British Airships, Past, Present, and Future · Chapter 42Naval Airships.--The Rigids--Rigid Airship No. 1
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The responsibility for the development the Rigid airship having been allotted to the Navy, with this object in view, in the years 1908 and 1909 a design was prepared by Messrs. Vickers Ltd., in conjunction with certain naval officers, for a purely experimental airship which should be as cheap as possible. The ship was to be known as Naval Airship No. 1, and though popularly called the Mayfly, this title was in no way official. In design the following main objects were aimed at:
1. The airship was to be capable of carrying out the duties of an aerial scout.
2. She was to be able to maintain a speed of 40 knots for twenty-four hours, if possible.
3. She was to be so designed that mooring to a mast on the water was to be feasible, to enable her to be independent of her shed except for docking purposes, as in the case with surface vessels.
4. She was to be fitted with wireless telegraphy.
5. Arrangements were to be made for the accommodation of the crew in reasonable comfort.
6. She was to be capable of ascending to a height of not less than 1,500 feet.
These conditions rendered it necessary that the airship should be of greater dimensions than any built at the time, together with larger horse-power, etc.
These stipulations having been settled by the Admiralty, the Admiralty officials, in conjunction with Messrs. Vickers Ltd., determined the size, shape, and materials for the airship required. The length of the ship was fixed at approximately 500 feet, with a diameter of 48 feet. Various shapes were considered, and the one adopted was that recommended by an American professor named Zahm. In this shape, a great proportion of the longitudinal huff framework is parallel sided with curved bow and stern portions, the radius of these curved portions being, in the case of the bow, twice the diameter of the hull, and in the case of the stern nine times the same diameter. Experiments proved that the resistance of a ship of this shape was only two-fifths of the resistance of a ship of the same dimensions, having the 1 1/2 calibre bow and stern of the Zeppelin airships at that time constructed.
A considerable difference of opinion existed as to the material to be chosen for the construction of the hull. Bamboo, wood, aluminium, or one of its alloys, were all considered. The first was rejected as unreliable. The second would have been much stronger than aluminium, and was urged by Messrs. Vickers. The Admiralty, however, considered that there was a certainty of better alloys being produced, and as the ship was regarded as an experiment and its value would be largely negatived if later ships were constructed of a totally different material, aluminium or an alloy was selected. The various alloys then in existence showed little advantage over the pure metal, so pure aluminium was specified and ordered. This metal was expected to have a strength of ten tons per square inch, but that which arrived was found to be very unreliable, and many sections had, on test, only half the strength required. The aluminium wire intended for the mesh wiring of the framework was also found to be extremely brittle. A section of the framework was, however, erected, and also one of wood, as a test for providing comparisons. In the tests, the wooden sections proved, beyond all comparison, the better, but the Admiralty persisted in their decision to adopt the metal.
Towards the end of 1909 a new aluminium alloy was discovered, known as duralumin. Tests were made which proved that this new metal possessed a strength of twenty-five tons per square inch, which was over twice as strong as the nominal strength of aluminium, and in practice was really five times stronger. The specific gravity of the new metal varied from 2.75 to 2.86, as opposed to the 2.56 of aluminium. As the weights were not much different it was possible to double the strength of the ship and save one ton in weight. Duralumin was therefore at once adopted.
The hull structure was composed of twelve longitudinal duralumin girders which ran fore and aft the length of the ship and followed the external shape. The girders were secured to a steel nose-piece at the bow and a pointed stern-piece aft. These girders, built of duralumin sections, were additionally braced wherever the greatest weights occurred. To support these girders in a thwartship direction a series of transverse frames were placed at 12 feet 6 inches centres throughout the length of the ship, and formed, when viewed cross-sectionally, a universal polygon of twelve sides. For bracing purposes mesh wiring stiffened each bay longitudinally, so formed by the junction of the running girder and the transverse frames, while the transverse frames between the gasbags were stiffened with radial wiring which formed structure similar to a wheel with its spokes. The frames where the gondolas occurred were strengthened to take the addition weight, while the longitudinals were also stiffened at the bow and stern.
Communication was provided between the gondolas by means of an external keel which was suspended from extra keel longitudinals. In this design the keel was provided for accommodation purposes only, and in no way increased the structural stability of the ship as in No. 9 and later ships. This keel, triangular in section, widened out amidships to form a space for a cabin and the wireless compartment. The fins and rudders, which were adopted, were based entirely on submarine experience, and the Zeppelin method was ignored. The fins were fitted at the stern of the ship only, and comprised port and starboard horizontal fins, which followed approximately the shape of the hull, and an upper and lower vertical fin. Attached to these fins were box rudders and elevators, instead of the balanced rudders first proposed. Auxiliary rudders were also fitted in case of a breakdown of the main steering gear abaft the after gondola. Elevators and rudders were controlled from the forward gondola and the auxiliary rudders from the after gondola.
The gasbags were seventeen in number and were twelve-sided in section, giving approximately a volume of 663,000 cubic feet when completely full. Continental fabric, as in use on the Zeppelin airships, was adopted, although the original intention was to use gold-beater's skin, but this was abandoned owing to shortage of material. These bags were fitted with the Parseval type of valve, which is situated at the top, contrary to the current Zeppelin practice, which had automatic valves at the bottom of the bags, and hand-operated valves on the top of a few bags for control purposes. Nets were laced to the framework to prevent the bags bulging through the girders.
The whole exterior of the hull was fitted with an outer cover; Zeppelin at this time used a plain light rubber-proofed fabric, but this was not considered suitable for a ship which was required to be moored in the open, as in wet weather the material would get saturated and water-logged. Various experiments were carried out with cotton, silk and ramie, and, as a result, silk treated with Ioco was finally selected. This cover was laced with cords to the girder work, and cover-strips rendered the whole impervious to wet. Fire-proofed fabric was fitted in wake of the gondolas for safety from the heat of the engines.
Two gondolas, each comprising a control compartment and engine-room, were suspended from the main framework of the hull. They were shaped to afford the least resistance possible to the air, and were made of Honduras mahogany, three-ply where the ballast tanks occurred, and two-ply elsewhere. The plies were sewn together with copper wire. The gondolas were designed to have sufficient strength to withstand the strain of alighting on the water. They were suspended from the hull by wooden struts streamline in shape, and fitted with internal steel-wire ropes; additional wire suspensions were also fitted to distribute the load over a greater length of the ship. The engines were carried in the gondolas on four hollow wooden struts, also fitted internally with wire. The wires were intended to support the gondolas in the event of the struts being broken in making a heavy landing.
Two engines were mounted, one in each gondola, the type used being the 8-cylinder vertical water-cooled Wolseley developing a horse-power of 160. The forward engine drove two wing propellers through the medium of bevel gearing, while the after engine drove a single large propeller aft through 4 gear box to reduce the propeller revolutions to half that of the engine. The estimated speed of the ship was calculated to be 42 miles per hour, petrol was carried in tanks, fitted in the keel, and the water ballast tanks were placed close to the keel and connected together by means of a pipe.
No. 1 was completed in May, 1911. She had been built at Barrow in a shed erected on the edge of Cavendish Dock. Arrangements were made that she should be towed out of the shed to test her efficiency at a mooring post which had been prepared in the middle of the dock. She was launched on May 22nd in a flat calm and was warped out of the shed and hauled to the post where she was secured without incident. The ship rode at the mooring post in a steady wind, which at one time increased to 36 miles per hour, until the afternoon of May 25th, and sustained no damage whatever. Various engine trials were carried out, but no attempt was made to fly, as owing to various reasons the ship was short of lift. Valuable information was, however, gained in handling the ship, and much was learnt of her behaviour at the mast. More trouble was experienced in getting her back into the shed, but she was eventually housed without sustaining any damage of importance.
Owing to the lack of disposable lift, the bags were deflated and various modifications were carried out to lighten the ship, of which the principal were the removal of the keel and cabin entirely, and the removal of the water-trimming services. Other minor alterations were made which gave the ship, on completion, a disposable lift of 3.21 tons. The transverse frames between the gasbags were strengthened, and a number of broken wires were replaced.
On September 22nd the ship was again completed, and on the 24th she was again to be taken out and tested at the mooring post. Unfortunately, while being hauled across the dock, the framework of the ship collapsed, and she was got back into the shed the same day.
Examination showed that it was hopeless to attempt to reconstruct her, and she was broken up at a later date. The failure of this ship was a most regrettable incident, and increased the prejudice against the rigid airship to such an extent that for some time the Navy refused to entertain any idea of attempting a second experiment.