CHAPTER V
ADVANTAGES AND DISADVANTAGES OF AIRSHIPS
The chief advantages of aircraft that are lighter than air over those that are heavier than air in warfare are:—
1. Their speed can be variable.
2. They can hover over a particular point.
3. They can be noiseless by cutting off motive power and drifting for a while with the wind.
4. They can from their possible size have long range of action.
5. They can carry considerable weights.
6. They are endowed with sustaining power and stability.
1. Their speed can be variable.
This advantage becomes apparent in cases where they are used both for scouting and offensive purposes.
In a later chapter it will be pointed out that though the aëroplane scout has often to make dashes over the enemy, and it would be thought that from his swift movements his impressions might be vague, still, in practice, most satisfactory work has been undoubtedly accomplished. Many, however, will maintain that there are circumstances when it may be advisable for observers to proceed at variable speeds. When at a safe height it may be an advantage for the observers to take their time and leisurely survey the country, observe, and take photographs. The airship can stealthily travel over camp and fortress and steal secret after secret of the enemy.
2. They can hover over a particular point.
The fact that the maintenance of the airship in the air does not depend upon a certain speed being maintained, as is the case with the heavier-than-air machine, endows it with the property of being able to hover in fairly calm weather. The hovering power is certainly an advantage for such offensive operations as dropping bombs.
3. They can be noiseless.
At night it may often be possible to approach over a fortress, camp or city quite noiselessly at a low altitude by shutting off the motive power and navigating by means of the natural forces alone.
4. They can from their possible size have long range of action.
From their size and the amount of fuel they can carry it is possible for them to travel for long distances.
This quality renders them specially fitted for naval purposes, though possibly in the not very distant future more highly developed hydroplanes will run them very close.
5. They can carry considerable weights.
The weights large airships can carry is an advantage in offensive operations. It enables larger stores of bombs to be carried than is at present possible with aëroplanes. Then several persons can be carried long distances in the larger airships.
6. They are endowed with sustaining power and stability.
As the envelopes of airships are filled with a gas which lifts and sustains, the great disadvantage of instability which is the bugbear of aëroplanists is absent. If engines break down or stop, it does not necessarily mean that the airship must immediately descend. It can often remain in the air while the machinery is being repaired.
But in spite of these advantages airships have very numerous counterbalancing disadvantages, so marked, indeed, that it seems a question whether, if the world decided to entirely use aëroplanes in their place, it would be much the loser.
The principal disadvantages would seem to be:
1. The resistance of the gas-bag.
2. Danger of fire from close combination of petroleum motor and gas-containing envelope.
3. Danger of fire from self-electrification of surface.
4. Difficulties in the way of applying the propulsive screws in the most effective position.
5. Difficulties of making gas envelope gas-proof.
6. Great cost of airships.
7. The great amount of personnel needed for the manipulation of large airships.
8. Great liability of being destroyed by aëroplanes in war.
9. Insufficient power of quickly rising.
1. The resistance of the gas-bag.
From a mechanical point of view it is in opposition to science to attempt aërial navigation by pushing such a large resisting surface as the envelope of an airship against the air. In navigating an airship against the wind, as the latter increases speed is diminished, until a limit is reached when the motive power will be unavailing. Thus there are weather limitations to the airship. Not that the aëroplane is unaffected by the weather. That also has its limits; but recent practice has shown that the proportion of days when aëroplanes can fly is considerably larger than those on which airships can venture forth from their sheds.
This disadvantage of the resistance of surface was very manifest in the earlier experiments with navigable balloons, when only feeble motive power was available. For instance, in Count Zeppelin’s experiments in 1900, his two motors of 16 h.p. could not combat a greater wind force than about three metres a second. Then airships could indeed only be called toys. It has only been possible to make them partially successful concerns by enormously increasing motive power. At the h.p. figures with which the latest made large airships have been endowed, the wind limit is much lower than in the case of the heavier-than-air constructions. Though now airships can encounter moderate winds, they are still fair-weather instruments. For the great records of distance established by Count Zeppelin favourable meteorological conditions have been wisely selected. It was M. Santos Dumont who first led the way in making airships something beyond toys. He, in his picturesque and world-alluring experiments, first dared to encounter winds which in force exceeded what would be called calm weather. It is exceedingly difficult to ascertain what are the exact wind forces overcome by a body moving in air. The measurements have to be taken from a point independent of the moving body. We generally find this one important figure omitted in accounts of airship voyages. M. Santos Dumont’s experiments gave especially favourable opportunity for ascertaining correct records of the wind forces overcome. Since M. Santos Dumont so frequently rounded the Eiffel Tower close to the storey where the meteorological instruments were placed, the writer obtained from the authorities of the Eiffel Tower a record of the wind forces registered on all the days of his experiments. A comparison of those records with those of M. Santos Dumont’s journeys made it possible to approximately ascertain the highest wind forces he combated on his journeys round the tower; these were about five metres a second. M. Santos Dumont, however, appears to have claimed six metres a second for his highest wind record.
The brothers Lebaudy in their earlier experiments about doubled the record of Santos Dumont in this respect. As time has gone on greater advance has been made, though the limit is still represented by moderate wind.
There is, perhaps, some consolation in this thought for those who fear raids of an inimical airship fleet. The proverbial windy nature of our favoured islands is perhaps even more protection than darkened cities and artillery shot, though it is well indeed not to neglect the two latter precautions.
Meteorologically speaking, to make a raid with bulky airships from a distance over these islands would be a very risky undertaking, fraught with the greatest danger to the occupants of the airships. It must be remembered that, chiefly owing to the weather, the history of the Zeppelin may well be called the history of disaster. For the very reason of its fragility over and over again it has been the victim of tempest and flame.
The use of aluminium for the framework of the Zeppelins has been largely responsible for Count Zeppelin’s repeated weather misfortunes. There has been a fascination about this brittle metal aluminium for aëronautical work on account of its lightness. Its employment for aircraft construction, except for trivial purposes, is, however, a fallacy. That most practical aëronautical engineer, M. Julliot, in working out his semi-rigid constructions, has never fallen into the snare of aluminium allurement, wisely using steel instead. Considering the aluminium framework of the first Zeppelin constructed was fairly wrecked by the trifling accident of its falling down from the ceiling of the shed to the floor, it is a wonder that this species of metal has been retained, to be crumpled up almost like paper in the many accidents that have occurred.
2. Danger of fire from close combination of petroleum motor and gas-containing envelope.
In airships of all three types—rigid, semi-rigid, and non-rigid—this danger is constantly present. There have been examples of airship conflagrations in mid-air, but the greatest danger of conflagrations is in descending when the airships have been overtaken by strong and gusty winds. As has before been stated, fire has been the great destroyer of the Zeppelins.
The nearer the car containing the motors is placed to the gas envelope, the greater the fire risk becomes. The Parsifal airship, in which the car is suspended a considerable distance from the gas-bag, should in this respect be the safest of all the types of airships yet constructed.
3. Danger of fire from self-electrification of surface.
This appears to be a great danger in the case of airships whose gas-bags are made with india-rubber surfaces. No less than two Zeppelins have been destroyed from this cause. In the case of the explosion of the gas in a Zeppelin of 1908, when it burst from its anchorage at Echterdingen, the destruction of the airship appears to have been caused by electric sparks produced by the friction of the material of which the gas-bag compartments were made. Colonel Moedebeck, in the Aëronautical Journal of October, 1908, gave an expert opinion as to the cause of this accident:—
The balloon material, which is india-rubber coated, has the peculiar property of becoming electrified in dry air. When rolled up or creased in any way it rustles, and gives out electric sparks, the latter being (as shown by the experiments undertaken by Professor Bonsteim and Captain Dele for the Berlin Aëronautical Society) clearly visible in the dark.
Now, the lower parts of the material of which the gas-cells are composed would, owing to the height to which the airship had ascended (1,100 m.) and the release of gas from the valves, become creased or folded upon each other, and the rubbing thus produced would be quite sufficient to generate the electric sparks above referred to. Under ordinary circumstances, when the space between the gas-cells and the outer envelope of the airship is full of atmospheric air, continually renewed, as when it is in full flight, these sparks would be harmless enough, but when the ship is at anchor, as at Echterdingen, this is not necessarily the case.
We know that the carefully made tissue of the Continental Caoutchouc Company resists the penetration of hydrogen very strongly, but some may have leaked through into the space between the cells and the outer envelope, while it seems very probable that when the mechanics opened the valves, and the long axis of the balloon became inclined, more hydrogen entered this space and an explosive mixture was formed.
According to the description given by eye-witnesses, the explosion took place after the forepart of the vessel (dragging its anchor) struck the ground. The shock thus caused would have been transmitted to the creased and wrinkled gas-cells, and the tearing of the material, already in an electrified condition, might easily have generated sufficient sparks to detonate the explosive mixture.
Again, in 1912, there was a repetition of this kind of disaster in the case of the destruction of another Zeppelin, the “Schwaben.” In this case the framework of the airship had got broken, being battered about in landing in an adverse wind. The india-rubber-coated bags were rubbed against each other, with the production of electric sparks. These either set fire to the gas issuing from one of the gas-bags or exploded the mixture of air and gas contained in the space between the gas-bags and outer covering of the airship. Perhaps it was on account of this accident that gold-beaters’ skin has sometimes been used for the gas containers of the Zeppelin airships.
4. Difficulties in the way of applying the propulsive screws in the most effective position.
Most airships are exceedingly defective in this respect, the screws being applied to the propulsion of the car and not to the whole system. The result is that the cumbersome gas-bag lags behind. Certainly, one of the best points in a Zeppelin was the attachment of the screws to the airship framework above the cars, thus securing more advantageous position. This, however, only amounted to something like half measures. In the case of the ill-fated airship “La Paix,” the Brazilian aëronaut Severo undoubtedly aimed at the ideal, though the experiment cost him his life. He devised the ingenious system of combining balloon and car in one symmetrical melon-shaped body, through the centre of which passed longitudinally the shaft which revolved the propelling screws at either end. The screws were therefore in the position in which to propel the whole system and not the car only. This, however, necessitated the introduction of a very small space between the car and balloon proper. By reason of this very small space the presence of the petroleum motor in the car could not fail to be dangerous, and was the cause of the fiery end of Severo’s balloon and the death of the inventor and engineer. On the morning of the ill-fated May 11th, 1902, Severo and Sachet ascended in “La Paix.” A few moments after the ascent the balloon exploded, in the words of an eye-witness, like a crash of thunder, and the occupants were precipitated to the ground.
In spite of the engineering advantages of Severo’s system no one has dared to revive the plan.
It has, however, been pointed out by the writer—and the suggestion elicited the keen interest of the late Professor Langley—that if electricity could be used as the motive power in an airship the Severo system could be reasonably revived. Then the electric motors could be inside the gas-bag. There, electric sparks and electric heating could do no harm. For it is only the borderland that is the place of danger, where there are oxygen atoms to combine with the hydrogen atoms. In the case of a balloon filled with gas it is surprising to what short distance the danger zone extends. In the case of the writer’s electric signalling balloons, on one occasion the ladder framework which supported the incandescent lamps was being hauled up into the balloon. Through some fault in the connections there was sparking at the framework just as it had passed over the dangerous borderland. The sparks went on with safety. An inch or two lower and there would have been an explosion!
But on account of the weight of the battery the practical application of electricity for propelling navigable balloons seems to be as far off as it was in the days of “La France,” and in airships we have to continue placing the screws in the wrong place.
5. Difficulties of making gas envelope gas-proof.
The absence of the knowledge how to obtain a really gas-proof envelope is, no doubt, one of the greatest difficulties of airship construction. As has already been pointed out, the gas-holding quality of gold-beaters’ skin is remarkable. Its cost, however, is fairly prohibitive in the case of large airships. A material which is a combination of india-rubber and cotton surfaces is now generally used for large airships, but this has undoubted disadvantages. India-rubber is a substance which time, low temperature, and certain climatic conditions deteriorate. All those who have worked with india-rubber experimental ballon-sondes (sounding balloons) can testify to its perishing qualities. Very much can be accomplished with a brand-new airship. Turned out of a factory it will retain its gas-holding qualities for a short time excellently. The lapse of time reveals deterioration and leakiness.
Considering the extreme importance of a varnish that will retain pure hydrogen for a reasonable time, it is a matter of surprise that chemists should have almost entirely neglected its production. Mr. W. F. Reid alone of British chemists seems to have given any serious thought to the question. In a paper which Mr. Reid read before the Aëronautical Society of Great Britain, he made some exceedingly important suggestions in the way of obtaining balloon and airship varnishes. In case this little volume should fall into the hands of any chemists who may like to devote their powers of original research to the production of one missing link in airship construction, the following quotation from Mr. Reid’s remarks are appended below.
Varnishes may be divided into two classes—those in which the film solidifies or “dries” by absorption of oxygen from the air, and those in which the varnish “sets” by the evaporation of a volatile solvent in which the solid ingredients have been dissolved. To the first class belong the drying oils, chiefly linseed oil, for, although there are a number of “drying” oils, but two or three of them are used commercially in the manufacture of varnishes. When exposed to the air, especially in warm weather, linseed oil absorbs oxygen and forms an elastic translucent mass termed by Mulder “linoxyn.” This linoxyn has completely lost its oily nature, does not soil the fingers, and is, next to india-rubber, one of the most elastic substances known. It possesses but little tensile strength, however, and can be crumbled between the fingers. It forms the basis of all linseed oil paint films, and is largely used in the manufacture of linoleum. Linoxyn, however, is not, as Mulder supposed, the final product of the oxidation of linseed oil. When exposed to the air it is still further oxidised, and then forms a sticky, viscid mass, of the consistency of treacle and of an acid reaction. This latter property is of importance because it is due to it that fabrics impregnated with linseed oil so soon become rotten. In order to hasten the oxidation of linseed oil it is usually heated with a small quantity of a lead or manganese compound, and is then ready for use. No method of preparation can prevent the super-oxidation of linseed oil, but experience has indicated two ways of diminishing the evil effects so far as paints and varnishes are concerned. The first is to mix the oil with substances of a basic character or with which the acid product of oxidation can combine. In the case of paints, white lead or zinc oxide are chiefly used for this purpose. The other method consists in mixing with the oil a gum resin which renders the film harder and prevents liquefaction. Such a mixture of linseed oil and Kauri gum forms an elastic, tough mass, which is much more durable than the linoxyn alone, and also possesses greater tensile strength. During oxidation the linseed oil absorbs about 12 per cent. of its weight of oxygen, and when the area exposed is very large in proportion to the weight of the oil the temperature may rise until the mass catches fire. At a high temperature the super-oxidation of the oil takes place more rapidly than in the winter, and I have seen fabrics that had only been impregnated with an oil varnish for a month cemented together in one sticky mass, and, of course, completely ruined. When the linseed oil is thickened by the addition of a gum resin, it is too thick for direct application, and is thinned down with a solvent, usually turpentine or a mixture of this with light petroleum. Many resins and gum resins are used in the manufacture of varnishes in conjunction with linseed oil, but none of them can deprive the oil of the defect referred to, and if used in too large a proportion they become too brittle for balloon purposes. Both scientific investigation and practical experience show that any varnish containing linseed oil must be looked upon with suspicion by the aëronaut, in spite of the glowing testimonials some manufacturers are always ready to give their own goods.
When we consider those varnishes which are solutions and which do not depend upon oxidation for their drying properties we enter upon a very wide field.
Practically any substance that is soluble in a neutral solvent and leaves an impermeable film on drying is included in this class. One of the simplest examples is gelatine in its various forms, with water as a solvent. Until recently glue or gelatine would have been useless for our purpose on account of its ready solubility in water, but now that we are able to render it insoluble by means of chromic acid or formaldehyde it comes within the limits of practical applicability. A fabric may be rendered almost impermeable to gas when coated on the inside with insoluble gelatine, and on the outside with a waterproof varnish. Animal membranes are far less permeable to gases than fabrics coated with varnishes of the usual kinds. A balloon of gold-beaters’ skin, if carefully constructed, will retain hydrogen gas for a long time, and if treated with gelatine that is afterwards rendered insoluble it becomes practically impermeable. Fabrics treated with linseed oil varnish, on the other hand, allow gas to pass with comparative ease. This is not a question of porosity or “pinholes,” as is sometimes imagined, but a property inherent to the material. Hydrogen or coal gas is absorbed on the one side of the film and given off on the other in the same way as carbonic oxide will pass through cast iron. An inert gas, such as nitrogen, does not appear to diffuse in this way, even when there is a considerable difference in pressure between the two sides of the film. Such a varnished fabric transmits hydrogen readily, but retains nitrogen, and is perfectly watertight. In filling up the interstices of a fabric composed of cellulose the most obvious substance to use would be cellulose itself, but until recently solutions of this kind were difficult to obtain. Toy balloons have long been made of collodion, and are fairly satisfactory, but a cotton fabric impregnated with pure collodion becomes hard and even brittle. Celluloid solution, which is collodion with camphor and a small quantity of castor oil, is more flexible, but, probably on account of the camphor, is more permeable to hydrogen than collodion. A variety of collodion known as flexile collodion is a solution of collodion cotton with a slight addition of castor oil, and is much to be preferred to any of the preceding forms. In using it great care must be taken to exclude moisture, as the presence of this renders the film opaque, in which case it is always more or less porous. A substance allied to collodion is velvril material, composed of collodion cotton and nitrated castor oil. It is tough and flexible, even in thick films, and gives a good coating to paper or cotton fabric. Unless very carefully prepared, however, acid products may be generated from the decomposition of the nitro-compounds present, in which case the strength of the fabric would suffer. Another form of cellulose in solution is viscous, which forms a good coating when applied in a very thin layer, but makes the fabric harsh and brittle if used in excess. The solutions of this substance do not keep well and are liable to spontaneous decomposition.
The difference in flexibility between thin and thick films of the same materials is very considerable.
Given an elastic, supple cement, such as is afforded by concentrated solutions of some of the above-mentioned substances, it is quite possible to cement a tough, close-grained paper to a cotton fabric of open mesh, and the compound material thus produced is much more easily rendered impermeable than the fine cotton fabric now used. An extremely tough paper made from silk, a recent invention of T. Oishi, a Japanese manufacturer, would be specially useful for such a purpose....
It will be noticed that the texture is very compact and free from pores, as might, indeed, be expected on account of the fineness of the silk fibres of which it is composed. It must not be forgotten that cotton fibres are tubes, and gas may pass through them even when they are embedded in an impermeable film. Silk fibres, on the other hand, are solid, as well as stronger than cotton.
Another way in which a tough, flexible cement may be utilised is to cement a metal foil to a textile fabric. Aluminium foil, for instance, cemented to cotton by means of flexile collodion, gives a completely impermeable fabric of much greater suppleness than the sheet aluminium hitherto used for balloons.
Fine aluminium flakes dusted upon the freshly varnished surface adds greatly to the impermeability of the fabric, and the same may be said of coarsely powdered mica.
It may be noted in this connection that an impermeable varnish does not only apply to balloon and airship construction, but will also have its use for impregnating the planes of the heavier-than-air machines.
6. Great cost of airships.
The cost of airships compared with that of aëroplanes certainly favours the extended use of the latter in war. It is easy to spend £50,000 on a very large airship. Supposing the cost of an aëroplane seating two persons is £1,000, it is a question from an economic point of view whether the possession of fifty aëroplanes is not far better military value for the money expended on the solitary airship. But in the case of the latter it is not only initial expense that has to be considered, but cost of housing, maintenance, and hydrogen gas. These items are very considerable. The upkeep of one large airship very much exceeds that incurred with fifty aëroplanes.
7. The great amount of personnel needed for the manipulation of large airships.
It is no exaggeration to say that the ground manipulation of large airships necessitates the attendance of quite an army. In the case of a Zeppelin the exigencies of wind may call for the assistance of 300 trained sappers on landing. This is the reason why it is so advisable to have the resting-places of large airships on water. In the case of rigid airships a slight bump on the earth may do considerable damage. Colonel Moedebeck has laid especial stress on the advisability of water landing.
In practice it is never possible, even by working the motor against the wind, to avoid a certain amount of bumping, since the aërostatical equilibrium is not easily judged and allowed for, especially in strong winds. On this account the safer water landing is always preferable.
An airship can be anchored more easily with the point against the wind on water. It is quite impossible to anchor on land when assistance is not forthcoming to hold down the airship. On water, also, the airship will give a little to side winds and to alterations in the direction of the wind, without overturning. On land this danger is not excluded, even with rigid airships. Of course, a watertight and seaworthy car is a necessary condition for landing on water.
The landing requires great attention, and rapid, decisive handling and management on the part of the aëronaut.
In the opinion of the same expert airship travelling on a large scale would not be possible without the publication of special charts, which would furnish information concerning natural airship harbours, and their relation to various winds, and also of the various airship sheds which may be erected. He states it would be highly dangerous to undertake airship voyages without the existence of suitable stations against storms, and where gas supplies, driving material, and ballast could be renewed.
8. Great liability of being destroyed by aëroplanes in war.
This is no doubt one of the greatest dangers the airship has to face in war. The aëroplane is the airship’s deadliest enemy. So terrible to the airship is this hornet of the air that the former has no chance of making an attack. It must ever remain on the defensive. The speed and quickly rising power of modern aëroplanes settles this question. When the aëroplane is advancing the airship cannot escape. Nor can it now any longer rise to safe altitude, for the nimbler heavier-than-air machines can easily outdo it.
The only salvation of the attacked airship is its mitrailleuse gun fixed on the platform at its topmost part, but the chance of hitting the swiftly advancing aëroplane is fairly remote.
There are more ways than one in which the fatal attack of aëroplane v. airship can be made. The airman can, indeed, ram the gas-bags by hurling himself and machine against it. Then destruction would be swift and sure, with the probable loss of the airman’s own life. Better tactics would be to fly above, and drop suitable weapons on the fragile gas-bag; a few sharp and jagged stones would probably suffice. Sharp darts of steel would be all-effective. So easy, indeed, would it be for one aëroplane skilfully handled to end the existence of the largest airship that one cannot refrain from asking the question whether on this account alone it can survive as the instrument of war?
9. Insufficient power of quickly rising.
This is a point which wants the attention of the aëronautical engineer. The old-fashioned spherical balloons were made to rise and fall by the alternate sacrifice of gas and ballast. Thus the very life-blood of the balloon became quickly exhausted. It was obvious that when airships supplanted balloons the former must be supplied with a less exhausting process of vertical movement.
As has already been mentioned, when treating of the Zeppelin airship, for the purpose of rising horizontal planes are now fitted to airships. Some engineers have thought these should be supplemented by a mechanical device, so that the speed of rising might be augmented. The late Baron de Bradsky provided his airship with a horizontal screw placed beneath the car. But one horizontal screw beneath an airship tends to twist it round—to convert it into an aërial top. To avoid this effect it would be necessary to have two horizontal screws rotating in opposite directions. This precaution was absent in de Bradsky’s construction, and it kept on twisting round, with the disastrous effect that the steel wires which held the car to the balloon snapped, with tragic results. But the idea of the horizontal screw is worth reviving. It has been a cherished plan of M. Julliot to include the principle in his designs, but on account of extra weight he has, I believe, hitherto not tried the interesting experiment.
The colour of most of the airships is a disadvantage, though this is a matter so easy of alteration that it has not been included in the list of disadvantages.
In military airships, and, it may be added, aëroplanes also, the colour should be a neutral tint that is as invisible as possible against the sky. Most of the airships have been made a glaring yellow, so that the india-rubber in the envelopes may be better preserved from the action of light. This protection may have to be sacrificed to the overpowering advantages of invisibility in the case of naval and military airships.