The conventional rigid airship has fabric-walled gas cells to hold the gas, i cord nettings to restrain the gas cells, many wires and substantial longitudinals to give strength—as a beam has strength—and finally, an outer cover to give a streamline form and keep out the weather. Why not combine all these parts of the airship into a single piece of material, that is, a metal skin? Thus, weight would be saved, tedious wiring work would be saved, and weathering and replacement of the outer cover would be eliminated. Such was the reasoning of the engineers of the Aircraft Development Corporation of Detroit (now a subsidiary of the Detroit Aircraft Corporation) in designing the metal-clad airship, which was ultimately built under contract for the Navy as the airship ZMC-2.
At first glance the theoretical advantages of a metal-clad ship appeared tremendous, but many practical difficulties soon presented themselves. How could an all-metal airship envelope be made gas-tight? How could the seams be made? Would it be light enough to be practical? Since the metal would be a continuous rather than an intermittent structure, how could it be supported while building? If it could be built, how could the air be removed from the hull to introduce the lifting gas?
In working out these problems a definite design was finally evolved in a small airship, or blimp, of 200,000 cubic feet capacity; length, about 150 feet; and maximum diameter 50 feet. There was a metal skin about 0.01 inch thick, with five main frames inside of the metal skin to maintain the circular form and distribute the concentrated loads from the car, the handling lines, and the control surfaces; and there were twenty- four light longitudinals and a number of intermediate frames, chiefly for support during construction and in the hangar. So much for the hull. In addition, there were eight sets of fixed and movable surfaces to give stability and control, with an enclosed all-metal car fastened close to the bottom of the hull to carry the twin engines and the crew. Inside of the metal hull were two rubberized fabric bags, called ballonets, which by being filled with air under pressure would maintain pressure within the entire hull as necessary during flight.
After an especially thin gage of aluminum alloy sheet had been procured, the next problem was the development of seams. Hand- riveted seams were made and tested, and found to have satisfactory strength. Since even a ship as small as this would require several million rivets, it was an economic necessity to develop an automatic machine to make the seams. The automatic riveting machine, as finally designed and used, operated much like a noisy sewing machine— it “stitched” along at the rate of a foot a minute—joining two sheets of metal by three rows of rivets about the size of ordinary pins. No punching or drilling of the sheets was necessary. The rivet material in the form of three wires was fed into the machine, the wires punching their own holes before being headed up. The final step in making the seams was to paint the slightly opened edge with a sticky solution, which crept into the joint and made an effective gas-tight seal.
After much calculating, the “fat cigar” shape was reduced to a mathematical curve from which the coordinates of any points could be calculated. From these, “patterns” were developed in the form of curved strips running transversely around the envelope. The method of construction finally used was to build the ship in two halves, starting with a small ring at each end and hanging it from the hangar roof so that the axis of the ship was vertical and the construction work was done from the floor. The curved strips were first riveted end to end and wound on a spool to such a length as would encircle the ship once. This strip was unwound from the spool as it was riveted to the part of the ship previously built. After a complete strip was riveted, the entire assembly was hoisted a foot or two higher and the process repeated. As the section of the hull grew larger, the riveting machine was mounted on a circular track on the floor so as to travel completely around the ship. (See p. XLI.) Finally, the two halves of the ship were completed with the longitudinals and structural rings riveted inside of each half. Then the problem was to tip these halves up to a horizontal position and joint the flimsy edges together. Imagine trying to join two huge sections of stove pipe, each about seventy-five feet long and fifty feet in diameter and weighing hundreds of pounds! The two circumferences were exactly equal, but on account of the flimsy nature of the material when not under pressure, the two ends were out of round and the irregularity occurred in different parts of the circumferences. Finally, after much work, the halves were clamped exactly together and the closing seam riveted— with one man working inside and another man outside. Although apparently flimsy, the thin metal skin is very strong when the airship carries pressure. A man with soft shoes can walk on top of the hull, or inside on the bottom, without doing any damage. As many as seventeen men have entered the bottom manholes and been at work at one time inside, on the bottom of the ship.
The next problem was inflation. After the hull had been completed, a heavy gas (C02) was introduced into the bottom— blowing the lighter air out of the top. When the air was reduced to a negligible amount, the light lifting gas (helium) was introduced at the top—blowing out the heavy C02 from the bottom. As a final stage the C02 was removed from the mixture of helium by passing through an especially designed and built scrubbing apparatus. An amusing incident, although serious at the time, happened when the scrubber was first turned on. It was found that the scrubbing solution became so hot as to produce steam which would enter the airship, finally condensing into some unwanted water ballast. The difficulty was finally overcome by slowing down the scrubbing operation and placing a large number of ice cakes in the scrubbing solution.
Meanwhile, the car had been finished and attached and the engines tested out, so that the ship was ready for its first flight. After watching the construction work from the first two-foot ring to the completed airship, it was an inspiring sight when the ship was first moved from the hangar. The test pilot gave a few curt orders to the ground crew and the new airship floated out of the hangar and soon flew away from the field on its maiden flight.
After a number of trial trips to check various features of the design, the ship was flown to the Naval Air Station, Lakehurst, New Jersey, where it is being subjected to long time trials as an experimental unit.
As an engineering development, the construction of this ship has proved that a metal airship hull can be built at least in a small size; that it will hold gas remarkably well; and that with due precaution the necessary pressure for safe operation can be maintained. However, handling under all conditions of weather, effect of continued vibration on the strength of the metal and tightness of the seams, necessity for local repairs, and means of making such repairs—all these must be checked to determine the final worth of the present experiment and to indicate what further possibilities of future development are justified in this interesting new type of airship construction.
STERN HALF OF ZMC-2 DURING CONSTRUCTION This ship was built in two halves, starting with a small ring at each end and hanging it from the hangar roof with the axis of the ship vertical. Note riveting machine on circular track in lower left-hand corner. (See page 742)
THE FINISHED METAL-CLAD ZMC-2, "THE TIN BALLOON"
The ZMC-2 (See page 730) is covered with 0.0095-in. sheet aluminum, which is "sewed” to the structure by about 3,500,000 rivets, 0.035 inch in diameter. The lifting gas is carried directly inside the hull with no internal gas bag except a bottom fabric membrane which allows for gas expansion and contraction.