Silentius (oz12381)

About this Plan

Silentius. Free flight sport model, for electric power. Wingspan 780 mm. Wing area 140 sq in.

Direct submission to Outerzone.

The Silentius was (as best we know) the world's first kit for an electric-powered free flight model airplane.

Update 06/08/2020 Added (later) article, from Modell Sport 5/2003, in German, thanks to Pit.

Quote: "On March 18, 1959, after 17 years of negative attempts, Fred Militky was finally successful and launched his first electric flight model. The FM-241 model, equipped with a new Faulhaber electric motor, disappeared after a few minutes of climbing, never to be seen again. This set a new milestone in the history of model flying. Nevertheless, it was still almost 15 years before the first remote-controlled electric flight models conquered the sky. With the Silentius FM-254, electric flight was made palatable to model makers in 1960. The dethermaliser device, equipped with a glow cord, should prevent the model from escaping. In addition to the absolute cleanliness and starting safety of the engine, the quietness of the engine is what makes the electric flight astonishing. The name 'Silentius' does justice to this special characteristic."

Apologies for my poor translation from the German (via Google translate). The above text is taken from the article in Modell Sport 5/2003, see https://www.rc-network.de/forum/modellflug/antike-flugmodelle/675468-graupner-silentius-1960

Update 07/08/2020: Added article "Flying the New Electric-Prop Planes" by Howard McEntee from Popular Science, March 1961, thanks to Pit.

Quote: "Here's an exciting breakthrough in the art of free-flight models. Flying the New Electric-Prop Planes. By Howard McEntee.

THERE'S a new thrill in store for pilots of free-flight model planes - the new electric-motor jobs. The motors are tiny jewels, smaller in diameter than a quarter. Penlight cells run them. The motors are sold for use with two imported model-plane kits, one from Germany and one from Japan. The planes are gauzy frame-and-tissue affairs, designed for low weight, with big gliderlike wings for high lift.

A new challenge in skill: Gas-prop planes have enough excess power to overcome sloppy construction; electric planes don't. Their much lower prop torque demands an absolute minimum of weight and a maximum of lift. This calls for real model-building skill - the same skill you needed years ago to build a successful rubber-band model.

Cement must be used sparingly to save weight; wings must be pinned flat during assembly to prevent power-robbing warps; the tissue covering must be free of lift-killing wrinkles. You may even want to forego the usual gleaming - and heavy - paint job.

Flight techniques also represent a return to the challenge of rubber-band days. Because the batteries last only a short time under constant drain, the objective is to get the plane to a high altitude and then cut the motor with a timed fuse. The prop then folds or disengages and the plane returns to earth in a flat, graceful glide - actually a longer powerless flight than the original climb.

The power plants: The tiny motors require only about four volts and weigh less than an ounce in the case of the German model and about 1-1/2 ounces for the Jap one. Their high-speed rotors, spinning up to 18,000 rpm, are geared down through miniature watch-like power trains to drive big-bladed propellers. The midget motors have been made possible only since the war, with the development of more powerful batteries and permanent-magnet motors that eliminate bulky field coils. The German motor, called the Mikromax T 03/15, has a 15:1 reduction and drives a whopping 12-3/4 in-long prop at about 1,200 rpm. Its design is unusual.

The permanent magnet is located inside the armature windings in place of an iron core. The outer casing is an iron core that completes the path for the magnet. The motor has such other exotic features as a silver commutator and gold-alloy brushes. It's said to have an efficiency of 75 percent without gears - an astonishing figure. A clever flexible coupling between the motor shaft and the prop shaft enables the motor to take up the load without shock.

The Jap motor, called the AP 35, has a gear reduction of about 7:1 and spins a shorter 9-1/2 in prop a little faster, at about 1,500 rpm. It's somewhat less powerful than the German motor, but it has sturdy bronze bearings and modem ceramic magnets.

The motors do not come with the model-plane kits and must be bought separately. It's important to get the right one for each model as there are other miniature power plants made for different purposes. In some cases, the propeller must also be bought extra.

What the planes are like: The German model, called Silentius, has a wingspan of 30 in and weighs less than five ounces, including power plant. It's the fancier of the two, having a true fuselage with a trim, gliderlike appearance. Total cost for the kit, motor, and prop is about $15. The Jap model, known as the TK-1, is bigger, with a 38 in wingspan, but has a more primitive stick-like fuselage reminiscent of early rubber-band models. This, however, makes it considerably cheaper - less than $6 including motor - and also simplifies assembly. It weighs about 6-3/4 ounces ready to fly.

The parts in the Jap kit are already cut out for you; those in the German kit aren't. Both kits come with complete instructions in English.

Electric power v gas: The new planes are no competition for glow-plug models and aren't intended to be. They have their own special charms. They're not temperamental; they start instantly in any weather. They fly as soundlessly as the breeze, never run out of fuel, and eliminate the fire hazard of working with hot fuels. Their flight is leisurely and enjoyable; you don't spin yourself dizzy on the end of a control line. When a mishap does occur, they don't screw themselves into the ground with the ferocity of a gas-prop job.

The safe and quiet operation of electric models is opening up new possibilities for model-plane sport in communities that frown on the gas-engine types. Their gentle nature permits meets to be held in gyms and armories without dependence on fair weather.

On the minus side, the planes require some pampering. They do not have enough power for ROG (rise-off-ground) takeoffs; you must launch them by hand. They also require calm air to fly succesfully - 'electric-flight weather' as hobbyists have dubbed it. Because the batteries run down so quickly, you're restricted to fairly short power-on periods - about 20 to 30 seconds - if you want the cells to last. The motors may also heat up and become damaged if they're allowed to run too long.

The German plane has a climbing rate of three feet a second, which its designer, Fred Militky, says is 'not spectacular' compared to what future developments in electric power will bring. Still, it seems adequate enough considering that Militky's first successful model flew clear away after climbing for five minutes. Militky, incidentally, tried more than 250 different designs over an 18-year period before finally coming up with a reliable model.

How power is controlled: The shut-off switches in both planes are held in the On position by a rubber band with a short fuse inserted in it. When the fuse burns down, it melts the rubber band and releases the switch, shutting off the motor.

The German plane uses a second fuse to operate a 'dethermalizer.' This is designed to keep the lightweight plane from being swept aloft by rising thermal air currents and becoming lost. The entire tail section is pivoted and is held down by a fused rubber band. When the fuse burns through the rubber band, the tail pops up at a steep angle. This destroys the plane's ability to climb, but without making it crash. It descends rapidly but gently, something like a parachute.

Fuses are included in both kits, and the length you use determines how long the power stays on. On one of the models shown here, a miniature pneumatic timer (Elmic Mini-Diesel, available at hobby shops) was used in place of the fuse arrangement. This adds a little extra weight, but is handier to use and more reliable than the fuses. The dethermalizer is usually needed only on very hot days or when you're trying to break altitude records.

Getting long, graceful glides: Both power plants have ingenious devices for reducing the drag of the prop after power has been shut off, so the planes will glide freely.

The giant German prop is a folder - its blades, pivoted at the hub, automatically swing back against the fuselage when power is cut so they offer less wind resistance. When the switch shuts off, it also shorts out the motor, stopping it immediately without any over-run. This gets the prop stopped and folded instantly for an unrestricted glide.

The Jap motor has a disengaging device that allows the prop to freewheel during a glide. The prop is spring-loaded so that it engages the drive gear during powered flight. When power is cut, wind pressure overcomes the spring, disengaging the prop and permitting it to spin freely without resistance.

What kind of batteries? Both planes will run satisfactorily on ordinary 1-1/2 volt penlight cells. The German one uses three, wired in series, for a total of 4-1/2 volts. The Jap model uses four cells in series to get six volts. In tests, the German motor drew 1.6 amps and the Jap 1.7 amps - a remarkably high drain to impose on tiny flashlight batteries.

Because of the high drain, several other types of batteries are recommended for longer life. Photoflash batteries in the penlight size will deliver high power in short bursts for brief flights. Still more durable are the newer alkaline cells, such as Eveready's E-91. These cost somewhat more than standard cells, but will give many times more flights.

The German plane, with more cockpit space, is designed to take another type of special battery. This is a lead-acid cell called Rulag RL-4, made for use in electric cigarette lighters. It costs about 75 cents and is available at large cigar, jewelry and department stores. Two Rulag RL-4s weigh 1-3/4 ounces and will provide 10 to 15 flights lasting about 20 seconds each.

Although the Rulag cells are not intended to be recharged, they actually can be charged a limited number of times in order to cut cost. Instructions for re-charging come with the German kit. The Jap plane's battery holder is designed to take only penlight cells, so a special holder would have to be devised to carry the Rulags.

The kits can be obtained at many local hobby shops or from Polk's Model Craft Hobbies, 314 Fifth Ave, NYC. Other sources are Wilshire Model Center, 1326 Wilshire Blvd, Santa Monica, Calif, for the German plane, and Associated Hobby Mfrs Co, 413 East Allegheny Ave, Philadelphia, for the Japanese model."

Update 10/10/2023: Added decals, thanks to Eric.

Quote: "Hi, I have an old Silentius kit - you have the plans, but I see that you do not have the original Decal Sheet supplied with the kit. For anyone who wants the original Graupner Decal Set here it is as attached, the outline frame is 12 x 19 cms and it prints at 100% scale on Letter (8 1/2 ins x 11 ins) paper. Thanks for your excellent work on this website, regards, Eric"

Supplementary file notes

Article (MS).
Article (PS).
Catalog page from Graupner (English).
Decals.
Instructions, in German.

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