keeps it hot. Diesels fire on compression alone.

When you get a new engine, you should first read the starting instructions carefully, even if you've had experience with engines before. Sometimes there's a trick or two which will save you a lot of cranking. After you've briefed yourself, set the engine up in a test stand for break-in. Usually you can make a test mount by notching out a piece of 1" hardwood for beam mounts, or by drilling a board or piece of metal for radial mounting. Clamp the mount to a bench, or hold in a vise. Don't hold the engine in a vise directly; you can easily warp it or even crack the crankcase casting that way. Put the recommended-size prop on the shaft, and tighten the nut down well. If there isn't a fuel tank on the engine, you'll have to buy or make a separate one and set it up at the level of the needle valve, and connect it to the engine with transparent tubing. Fill the tank using a pump can, eye-dropper, or a hypodermic syringe until fuel comes out of the overflow. Put a finger over the engine air intake (choking) and flip over the prop until fuel fills the line; then stop, or you'll flood the engine.

Set the needle valve at the position recommended, usually about three turns open. If you have no instructions, you can gauge the approximate setting by opening it just far enough so that fuel comes readily through the line when you choke the engine and flip the prop. Recommended setting for the compression screw on a diesel is usually supplied too; if not, try it with the setting as is. If that's no good, set the piston at the top of its stroke, and screw the lever down until the counter-piston touches the piston. (Be careful - don't force it.) Then open the screw a turn.

Up to this point, glow and diesel procedure are about the same; now the differences start. For glow, connect the battery leads; tap the connection against the plug to be sure you're getting a spark as it touches, or look in the exhaust port for a glow. A hot plug will light up the interior of the engine like a dim light bulb. The glow of a cold-type plug is barely discernible, but it will work where recommended.

When you're sure you have fire, use your eye-dropper or hypo to place a drop or two of fuel in the cylinder (priming) through the exhaust port. Now turn the prop over once to be sure you don't have too much fuel in the chamber, resulting in blockage. If it turns over O.K., start flipping the prop. Snap it over with a quick flip, and keep your finger moving on out of the line of action of the prop. The engine should fire after a few flips. If it merely pops, keep flipping. If it continues to pop but won't catch, close the needle valve a half turn and go on flipping. If it pops a couple of times and then no more, prime it again and flip the prop some more. If after two or three primes it's still popping without firing a burst or running, open the needle valve a half turn, and continue.

Soon, you'll be rewarded by at least a ragged burst. You have to listen carefully at this point in order to decide what to do next. If the burst starts relatively slowly, runs up the scale and cuts abruptly - whaaap! - it's too lean; open the needle valve another half turn. If it runs unevenly - slowing down and petering out - it's too rich; close half a turn. Sometimes, if an engine is too tight, it will fire a long tight burst, and run down, sounding rich. It's hard to tell every time which is which, but a few minutes of flipping and firing will loosen up a tight engine enough to run. Put a few drops of fuel on the head after firing; if it boils away furiously, it's probably overheated.

Once the engine fires and keeps running, start closing the needle valve slowly; the engine should pick up speed. It's almost never necessary to open the N.V. after starting; a too-lean engine won't keep running, unless it's well broken in; then it may four-cycle, running with a ragged, flat barking sound.

If the engine quits as you lean it, open to the starting setting and try again. It may be heating as it speeds up, and seizing. Be careful not to run a new engine wide open for the first few minutes. Sometimes it helps to add a little of the proper oil (castor for glow - mineral for diesel) to the fuel on the first few runs. Disconnect the power leads as soon as the engine is running smoothly - not before - to save batteries. Be careful not to let the leads cross - your battery will be dead in short order!

Diesels depend much more on the compression setting for starting than on the needle-valve adjustment. After setting the N.V. somewhere near the proper spot as outlined above for glow, prime the engine and start flipping. If the prop meets stiff resistance as it goes over, slack off the compression until it flips over easily. Don't overdo it; you should feel a definite bounce as the piston passes center and is pushed down. If you set the prop on dead center, with the piston at the top of the stroke, then barely nudge the prop tip, it should snap over by itself.

If the engine fires weakly and dies out, you probably need more compression; tighten it down a quarter turn and try again. If it fires, but instead of running, starts rocking back and forth - a weird trick typical of long-stroke diesels - slack off the compression, or close the needle valve a bit. Rich and lean conditions sound much the same as with glow engines, except that the diesel is more tolerant; and remember that excessive compression can sound like a lean mixture, while inadequate compression has symptoms similar to a rich mixture. Actually, the two adjustments must strike a balance; the higher the compression, the leaner the N.V. setting required, and vice versa. You can run a diesel very slowly by backing off compression and opening the N.V. Keep flipping and adjusting until you get a start, then tighten down the compression lever for smooth running, leaning the N.V. if necessary.

Sometimes a new diesel, after starting, will begin to heat up and slow down. In this case, let off on the compression, a little at a time, until it settles down. Opening the N.V. will have the same effect on diesel or glow.

Many engines of either type will run equally well in either direction; this is a very handy feature since you can use regular props to power a pusher, or use counter-rotating props on a twin-engined job to balance torque. But be sure to note which way your prop is turning before launching a model; a backward flight can be very embarrassing.

Rather than change running settings, you can stop a running engine by dropping a rag over the prop, pulling the fuel line off the spray bar, pinching the fuel line, or by sticking a knuckle in the prop blades. The latter method has been thoroughly tested by modelers, and is not recommended, though it's more painful than dangerous. Don't crank engines with loose cuffs hanging down; the effect can be gripping indeed. And keep out of the line of fire of any flying prop blades; they very rarely come off - but they could!

Always test-run an engine before installing it in a model. It's a lot easier to tinker with on the bench, and it's always discouraging to get out in the field with a cowled-in engine before discovering that for some reason it won't go. Although manufacturers test engines and guarantee them, occasionally you'll get a defective one. Don't fret; take it back and get a new one for it. But don't assume you've got troubles until you're sure. Some engines are reluctant to start the first time. Get someone wth modeling experience to help you, if necessary. After a run-in, it will be a lot easier. After running, it will usually be necessary to open the needle valve on a glow engine, or to slack off compression on a diesel, for the next start. Be sure you know these settings, and once you've determined them, don't lose them.

Engines come in a wide range of sizes, with displacements starting at 1/100 of a cubic inch (.010) and ranging up to .60 cubic inch. By far the most popular engines, and the most numerous, are the 1/2 A (also designated AA) engines of .049 cu. in. or less. These will fly free-flight models up to three feet in span with ease, and will handle control-line jobs of about half that span satisfactorily. To gauge the correct engine size for a given model, you can figure that the recommended prop length should be about one-sixth of the span for a free-flight model.

Engines may be either beam or radial mounted; some have fittings for both. Beam mounts are a little more trouble to build, but they're stronger and more rigid. It's a good idea to use bolts with washers, lock washers, and nuts, rather than wood screws, since the latter can be loosened by use or hard knocks, and there's no way to retighten them satisfactorily, though a piece of wood stuffed into an enlarged screw hole will help in an emergency. Sometimes wood screws can be helpful when changing engines in the field, or if a mounting nut comes loose.

There are a lot of accessories you can buy for engines. You can get a muffler to keep the noise down to a low roar; in fact, in some areas this is now required by law. To extend the length of the propeller shaft, or to mount a spinner, special union prop nuts can be had. Spinners, which serve to streamline the front of a model and carry out fuselage lines, are manufactured in many sizes and shapes, from plastics and aluminum. Special fittings for speed control, marine operation, etc., are also available. You can even buy a conversion unit which will enable you to mount two or more engines in tandem, driving a single propeller through a system of gears, or, contrarily, equipment for driving two propellers with a single engine.

Fuel tanks of all sizes can be had in specialized shapes for various uses. Rigid-type fuel tanks usually have three tubes projecting from them. One is a filler tube, which extends through almost to the bottom of the tank, so that fuel won't pour out in inverted flight. The overflow tube projects from the bottom; inside, it extends nearly to the top of the tank so as to release air as the tank is filled, and to indicate filling by overflowing. The fuel feed tube to the engine projects at right angles to the other two and is placed to pick up the last drop of fuel - if the tank is mounted in the proper position. C/L jobs usually use a wedge or other non-rectangular shape, designed to feed fuel to the engine as the model circles. This type tank must be mounted with the pick-up line to the outside of the circle. A clunk or bottle-type tank with a weighted fuel pick-up helps do the same job in R/C ships. Unless an integral tank is included with the engine, free-flight models usually use a small rectangular tank, either in conjunction with a timer or containing a metered amount of fuel, to control engine run. To insure a steady flow of fuel with a stunt job, or where several engines are fed from a single tank, bladder-type pressure tanks with regulators are used.

To harness the power of the model plane engine, a propeller is necessary. Various hardwood and plastic propellers are made in a complete range of sizes and pitches for all engines. Nylon propellers are the most satisfactory for most uses - although they have a tendency to disintegrate at the extremely high tip-velocity attained by the bigger racing engines. For speed competition, mylar props are superior. Wooden props are fine, but brittle. Be careful when drilling the mounting hole on a nylon prop to a larger size to fit a big shaft; with a power drill the bit may grab in the tough nylon and shatter the prop (and your composure).

Prop sizes are given in inches, representing the length, followed by a figure indicating the pitch. Be sure to observe the manufacturer's recommendation as to the size prop to use with your engine. A prop with a pitch equal to half its diameter or less is a low-pitch prop. Longer, low-pitch props are used for free-flight; short, high-pitch props are for C/L only. Multi-blade props are made, mostly, for use on scale jobs; these should be a little shorter than a two-bladed prop for the same engine. A diesel generally requires one size larger prop than a glow engine of the same displacement. Always keep your props in balance by sanding a bit off one end or the other, as needed; if a blade receives more than a minor chip, discard it, or trim it down to a smaller size to eliminate the flaw. Lopsided props cause fast engine wear. When mounting the prop on the engine, take care to set it so that it rests horizontally when the engine stops, to save damage in landings.

You can get extra use from broken wood props, by matching up halves and trimming them at an angle through the mounting hole so that each part has half of the hole (Fig. 9). Cement them together with the holes in alignment, and the pressure of the prop washer when tightened up on the engine will hold them firmly in place.

fig.9 - Splicing a prop.
fig.9 - Splicing a prop.

To keep those models rolling, you can equip their undercarriages with wheels of any desired size, in a choice of materials and styles. Small rubber models usually use hardwood wheels, or light-weight inflatable air wheels with wooden hubs. Engine-powered jobs usually carry aluminum-hub wheels with rubber tires; these can be solid, semi-pneumatic (hollow), or fully pneumatic air wheels. They also come with doughnut tires or slim streamlined ones for contest jobs. You can even get perfect replicas of full-scale aircraft wheels for scale models.

When mounting wheels, be sure the axle holes are roomy enough for free rotation. Drill them out to a larger size if necessary. Plastic or metal wheel collars for retaining wheels may be used in place of solder; this is simple and allows wheels to be removed easily if necessary. Collars are heavier and less secure than solder and on small models are unattractively large. Still, you don't need a soldering iron to use them, and they're fine for temporary field use if a soldered retainer comes loose a long way from the workbench. They can also be used for other purposes, such as retaining a pendulum or securing a C/L push-rod.

For control-line models, hobby shops offer a complete line of fittings. The heart of the control system is the bellcrank. These are made in a number of sizes, and are equipped with bearings, mounting bolts, washers, etc. Elevator horns, to link the pushrod to the movable surface, come in several types and sizes, as do hinges. There are also special eyelets to line the holes through which the lines emerge from the fuselage, retainers for use in keeping lead-out wires secure in the bellcrank, guides for lead-out wires, etc. You can get a gadget to hold your C/L ship in position while you scramble for the lines, and pull the trigger to release it. And how about an outside-the-circle control handle and lines, so you can sit down and fly the ship from the shade of a tree without having to rotate - a boon to vertiginous modelers.

To dress up the external appearance of your ship, there are clear plastic canopies in various sizes, miniature pilots' heads so your plane doesn't have to fly around unoccupied, colorful decals, and other items. Look over a display and you'll see lots of useful items. Of course, some of these are mere luxuries, which you can either improvise yourself, or fly without very satisfactorily. Some gadgets haven't gotten to the manufacturing stage yet - like power panels for starting multi-engined jobs, enabling you to dial juice to each engine in turn. When you get around to installing operating mechanisms in your ships, you'll be pleased to find small electric motors available, which turn up several thousand rpm, on 1-1/2 to 6 volts. You can get battery holders for any number of batteries of any standard size.

Many of the accessories mentioned above you can make yourself if you prefer, for reasons of convenience or economy. Use sheet aluminum to form such fittings as battery boxes, motor mounts, and bellcranks. Follow the construction of the manufactured item to be sure it will work. You can make up battery leads by using a length of old two-wire electric cord and soldering fittings onto the ends. Make an extension needle valve by soldering a length of coil spring to the needle valve, and add a knob at the tip to turn with. Spinners can be turned on the lathe or in a drill press from aluminum, wood, or plastic.

You can make a canopy by heating a thermoplastic like plexiglass in the oven, then pulling it over a carved form while it cools (Fig. 10). Tanks are easy to make from tin-can stock or shim brass. Cut a rectangle to form the body of the tank and solder the ends to form a hollow square, oval, wedge, or whatever you have in mind. Then add a plate to each end and trim off the excess. Drill three holes and install the filler, overflow, and feed tubes. Bevel the ends of the tubes, and extend them all the way through to the opposite side. Place the filler on top, the overflow on the bottom, and the feed line in front (Fig. 11). Check the tank for leaks by closing two openings and sucking the air out; if your tongue sticks to the tube and comes away with a pop, it's tight. If not, start looking for the leak and plug it with solder.



Without question, the ultimate in accessory equipment for model planes is a radio control installation. For a long time, radio control was the private domain of a few electronic experts. Equipment was complex, heavy, and unreliable, and the necessary licenses were hedged about with elaborate and unrealistic requirements. Not so long ago you had to know Morse code to get a permit to operate your R/C model.

Today all this is changed. Receivers have grown feather-light - so small they can be carried in models of the smallest size, powered by 1/2 A engines. Instead of a maze of wiring, a simple plug-in connector is supplied. Ingenious escapements permit up to four different controls from a simple single-channel radio. And along with all the improvements, the price has dropped steadily. R/C flying constitutes a whole field of modeling in itself, and before going into it, you should devote some time to study of a good recent book dealing exclusively with the subject. Developments in the field have been so rapid that a book over a year old would be obsolete now.

Once you have a model performing smoothly, flight after flight, you may get the urge to liven things up with a gadget or two. Some of the mechanical marvels you can install in your ship will add to its performance - like retracting landing gear, operable wing flaps, or lights for night flying, all of which are covered in a separate chapter. Other gimmicks are just for fun. You use a third line for C/L or a timer with F/F for delayed action operations, and rubber, springs, or gravity for power.

For example, it's easy to rig up a device to drop "bombs" (consisting of cornstarch or similar material in tissue-paper bags) which make a satisfactory cloud of dust when they hit. Simply build a chute from balsa, or use a stiff paper tube with a cover at the bottom which can be opened to drop one bomb at a time. You can practice precision bombing by marking a target on the ground.

Another way of using cornstarch is to let it feed out through a small hole in the bottom of the container, to lay a smoke trail. This idea can even help you find a wandering F/F job; just follow the white line. An air scoop to pressurize the starch compartment will insure a steady flow. You can also lay a trail using a smoke bomb made especially for model use.

A small glider, or even a big one, can easily be clamped under the fuselage of a powered model and released in flight; or try towing a glider behind the mother ship with a towline. It will release automatically when the motor cuts and the pull on the line is relaxed.

The experimental-minded builder can try some interesting mid-air tests by arranging for a slow-flying biplane to shed a wing aloft - or a fast job can sprout an extra wing when the motor cuts for a safe glide-in. This isn't as tough as it may sound. The extra wing can be made from sheet balsa, sanded to an airfoil section, and can be folded alongside the fuselage, clamped flat under the regular wing (or on top of it), or fitted inside the wing to emerge from the tips, being snapped into place by a rubber band when the release acts.

Try an ejection seat in a C/L fighter. Blow the canopy in mid-flight and kick the balsa pilot free - and don't forget his parachute, made of the lightest nylon or covering tissue. Try it out as a surprise on a combat ship. When you lose your streamer - bail out! It'll be worth losing to catch audience reaction! The parachute idea can be used with a F/F job, to liven up a flight; as the model circles overhead, a trap door opens in the side or bottom, and the chute blossoms below. Very effective!

Not everything that is carried has to be dropped. Try fitting a small light-weight camera into the fuselage of a F/F model, and use the timer to trip the shutter, making a nice aerial shot of the flying field. Or put the camera in a C/L job, and practice aerial photography; see if you can catch another model flying in the same circle.

A semi-scale pendulum control model fitted with floats.
A semi-scale pendulum control model fitted with floats.

If you want to see some real action, fit a reliable F/F ship with an acrobatic sequence attachment. Rig up a movable control surface, along the lines of an R/C rudder or ruddervator; however, instead of actuating it by a radio impulse, trip the gimmick with a timer. Use a small spring motor from a toy, or gear down a few strands of rubber to produce a slow rotation; mount an eccentric on the rotating shaft, with the operating arm for the surface held against it. The shape of the eccentric will determine the movement of the controls. Use a timer (or third line for C/L) to trip the device. When released, the movable surface (or surfaces) will cycle to put the ship through a stunt pattern. For example, a movable elevator could be first held half-down for two or three seconds to put the ship in a steep dive, then full-up to bring her up and over in a loop, then back to neutral to continue straight and level. If you have a big field, a steady-flying model and no wind, you can gas up and enjoy long motor runs with your acrobatic ship.

fig.12 - Pendulum control.
fig.12 - Pendulum control.

A very practical installation you can use in sport and scale F/F jobs is pendulum control - and the idea is also applicable to R/C in conjunction with radio-operated controls. A pendulum, consisting of a lead weight at the end of an arm, is suspended near the G.G. of the model. The upper extension of the arm operates the control (Fig. 12). For elevator, a setup identical with C/L can be used substituting a fore-and-aft swinging pendulum for the bellcrank. A side-mounted pendulum can operate rudder and/or ailerons - or try a universal joint for complete control. Always mount the pendulums so that controls are at neutral when the model sits level, in flight attitude. Be sure that all joints work freely, or disaster's bound to strike.

When using a pendulum, trim the model with the controls locked, then unlock and test. Nose dives are the result of a too-light pendulum; a fluttery motion of the tail means tail-heavy trim. Be careful on launch not to heave too hard or you'll swing back the pendulum and dive in!

These are only a few of the weird ideas possible to the modeler with a yen for the unexpected; with a little thought you can dream up even wackier ideas to amaze your friends. And remember - you don't have to be crazy to be a model builder - but it helps!

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