The Airship and Its Problems

THE AIRSHIP AND ITS PROBLEMS Types of Airship.—Ingeneral there have been two distinct parallel lines of development from the free balloon. The non rigid type of ship is in effect merely an elongated free balloon with car and engine suspended beneath by a suitable arrangement of rigging. In this type of ship the shape is preserved by main taining the buoyant gas inside the envelope at a definite pressure above the air outside, usually through the agency of air ballonets or bags inside, which, being only partly inflated, are free to ex pand and in which the air pressure is maintained either by air scoops and valves or by an independent air compressor in the car. The semi-rigid type of ship, often treated as a distinct class, is in reality merely a variation of the non-rigid type. In this, some sort of elongated framework is provided, usually in the form of a stiff keel, carrying the power unit, for the purpose of better distributing the weight of the latter over the length of the en velope. In the rigid type of ship, on the other hand, the shape is maintained by a stiff framework built up of girders over which fabric is stretched to produce the required shape or form and the buoyant gas is carried in independent bags or containers inside. The gas in these gasbags is not maintained at an excess pressure above that of the surrounding air; in fact, the gasbags may not necessarily be fully inflated.

Lifting Power.

Allairships obtain their ascensional power by the displacement of air by a lighter gas in exactly the same way that a bladder filled with air and submerged in water will rise to the surface of the water. The magnitude of the "lift" is determined by the kind and purity of the gas, the size of the gas container, and the condition of the atmosphere. The con ditions which affect the densities of the atmosphere and any given lifting gas are temperature, pressure and humidity.

At sea level, under normal conditions i,000 cu.ft. of air weighs approximately 8o lb., i,000 cu.ft. of pure hydrogen under the same conditions weighs only 5 lb. and i,000 cu.ft. of pure helium weighs 12 lb. If, therefore, a small balloon is filled with pure hy drogen it will have a "lift" of 8o-5 or 75 lb., and with pure helium 8o—I 2 or 68 lb. for each i,000 cu.ft. of capacity.

In practice, owing to the presence of impurities, the lift of hydrogen in airships is usually about 7o lb. and of helium 6o to 64 lb. per i,000 cu.ft. Hence in the case of an airship having a total displacement of 5,000,00o cu.ft. the effective buoyancy or lift when filled with hydrogen will be 5,00o X 7o lb. or 156 tons, and with helium from 134 to 143 tons only.

These figures give the total lift available and therefore, the hull, engines, fuel, ballast, crew, etc., or "fixed weights" cannot be allowed to total more than this amount. "Disposable" lift is the amount of lift available after all fixed weights have been deducted and is usually given as a percentage of the total lift.

Helium Versus Hydrogen.

For a time after the United States Government adopted helium in place of hydrogen for the lifting gas of its airships there was considerable controversy as to the relative merits of the two gases. The new German "LZ-I29," however, was designed for helium after several hydrogen craft ran into disasters.

The advantage of helium is its non-inflammability, and it is idle to pretend that hydrogen is in any way comparable to it in this respect. On the other hand if the above figures for the 5,000,00o cu.ft. ship are examined and it is assumed that -in each case the fixed weights total I 1 o tons, the disposable lift in the case of the hydrogen-filled ship will be 46 tons, while in the case of the helium ship only from 24 to 33 tons. This represents a very considerable percentage loss in performance, as compared with the same vessel operated upon hydrogen, but the difference is written off to safety. Helium is $10 per i,000 cu.ft. against $1.25 for hydrogen, and is obtainable only in the United States. (See