The displacements of present dirigibles vary from 20,000 cubic feet (in
the United States Signal Corps airship) up to 460,000 cubic feet (in
the Zeppelin). The former balloon has a carrying capacity only about
equivalent to that of a Wright biplane. While anchored or drifting
balloons are usually spherical, all dirigibles are elongated, with
a length of from four to eleven diameters. The Zeppelin represents
an extreme elongation, the length being 450 feet and the diameter
forty-two feet. At the other extreme, some of the English military
dirigibles are thirty-one feet in diameter and only 112 feet long.
Ballonet capacities may run up to one-fifth the gas volume. All present
dirigibles have gasoline engines driving propellers from eight to
twenty feet in diameter. The larger propellers are connected with
the motors by gearing, and make from 250 to 700 turns per minute.
The smaller propellers are direct connected and make about 1200
revolutions. Speeds are usually from fifteen to thirty miles per hour.
[Illustration: THE BALDWIN
Dirigible of the United States Signal Corps]
The present-day elongated shape is the result of the effort to
decrease the proportion of propulsion resistance due to the pressure
of the air against the head of the balloon. This has led also to the
pointed ends now universal; and to avoid eddy resistance about the
rear it is just as important to point the stern as the bow. As far as
head end resistance alone is concerned, the longer the balloon the
better: but the friction of the air along the side of the envelope
also produces resistance, so that the balloon must not be too much
elongated. Excessive elongation also produces structural weakness. From
the standpoint of stress on the fabric of the envelope, the greatest
strain is that which tends to break the material along a longitudinal
line, and this is true no matter what the length, as long as the seams
are equally strong in both directions and the load is so suspended as
not to produce excessive bending strain on the whole balloon. In the
_Patrie_ (page 77), some distortion due to loading is apparent. The
stress per lineal inch of fabric is obtained by multiplying the net
pressure by half the diameter of the envelope (in inches).
[Illustration: THE ZEPPELIN ENTERING ITS HANGAR ON LAKE CONSTANCE]
Ample steering power (provided by vertical planes, as in
heavier-than-air machines) is absolutely necessary in dirigibles: else
the head could not be held up to the wind and the propelling machinery
would become ineffective.