Zipper (oz16000)

About this Plan

Comet Zipper. Free flight power model. Wingspan 54 in, wing area 495 sq in. For 1/4 to 1/6 hp engines. Plan shows installation for Brown and Denny engines.

The Zipper was first described in July 1939 Air Trails (in an article titled: 'The 1939 Gas Model'). This here is the Comet kit plan ref T-10, dated 1939.

Scanned at 400 dpi.

Direct submission to Outerzone.

Quote (from ad): "The Comet Zipper. What a climb! 2,000 ft per min! Presented to a startled world by Comet and Carl Goldberg. More features than you've ever seen packed into a gas model. Features that create new standards of performance. Features proved by hundreds of test flights. The Zipper's breathtaking climb and floating glide will thrill and amaze you! And look at this partial list of contents..."

Note see Zipper (oz387) for an alternate version of this plan (redrawn, with some modifications, at a later date).

Update 2/5/2025: Added article (from AT, July 1939) thanks to RFJ.

Quote: "The 1939 Gas Model. By Carl Goldberg. WHEN the thirty-second motor run rule was adopted early in 1938, the doom of large gas models for competitive work was sounded. Before long, experts began to agree with the prediction made in an article by the author in 1937 that a limited motor run would lead to small high-powered ships with unusual rate of climb. In January 1938, the Gas Model Aeronauts of Chicago scheduled a contest for models with wingspan not to exceed 4-1/2 feet, with thirty-second run. The writer was challenged to power such a model with the 1/3-horsepower Forster motor, on the theory that so much torque could not be controlled on such a short span. The ship was built and won the contest. In flight, torque wasn't noticeable, the ship handling itself smoothly.

At the same meet, a noted Midwestern model builder flew two 4-1/2 foot ships powered with Browns. This design was a good-looking representative of conventional ideas in gas models. The thrust line was high, the center of lateral area 'properly' located, and the wing had a normal dihedral angle of 1 or 1-1/4 inch per foot span. Under low power, the models acted nicely, climbing well. But every time the motor was opened tip, all control seemed to be lost and a crash ensued. Because of the extreme ruggedness of the construction, it was possible to make several dozen attempts before the models became too badly damaged for further repair at the field. The slightest adjustment caused large variation in flight. It was evident that the reserve stability was zero.

At this time, the writer began work on a new design which had been under consideration for some six months, incorporating all the lessons in stability design learned from many years of flying indoor models. To draw up the ship and help design the structure, I sought the aid of Alvin Anderson, winner of the 1937 Berryloid finish contest. His broader experience with outdoor models was of considerable help, and later reflected in the ruggedness of the ship.

The basic thought behind the design was simply this: How low a horsepower loading can you get, and still keep close to the wing-loading rule? For example, if your ship weighs three pounds and has a 1/5-horsepower engine, the horsepower loading is 3 ÷ 1/5, which equals fifteen pounds per horsepower. The lower the number of pounds per horsepower, the greater is the possible performance.

By cutting the battery down to the minimum of one ounce - using two pen light cells which would last a dozen flights or more - and building a small ship, it looked as though it would be practical to get the flying weight down to 1-3/4 pounds. Using the 1/4-horsepower Dennymite Airstream, this meant a horsepower loading of 1-3/4 ÷ 1/4, or seven pounds per horsepower. Since the thrust of the Denny is over three pounds, it was obvious that such a ship could very easily climb straight up, even if released for flight in a vertical position.

At the time, the wing-loading rule was ten ounces per square foot. For 1-3/4 pounds or twenty-eight ounces total weight this meant an allowable maximum of 2.8-square-foot wing area. To be on the safe side in avoiding differences in weighing scales at contests, I used a little less area, about 2.7 square feet so that a twenty-eight-ounce ship would be one ounce over the minimum.

For strength, an aspect ratio of six was decided upon. This worked out at around four-foot span. Other parts of the general specifications followed usual rules for design: overall length two thirds of the wing span; lifting stabilizer one third of the wing area; rudder area six percent of wing area. The wing section was of the bird type used by discriminating builders for the past ten years. (For the convenience of more builders who might like to use it, several sizes of my version of this section are shown.)

The great question, of course, was how to handle the tremendous engine torque on such a small span, and how to have sufficient reserve stability so that small adjustments would not affect the flight to a great extent. This was solved by simply raising the wing about six inches above the center of the fuselage, and using a fair amount of polyhedral. This may be in defiance of certain theories regarding careful location of the center of lateral area, but every ship built to this design has been found to handle engine torque quite easily, and to be capable of proper adjustment. This will be shown later.

There wasn't any question about the need for quick accessibility of the ignition parts - many contests are won and lost on the ability to find and repair a broken wire, or replace batteries in a hurry. So of course the ship was equipped with an instantly removable fire wall to which was attached a simple battery rack carrying everything but the automatic tinier. For low drag and light weight, the single-strut landing gear with balsa wheels was employed. Mounting the landing gear on the fire wall saved having to increase the strength and weight of some portion of the fuselage to take landing shocks.

The fuselage structure was designed according to a principle fairly well known but not often used. In a crash the stress grows greater and greater toward the nose because of the cumulative weight of the tail and each portion of the fuselage all pushing toward the point of contact with the ground. Therefore the portion on which the fire wall rested was made of a plywood ring to bind the balsa firmly together. Immediately behind was heavy sheet balsa, followed with medium balsa, et cetera. The idea was to make the fuselage progressively stronger from the tail to the nose.

Lastly, the wing attachment was the same as used on my old Valkyrie (oz6156) and Clipper (oz7380), and which now is coming into wider use. This is the method employing a rubber band stretched tightly across the center of the wing, and attached to straight wires in the wing mount. The most important advantage is that the wing detaches from the fuselage on any hard impact by pushing the rubber band off one of the wires, which saves both wing and fuselage from a great deal of damage. The tail was attached in this way also, and for the same reason, as well as another. To avoid damage to this unit, it has to be able to slide off forward, because landing on rough ground occasionally causes a 'cartwheel' onto the tail, smashing it if it is very firmly fixed.

That, briefly, is the gist of the thought that went into the model which later began to be called the Zipper. Several hundred flights were made with a number of test models, and naturally we found plenty of things to improve. The greatest source of trouble was broken ignition wire..."

Note see also Diamond Zipper (oz4753) for a drawing of the earlier 1938 version of this design.

Supplementary file notes

Advert (from Aug 1939, Air Trails).
Article (AT, July 1939)
Printwood.
Templates.

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