Translated by LIEUTENANT G. M. BAUM, U. S. Navy
(Marine Rundschau, October, 1913.)
The general usefulness of airships has been demonstrated. The deplorable loss of the L 1 brings up discussion as to their utility for the Navy.
Naval airships should serve principally for scouting.
1EDITOR'S NOTE.—The United States Navy has done nothing so far in regard to dirigibles other than to convene a special board on aeronautics in December, 1913. This board recommended in their report that dirigibles be purchased at an early date. (See page 261, PROCEEDINGS, Whole Number 149, Jan.-Feb., 1914.)
This article (translation), was written by a naval constructor of the German navy, who was actively associated with the development of naval dirigibles; he lost his life in the L 2 disaster. As the Germans lead the world in dirigibles, it would seem well to seriously consider what Naval Constructor Pietzker has to say about them.
The Germans have met with accident after accident in their attempts to develop the dirigible into a practical and reliable vehicle of the air. The many Zeppelin dirigibles have all been wrecked and the naval Zeppelins have met a like fate; the L 1 foundered in a gale near Helgoland late last summer (see illustration of L 1 in frontispiece); the L 2 caught on fire and blew up during her acceptance trials on October 7, 1913. (See page 265, PROCEEDINGS, Whole Number 149, Jan.-Feb., 1914)
They are especially qualified for scouting at sea, inasmuch as, contrary to the conditions over the land, there is no possibility of hiding at sea, and the deceptions that trees, villages and the like can offer, are excluded. A slight elevation over the surface of the sea decidedly increases the range of vision. The visible horizon radius is:
50 m 23 km.
100 " 33 "
200 " 47 "
500 " 74 "
1000 " 104 "
Naturally, this total field of vision will not always be available. Foggy or hazy weather reduces it. But on clear days, this vast field comes fully into view. What this means in a military sense need not be inquired into further. It is clear that such scouting may be of great importance in operations, since it can not be easily interfered with by the enemy.
In order to be able to perform this service, the airships must possess the technical ability to undertake over-sea trips under war conditions with sufficient safety in such a number of days that their employment will not be the exception but the general rule, even though it is not to be expected and not to be presumed that the duty assigned can be accomplished under all circumstances. The radius of action shows the principal difference as compared to the employment over land, because at sea the fighting forces move much more quickly and any intermediate landing of the airship is out of the question.
We had best analyze their ability to perform these services from two points of view. First, are airships in general "capable," under normal conditions, and does the status of aerial navigation permit in other respects? and second, do not the special dangers and sources of accidents represent too great a reduction of their ordinary usefulness under normal conditions?
- ABILITY UNDER NORMAL CONDITIONS.
Extended over-sea trips under war conditions require in addition to general utility for aerial navigation:
1. The practicability of carrying on board a sufficient quantity of fuel for long trips, and of ballast necessary for altitude flights; sufficient certainty of the operation of the engines; a speed which enables them to make headway even in a strong wind and even when flying against the wind; equipment with searchlights, machine guns for repelling aeroplanes, and a radio set, which is indispensable for sending out military information and receiving weather reports and adds to safety; finally, the practicability of carrying a double crew (2 watches) so that on long trips the crew may have a relief, as well as sufficient comfort so that the watch off duty can recuperate.
2. Points not directly connected with the ship:
(a) Perfecting of the hangars so that the running out from the interior is possible in all winds in which the airship can travel.
(b) A meteorological service which will guarantee that a trip is started only in such weather that the return can be assured and that too strong contrary winds will not be encountered.
(c) The possibility of sufficient orientation.
The extent to which these points are already realized is pointed out briefly in the following:
I. The Development of the Ship.—It is evident in advance that the increase of the requirements as compared to ordinary air travel, must lead to large displacement, that is, large gas space, of airships. The weights of building material of the hull which are useless in other respects but can not be dispensed with, and of the machinery necessary to attain a specified speed, do not increase in the same ratio as the displacement but in a lesser ratio, so that with an increase of displacement, there is unused lifting power available for increasing the efficiency. The performances expected require so much weight and displacement that the adoption of non-rigid airships for the navy is out of the question for the time being, since in them the large displacements are not yet practicable, and they are still confronted with great difficulties. A principal advantage of the non-rigid airship almost entirely disappears also with increase of size, namely, the lightness of the hull for the displacement, and its other advantage, easy demountability and transportability by railway, is no attraction for the navy. Therefore, only rigid airships enter into the question.
In general, the basic principles for the carrying power of airships are little known and are therefore briefly set forth in the following:
Inasmuch as the airship floats and must therefore first rise in the air on departure, the lifting power of its gas volume must be a little greater than its weight. The weight is made up of dead weight (weight of building material and permanently installed or necessary equipments, for example, radio, searchlight, anchor gear), and loose cargo, ordinarily called the useful load (crew, supplies; fuel, ballast). The total of this variable, useful load, which can be carried at one time depends upon the amount of the lifting power on departure, which varies and may be of very different amounts. The lifting power is equal to the weight of the displaced air, less the weight of the gas contained in the cells. The weight of the air is directly proportional to the barometric pressure, inversely proportional to the absolute temperature, varying slightly according to the moisture of the air. The weight of the gas (hydrogen) depends practically on its grade of purity or its mixture with air, which is essentially a question of the practical handling and the quality of the fabric of the cells. How great the difference may be under conditions that are likely to occur in practice, is shown by the following comparison:
I cubic meter of gas space represents a lifting power:
of 1.22 kg. at 770 mm. barometric pressure, —10°C.,
specific weight of the gas 0.1, and
of 1.01. kg. at 740 mm. barometric pressure, 20°C.,
specific weight of the gas 0.14.
What the airship can carry on departure depends on these figures. During the flight, a great change in the conditions takes place. The airship travels, not like a submarine with practically uniform propulsion, but the weight and the lifting power change so that the changing conditions of the trip may be entirely different. The weight diminishes through the consumption of fuel and increases in conditions of rain or snow, whose added weight may reach 1000 to 1500 kg. The lifting power of the gas decreases through the diffusion of the gas through the cell walls which, however, does not amount to a great deal practically, and secondly, through the blowing off of the cells through the valves. The capacity of the cells is fixed and can not increase through elastic expansion of the cells because these are already small and are, above all, limited by the framework against which they press. When the gas expands, some of it escapes through the valve and is lost. When the cause of the expansion ceases later and the gas again contracts, the cells are left only partly filled. The unfilled portion, in the meantime, loses in weight of gas and lifting power. Making up for the escaped gas by carrying along reserve hydrogen, etc. is out of the question in practice, since the weights necessary for this are greater than the advantages to be obtained. The causes for the expansion of the gas are:
(a) Ascending to greater altitudes. An ascension of about 80 to 100 meters represents a reduction of the outside pressure of one per cent, resulting in an expansion of one per cent of the gas, The value to be inserted for the calculation of the loss of gas, will be strongly influenced by the mixture of the gas with air. The airship does not regain any of its lost lifting force by reducing its altitude. It comes into heavier and more buoyant air strata causing the gas to contract and occupy a reduced volume; the product of pressure and volume remains constant.
(b) Increase of the gas temperature. One degree represents an expansion of about 0.36 per cent. The rise in temperature depends first on the different heat strata of the air in which very unusual reverses of the ordinary strata are possible, and whose influence can not be followed here in detail, since it depends on the rapidity with which the gas temperature follows the air temperature. It thus makes a whole series of combinations possible.
Secondly and principally, it depends on the rise in temperature of the gas due to the influence of the sun's rays (up to about 15°C.) as compared to the surrounding air. The loss in buoyancy becomes effective as soon as the gas again cools off, after the sun's rays have been cut off (clouds, night). In unusual circumstances, when the cells are not fully inflated beforehand and the expansion due to the temperature does not necessitate blowing off the gas cells, the heating by the sun's rays can cause a temporary increase above the lifting power at an intermediate stage of the flight (however, not above the conditions at the beginning of the flight). In general, however, it is only the cause of the loss of gas, as in rising in altitudes, that is brought out.
Should this loss (near the end of a long trip) balance the lightening of the load due to the consumption of fuel, then it has no effect. If this is not the case, however, as at the beginning of a trip, then the balance must be established dynamically by the use of the altitude rudders. This dynamical operation is considerable in still air conditions; for an airship like the L 1 it can amount to from 1500 to 2000 kg.; it will be substantially reduced when the squally air structure causes differences of velocities of separate air strata and suddenly produces loss of its supporting quality (the famous air pockets of the aviator) or when the ship answers the rudder badly. If the dynamic power is no longer sufficient, or if a landing must be made (at the stopping of the engines the dynamic power naturally ceases) then sufficient ballast must be thrown overboard. Herein lies the principal function of the ballast. Aside from this it temporarily aids in steering when making a landing. Only water can be considered as such steering ballast; the fuel must be drawn on to furnish the large amounts of ballast to be given off in high altitude flights, since the great weights necessary can be obtained in this way only; later on in the discussion of the vertical currents, this will be gone into more fully.
How different the conditions may be, is shown by the two following examples of a flight of the L 1 in mean conditions (height of barometer 760 mm., temperature 10° C., specific weight of the gas 0.125). At the beginning of the flight (after 3 hours) the ship was compelled to at once seek an altitude of 1000 meters on account of a simultaneous higher gas temperature of 12°C.; on the second occasion, not until toward the end of the flight (after 30 hours) without a simultaneous increase of gas temperature. In the first case, the loss in lifting power is 3600 kg. against which the weight reduction in fuel consumption is only 360 kg.; thus there arises a total loss of about 3200 kg. in lifting power. In the second case, the loss in lifting power is 2500 kg., and corresponding to this we have a lightening of 3500 kg. in fuel consumed. Thus there remains a lifting force of 1000 kg.
From these examples it is seen that the seeking of greater altitudes is undoubtedly practicable under certain circumstances; however, such altitudes require too much of the weights provided as ballast, and are therefore not compatible with the amount of fuel that must be carried for long flights. Therefore, it must be remembered that the altitude flight, the travelling at a so called battle-altitude, is not nearly so necessary for the navy as it is for the army (which, moreover, needs only shorter flights). In army operations, the enemy may be hidden in any forest, or in any village, firing upon the airship, and in clear weather conditions the ship must be able to avoid these dangers by flying at high altitudes. At sea, except in case of a fog, the airship will sight the enemy in sufficient time to be able to make use of its speed when rising to a higher altitude is too difficult.
The two following examples will show how the carrying capacity of the L 1 was apportioned under different conditions in accordance with these foregoing principles, if we disregard the possibility of making an altitude flight at any moment:
Under very favorable Under very unfavorable
Barometer, 770 mm.; Barometer, 740 mm.;
temperature, —10° C; temperature, 20° C;
specific weight of gas, specific weight of gas,
Carrying capacity 27,518 kg. 23,635 kg.
Ship with permanent
installations 116,372 kg.
power plant 314 kg.
Radio outfit 210 kg.
Equipment 960 kg.
Crew of 17 men with
effects and provisions 1,870 kg.
12 ballast bags, about
80 kg. each 960 kg. 20,686 kg. 20,686 kg.
Therefore there remains for
fuel and oil 6,832 kg. 1,949 kg.
Supply of fuel lasts in hours,
using one motor 171 kg. 49 kg.
Supply of fuel lasts, using two
motors, in hours of flight 85½ kg. 24½ kg.
The foregoing shows clearly that the carrying capacity of the L 1 was capable of the prescribed requirements of lifting power, and that the airship was qualified for military work. At the beginning of the development, the navy was already satisfied that only airships of large displacement could be considered for employment over the sea, and had therefore ordered the construction of the L 1 to be decidedly larger than any previous airships, in fact, as large as it was then possible to build at the construction plants. Thus the L 1 had a capacity of 22,500 cubic meters as compared to 19,000 of the ships of that time. It was believed that the construction plants could not exceed this. In later ships it was found possible to exceed this; the L 2 has a capacity of 27,000 cubic meters and the following ones will be still larger. However, the progress must be gradual and not in too large single steps, since two considerations are opposed to too large single advances. First, the security of the framework can not be obtained entirely by calculation, but in addition to the calculation, trials and careful subsequent building on the basis of tested construction are involved, since on account of the lightness, it is necessary to use much broader breaking limits than is otherwise customary. One obtains an idea of what has been accomplished in progressive construction when one recalls that such an airship has a capacity equal in size to that of an average warship and a weight of material equal to that of one of the larger ship's boats. Secondly, the increase in the size of airships, with which arises increased difficulty of handling them on land upon landing and departure, forms an important point because in these maneuvers, the airships are tended principally by hand; the extent of our progress in this must likewise be developed step by step.
After this principal subject of carrying capacity, come the remaining requirements previously stated.
The reliability of the motors has been perfected lately by the Maybach Motor Factory at Friederichshafen and brought to a high grade of efficiency. The motors are given a shop test of 6 hours uninterrupted operation. However, uninterrupted operation of the motors lasting 12 and more hours has actually taken place in airships without any mishap. That entirely suffices since, as a rule, a motor is occasionally disconnected and looked after.
The greatest speed of the L 1 was about 20 meters per second. In the practical endurance run, when it was reduced by the shifting of the rudders and the canting of the ship, it was somewhat smaller. Practical experience shows that in most days of the year, this speed suffices, so that a fixed destination can be reached with certainty. These practical experiences of airship flight thus give a clearer idea than various tables of prevailing winds and the like, since these do not ordinarily give the individual strength of the currents and, generally, no clear idea of conditions favorable for airship flight is given. The L 2 has a still greater speed, about 22 meters per second, and further increases will follow.
The equipment with searchlights and machine guns presents no difficulties. It can be so arranged that the open flame occurs in places where none of the gas can enter. This is a little more difficult with the radio outfit, but even so, the problem is practically solved. The operation of the power plant is possible without endangering the ship; only a few minor questions remain to be cleared up in this. The radio is furthermore one of the essential parts of the ship, partly because of its employment in scouting and partly because it is needed to receive orders and weather notices.
2. Status of Air Travel Aside from the Ship Itself.
(a) The question of sheds. Until recently, this was the principal cause for the restriction of the employment. The rigid sheds used almost exclusively up to the present time, allow the running in and out of the airship only in winds parallel to the shed, while cross winds of only moderate strength make it impossible. Various means have been tried to remove this evil; broadening the sheds, using cars to which the airship was secured and run out; but all such arrangements signify only a limited increase of safety, especially since the cars for which the laymen promise much and for whose mechanical structure many proposals have been submitted presupposing a complete utilization of the strength of the airship's framework which will not be attained for some time to come. Aside from this, even under favorable conditions with the wind blowing parallel to the length of the shed, it is still possible that the airship may be in the shed with stern to the wind, and must be run out thus. In this case, it must be turned around in order to take its departure, because it can take its departure with the wind only under great difficulties. The turning around thus necessitated is a dangerous operation even in a moderate breeze.
Lately, and in fact through the lead taken by the Admiralty, this difficulty has been almost entirely removed. In the navy installations, and in addition, in all the future installations, revolving sheds, which can at all times be turned into the wind and out of which the airship can be run safely in any wind in which it can possibly fly, will be provided. The question whether it is really more advantageous to build single revolving sheds, double revolving sheds, or a combination of a shed with fixed foundation as a storehouse with a revolving shed to be used as a slip, is still to be cleared up.
(b) The weather forecast. A whole series of organizations which will furnish the weather forecast for the practical airship service with increasing frequency has been provided and still more will be provided. Even the science itself is becoming more and more developed through aeronautics, which has decidedly broadened our knowledge and continues to do so daily. On the other hand, one must not rate the weather forecasts based on weather reports from all stations too highly in their significance in war. For this purpose there must be, in addition, as complete a development of pilots in judging weather conditions as possible, conditions which they themselves can observe; in this field also, constant progress is noted.
(c) Orientation. Orientation over the open sea with unknown wind conditions which make dead reckoning impossible, still presents difficulties in foggy weather and on days when two simultaneous astronomical observations are impossible. This condition, however, is not an absolute hindrance, since in most cases an approximate plotting of the general bearings is possible and there are already encouraging attempts under way to make orientation possible at all times.
The general outlook of the normal conditions shows that the present day airships are in a position to commence fulfilling the prescribed requirements, and that they are therefore practicable for the navy. It is conceded that they are very dependent upon the weather and that they cannot always accomplish flights at high altitudes; the scouting service performed by them will be looked upon as supplementary to the ordinary means of scouting, and to dispense with any of these other means of scouting in any way would signify a misunderstanding of the actual conditions; but the value that the airship can have under certain circumstances is so large, it can have such decisive significance, that its employment is required in spite of this. The further development must, above all, lead to larger displacements gradually, since it must include the radio station, the meteorological improvements and the orientation. Development is in full progress in all these lines.
- THE SPECIAL DANGERS OF AIR FLIGHT
The normal ability of airships will be strongly influenced by the special dangers. For this reason, a special examination must be made to determine if they influence the conclusions drawn in II.
1. Storms.—As long as we consider the word "Storms" to signify strong winds in a horizontal plane, as is generally accepted, they represent a small danger. Such weather conditions can generally be recognized in advance with certainty as shown by the long past record of the storm signal-service on the coast. Should the airship, in spite of this information, get into a wind to which it is not equal, then under certain circumstances its non-arrival in port may result. In this respect conditions are becoming continually better due to the increasing speed of the airships and the increasing number of sheds.
2. Vertical Currents.—The vertical movement of the air such as came up strongest and most fatally in the storm-like, passing squalls which brought about the destruction of the L 1 is actually one of the greatest dangers of aeronautics. In extreme cases, as for example in the case of the L 1, it can render the steering gear useless, and it represents a great danger under all circumstances inasmuch as it can blow the airship to such an undesired altitude as to cause great loss of gas. I have stated previously that the flight at high altitudes is one of the chief difficulties and can not always be accomplished. Therefore it is clear that an undesired altitude under unfavorable circumstances may have serious consequences. The chief remedy is naturally the carrying of as large a weight of ballast as possible in such a form that it is possible to release it. Solid weights for this purpose enter the question only slightly as compared to the large amount of fuel. It has been tried—and it will be still further developed—to make it possible to throw overboard the greater part of the fuel supply, that is, the explosive fuel, in addition to a certain amount of water ballast which must remain on hand to assist in steering when landing. This has been done by the L 1 and has been carried out to a further extent by the L 2. Naturally, one cannot carry this too far, otherwise the operation of the main engines is impaired or made practically impossible; a certain amount of reserve fuel must remain on board to develop the doubly valuable dynamical power in case of a squall and to resume the flight after it is over. The aim of the development which, as already stated, can take place only gradually, must be to construct the airship with sufficient ballast from the very beginning so that it will have sufficient excess fluid weight on hand for even the most difficult weather conditions. For the time being we must still make it a rule to avoid the possibility of encountering heavy squalls. If the squall cannot be avoided, it is necessary to ride it out at as low an altitude as possible where the air currents are horizontal and the danger of being blown to a higher altitude is reduced. Above all, however, the increase of the displacement presents the best solution because, even though it carries with it a greater loss of gas at high altitudes, the increase of carrying capacity is still greater, and with its increase, the efficiency of the machinery and dynamic power also grows, enabling the airship to prevent descending.
3. Thunder Storms.—A large part of the dangers from thunder-storms is covered by the previously discussed dangers due to vertical squalls inasmuch as thunder storms are full of such squalls and the squalls nearly always have a stormy character. The danger from lightning has not yet been cleared up. As we proceed, this seems smaller than was at first believed. Probably the framework of the airship furnishes an excellent conducting medium for this electrical phenomenon. The charging of the airship itself is made difficult by the exhaust gas and by the propeller tips, which are very good discharging mediums.
4. Fire and Ignition Danger.—Unfortunately, hydrogen is the only gas that up to the present time is adaptable. It is very inflammable as soon as its presence in air exceeds 9.45 per cent. Thus an explosion follows ignition up to a percentage of 66.4 (oxyhydrogen). For this reason all unscreened fire must be excluded from all parts of the ship where hydrogen may be present. This question is solved for the installation of motors, machine guns, searchlight, and radio. There still remains a certain danger in accidents to the ship in which sparks can be caused by breaking metal, and perhaps in still another way, for example, when there is a great deal of friction of balloon fabric. This matter still requires a final clearing up, in which the navy is zealously cooperating. Most probably, these conditions can not come up while the airship is in the air, but only when it is on the ground, in which case the scope of the danger is decidedly reduced.
5. The Frailness of the Framework.—It has been previously stated that the ordinary requirements for strength could not be made of the framework of an airship. This becomes uncomfortably apparent in maneuvering the ship by hand on the ground, and in addition, when lying at anchor in a storm. In the meantime the resistance continues to increase with the increase of the displacement, for which more weight for solid fittings and the principal parts coming under observation is continually required.
6. Special Conditions in Over-Sea Flying.—Contrary to the flying over land, for which the conditions can now be generally considered as cleared up, the flying over-sea has been tested very little up to the present, and it is not as yet possible to make any definite statements regarding it. On the one hand, the uniformity of the lower stratum is, as a rule, constant and thus provides good wind conditions from the aeronautic point of view; on the other hand, however, it is certain that the long flights necessary, and the impossibility of an intermediate landing, both lead constructively to large displacement, as previously stated; they will also present new problems in the science of airship flight. Presumably the time will soon come when the experiences derived through over-sea flights will influence construction favorably for them; to this end, as much flying over-sea as possible must be done in a gradual progression of performances.
Finally, as the result of the analysis of these particular sources of danger, it can be said that even they do not suffice to upset or in any way change the opinion which would be formed of the ability of the airship under normal conditions. It has been more clearly brought out that the efficiency of the airships is very dependent on the weather and that conditions may arise in which the airship is not equal to the weather. However, as we failed to come to the conclusion to dispense with naval warfare in the days of the sailing ship, when even frigates were not equal to all storms, so little may we say that the existence of certain dangers justifies dispensing with the development of a weapon which is progressing to full efficiency and can then render great service. The points which come under observation for a methodical development, and the approximate lines which this must follow, are discussed separately in the foregoing. Intensive work is being done on all these points, and there can be no doubt that the safety will be decidedly increased in the extremely rapid technical development to which we are nowadays accustomed in all services.