For this reason height was of primary importance and speed was
quite a secondary consideration, owing to the low velocity of
prevailing winds in that country. Flights were never of long
duration compared with those carried out by our airships. Height
was always of the utmost importance, as the Italian ships were
used for bombing enemy towns and must evade hostile gunfire. For
this reason weight was saved in every possible manner, to
increase the height of the "ceiling."
In addition to the types already mentioned, three other varieties
have been constructed since the war--the Usuelli D.E. type and G
class. The G class was a rigid design which has not been
proceeded with, and, with this single exception, all are of a
semirigid type in which an essentially non-rigid envelope is
reinforced by a metal keel. In the Forlanini and Usuelli types
the keel is completely rigid and assists in maintaining the shape
of the envelopes, and in the Forlanini is enclosed within the
envelope. In the other types the keel is in reality a chain of
rigid links similar to that of a bicycle. The form of the
envelope is maintained by the internal pressure and not by the
keel, but the resistance of the latter to compression enables a
lower pressure to be maintained than would be possible in a
purely non-rigid ship.
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