Zipper (oz387)


Zipper - plan thumbnail image

About this Plan

Zipper. Free flight gas model by Comet. Kit no T-10.

The Zipper was the first, the original 'pylon' model. When Carl Goldberg introduced this design in the 1930s it was the start of a radical new direction.

Quote: "The following was sent to me from a friend. It is an article by Bob Larsh that was in the April 1981 issue of Model Airplane News about the background of the famous Cornet Zipper. I thought it should accompany the plan so I have added it.

Rufus Carswell ("Planeman")

by Bob Larsh

In his work on airfoil sections, Carl Goldberg developed a system by building a number of wings that were identical except for these sections and would make comparisons of the performance potentials. He designed a profile-type glider body on which he could interchange the wings and would then hand-glide the test unit off a balcony in a large armory. After each configuration was trimmed for maximum performance in a straight line flight, the linear distance of the flight and the time duration achieved were carefully measured and recorded. The results were studied and conclusions drawn. The more promising airfoil sections were then tried on powered models. Carl found that a wing which indicated definite potential on one model design did not fare nearly as well on another model of dissimilar design. It therefore readily became apparent that all components had to be carefully matched for the best results. These findings were evaluated, and only the best combinations were incorporated into the final kit configuration.

In June 1941, Comet advertised that the Zipper had been redesigned by improving the airfoil, streamlining the engine cowling to obtain a cleaner frontal area, reducing the wing area to 465 square inches but maintaining the 54-inch wingspan, and rearranging the ignition components. Apparently these modifications were never incorporated into the production kits, as pre-war plans checked to date have not revealed these changes; therefore, it is felt that Comet was premature in making this announcement. However, Goldberg did build at least one Zipper that sported a new flat-bottom, thinned-down airfoil; a new wing mount; and a redesigned engine cowl. The wing area remained the same as the original 1939 kit design. This particular combination did not live up to expectations, and the flat-bottom foil was dropped for the Zipper.

World War II then came along, which of course curtailed gas model kit production, but Comet did manage to produce and market the Interceptor before the end of 1941. Carl had spent about two and a half years on this design before he was satisfied with its competitive potential, as he had flown slightly different versions of it at the '39 and '40 Nationals. The model flown at the '40 Nats accumulated a number of important wins over the 1940 and 1941 contest seasons in Class A, and bore an exact resemblance to the kitted model.

With the war finally over in 1945, modeling took up where it left off. Goldberg and Comet updated the Zipper and released a new revision for 1946. The basic layout remained pretty much the same as the original production Zipper except for a general streamlining as in the 1941 version. The wing airfoil was thinned considerably, but retained undercamber and was designated the 'Goldberg G610.' The plans now featured a Forster .29 up front. The wing mount on top of the pylon was redesigned, as was the rudder, which was enlarged and the outline reshaped. The planform wing area remained the same as the 1939 model, 488 square inches. The fuselage cross-section also remained the same, but the construction was beefed up somewhat.

Had Comet resumed production of its 1941 engine, it is reasonable to assume that this would be the motor shown on the post-war plan; but as engine buffs already know, the Comet .35 evolved into the Vivell series, which was marketed by this company on the West Coast. However, the Forster .29 was a natural because the designer, Bob Forster, lived and manufactured his engines in the Chicago area. It is interesting to note, however, that a close scrutiny of the Zipper picture on the front of the 1946 kit box reveals that the motor clearly resembles a Comet .35, less the front intake tube.

The third and final production Zipper kit was released in 1949. Comet, in keeping with the times, altered the design to accept glow engines. This basically amounted to extending the firewall forward to maintain the center of gravity due to the absence of the coil and batteries. The engine now shown on the plans was a glow front rotary Ohlsson .23 and the firewall was no longer removable.

The Zipper continued to be a popular contest model after World War II and remained competitive throughout the Forties. As the new glow engines improved and models became larger and cross-sections smaller, the familiar Zipper began to disappear from the flying fields and the contest scene. Down through the years, however, the Zipper influenced many successful model designs that were published and kitted and are seen on the contest field today. With the advent of the SAM organization and interest shown in the early gas model designs, the Zipper is back with us, reaffirming its dominance and demonstrating its inherent stability in harnessing the power of even today's exotic powerplants.

The strange part of this story is that it may never end. Should you be an oldster wishing to become involved again, or a youngster who feels that he may have missed out on the 'good old days,' you need only to obtain a copy of the Zipper plans from Model Airplane News.

Many of the Old-Timer enthusiasts prefer to power their models with original ignition engines and the required electrical components. One may obtain a good ignition motor with a little diligent sleuthing. The best method is to locate the older modelers of yesteryear who just may have an old gem salted away and will part with it. Placing ads in neighborhood newspapers and checking out local garage sales many also prove fruitful.

By building Carl Goldberg's immortal design, you too can fly in Old-Timer contests and add your own personal experiences to the ongoing Zipper story.

This report is not an attempt to present a complete biography of Carl Goldberg; that would require more than just a few pages, written by someone closer to Carl and more qualified than myself. Who knows - Carl may write his autobiography for us someday and it probably will outsell all his Zipper kits. My purpose was to present an in-depth study of the birth of the famous Zipper and its justification as 'THE' revolutionary gas model aircraft of all time, to offer a little insight into the man and his associates who created it, and to establish the chronological order of all these memorable events."

Supplementary file notes

Planfile includes article text (from MAN April 1981).


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