Lighter-than-air craft consist of three distinct types: Airships, which are by far the most important, Free Balloons, and Kite Balloons, which are attached to the ground or to a ship by a cable. They derive their appellation from the fact that when charged with hydrogen, or some other form of gas, they are lighter than the air which they displace. Of these three types the free balloon is by far the oldest and the simplest, but it is entirely at the mercy of the wind and other elements, and cannot be controlled for direction, but must drift whithersoever the wind or air currents take it. On the other hand, the airship, being provided with engines to propel it through the air, and with rudders and elevators to control it for direction and height, can be steered in whatever direction is desired, and voyages can be made from one place to another--always provided that the force of the wind is not sufficiently strong to overcome the power of the engines. The airship is, therefore, nothing else than a dirigible balloon, for the engines and other weights connected with the structure are supported in the air by an envelope or balloon, or a series of such chambers, according to design, filled with hydrogen or gas of some other nature.
It is not proposed, in this book, to embark upon a lengthy and highly technical dissertation on aerostatics, although it is an intricate science which must be thoroughly grasped by anyone who wishes to possess a full knowledge of airships and the various problems which occur in their design. Certain technical expressions and terms are, however, bound to occur, even in the most rudimentary work on airships, and the main principles underlying airship construction will be described as briefly and as simply as is possible.
The term "lift" will appear many times in the following pages, and it is necessary to understand what it really means. The difference between the weight of air displaced and the weight of gas in a balloon or airship is called the "gross lift." The term "disposable," or "nett" lift, is obtained by deducting the weight of the structure, cars, machinery and other fixed weights from the gross lift. The resultant weight obtained by this calculation determines the crew, ballast, fuel and other necessities which can be carried by the balloon or airship.
The amount of air displaced by an airship can be accurately weighed, and varies according to barometric pressure and the temperature; but for the purposes of this example we may take it that under normal conditions air weighs 75 lb. per 1,000 cubic feet. Therefore, if a balloon of 1,000 cubic feet volume is charged with air, this air contained will weigh 75 lb. It is then manifest that a balloon filled with air would not lift, because the air is not displaced with a lighter gas.
Hydrogen is the lightest gas known to science, and is used in airships to displace the air and raise them from the ground. Hydrogen weighs about one-fifteenth as much as air, and under normal conditions 1,000 cubic feet weighs 5 lb. Pursuing our analogy, if we fill our balloon of 1,000 cubic feet with hydrogen we find the gross lift is as follows:
1,000 cubic feet of air weighs 75 lb.
1,000 cubic feet of hydrogen weighs 5 lb.
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The balance is the gross lift of the balloon 70 lb.
It follows, then, that apart from the weight of the structure itself the balloon is 70 lb. lighter than the air it displaces, and provided that it weighs less than 70 lb. it will ascend into the air.
As the balloon or airship ascends the density of the air decreases as the height is increased. As an illustration of this the barometer falls, as everyone knows, the higher it is taken, and it is accurate to say that up to an elevation of 10,000 feet it falls one inch for every 1,000 feet rise. It follows that as the pressure of the air decreases, the volume of the gas contained expands at a corresponding rate. It has been shown that a balloon filled with 1,000 feet of hydrogen has a lift of 70 lb. under normal conditions, that is to say, at a barometric pressure of 80 inches. Taking the barometric pressure at 2 inches lower, namely 28, we get the following figures:
1,000 cubic feet of air weighs 70 lb.
1,000 cubic feet of hydrogen weighs 4.67 "
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65.33 lb.
It is therefore seen that the very considerable loss of lift, 4.67 lb. per 1,000 cubic feet, takes place with the barometric pressure 2 inches lower, from which it may be taken approximately that 1/30 of the volume gross lift and weight is lost for every 1,000 feet rise. From this example it is obvious that the greater the pressure of the atmosphere, as indicated by the barometer, the greater will be the lift of the airship or balloon.
Temperature is another factor which must be considered while discussing lift. The volume of gas is affected by temperature, as gases expand or contract about 1/500 part for every degree Fahrenheit rise or fall in temperature.
In the case of the 1,000 cubic feet balloon, the air at 30 inches barometric pressure and 60 degrees Fahrenheit weighs 75 lb., and the hydrogen weighs 5 lb.
At the same pressure, but with the temperature increased to 90 degrees Fahrenheit, the air will be expanded and 1,000 cubic feet of air will weigh only 70.9 lb., while 1,000 cubic feet of hydrogen will weigh 4.7 lb.
The lift being the difference between the weight of the volume of air and the weight of the hydrogen contained in the balloon, it will be seen that with the temperature at 60 degrees Fahrenheit the lift is 75 lb. - 5 lb. = 70 lb., while the temperature, having risen to 90 degrees, the lift now becomes 70.9 lb. - 4.7 lb. = 66.2 lb.
Conversely, with a fall in the temperature the lift is increased.
We accordingly find from the foregoing observations that at the start of a voyage the lift of an airship may be expected to be greater when the temperature is colder, and the greater the barometric pressure so will also the lift be greater. To put this into other words, the most favourable conditions for the lift of an airship are when the weather is cold and the barometer is high.
It must be mentioned that the air and hydrogen are not subject in the same way to changes of temperature. Important variations in lift may occur when the temperature of the gas inside the envelope becomes higher, owing to the action of the sun, than the air which surrounds it. A difference of some 20 degrees Fahrenheit may result between the gas and the air temperatures; this renders it highly necessary that the pilot should by able to tell at any moment the relative temperatures of gas and air, as otherwise a false impression will be gained of the lifting capacity of the airship.
The lift of an airship is also affected by flying through snow and rain. A considerable amount of moisture can be taken up by the fabric and suspensions of a large airship which, however, may be largely neutralized by the waterproofing of the envelope. Snow, as a rule, is brushed off the surface by the passage of the ship through the air, though in the event of its freezing suddenly, while in a melting state, a very considerable addition of weight might be caused. There have been many instances of airships flying through snow, and as far as is known no serious difficulty has been encountered through the adhesion of this substance. The humidity of the air may also cause slight variations in lift, but for rough calculations it may be ignored, as the difference in lift is not likely to amount to more than 0.3 lb. per 1,000 cubic feet of gas.
The purity of hydrogen has an important effect upon the lift of an airship. One of the greatest difficulties to be contended with is maintaining the hydrogen pure in the envelope or gasbags for any length of time. Owing to diffusion gas escapes with extraordinary rapidity, and if the fabric used is not absolutely gastight the air finds its way in where the gas has escaped. The maximum purity of gas in an airship never exceeds 98 per cent by volume, and the following example shows how greatly lift can be reduced:
Under mean atmospheric conditions, which are taken at a temperature of 55 degrees Fahrenheit, and the barometer at 29.5 inches, the lift of 1,000 cubic feet of hydrogen at 98 per cent purity is 69.6 lb. Under same conditions at 80 per cent purity the lift of 1,000 cubic feet of hydrogen is 56.9 lb., a resultant loss of 12.9 lb. per 1,000 cubic feet.
The whole of this statement on "lift" can now be condensed into three absolute laws:
1. Lift is directly proportional to barometric pressure.
2. Lift is inversely proportional to absolute temperature.
3. Lift is directly proportional to purity.
The design of airships has been developed under three distinct types, the Rigid, the Semi-Rigid, and the Non-Rigid.
The rigid, of which the German Zeppelin is the leading example, consists of a framework, or hull composed of aluminium, wood, or other materials from which are suspended the cars, machinery and other weights, and which of itself is sufficiently strong to support its own weight. Enclosed within this structure are a number of gas chambers or bags filled with hydrogen, which provide the necessary buoyancy. The hull is completely encased within a fabric outer cover to protect the hull framework and bags from the effects of weather, and also to temper the rays of the sun.
The semi-rigid, which has been exploited principally by the Italians with their Forlanini airships, and in France by Lebaudy, has an envelope, in some cases divided into separate compartments, to which is attached close underneath a long girder or keel. This supports the car and other weights and prevents the whole ship from buckling in the event of losing gas. The semi-rigid type has been practically undeveloped in this country.
The non-rigid, of which we may now claim to be the leading builders, is of many varieties, and has been developed in several countries. In Germany the chief production has been that of Major von Parseval, and of which one ship was purchased by the Navy shortly before the outbreak of war. In the earliest examples of this type the car was slung a long way from the envelope and was supported by wires from all parts. This necessitated a lofty shed for its accommodation as the ship was of great overall height; but this difficulty was overcome by the employment of the elliptical and trajectory bands, and is described in the chapter dealing with No. 4.
A second system is that of the Astra-Torres. This envelope is trilobe in section, with internal rigging, which enables the car to be slung very close up to the envelope. The inventor of these envelopes was a Spaniard, Senor Torres Quevedo, who manufactured them in conjunction with the Astra Company in Paris. This type of envelope has been employed in this country in the Coastal, C Star, and North Sea airships, and has been found on the whole to give good results. It is questionable if an envelope of streamline shape would not be easier to handle, both in the air and on the landing ground, and at present there are partisans of both types.
Thirdly, there is the streamline envelope with tangential suspensions, which has been adopted for all classes of the S.S. airship, and which has proved for its purpose in every way highly satisfactory.
Of these three types the rigid has the inherent disadvantage of not being able to be dismantled, if it should become compelled to make a forced landing away from its base. Even if it were so fortunate as to escape damage in the actual landing, there is the practical certainty that it would be completely wrecked immediately any increase occurred in the force of the wind. On the other hand, for military purposes, it possesses the advantage of having several gas compartments, and is in consequence less susceptible to damage from shell fire and other causes.
Both the semi-rigid and the non-rigid have the very great advantage of being easily deflated and packed up. In addition to the valves, these ships have a ripping panel incorporated in the envelope which can easily be torn away and allows the gas to escape with considerable rapidity. Innumerable instances have occurred of ships being compelled to land in out-of-the-way places owing to engine failure or other reasons; they have been ripped and deflated and brought back to the station without incurring any but the most trifling damage.
Experience in the war has proved that for military purposes the large rigid, capable of long hours of endurances and the small non-rigid made thoroughly reliable, are the most valuable types for future development. The larger non-rigids, with the possible exception of the North Sea, do not appear to be likely to fulfil any very useful function.
Airship design introduces so many problems which are not met with in the ordinary theory of structures, that a whole volume could easily be devoted to the subject, and even then much valuable information would have to be omitted from lack of space. It is, therefore, impossible, in only a section of a chapter, to do more than indicate in the briefest manner a few salient features concerning these problems. The suspension of weights from the lightest possible gas compartment must be based on the ordinary principles of calculating the distribution loads as in ships and other structures. In the non-rigid, the envelope being made of flexible fabric has, in itself, no rigidity whatsoever, and its shape must be maintained by the internal pressure kept slightly in excess of the pressure outside. Fabric is capable of resisting tension, but is naturally not able to resist compression. If the car was rigged beneath the centre of the envelope with vertical suspensions it would tend to produce compression in the underside of the envelope, owing to the load not being fully distributed. This would cause, in practice, the centre portion of the envelope to sag downwards, while the ends would have a tendency to rise. The principle which has been found to be most satisfactory is to fix the points of suspension distributed over the greatest length of envelope possible proportional to the lift of gas at each section thus formed. From these points the wires are led to the car. If the car is placed close to the envelope it will be seen that the suspensions of necessity lie at a very flat angle and exert a serious longitudinal compression. This must be resisted by a high internal pressure, which demands a stouter fabric for the envelope and, therefore, increased weight. It follows that the tendency of the envelope to deform is decreased as the distance of the car from the gas compartment is increased.
One method of overcoming this difficulty is found by using the Astra-Torres design. As will be seen from the diagram of the North Sea airship, the loads are excellently distributed by the several fans of internal rigging, while external head resistance is reduced to a minimum, as the car can be slung close underneath the envelope. Moreover, the direct longitudinal compression due to the rigging is applied to a point considerably above the axis of the ship. In a large non-rigid many of these difficulties can be overcome by distributing the weight into separate cars along the envelope itself.
We have seen that as an airship rises the gas contained in the envelope expands. If the envelope were hermetically sealed, the higher the ship rose the greater would become the internal pressure, until the envelope finally burst. To avoid this difficulty in a balloon, a valve is provided through which the gas can escape. In a balloon, therefore, which ascends from the ground full, gas is lost throughout its upward journey, and when it comes down again it is partially empty or flabby. This would be an impossible situation in the case of the airship, for she would become unmanageable, owing to the buckling of the envelope and the sagging of the planes. Ballonets are therefore fitted to prevent this happening.
Ballonets are internal balloons or air compartments fitted inside the main envelope, and were originally filled with air by a blower driven either by the main engines or an auxiliary motor. These blowers were a continual source of trouble, and at the present day it has been arranged to collect air from the slip-stream of the propeller through a metal air scoop or blower-pipe and discharge it into an air duct which distributes it to the ballonets.
The following example will explain their functions:
An airship ascends from the ground full to 1,000 feet. The ballonets are empty, and remain so throughout the ascent. By the time the airship reaches 1,000 feet it will have lost 1/30th of its volume of gas which will have escaped through the valves. If the ship has a capacity of 300,000 cubic feet it will have lost 10,000 cubic feet of gas. The airship now commences to descend; as it descends the gas within contracts and air is blown into the ballonets. By the time the ground is reached 10,000 cubic feet of air will have been blown into the ballonets and the airship will have retained its shape and not be flabby.
On making a second ascent, as the airship rises the air must be let out of the ballonet instead of gas from the envelope, and by the time 1,000 feet is reached the ballonets will be empty. To ensure that this is always done the ballonet valves are set to open at less pressure than the gas valves.
It therefore follows in the example under consideration that it will not be necessary to lose gas during flight, provided that an ascent is not made over 1,000 feet.
Valves are provided to prevent the pressure in the envelope from exceeding a certain determined maximum and are fitted both to ballonets and the gaschamber. They are automatic in action, and, as we have said, the gas valve is set to blow off at a pressure in excess of that for the air valve.
In rigid airships ballonets are not provided for the gasbags, and as a consequence a long flight results in a considerable expenditure of gas. If great heights are required to be reached, it is obvious that the wastage of gas would be enormous, and it is understood that the Germans on starting for a raid on England, where the highest altitudes were necessary, commenced the flight with the gasbags only about 60 per cent full.
To stabilize the ship in flight, fins or planes are fitted to the after end of the envelope or hull. Without the horizontal planes the ship will continually pitch up and down, and without the vertical planes it will be found impossible to keep the ship on a straight course. The planes are composed of a framework covered with fabric and are attached to the envelope by means of stay wires fixed to suitable points, in the case of non-rigid ships skids being employed to prevent the edge of the plane forcing its way through the surface of the fabric. The rudder and elevator flaps in modern practice are hinged to the after edges of the planes.
The airship car contains all instruments and controls required for navigating the ship and also provides a housing for the engines. In the early days swivelling propellers were considered a great adjunct, as with their upward and downward thrust they proved of great value in landing. Nowadays, owing to greater experience, landing does not possess the same difficulty as in the past, and swivelling propellers have been abandoned except in rigid airships, and even in the later types of these they have been dispensed with.
Owing to the great range of an airship a thoroughly reliable engine is a paramount necessity. The main requirements are--firstly, that it must be capable of running for long periods without a breakdown; secondly, that it must be so arranged that minor repairs can be effected in the air; and thirdly, that economy of oil and fuel is of far greater importance to an airship than the initial weight of the engine itself.
The arrangements made for handling airships on the ground and while landing, and also for moving them in the open, provide scope for great ingenuity. An airship when about to land is brought over the aerodrome and is "ballasted up" so that she becomes considerably lighter than the air which she displaces. The handling party needs considerable training, as in gusty weather the safety of the ship depends to a great extent upon its skill in handling her. The ship approaches the handling party head to wind and the trail rope is dropped; it is taken by the handling party and led through a block secured to the ground and the ship is slowly hauled down. When near the ground the handling party seize the guys which are attached to the ship at suitable points, other detachments also support the car or cars, as the case may be, and the ship can then be taken into the shed.
In the case of large airships the size of the handling party has to be increased and mechanical traction is also at times employed.
As long as the airship is kept head to wind, handling on the ground presents little difficulty; on many occasions, however, unless the shed is revolving, as is the case on certain stations in Germany, the wind will be found to be blowing across the entrance to the shed. The ship will then have to be turned, and during this operation, unless great discretion is used, serious trouble may be experienced.
Many experiments have been and are still being conducted to determine the best method of mooring airships in the open. These will be described and discussed at some length in the chapter devoted to the airship of the future.
During flight certain details require attention, and carelessness on the pilot's part, even on the calmest of days, may lead to disaster. The valves and especially the gas valves should be continually tested, as on occasions they have been known to jam, and the loss of gas has not been discovered until the ship had become unduly heavy.
Pressure should be kept as constant as possible. Most airships work up to 30 millimetres as a maximum and 15 millimetres as a minimum flying pressure. During a descent the pressure should be watched continuously, as it may fall so low as to cause the nose to blow in. This will right itself when the speed is reduced or the pressure is raised, but there is always the danger of the envelope becoming punctured by the bow stiffeners when this occurs.
During the early days of the war, when stations were being equipped, the small type of airship was the only one we possessed. The sheds to accommodate them were constructed of wood both for cheapness and speed of construction and erection. These early sheds were all of very similar design, and were composed of trestles with some ordinary form of roof-truss. They were covered externally with corrugated sheeting. The doors have always been a source of difficulty, as they are compelled to open for the full width of the shed and have to stand alone without support. They are fitted with wheels which run on guide rails, and are opened by means of winches and winding gear.
The later sheds built to accommodate the rigid airship are of much greater dimensions, and are constructed of steel, but otherwise are of much the same design.
The sheds are always constructed with sliding doors at either end, to enable the ship to be taken out of the lee end according to the direction of the wind.
It has been the practice in this country to erect windscreens in order to break the force of the wind at the mouth of the shed. These screens are covered with corrugated sheeting, but it is a debatable point as to whether the comparative shelter found at the actual opening of the shed is compensated for by the eddies and air currents which are found between the screens themselves. Experiments have been carried out to reduce these disturbances, in some cases by removing alternate bays of the sheeting and in other cases by substituting expanded metal for the original corrugated sheets.
It must be acknowledged that where this has been done, the airships have been found easier to handle.
At the outbreak of war, with the exception of a silicol plant at Kingsnorth, now of obsolete type, and a small electrolytic plant at Farnborough, there was no facility for the production of hydrogen in this country for the airship service.
When the new stations were being equipped, small portable silicol plants were supplied capable of a small output of hydrogen. These were replaced at a later date by larger plants of a fixed type, and a permanent gas plant, complete with gasholders and high pressure storage tanks was erected at each station, the capacity being 5,000 or 10,000 cubic feet per hour according to the needs of the station.
With the development of the rigid building programme, and the consequent large requirements of gas, it was necessary to reconsider the whole hydrogen situation, and after preliminary experimental work it was decided to adopt the water gas contact process, and plants of this kind with a large capacity of production were erected at most of the larger stations. At others electrolytic plants were put down. Hydrogen was also found to be the bye-product of certain industries, and considerable supplies were obtained from commercial firms, the hydrogen being compressed into steel cylinders and dispatched to the various stations.
Before concluding this chapter, certain words must be written on parachutes. A considerable controversy raged in the press and elsewhere a few months before the cessation of hostilities on the subject of equipping the aeroplane with parachutes as a life-saving device. In the airship service this had been done for two years. The best type of parachute available was selected, and these were fitted according to circumstances in each type of ship. The usual method is to insert the parachute, properly folded for use, in a containing case which is fastened either in the car or on the side of the envelope as is most convenient. In a small ship the crew are all the time attached to their parachutes and in the event of the ship catching fire have only to jump overboard and possess an excellent chance of being saved. In rigid airships where members of the crew have to move from one end of the ship to the other, the harness is worn and parachutes are disposed in the keel and cars as are lifebuoys in seagoing vessels. Should an emergency arise, the nearest parachute can be attached to the harness by means of a spring hook, which is the work of a second, and a descent can be made.
It is worthy of note that there has never been a fatal accident or any case of a parachute failing to open properly with a man attached.
The material embodied in this chapter, brief and inadequate as it is, should enable the process of the development of the airship to be easily followed. Much has been omitted that ought by right to have been included, but, on the other hand, intricate calculations are apt to be tedious except to mathematicians, and these have been avoided as far as possible in the following pages.
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