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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.