1. The resistance of the gas-bag.
From a mechanical point of view it is in opposition to science to attempt aërial navigation by pushing such a large resisting surface as the envelope of an airship against the air. In navigating an airship against the wind, as the latter increases speed is diminished, until a limit is reached when the motive power will be unavailing. Thus there are weather limitations to the airship. Not that the aëroplane is unaffected by the weather. That also has its limits; but recent practice has shown that the proportion of days when aëroplanes can fly is considerably larger than those on which airships can venture forth from their sheds.
This disadvantage of the resistance of surface was very manifest in the earlier experiments with navigable balloons, when only feeble motive power was available. For instance, in Count Zeppelin’s experiments in 1900, his two motors of 16 h.p. could not combat a greater wind force than about three metres a second. Then airships could indeed only be called toys. It has only been possible to make them partially successful concerns by enormously increasing motive power. At the h.p. figures with which the latest made large airships have been endowed, the wind limit is much lower than in the case of the heavier-than-air constructions. Though now airships can encounter moderate winds, they are still fair-weather instruments. For the great records of distance established by Count Zeppelin favourable meteorological conditions have been wisely selected. It was M. Santos Dumont who first led the way in making airships something beyond toys. He, in his picturesque and world-alluring experiments, first dared to encounter winds which in force exceeded what would be called calm weather. It is exceedingly difficult to ascertain what are the exact wind forces overcome by a body moving in air. The measurements have to be taken from a point independent of the moving body. We generally find this one important figure omitted in accounts of airship voyages. M. Santos Dumont’s experiments gave especially favourable opportunity for ascertaining correct records of the wind forces overcome. Since M. Santos Dumont so frequently rounded the Eiffel Tower close to the storey where the meteorological instruments were placed, the writer obtained from the authorities of the Eiffel Tower a record of the wind forces registered on all the days of his experiments. A comparison of those records with those of M. Santos Dumont’s journeys made it possible to approximately ascertain the highest wind forces he combated on his journeys round the tower; these were about five metres a second. M. Santos Dumont, however, appears to have claimed six metres a second for his highest wind record.
The brothers Lebaudy in their earlier experiments about doubled the record of Santos Dumont in this respect. As time has gone on greater advance has been made, though the limit is still represented by moderate wind.
There is, perhaps, some consolation in this thought for those who fear raids of an inimical airship fleet. The proverbial windy nature of our favoured islands is perhaps even more protection than darkened cities and artillery shot, though it is well indeed not to neglect the two latter precautions.
Meteorologically speaking, to make a raid with bulky airships from a distance over these islands would be a very risky undertaking, fraught with the greatest danger to the occupants of the airships. It must be remembered that, chiefly owing to the weather, the history of the Zeppelin may well be called the history of disaster. For the very reason of its fragility over and over again it has been the victim of tempest and flame.
The use of aluminium for the framework of the Zeppelins has been largely responsible for Count Zeppelin’s repeated weather misfortunes. There has been a fascination about this brittle metal aluminium for aëronautical work on account of its lightness. Its employment for aircraft construction, except for trivial purposes, is, however, a fallacy. That most practical aëronautical engineer, M. Julliot, in working out his semi-rigid constructions, has never fallen into the snare of aluminium allurement, wisely using steel instead. Considering the aluminium framework of the first Zeppelin constructed was fairly wrecked by the trifling accident of its falling down from the ceiling of the shed to the floor, it is a wonder that this species of metal has been retained, to be crumpled up almost like paper in the many accidents that have occurred.
2. Danger of fire from close combination of petroleum motor and gas-containing envelope.
In airships of all three types—rigid, semi-rigid, and non-rigid—this danger is constantly present. There have been examples of airship conflagrations in mid-air, but the greatest danger of conflagrations is in descending when the airships have been overtaken by strong and gusty winds. As has before been stated, fire has been the great destroyer of the Zeppelins.
The nearer the car containing the motors is placed to the gas envelope, the greater the fire risk becomes. The Parsifal airship, in which the car is suspended a considerable distance from the gas-bag, should in this respect be the safest of all the types of airships yet constructed.