THE CONTROL CAR OF THE HINDENBURG. Among the more important instruments and controls were (1) gas pressure gauge; (2) inclinometer; (3) climb and fall indicator; (4) altimeter; (5) gas and air thermometers; (6) elevator wheel; (7) gyro-compass; (8) magnetic compass; (9) rudder wheel;
(10) engine telegraphs; (11) revolution counter. Should the steering controls have become damaged, the airship could still have been steered by means of auxiliary controls inside the lower rudder fin.
ALTHOUGH the physicist Jacques Charles proved as early as 1783 that a gas-filled balloon would go aloft, and Henri Giffard, in 1852, proved that a balloon’s movements in the air could be directed, demonstration of the airship as a practical carrier was to wait for the internal combustion engine and for the sturdy lightweight metal duralumin.
The airship principle was not definitely accepted until the beginning of the nineteenth century and fewer than 160 rigid airships have yet been built in the whole world. It is remarkable, therefore, not that the airship has not come into its own, but that so much progress has been made. Scores of improvements have been effected in design, in engines, in communication systems, in stress calculations and in materials. Today the building of the modern airship is a most interesting study.
The description of airship construction in this chapter is largely based on that of the German airship Hindenburg. From a general point of view the methods described are applicable to any modern airship, but where figures are given these apply specificially to the Hindenburg.
As the work begins with the metal skeleton, trucks of sheet duralumin arrive at the airship workshops alongside the main shed. In the shops strips of duralumin, from 6 in. to 9 in. wide, are fed to great presses which turn up the edges and punch in the metal holes of various sizes. As the punching of these holes cuts out half the metal, the dead weight is halved. Yet when small flanges have been machine-turned round each hole, the perforated strip is left stronger than if it had remained one solid piece. These channel sections, as they are called, are then ready to be riveted together, to form a girder section.
Airships must fly in all weathers, and one of the useful properties of duralumin is that it is highly resistant to corrosion. To make the ships still more immune to damage from weather and to give them longer life, the channel sections, before being assembled into girders, are electrically treated and then varnished.
Next comes the assembly of the girders to form giant rings, or frames; the largest of these being over 130 feet in diameter. The frames are not round, but polygonal, with their comers connected by longitudinal girders. Over most of the airship’s length, the polygon is one of thirty-six sides, but near the stern the number of sides is reduced to twenty-four.
The former practice in building was to erect a great cradle or steel scaffolding into which the ship was built. Now the main frames are built flat on the floor of the airship shed; when they have been completed they are hoisted into their upright position in the ship, in the following manner. First the lighter and intermediate frames are lashed to the main frames. Then ropes are attached at many points round the circle to distribute the weight; these ropes are fastened to a cable, which reaches to a pulley in the roof, and thence to a winch below. Other lines are attached to guide the frame. When all is ready, the frame is gently raised into an upright position and set in place without the slightest jar.
The main frames are inherently strong, and consist of two outer rings, connected by cross girders in zigzag fashion to an inner ring, thus forming a triangular section. The main frames are spaced approximately 74 feet apart, and are large enough to allow members of the crew to climb entirely round the circumference of the ship, thus facilitating inspection and maintenance.
The intermediate frames are of the single-girder type, and are spaced between the main frames, usually three intermediates between two main frames. Their function is merely to stiffen the longitudinal girders, the principal loads being carried by the main frames. A network of diagonal wires braces the outside panels; another system of wires and cord netting transfers the gasbag pressure to the hull structure.
The longitudinal girders connect the transverse frames and run the entire length of the ship, forming the fore-and-aft ridges discernible through the outer cover. More than 7,000,000 duralumin rivets are used in the construction of the hull of the ship.
After the first frames are in place, a test gasbag is made and inserted in the structure, where it is filled with gas and floated into position. This is done to make sure that it will fit snugly between the bulwarks of the duralumin framework and the network of wires. After the test has been completed the bag is deflated and removed until the hull is built.
The modern ship has from twelve to sixteen gasbags ranging in capacity from approximately 100,000 cubic feet to 1,000,000 cubic feet. Each gasbag is a fabric cylinder of a size to fit its particular place in the ship. The largest bag is amidships.
The search for improvement has gone into the basic materials of construction. In the past, gasbags were made of goldbeater’s skin, laid down in two piles on thin fabric. Goldbeater’s skin is a small section of the intestines of a steer; it has been used since the Middle Ages by goldsmiths for the hammering out of gold leaf because of its thinness and extraordinary strength. It has been estimated that goldbeater’s skin from more than 2,000,000 cattle would be required for the gasbags of one modern airship. Each skin would have to be laid by hand and with great precision.
Twelve Acres of Fabric Needed
A great deal of research has gone into the matter of finding a substitute which would avoid this vast amount of labour. There has eventually been produced a gelatine-latex fabric in which many coats of latex and gelatine are spread over fabric; this matches the goldbeater’s skin in weight and shows considerably higher resistance to gas diffusion. More than twelve acres of this fabric are required for an airship’s gasbags.
Each pair of gasbags has, between the two gasbags, a gas manifold or shaft into which the gas release valves are made to discharge. The gas escapes through a special discharge fitting on the outer cover of the ship. The gas manifolds serve also to ventilate the interior of the ship, not only the parts that lie above the central gangway, (above which all the gas valves are arranged) but also those lower parts of the ship down to the keel gangway.
The fins are placed near the stern of the ship; two of the fins are horizontal and two vertical. The fixed surfaces are approximately 105 feet long, 40 feet wide and 12 feet thick at the base adjacent to the hull. The lower vertical fin is slightly smaller in area than the upper vertical and two horizontal fins.
The fins are built up of duralumin girders, called transverse frames and longitudinal ribs, that leave comparatively small areas of fabric unsupported. Because of the immense size of the surfaces, members of the crew can enter the interior for inspection of the fabric and make any repairs that may be necessary. The lower vertical fin is provided with a landing wheel to take up the shock of landing.
CONSTRUCTION OF THE STERN of the Hindenburg. The vertical and horizontal fins were secured in position by a cruciform girder, I, which passed right through the envelope of the airship. 2 indicates the netting which held the gas cells in position ; 3, the upper fin ; 4, the control lines which ran to the control-line bracket, 5, attached to the upper rudder 6 ; 7 and 8, the elevators ; 9, the lower rudder ; 10, the lower fin ; II, the landing wheel ; 12, the auxiliary control room ; 13, port elevator fin ; 14, control lines; 15, fuel tanks.
The rudders and elevators, or movable surfaces, which control the movements of the airship are fixed respectively to the vertical and horizontal fins. Each movable surface is attached to its fixed surface by four pins and gudgeons and in also fitted with a set of balancing vanes to provide easier operation of the controls. The leading edges of the fins, nearly four feet wide, are among the few pieces of solid metal, apart from the nose cone and tail cap, in the ship’s structure.
In the construction of rigid airships it was formerly the practice to have one “keel” or backbone, in which was contained a gangway or corridor through which members of the crew might move from one end of the craft to the other. This keel was the airship’s major reinforcement against vertical thrusts of the wind which might be of uneven force along a vessel hundreds of feet long. Modern ships have two, if not three such keels. Some idea of the increased strength may be gained from the act of lashing two or three lead pencils together and trying to break them, and comparing the effort required with that needed in breaking a single pencil.
One of the longitudinal gangways, commonly known as “catwalks”, extends along the top centre line of the ship, from a point about 85 feet from the bow to a point about 135 feet from the stern. Access to the top catwalk is provided through any of the main frames and by three access ladders from the lower gangway. This lower or keel catwalk runs from the bow cap to the stern, with side gangways to the four engine gondolas.
The forward keel catwalk leads from the officers’ quarters above the control car, or bridge, to the mooring winch platform (see the chapter “The Mooring of Airships”) and to the “gangplank” which is lowered to the mooring mast platform for the transfer of personnel from the ship to the ground through the mast.
Tanks for Trimming
Abaft the officers’ accommodation, and running almost the full length of the airship, a series of water and fuel tanks is placed at intervals on either side of the catwalk. Some of the water tanks contain water for the use of personnel; others are ballast tanks.
The fuel tanks, which generally contain 130,000 lb. of gas oil for the modern airship’s diesel engines, are interconnected; thus it is possible to trim the ship by pumping oil from one set of fuel tanks to the other.
For this purpose there is a pump driven by a propeller working in the airstream of the airship. The fuel is also pumped from the main supply to service tanks in the engine gondolas. Each main tank is suspended by wire which may be cut in an emergency, and thus allow the tank to drop completely from the ship.
Triangular patches of the outer cover are laced below these tanks, so that only a certain amount of the fabric will be torn away if a tank has to be dropped. An extensive system of piping permits fuel to be received through the bow, if the ship is at the mast, or near amidships; the fuel can be pumped to any desired tank. Lubricating oil tanks are placed near each engine gondola.
The ballast system consists of eight rubberized storage bags, each of 1,100 lb. capacity and connected by piping. Four of the bags are fixed in the bow and four in the stern. Each of the bags is equipped with a quick-discharge valve operated through a wire pull leading to the control car.
The ballast tanks arranged on either side of the lower catwalk are for recovered water ballast. The water ballast recovery system, introduced by British airships in 1911, is used to condense into water the water vapour in the exhaust gases of the engines; this water serves as ballast in preserving the equilibrium of the ship.
AN AIRSHIP’S GENERAL DESIGN is illustrated by this sectionalized diagram of the Hindenburg. The illustration shows (1) inter-girder bracing wires; (2) gasbags; (3) longitudinal girder; (4) and (7) main frames; (5) and (6) intermediate frames; (8) axial girder; (9) gas and ventilation duct; (10) gas-release valve; (11) gangway built in axial girder; (12) resilient wire bulkhead between gasbags; (13) radio room; (14) upper deck; (15) lower deck.
As fuel is consumed, the load being carried by the airship becomes lighter. If no means were available for the compensation of weight lost, it would be necessary to valve off valuable lifting gas before landing. The apparatus for each engine consists of five groups of panels mounted in the hull above each engine, each panel being made up of horizontal aluminium tubes connected by vertical headers. The flow of exhaust gas is initially upward, and the condensed water is drawn off through bypass pipes and circulated to the ballast tanks throughout the ship.
It is now possible to recover 85 lb. of water through the water ballast recovery system for every 100 lb. of fuel consumed. With developments now taking place it is anticipated that in the near future the amount recovered will equal that of the fuel consumed.
In a commercial airship, and at about a third of the ship’s length from the bow, is placed the passenger accommodation. Here are staterooms, lounges, dining-room, reading and writing-room, smoke-room, bar, and so forth, with a total floor space of 5,380 square feet. The accommodation is arranged on two decks. The lower deck contains a large modern kitchen with an electric galley stove 6 ft. 6 in. long, with the necessary working tables and cooling apparatus. On this deck also are the officers’ dining-room, the crew’s mess room, the stewards’ room and bathrooms.
In the naval airship space is allocated for the ship’s six fast fighting aeroplanes, and here the aeroplane hangar is situated. To provide entrance and exit for the aeroplanes, a T-shaped opening is made; this may be closed by part-sliding and part-folding hatches. The aeroplanes are released and picked up by a trapeze arrangement while the airship is in flight, and they are hauled into the hangar by a winch. Instead of resting on a deck the machines are suspended from tracks carried by the overhead structure.
The lines carrying electric current throughout the ship are contained in over a mile of gastight conduit, placed inside the ship’s girders, where it is easily accessible but out of the way. There are more than 200 electric lights in the airship, ranging from a powerful searchlight situated near the generator room to 10-watts and 40-watts bulbs, and from ordinary ceiling lights to desk lamps. In addition to lights within the ship there are eight red, green and white navigation lights which must be kept burning when the ship is under way at night.
Gastight receptacles, or outlets, for plugging in electrical equipment, similar to those found in a house, are fixed in many places throughout the ship. All plugs for use in these outlets are of
aluminium and are waterproof and vapour-proof. The plugs are screwed in and cannot be accidentally pulled out. Near the generator room is the chief engineer’s office, with its array of instruments and telegraphs which enables him to see exactly how each engine is operating.
THE OUTER COVERING of an airship is made of cotton cloth cut into carefully shaped panels, a tolerance of only one-thirty-second of an inch being allowed on each seam. The cloth is first laced into position as tightly as possible and allowed to stand for a time. The lacing is then tightened, a process which is again repeated after the first coat of dope has been applied. This photograph shows part of the covering of the Hindenburg in position.
Near the stern a ladder leads from the catwalk into the lower vertical fin, and here auxiliary elevator and rudder wheels are installed. This auxiliary control station is about 15 feet long and 3 feet wide; it provides facilities for directing the ship’s movements should the control cables from the control car be shot away or disabled. Even with all controls broken the airship’s movements can still be directed (see the chapter “How Airships are Flown”). With the completion of the hull the engine gondolas, which have been erected in the shops, are now hoisted into their respective positions and attached to the ship. The engines are then built into the gondolas and the propellers fixed.
The propelling machinery of a modern airship consists of four sixteen-cylinder V-type heavy oil engines. Each engine has a maximum designed output of 1,100-1,200 brake horse-power when running at 1,500 revolutions a minute, and a continuous cruising rating of 800-900 brake horse-power at about 1,350 revolutions.
The engines are mounted on a duralumin supporting base and the upper and lower parts of the crankcase are made of “Alpax-Gamma” modified aluminium silicon alloy. Cooling ribs on the underside of the crankcase are provided. All the working parts are enclosed. The cylinders have two inlet and two exhaust valves, and the water-cooled liners are of steel. The connecting-rods are provided with roller bearing ends, and great care has been taken to produce an engine free from torsional vibration.
Coolers for the circulating water and the lubricating oil are built into the forward part of the engine gondola. By means of adjustable inlets, the temperature of the water and of the oil can be varied as required. Each gondola is fitted with the necessary oil and cooler connexions but the engines can be supplied with warmed water for starting in cold weather.
There is a starting air cylinder in each gondola; this cylinder is kept charged by an engine-driven two-stage air compressor, at a working pressure of 735 lb. per square inch. This compressor also supplies the necessary air for the springing arrangement on the landing wheels under the control car and lower vertical fin.
The control car, from which the ship is flown and navigated (see the chapter “Types of Airship”), is built on to the forepart of the hull. Throughout the construction hundreds of tests of small magnitude are made on the girders, joints, fittings, fins, rudders and various items of installation and equipment.
When the hull has been completed the gasbags are inserted in their respective places and duly inflated. These gasbags are fitted with automatic and hand-operated release valves. The automatic valves will release gas of their own accord when the lifting gas has expanded to the full extent of the gasbag. The other valves are opened manually by means of a wire pull leading to the control car. The valves, 32 inches in diameter, allow the airship to rise at a rate of 4,000 feet
a minute without causing a serious increase in the internal pressure.
In the plane of each of the main frames is placed a resilient bulkhead of hard wires which resembles a spider’s web. These bulkheads are situated between the gasbags and each is fitted with resiliency devices, consisting of gas-filled cylinders, so that some bulging of the bulkhead may take place in the event of a gasbag pushing against it, and serious torsional loads in the main frames will not therefore be built up. To deal with the possible bulging of the gasbags, extra material is provided in the circular ends of the gasbags abreast of the resiliency devices.
The outer cover of the ship is finally put on. This cover is made of cotton cloth having a weight of 2.8 ounces per square yard, and covering an area of more than seven acres. The cloth is made into carefully tailored panels, the majority of which are 74 feet long and about 12 feet to 24 feet wide. More than a thousand yards of thread are necessary to sew the panels.
The panels are first laced to the structure of the ship; each panel has its appointed place. To facilitate the lacing, duralumin eyelets, averaging more than 800 to the panel, are placed along the edges of each panel. The panels are laced as tightly as possible, and allowed to stand for a time, after which the lacing is again “pulled” or tightened. This process is repeated before the first coat of clear acetate dope is applied by hand.
On the sides near the propellers the outer cover is specially strengthened so that hard substances, such as ice thrown against the hull, will not damage the gasbags. Application of the first coat of dope by brush allows every part of the fabric to become permeated. The effect is to shrink the cover and make the panel taut.
To diminish the effect of ultra-violet radiation, the top side of the outer cover is treated with a special red paint. After this the next coat of dope is sprayed on; the last two coats contain aluminium powder, which gives the ship her silvery appearance.
The airship is now ready for her “lift and trim” tests (see the chapter “How Airships are Flown”), after which she will be prepared for flight and taken out for her official trials.
FIXING AN AFTER ENGINE GONDOLA to the Hindenburg. Gondolas are fitted to the main frame of an airship before the adjacent covering is put into position. A ladder is provided which allows engineers to pass from the hull of the airship to the gondola while the airship is in flight. In the front of the gondola adjustable ventilators are fitted which control the flow of air to coolers for the engine water and oil.