Helium

HELIUM.) Variation of Volume.—Since the volume of a gas depends upon its pressure and temperature and since with a rise in height the pressure becomes less, it is clear that if the gasbags were full at sea level they would be overflowing at, say, 2,000f t. One of two things must happen in such a case, either the bag must be so strong that the difference in atmospheric pressure between sea level and 2,000ft. can be withstood by the bag or some of the gas must be allowed to escape. In practice it is found undesirable to make gasbags to withstand the difference in pressure due to an alteration of more than about 2ooft. in altitude, and accord ingly, automatic gas valves have to be fitted to all gasbags. The height at which, with a given amount of gas, the gasbags are just full is known as the "pressure height." An airship can alter its altitude substantially below this height without change of lift, since as the ship descends the surrounding air being denser com presses the gas automatically, but if it exceeds this height it must lose gas, and a new "pressure height" is established. It is clear from this that the higher an airship flies, the less becomes its useful lift, as the gross lift of the gas is less by the amount that has to be allowed to escape through the automatic valves to es tablish the new "pressure height" at the higher level.

Temperature Variations.

The first flight ever made de pended, as has been described, upon the fact that heated air is less dense than cold air. Similarly, heated hydrogen or heated helium is lighter than cold hydrogen or cold helium. Now in an airship it is possible to obtain much the same effect as that notice able in a glass-house on a sunny day. The sun's rays striking upon the envelope of the ship heat the gas in the gasbags to a higher temperature than that of the surrounding air, with the result that the gas expands and an increased lift is given. This is called superheating. The reverse effect is often found to occur when flying through the clouds.

Height Control.

The control of the flying level of an air ship is effected in two ways : for minor alterations of height by the elevators or horizontal rudders and for large alterations by releasing ballast or gas. The action of the elevators is dynamic and can only operate when the ship is moving, and the magnitude of their effect depends upon the speed of the vessel. In the "R. i oo" at full speed, the "dynamic lift" was estimated to be as much as 17 tons and except for mooring and landing was mainly depended upon to keep a constant height level.

Hull Structure.

The most striking development in the more modern ships as compared with the earlier war-time "Zeppelins" lies in the changes made in the shape of the hull. The early "Zeppelins" had long, narrow, cigar-shaped bodies, the ratio of length to diameter being about I o–I and embodied a very long parallel portion with a relatively short conical bow and tail. This original shape was adopted partly from the desirability of keeping the diameter as low as possible to meet shed difficulties, partly because it may have seemed that the obvious way to re duce head resistance was to make the ship as long and narrow as possible. Experience has shown, however, that, providing the hull is correctly streamlined, the head resistance per unit of volume of a hull, having a length-diameter ratio of only 5-1, may be very considerably less than of these old shapes, which were extremely poor aerodynamically. Again, the vast amount of investigation into the distribution of stresses in the hull frame work carried out during and since the war has shown that with the shorter ships it is possible to obtain a strength and factor of safety double that allowed in the older ships for the same pro portional weight of structure.

The hull itself consists of a metal framework, divided into 15 or more compartments into each of which a gasbag is placed. The gasbag consists of linen fabric coated with gold-beater skin or a specially fabricated gelatinoid film material, and made up in the shape of a cylindrical container. Surrounding and con taining each gasbag is a network of wires, distributing and con veying the lift from the bag to the hull structure. The whole structure is then covered with an envelope of fabric called the "outer cover," the function of this outer cover being to give a smooth surface to the vessel, protect the gasbags and, being specially treated with aluminium powder, to reflect the sun's rays and minimize the absorption of heat.

The engines of the ship are usually carried in separate cars or gondolas suspended from the metal framework of the hull, al though, in order still further to reduce resistance, the engines of the "Akron" and the "Macon" were enclosed in the hull, the only parts without being the propellers. The fuel tanks are carried inside the hull suitably suspended from the girder work, and the control cars, passenger quarters, etc., are either built into the hull or suspended from it. The enclosure of the passenger quarters entirely within the hull, as developed for the first time in the "R-ioo" design, formed a precedent that is expected to be universally accepted, owing to the reduction of resistance. It has also been proposed to situate the control car in the extreme bow.

Performance.

If the atmosphere was always calm the per formance of an airship would be almost incredible, since the power required to drive her at a given speed varies as the cube of the speed, and consequently at low speeds her range would be immense. Under practical conditions, however, that is not the case as average winds that may be encountered upon any flight range up to 3o m.p.h. Under adverse conditions the winds may be as much as 6o m.p.h. or more. If, therefore, the airship is to be a dependable means of transport her speed must be high as compared to the winds she may encounter, and for practical purposes, especially from the commercial point of view, a cruis ing speed of less than 75 m.p.h. is not of much value, and in the opinion of those who have studied the problem a cruising speed of ma m.p.h. will be required before the airship will be able to maintain a frequent commercial transport service. For commercial work, an equal division between fuel and paying load gives the maximum ton-miles and determines the distance between stopping and consequently refuelling bases.

Consider the performance of the "R-Ioo" as an example, as she was the first really commercially-designed vessel. The useful lift was 73 tons. From this was subtracted 21 tons for crew, ballast, loss of lift to 2,000 ft., etc., leaving 52 tons for fuel, passengers and freight. Half of this was 26 tons. The consumption of fuel at 75 m.p.h. was half a ton per hour, thus giving a total endurance of 52 hours at 7 5 m.p.h., or 3,910 m.

To allow for contrary winds fuel for 50% more than the geo graphical distance must be carried, so the performance of the "R-Ioo" could be said to be 2,600m. at 75 m.p.h., carrying 26 tons of paying load.

Strength and Factor of Safety.

The development of the airship, unlike that of the aeroplane, was hindered rather than helped by the World War, owing to the much longer period of time involved in producing a new design. The necessity for the rapid increase of the number of airships after the outbreak of war prevented the natural improvement in shape which would otherwise have developed, and furthermore, the height of the sheds or hangars limited an increase of diameter. The result was that both in Germany and England long, parallel-bodied and cigar shaped ships, having an extremely high head resistance per unit volume were to a certain extent standardized even although their defects were obvious. These vessels, owing to their great length and small effective depth considered as a beam, were subjected to excessive bending moments and stresses when turned rapidly or manoeuvred under gusty atmospheric conditions. In these cir cumstances, with the small total lift available, it was impossible to provide a really adequate margin of strength, and structural failures followed.

In the case of an airship, structure strength has to be provided to meet two distinct sets of conditions; the first, definite and readily calculable, arising from the tendency of the ship to de form under the pressure action of the buoyant gas in the hull, and secondly, the bending condition arising out of rudder action, gust action, etc., which are known with much less exactitude. In the case of the latter an assumption has to be made as to the character and magnitude of the worst possible condition the ship will have to meet, and an arbitrary condition is used of some what similar character to that employed by naval architects in the design of marine vessels. When the magnitude of the worst condi tion is fixed the distribution of the corresponding stresses through out the structure can readily be calculated, since the airship struc ture is an open lattice structure amenable to exact calculation.

Air Currents and Storms.

The airship has to encounter several general types of air-currents. One, the ordinary depression, which may be anything, in temperate zones from 700 to 1,5oom. across. The wind will be steady and blowing in a horizontal direc tion at speeds varying from a few miles per hour to 3o-4o m.p.h. at low altitudes. An airship is not much affected by such a wind in so far as the structure is concerned, and although such a wind would create a rough sea, the air-borne vessel is flying almost as calmly as in still air. The distance the vessel makes good over the ground will, of course, be altered by an amount corresponding to the strength and direction of the wind.

Again, there is what is known as a "line squall." This mete orological phenomenon is caused by cold currents meeting and overlapping hot currents of air. These line squalls may be several hundred miles across and generate dangerous vertical currents. These vertical currents will affect the airship in much the same way as the vertical motion of the waves affects a marine vessel. In certain cases the vertical velocity of these gusts is very high and, moreover, being in some cases of small diameter may be likened to a stream of air being discharged from the nozzle of a hose. If an airship strikes one of these vertical gusts when flying at high speed, she will be thrown violently out of her horizontal path and be subjected to severe stresses. It is computed that the vertical gust which destroyed the "Shenandoah" had a velocity not less than 1,400ft. per minute. The effect of these gusts upon an airship structure was largely neglected in all vessels prior to 1925.

Engines.

For many years no efficient and satisfactory airship engine was evolved. The general effort had been to avoid the use of petrol, with its attendant dangers in connection with hydro gen, and attempts were made to develop a light diesel engine using heavy oil, or a sleeve-valve engine using hydrogen and kero sene as fuel.

The Maybach motors of the "Graf Zeppelin," however, per formed consistently on "blau" gas, which, having approximately the same weight as air, did not so much require the release of lifting gas from time to time to compensate for the steadily decreasing weight of the ship due to the consumption of fuel, which would have been the case otherwise in the instance of gasolene.

Three years earlier, American engineers solved the weight corn pensation problem in another way, being especially impelled to do so because the American-produced helium gas was more ex pensive than hydrogen. They devised exhaust condensers and installed them on the engines. Ballast-recovery devices, as they were called, condensed the water vapour in the exhaust gases into water which was actually a trifle heavier than the gasolene burned in the Maybachs of the "Akron" and "Macon." This not only conserved the helium, but had an operating advantage in demon strating that a modern airship can start on a journey with a min imum of ballast, building up its supply as it goes along, thus increasing its cruising radius by permitting it to start off with more fuel in place of the heavier ballast load formerly carried. The condensers, however, added somewhat to the parasitical resistance, but since the "Akron" and "Macon" motors were in side the hull, a condenser system was devised composed of small ribbed fins along the side of the ship, something like the louvres in the hood of an automobile, getting a maximum cooling result with minimum head resistance.

Mooring Masts.

A large airship cannot be moved into or out of its shed or hangar except in a comparative calm, unless the wind is blowing in the line of the length of the shed. In Germany a revolving shed was made, so that, whatever the direction of the wind, the length of the shed could always be made to coincide with the direction of the wind. When it is realized that a modern shed may be I ,17 5 f t. long, 3 2 5 f t. wide and 2 I I ft. high, weighing many thousand tons, it is obvious that the engineering problems involved in rotating so vast a structure are so great as to render a simpler and cheaper solution to the problem imperative.

One specification of the first British rigid, the "Mayfly," in 1909, included a mooring tower or mast. Prior to that vessel's destruction she was moored to a mast and remained safe during winds with gusts up to 48 m.p.h. In 1912 this idea was adapted by the Royal Aircraft Factory at Farnborough to the mooring of a non-rigid vessel. Many of the essential features employed at Farnborough, such as the fixation point being free to rotate about the axis of the mast, have remained in all masts hitherto con structed.

While further experiments with mast mooring were carried on in England, the U.S. Navy achieved considerable comparative efficiency in airship handling by the construction of a 6o-foot mast at Lakehurst. Earlier masts in the United States and Eng land had been built up to 18o and 200 feet, but the theory of and practical experience with the short mast was that by tying up the airship low it would be less subject to the effects of vertical wind currents.

Supplementing the American masts, a carriage has been de vised to which the after end of an airship may be attached. The carriage is castered so that the ship can swing on a circular track in any direction. The mast and aft attachments are mobile, so that once a ship is tied up outside its dock it can be run into the hangar as on a straight track. For operations at sea, the U.S. Navy has employed the converted tanker, Patoka, which with a mast astern was used considerably as a temporary base by the "Shenandoah" and the "Los Angeles." BIBLIOGRAPHY.-T. Cavallo, Treatise on the Nature and Properties Bibliography.-T. Cavallo, Treatise on the Nature and Properties of Air (1 781) ; V. Lunardi, Account of the First Aerial Voyage in England (1784) ; T. Monck Masen, Aeronautica (1838) ; J. Wise, A System of Aeronautics (Phil., 185o) ; Hatton Turnor, Astra Castra, Experiments and Adventures in the Atmosphere (1865) ; C. Flam marion, Voyages Aeriens (187o) , trans. and ed. by J. Glaisher, as Travels in the Air (1871) ; F. Linke, Moderne Luftschiffahrt (1902) ; and Die Luftschiffahrt von Montgolfier bis Graf Zeppelin (191o) ; J. Lecornu, Navigation aerienne (1903) ; A. Santos Dumont, My Airships (1904) ; M. L. Marchis, Lesons sur la navigation aerienne, Bibl. (19o4) ; W. de Fonvielle, Histoire de la Navigation (1907) ; H. Moedebeck, Pocket-book of Aeronautics (19o7) ; Navigating the Air, papers collected by the Aero Club of America (19o7) ; A. Hildebrandt, Airships Past and Present (1908) ; F. Walker, Aerial Navigation, end. ed. rev. (Ig1o) ; M. de Nansouty, Aerostation Aviation (1911) ; P. Neumann, Die internationalen Luftschiffe and Flugdrachen (Oldenberg, 1912) ; J. Reuper, Graf Zeppelin and sein Werk (1917) ; G. Whale, British Airships: Past, Present, and Future (1919) ; E. O. H. Vivian and W. Lockwood Marsh, A History of Aeronautics, Bibl. (1921) ; C. F. Lafon, Etude sur le ballon captif et les aerone f s marin (192 2) ; R. T. Glazebrook, Dict. of Applied Physics (1922, etc.) ; V. C. Richmond, "The Hulls of Rigid Airships," Proc. Internat. Congress (1923) ; G. H. Scott, "The Commercial Aspect of Airship Transport," Proc. Internat. Congress (1923) ; B. M. Jones, "Aerodynamical Characteristics of the Airship," Jour. Roy. Aero. Soc. (Feb. 1924) ; G. H. Scott and V. C. Richmond, "The Effect of Meteorological Conditions on Airships," Jour. Roy. Aero. Soc. (March 1924) ; C. P. Burgess, J. C. Hunsaker, and S. Truscott, "The Strength of Rigid Airships," Jour. Roy. Aero. Soc. (June 1924) ; J. E. Hodgson, History of Aeronautics in Great Britain (1924) ; L. Crosana, Cronologia aeronautics (1924, etc.) ; J. du Plessis de Grenedan, Les Grands Dirigeables dans la paix et dans la guerre (1925, etc.) ; see also F. T. Jane's All the World's Aircraft Annual (Iqo9, etc.) ; Aeronautical Research Committee, Reports and Memoranda (1911, etc.) ; Hugh Allen, The Story of the Airship.

(C. D. B.; X.)