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Aviation History
1913
1913 - 0644.PDF
I/jjGHT JUNE 21, 1913. THE ASTRA TORRES AIRSHIP THE first trial of the Astra Torres airship, which has been in process of construction in one of the sheds of the Military Wing of the R.F.C. because the Naval Wing, to which it will ultimately belong, has at present no suitable accommodation for it, was not altogether a success, but it would be absurd to make too much of any mishap to which any airship succumbed in an experimental flight. It is far from easy to bring any piece of machinery to a state of working perfection, and when the machine is an airship the problem is somewhat more difficult than usual. To have any apparatus work perfectly the first time after its erection is naturally claimed as meritorious by those concerned, but it is equally recog nised by all who have any experience of similar undertakings that such success pertains almost as much to the nature of an " accident " as does a mishap. There is of course no longer any such excuse after the preliminary trials have once been held, and those responsible for the charge of the machine have become acquainted with all the details of its operation. The Astra Torres at the time was still in the charge of the French company who are supplying the airship to the Navy, and whose mechanics have been respontible for its erection at Farnborough. From the photographs, it will be apparent that the envelope buckled downwards in the centre. The cause indicating the fault on this occasion may have been trivial and was certainly temporary, but the problem of preventing airship envelopes in general from buckling constitutes, one may say, the basic problem of airship design. The essential speciality of each constructor who has engaged upon airship manufacture resolves itself, to all intents and purposes, into some particular principle of attachment between the envelope and the car. In an elementary airship, if the car is hung from the centre, the ends tends to rise. If the car is made longer and hung from the extremities of the envelope, they tend to sag. To avoid either of these extremes is not easy, because although it is simple enough to say that the load of the car must be distributed, the proper distribu tion is less readily accomplished. A car that extends the full length of the airship envelope is larger than could reasonably be required for any useful purpose other than that of uniformly distributing the load. For this purpose, the rigid or semi-rigid type, in which the envelope is artificially strengthened by some form of independent engineering structure, offers greater economy of material. Where the car is only made so large as is required for the accom modation of the engines, fuel and crew, to the amount that can be lifted by the gas vessel, it is invariably shorter than the envelope itself, and any attempt to distribute its weight along the envelope necessarily calls for the use of a number of wires that pull upon the envelope fabric at various degrees of obliquity. To adjust these tensions so that they may be resisted by the internal gas pressure of the envelope without any predisposition to buckling is a matter requiring considerable thought. Incidentally it is a problem that affords a fertile field for originality in design. The Astra Torres is a case in point. It will be observed that the envelope is not cylindrical. It is variously described as being of trefoil section, or as resembling a bunch of three bananas. In section, each of these lobes forms a semicircle described upon a side of an equilateral triangle for its base. In actual fact, this equilateral triangle finds a material existence inside the envelope in the form of fabric. Thus, the construction of the envelope comprises a fabric " girder " of triangular section with its apex inverted ; around this are the three semi-cylindrical lobes. The interior fabric is porous ; consequently the gas has the same free motion as in an undivided chamber. We understand that a principal claim of the constructors in respect to the design is that the system permits of suitably dis tributing the load with proportionately less weight of material. The fabric of the outer envelope is said to be appreciably lighter than would ordinarily be employed for the purpose, partly because it is not stressed by the attachments as much as would be the case if they were fastened to its exterior surface in the usual way, and partly because the bursting pressure to which they are subjected by the gas is reduced on account of their small diameter as com pared with the diameter of a single cylindrical gas-vessel of equal capacity. These arguments are interesting but scarcely conclusive in the absence of numerical data and comparative calculations. A feature of the Astra Torres design in this particular that is more apparently meritorious, however, is that a considerable amount of the rigging being inside the envelope instead of on its surface, is excluded from augmenting the air-resistance. So much of the rigging as is outside is itself somewhat peculiar from the very nature and position of the attachment, but it is not less interesting in respect to the arrangement of the wires at then- points of attachment to the car. There are various ways of causing an airship to ascend and descend, but that adopted by the designers of the Astra Torres is the travelling car system, in which the fore and aft trim of the ship is adjusted by means of mechanism that enables the car itself to be moved bodily to and fro beneath the envelope. To facilitate this movement, the cables are attached to the car by means of pulleys, and a set of them are connected to a roller chain that forms a kind of rack rail, along which the car can move or be held steady at any point as required. A sprocket, forming part of the elevating gear under the pilot s control, engages with the chain. When the sprocket is rotated, the car is caused to move along the chain, and it is enabled to do so because all the other cables from which it is slung also pass fore and aft under pulleys. In the patent, the Astra Co. calls its suspension " elliptic, for the reason that the free motion of any one pulley considered separately is an ellipse. The two points of attachment between its cable and the envelope are its foci. The system may be visualised in an instant by recalling the geometry of the ellipse that was so popular at school, when one was permitted to describe the said curve by the aid of two pins and a piece of cotton. The two pins correspond to the points of attach ment to the envelope. The piece of cotton is the cable supporting the car, and the pencil point that engages in a loop of the cotton to describe the ellipse, represents the pulley by which the cable is attached to the car of the Astra Torres airship. A series of five such cords with pulleys constitutes, in principle, the external portion of the suspension of each side of the car from the envelope. The cords from the car, instead of diverging to the equator of the gas-vessel, as is usually the case, converge to the lowermost meridian. At each point where they meet the envelope they are attached to fastenings that pass through the envelope into the gas-vessel, which contains an internal system of radiating cords to distribute the load fore and aft to the upper corners of the interior triangle of fabric. The car of the Astra Torres is a tubular steel structure between 50 and 60 ft. long and about 8 ft. broad. It is sheathed with aluminium plating. Its characteristic feature is its roominess, and although some of the members seem to be somewhat unnecessarily spread about, the general arrangement has at least the merit of accessibility. The bows afford a large compartment with a wind screen in front, at such a height as to protect the "look-out." The second section comprises two decks, on the upper of which are the controls. Be neath are the fans for inflating the balloonettes, the water ballast bags and two guide ropes. The third compartment in the bows is for the engineers, and the fourth section contains the engines. Aft of the engines are the fuel tanks, disposed in two groups of three vertical cylinders, arranged on either side of a central gangway that runs between the engines into the engine control room. A pair of 6-cylinder Chenu engines, each driving its own two- bladed propeller, comprise the lower plant. These engines have their cylinders cast in pairs, and in general design they follow standard automobile practice on a large scale. They are supported on four quarter-elliptic leaf springs, placed transversely in pairs. Each pair virtually constitutes a semi-elliptic spring, but the attachment to the engine frame has a more rigid base by the method adopted. The ends of the springs rest on a system of links, so arranged as to tend to preserve the alignment of the shafts without interfering with the suspension. On each engine, a single float chamber feeds jets in the twin induction-pipes, and a single high-tension magneto is connected to a single set of plugs. The cylinders are, of course, water-cooled, and the radiators are mounted in a similar relative position to the engine to what they would be on a motor car. Honeycomb radiators are employed, and each is cooled by a fan-induced draught that is quite independent of the speed of the machine through the air. The fans are driven by broad belts from pulleys situated immediately behind the flywheels. Transmission from each engine to its propeller is effected through a Hele Shaw clutch and universal joint to a bevel gear box that also provides a reverse. An oblique shaft, supported by ball bearings in a derrick-like outrigger, transmits the power to another bevel gear and so to the propeller. The propellers do not swivel as they do on the airships constructed at the R.A.F., and the Astra Torres has not the advantage of utilising their thrust for the purpose of ascent and descent. By means of its reverse gear, however, it can use them to check the motion of the airship through the air. A brake mechanism is fitted to the propeller-shaft so that it may be brought to rest more quickly prior to reversing. 67O
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