Part 3

*Location of Propeller.* The final factor of importance in the design of the successful dirigible is the proper location of the propulsive effort with relation to the balloon. Theoretically, this should be applied to the axis of the balloon itself, as the latter represents the greater part of the resistance offered to the air. At least one attempt to carry this out in practice resulted disastrously, that of the Brazilian airship Pax, while the form adopted by Rose, in which the propeller was placed between the twin balloons in a plane parallel with their horizontal axes, was not a success. In theory, the balloon offers such a substantial percentage of the total resistance to the air that the area of the car and the rigging were originally considered practically negligible by comparison. Actually, however, this is not the case. Calculation shows that in the case of any of the typical French airships mentioned, the sum of the surface of the suspending rigging alone is easily the equivalent of 2 square meters, or about 21 square feet, without taking into consideration the numerous knots, splices, pulleys, and ropes employed in the working of the vessels, air tubes communicating with the air balloonets, and the like. Add to this equivalent area that of the passengers, the air pump, other transverse members and exposed surfaces, and the total will be found equivalent to a quarter or even a third of the transverse section of the balloon itself.

To insure the permanently horizontal position of the ship under the combined action of the motor and the air resistance, a position of the propeller at a point about one-third of the diameter of the balloon below its horizontal axis will be necessary. Without employing a rigid frame like that of the Zeppelin and the Pax, however, such a location of the shaft is a difficult matter for constructional reasons. Consequently, it has become customary to apply the driving effort to the car itself, as no other solution of the problem is apparent. This accounts for the tendency common in the dirigible to "float high forward," and this tilting becomes more pronounced in proportion to the distance the car is hung beneath the balloon. The term "deviation" is employed to describe this tilting effect produced by the action of the propeller. Conflicting requirements are met with in attempting to reduce this by bringing the car closer to the balloon as this approximation is limited by the danger of operating the gasoline motor too close to the huge volume of inflammable gas. The importance of this factor may be appreciated from the fact that if the car were placed too far from the balloon, the propulsive effect would tend to hold the latter at an angle without advancing much, owing to the vastly increased air resistance of the much larger surface thus presented.

*Relations of Speed and Radius of Travel.* The various factors influencing the speed of a dirigible have already been referred to, but it will be apparent that the radius of action is of equally great importance. It is likewise something that has a very direct bearing upon the speed and, in consequence, upon the design as a whole. It will be apparent that to be of any great value for military or other purposes, the dirigible must possess not only sufficient speed to enable it to travel to any point of the compass under ordinarily prevailing conditions of wind and weather but also to enable it to remain in the air for some time and cover considerable distance under its own power.

_Total Weight per Horsepower Hour._ As is the case in almost every point in the design of the dirigible, conflicting conditions must be reconciled in order to provide it with a power plant affording sufficient speed with ample radius of action. It has already been pointed out that power requirements increase as the _cube of the speed_, making a tremendous addition necessary to the amount of power to obtain a disproportionately small increase in velocity. In this connection there is a phase of the motor question that has not received the attention it merits up to the present time. The struggle to reduce weight to the attainable minimum has made weight per horsepower apparently the paramount consideration—a factor to which other things could be sacrificed. And this is quite as true of aeroplane motors as those designed for use in the dirigible. But it is quite as important to make the machine go as it is to make it rise in the air, so that the question of _total weight per horsepower hour_ has led to the abandonment of extremely light engines requiring a great deal of fuel.

Speed is quite as costly in an airship as it is in an Atlantic liner. To double it, the motor power must be multiplied by 8, and the machine must carry 8 times as much fuel. But by cutting the power in half, the speed is reduced only one-fifth. The problem of long voyages in the dirigible is, accordingly, how to reconcile best the minimum speed which will enable it to make way effectively against the prevailing winds, with the reduction in power necessary to cut the fuel consumption down to a point that will insure a long period of running.

When the speed of the dirigible is greater than that of the prevailing wind, it may travel in any direction; when it is considerably less, it can travel only with the wind; when it is equal to the speed of the latter, it may travel at an angle with the wind—in other words, tack, as a ship does, utilizing the pressure of the contrary wind to force the ship against it. But as the air does not offer to the hull of the airship, the same hold that water does to that of the seagoing ship, the amount of leeway or drift in such a manoeuver is excessive. This applies quite as much to the aeroplane as it does to the dirigible.

FRENCH DIRIGIBLES

*The First Lebaudy.* The interest evidenced by the German War Department in Zeppelin’s airship was more than duplicated by that aroused in French military circles by the success of the Lebaudy Brothers. Since 1900 these two brothers had been experimenting with dirigible balloons. Their first dirigible—built by the engineer Juillot—made thirty flights, in all but two of which it succeeded in returning to its starting point. This machine was somewhat similar to the later types built by Santos-Dumont and carried a 40-horsepower Daimler motor. A speed of 36 feet per second, or about 25 miles per hour, was obtained. During tests in the summer of 1904, the balloon was dashed against a tree and almost entirely destroyed.

*Lebaudy 1904.* The next year the "Lebaudy 1904" appeared. This was 190 feet long and had a capacity of 94,000 cubic feet of gas. The air bag was divided into three parts and contained 17,600 cubic feet of air. It was supplied with air from a fan driven by the engine, and an auxiliary electric motor and storage battery were carried to drive the fan when the gas engine was not working. The storage battery was also used to furnish electric lights for the airship. A horizontal sail of silk was stretched between the car and the gas bag, which had an area of something over 1,000 square feet, and a sort of keel of silk was stretched below it. A horizontal rudder, shaped like a pigeon’s tail, was used at the rear, and immediately behind it were two V-shaped vertical rudders. A small vertical sail was carried, which could be used to assist in guiding the airship. The car was 16 feet long and was rigidly hung 10 feet below the bag. It was provided with an inverted pyramid of steel tubes meeting at an apex below the car to prevent injury in alighting. Sixty-three ascents were made in 1904 with this balloon, all of them comparatively successful, the longest being a journey of 60 miles in two hours and forty-five minutes.

The next year a new and larger balloon equipped with a more powerful motor was used. Many flights were made in tests for the French War Department.

*La Patrie.* La Patrie was then built for the French government by the Lebaudy Brothers and was of the same design as their earlier airships. In speed it was nearly equal to Zeppelin’s, and its dirigibility was nearly perfect. Fig. 9 shows a view of this airship in flight.

It was 200 feet long, and the 70-horsepower engine drove two propellers. It could carry seven people and one-half ton of ballast. It carried four people at a speed of 30 miles per hour. On its last trip it covered 175 miles in seven hours. A few days afterward, a heavy wind tore it away from its moorings and it was blown out to sea and lost.

*La Republique and Le Jaune.* Two more airships of the same type, La Republique and Le Jaune, followed this. These were tried by the French government, in 1908, and both proved successful. La Republique is illustrated in Fig. 10. The shape and equipment of the car are shown in Fig. 11. The automobile type of radiator may be seen attached to the side of the car. During a flight in the fall of 1909, a propeller blade broke and was thrown clear through the balloon envelope, causing the balloon to fall from a height of 500 feet. The four officers who formed the crew of the dirigible were killed instantly.

*Clement-Bayard II.* The numerous factors that must be considered in the design of a successful dirigible balloon as well as the many conflicting conditions that must be reconciled have already been referred to in detail. How these are carried out in practice may best be made clear by a description of what may be considered as an advanced type of dirigible, the Clement-Bayard II, Fig. 12, of French design, and the most successful of the French military air fleet. Its predecessor, the Clement-Bayard I, Fig. 13, made thirty voyages, some of them of considerable distances, without suffering any damage, but a study of its shortcomings led to their elimination in the following model.

The pisciform shape of the first Clement-Bayard was retained but given more taper, the dimensions being 248.6 feet overall by 42.9 greatest diameter, this being but a short distance back of the bow. This gives it a ratio of length to diameter of 5.76. The gas balloonet stabilizers were eliminated altogether, Fig. 12. The total gas capacity is approximately 80,000 cubic feet. Like all French dirigibles it is of the true flexible type, the only rigid construction being that of the framework of the car itself. To the latter are attached all rudders and stabilizing devices, instead of making them a part of the envelope as formerly. The latter is made of continental rubber cloth.

Light steel and aluminum tubing are employed in the construction of the frame supplemented by numerous piano-wire stays. This frame extends almost the entire length of the envelope and carries at its rear end a cellular, or box-kite, type of stabilizing rudder, instead of the former gas balloonets employed on the Clement-Bayard I, Fig. 13. This cellular rudder is in two parts, consisting of two units of four cells each, the two groups being joined at the top, with a space between them. In addition to acting as a stabilizer, this is also the direction rudder, its leverage being increased by making the end planes somewhat larger than the partitions of the cells. Between the cellular stabilizing rudder and the envelope is placed the horizontal rudder for ascending or descending. In the illustration this appears to be a flag, but it is in reality a long rectangular plane, which may be tilted on its longitudinal axis, the latter being at right angles to that of the balloon. There are two air balloonets of about one-third the total capacity of the balloon itself, and they are designed to be inflated by large aluminum centrifugal blowers driven from the main engines themselves.

There are two motors, each of 125 horsepower, both being of the same conventional design, _i.e._, four cylinder four cycle vertical water cooled. In fact, they are merely light automobile motors. The cylinders have separate copper water jackets and the motors themselves are muffled, which is a departure from the usual custom. Each drives a separate propeller carried on top of the main frame through bevel gearing.

The Clement-Bayard II made itself famous by its rapid and successful flight from the suburbs of Paris across the Channel to London, in October, 1910.

*Astra-Torres.* In reviewing the specifications of any of the big dirigibles, the observer cannot fail to be struck by the excessive amount of power necessary to drive them at speeds which are lower than the minimum, or landing speeds, of many aeroplanes. When a speed of 45 miles per hour was first reached by a dirigible, it was acclaimed as a great feat. But this comparatively moderate rate of travel was surpassed only by increasing the number of motors and their horsepower until the fuel consumption became exceedingly high. This necessitated the carrying of a great weight of fuel and cut down correspondingly the useful load that the dirigible was capable of lifting as well as restricted its radius of flight at full speed. Until aerodynamic research had demonstrated the contrary, the necessity for such a tremendous amount of power was considered necessary to overcome the head resistance of the balloon itself. Research brought out in a striking manner how great a proportion of the total head resistance of an aeroplane was due to the struts and bracing wires. In the construction of the different types of airships illustrated, it will be noted that the gear provided for suspending the car or cars below the balloon requires a great number of cables. Later developments showed that by eliminating the great amount of head resistance caused by these numerous surfaces, the speed of a dirigible could be increased by over 50 per cent with the same amount of power.

_Improved Suspension._ The shortcoming of the dirigible with reference to suspension was realized more than ten years previous by a Spaniard—Torres—but owing to lack of financial support, he was unable to put his idea into execution. The principle he evolved is made clear by Fig. 14, which gives a section of an Astra-Torres dirigible illustrating the method of suspension. Instead of the ropes _SR_ used to suspend the car being attached to bands passing around the envelope, these reinforcing bands _CB_ and also the ropes fastened to them are placed inside the envelope, thus eliminating head resistance from those sources.

_Performance._ Failing to obtain any encouragement in Spain, Torres finally succeeded in interesting the French Astra Company, which built a vedette, or scouting airship, of a little over 50,000 cubic feet capacity. It was pitted against the Colonel Renard, at that time the leading unit in the French aerial navy and the fastest airship in commission. The small Torres dirigible so completely outclassed its huge competitor that another of close to 300,000 cubic feet capacity was built and tried against the Parseval with similar results. An Astra-Torres dirigible built for the British government showed a speed in excess of 50 miles per hour. This particular dirigible has been at the front in France almost since the outbreak of hostilities and has rendered considerable valuable service. Its success led the French Government to order a huge replica of it, having a capacity of over 800,000 cubic feet and with motors developing 1,000 horsepower, which would give it an indicated speed of 60 miles per hour. So confident were its builders of attaining or even exceeding this, that an order for a second and even larger airship of the Astra-Torres design was placed before the first one was finished. This is also fitted with motors aggregating 1,000 horsepower and displaces 38 tons, making it larger than any Zeppelin that had been constructed up to the time it was built. As its construction and trials were undertaken during the war, no details have been published, but it is said on good authority that its speed exceeds 60 miles per hour, so that it is faster than any of the German dirigibles.

_Construction._ Unlike the German dirigibles, the larger types of which have been characterized by a rigid frame, the Astra-Torres is a flexible airship and, owing to its method of suspension, its external appearance is decidedly unconventional, since the envelope instead of being of the usual cigar shape is more like a triangular bundle of three cigars with the third one on top. At the point where the three envelopes join, as shown in section, Fig. 14, heavy cloth bands _CB_ are stretched across the arcs, forming a chord across each arc, the three chords comprising an inverted triangle. The suspension ropes _SR_ are attached to the opposite ends of the base of this inverted triangle and converge in straight lines downward through the gas space, so that the air resistance offered by the ropes is practically eliminated since only a very small part of the suspension system appears outside the envelope. This external part consists of vertical cables _A_ attached to the collecting rings of the bracing system and extending downward through special accordion sleeves _S_ which permit the free play necessary at the points where they pass through the outer wall of the envelope. These sleeves also have another function—that of permitting the escape of gas under the pressure of expansion. A short distance below the envelope _E_ each of these cables splits into two parts _C_ and _C’_ attached to opposite sides of the car.

The British airship mentioned is provided with but one car, but the larger French ships have two placed tandem, each of which carries a 500-horsepower motor driving two two-bladed propellers of large diameter. While the form of envelope made necessary by this construction increases the frictional resistance, this is negligible in comparison with the great saving in power effected by the method of suspension, not to mention the greater simplicity of construction.

GERMAN DIRIGIBLES

*Early Zeppelin Airships.* At the same time that Santos-Dumont was carrying on his hazardous experiments, the problem was being attacked along slightly different lines by Count Zeppelin.

It will be remembered that Dumont experienced much trouble on account of the envelope of his balloon being too flexible, causing it to crumple in the middle and to become distorted in shape from the pressure of the air. His efforts to overcome this by the employment of air bags did not meet with great success, even in his later types.

_Construction._ Zeppelin employed a very rigid construction. His first balloon, which was built in 1898, was the largest which had ever been made. It is illustrated in Fig. 15, which shows his first design slightly improved. It was about 40 feet in diameter and 420 feet long—an air craft as large as many an ocean vessel. The envelope consisted of two distinct bags, an outer and an inner one, with an air space between. The air space between the inner and outer envelopes acted as a heat insulator and prevented the gas within from being affected by rapid changes of temperature. The inner bag contained the gas, and the outer one served as a protective covering. In the construction of this outer bag lies the novelty of Zeppelin’s design. A rigid framework of strongly braced aluminum rings was provided and this was covered with linen and silk which had been specially treated to prevent leakage of gas. The inner envelope consisted of seventeen gas-tight compartments which could be filled or emptied separately. In the event of the puncture of one of them, the balloon would remain afloat. An aluminum keel was provided to further increase the rigidity. A sliding weight could be moved backward or forward along the keel and cause the nose of the airship to point upward or downward as desired. This would make the craft move upward or downward without throwing out ballast or losing gas. Lender each end of the balloon a light aluminum car was rigidly fastened and in each was a 16-horsepower Daimler gasoline engine. The two engines could be worked either independently of each other or together. Each engine drove a vertical and horizontal propeller. The propellers each had four aluminum blades. As will be seen from Fig. 15, the ears were too far apart for ordinary means of communication and so speaking tubes, electric bells, and an electric telegraph system were installed.

_First Trials._ Very little was known as to the effects of alighting on the ground with such a rigid affair as this vessel, therefore the cars were made like boats so that the airship could alight and float on the water. The first trials were made over Lake Constance in July, 1900. The mammoth craft was housed in a huge floating shed, and the vessel emerged from it with the gas bag floating above and the two cars touching the water. She rose easily from the water, and then began a series of mishaps such as usually fall to the lot of experimenters. The upper cross stay proved too weak for the long body of the balloon and bent upward about 10 inches during the flight. This prevented the propeller shafts from working properly. Then the winch which worked the sliding weight was broken and, finally, the steering ropes to the rudders became entangled. In spite of all this, a speed of 13 feet per second, or about 9 miles per hour, was obtained. These breakages made it necessary to descend to the lake for repairs and in alighting the framework was further damaged by running into a pile in the lake. The airship was repaired and another flight was made later in the year, during which a speed of 30 feet per second, or 20 miles per hour, was obtained.

_Second Airship._ Zeppelin had sunk his own private fortune and that of his supporters in his first venture, and it was not till five years later that he succeeded in raising enough money to construct a second airship. No radical changes in construction were made in the new model, but there were slight improvements made in all its details. The balloon was about 8 feet shorter than the original and the propellers were enlarged. Three vertical rudders were placed in front and three behind the balloon, and below the end of the craft horizontal rudders were installed to assist in steering upward or downward. The steering was taken care of from the front car.

The most important change was made possible by the improvement in gasoline engines during the preceding five years. Where, in the earlier model, he had two 16-horsepower engines, he now used an 85-horsepower engine in each car, with practically the same weight. In fact, the total weight of the vessel was only 9 tons, while his first airship weighed 10 tons.

His new craft made many successful flights. One was made at the rate of 38 miles per hour and continued for seven hours, covering a total distance of 266 miles.

*Later Zeppelins.* The later Zeppelins embody no remarkable changes in design, the principal alteration being in size. One of these is illustrated in Fig. 16. In this the gas bag was increased to 446 feet in length and it held over 460,000 cubic feet of gas. This gave it a total lifting power of 16 tons. With this, Zeppelin made a voyage of over 375 miles. He was in the air for twenty hours on this trip and carried eleven passengers with him.