Engineering Wonders of the World
Volume III

48 ENGINEERING WONDERS OF THE WORLD. travel at the same speed with the expenditure of about one-eighth of the power. “ As there are practically always currents in the air reaching at times a velocity of many miles per hour, a dirigible balloon should be con- structed with sufficient power to be able to travel at a speed of about 50 miles per hour, in order that it may be available under prac- tical conditions of weather. In other words, it should have substantially as much power as would drive a boat, carrying the same weight, 25 miles an hour, or should have the same ratio of power to size as the Lusitania The pressure on the envelope of a balloon, when the latter is moving at high velocity relatively to the air, must indent it and cause great increase of resistance un- i Pressure on ]ess the envelope be either kept the Envelope. inflation or supported by a rigid framework of some kind. As high inflation is prevented by the comparative weakness of the fabric, and even, if feasible, would mean a sufficient compression of the gas to cause a serious loss of buoyancy, the “ rigid ” school, whose great exponent is, of course, Count von Zeppelin, makes use of an internal skeleton, a light polygonal girder running from stem to stern. The weight of the girder makes great volume necessary, and to obtain this without increasing the head resistance unduly, the body is given a length of rather Zeppelin moro than ten diameters. A Principle. gjngje container of this shape would be subjected to dangerous surgings of gas to and fro as either end rose and fell, so Zeppelin has adopted a number of small balloons separated from one another by parti- tions, and from the external covering of the balloon by an air-space which serves to insulate th© gas from the changes in temperature of the atmosphere. This subdivision has the further advantage of localizing damage to the balloon. Had the ill-fated République not had a single chamber, she might have come to ground without fatal results. For non-rigid dirigibles one or more internal air ballonets are used. Air is pumped con- stantly into them, escaping again through a valve if the pressure rises . Ballonets. above a certain point. The gas chamber also is provided with a valve, acting at a somewhat higher pressure, so that under no conditions can the distension of the ballonets cause a loss of gas. If the gas is expanded by a rise in temperature, the ballonet is squeezed until the pressure is normal. If, on the other hand, the gas contracts or leaks, the ballonet swells out until equilibrium is restored. The distribution of the load over the gas holder in such a way as not to strain any part unduly is, in the case of a Zeppelin airship, simplified by the employment of a girder keel. Unless the distribution is made properly over a non-rigid envelope, there must be a danger of the balloon collapsing. To simplify the problem a keel or frame fitting the lower side of the envelope is used, and from it are slung the car, motor, etc. Dirigibles thus provided are known as semi-rigid, and have some of the stiffness of the Zeppelin type, while being capable of deflation like the non- rigid type, though less convenient for trans- port by land. The German Gross and the French Lebaudy and République belong to this class. The rigid airship has a further advantage over the non-rigid in that the propellers can be attached to the gas-holder frame and deliver their thrust at tho same elevation as that of the APPj*cation of Power, centre of air pressure. In the case of a non-rigid or semi-rigid airship, tho propellers are mounted far below the centre of pressure, and this produces a tilting action and less efficient drive. Renard, during his experiments in 1884 and 1885, found that his airship began to pitch—tilt up and down longitudinally—as ■■■■■