3. Danger of fire from self-electrification of surface.
This appears to be a great danger in the case of airships whose gas-bags are made with india-rubber surfaces. No less than two Zeppelins have been destroyed from this cause. In the case of the explosion of the gas in a Zeppelin of 1908, when it burst from its anchorage at Echterdingen, the destruction of the airship appears to have been caused by electric sparks produced by the friction of the material of which the gas-bag compartments were made. Colonel Moedebeck, in the Aëronautical Journal of October, 1908, gave an expert opinion as to the cause of this accident:—
The balloon material, which is india-rubber coated, has the peculiar property of becoming electrified in dry air. When rolled up or creased in any way it rustles, and gives out electric sparks, the latter being (as shown by the experiments undertaken by Professor Bonsteim and Captain Dele for the Berlin Aëronautical Society) clearly visible in the dark.
Now, the lower parts of the material of which the gas-cells are composed would, owing to the height to which the airship had ascended (1,100 m.) and the release of gas from the valves, become creased or folded upon each other, and the rubbing thus produced would be quite sufficient to generate the electric sparks above referred to. Under ordinary circumstances, when the space between the gas-cells and the outer envelope of the airship is full of atmospheric air, continually renewed, as when it is in full flight, these sparks would be harmless enough, but when the ship is at anchor, as at Echterdingen, this is not necessarily the case.
We know that the carefully made tissue of the Continental Caoutchouc Company resists the penetration of hydrogen very strongly, but some may have leaked through into the space between the cells and the outer envelope, while it seems very probable that when the mechanics opened the valves, and the long axis of the balloon became inclined, more hydrogen entered this space and an explosive mixture was formed.
According to the description given by eye-witnesses, the explosion took place after the forepart of the vessel (dragging its anchor) struck the ground. The shock thus caused would have been transmitted to the creased and wrinkled gas-cells, and the tearing of the material, already in an electrified condition, might easily have generated sufficient sparks to detonate the explosive mixture.
Again, in 1912, there was a repetition of this kind of disaster in the case of the destruction of another Zeppelin, the “Schwaben.” In this case the framework of the airship had got broken, being battered about in landing in an adverse wind. The india-rubber-coated bags were rubbed against each other, with the production of electric sparks. These either set fire to the gas issuing from one of the gas-bags or exploded the mixture of air and gas contained in the space between the gas-bags and outer covering of the airship. Perhaps it was on account of this accident that gold-beaters’ skin has sometimes been used for the gas containers of the Zeppelin airships.
4. Difficulties in the way of applying the propulsive screws in the most effective position.
Most airships are exceedingly defective in this respect, the screws being applied to the propulsion of the car and not to the whole system. The result is that the cumbersome gas-bag lags behind. Certainly, one of the best points in a Zeppelin was the attachment of the screws to the airship framework above the cars, thus securing more advantageous position. This, however, only amounted to something like half measures. In the case of the ill-fated airship “La Paix,” the Brazilian aëronaut Severo undoubtedly aimed at the ideal, though the experiment cost him his life. He devised the ingenious system of combining balloon and car in one symmetrical melon-shaped body, through the centre of which passed longitudinally the shaft which revolved the propelling screws at either end. The screws were therefore in the position in which to propel the whole system and not the car only. This, however, necessitated the introduction of a very small space between the car and balloon proper. By reason of this very small space the presence of the petroleum motor in the car could not fail to be dangerous, and was the cause of the fiery end of Severo’s balloon and the death of the inventor and engineer. On the morning of the ill-fated May 11th, 1902, Severo and Sachet ascended in “La Paix.” A few moments after the ascent the balloon exploded, in the words of an eye-witness, like a crash of thunder, and the occupants were precipitated to the ground.
In spite of the engineering advantages of Severo’s system no one has dared to revive the plan.