Besides proper distribution of the loads, correct vertical location of
the propeller is important if the balloon is to travel on a level keel.
In some early balloons, two envelopes side by side had the propeller
at the height of the axes of the gas bags and midway between them. The
modern forms carry the car, motor, and propeller below the balloon
proper. The air resistance is mostly that of the bow of the envelope:
but there is some resistance due to the car, and the propeller shaft
should properly be at the equivalent center of all resistance, which
will be between car and axis of gas bag and nearer the latter than
the former. With a single envelope and propeller, this arrangement
is impracticable. By using four (or even two) propellers, as in the
_Zeppelin_ machine (page 68), it can be accomplished. If only one
propeller is employed, horizontal rudder planes must be disposed at
such angles and in such positions as to compensate for the improper
position of the tractive force. Even on the _Zeppelin_, such planes
were employed with advantage (pages 66 and 73).
Perfect stability also involves freedom from rolling. This is usually
inherent in a balloon, because the center of mass is well below the
center of buoyancy: but in machines of the non-rigid type the absence
of a ballonet might lead to both rolling and pitching when the gas was
partially exhausted.
[Illustration: STERN VIEW OF THE ZEPPELIN]
What is called "route stability" describes the condition of straight
flight. The balloon must point directly in its (independent) course.
This involves the use of a steering rudder, and, in addition, of fixed
vertical planes, which, on the principle of the vertical partitions
of Voisin, probably give some automatic steadiness to the course. To
avoid the difficulty or impossibility of holding the head up to the
wind at high speeds, an _empennage_ or feathering tail is a feature of
all present balloons. The empennage of the _Patrie_ (page 77) consisted
of pairs of vertical and horizontal planes at the extreme stern. In
the _France_, thirty-two feet in maximum diameter and nearly 200 feet
long, empennage planes aggregating about 400 square feet were placed
somewhat forward of the stern. In the _Clément-Bayard_, the empennage
consisted of cylindro-conical ballonets projecting aft from the stern.
A rather peculiar grouping of such ballonets was used about the
prolonged stern of the _Ville de Paris_.
[Illustration: THE "CLÉMENT-BAYARD"]
[Illustration: THE "VILLE DE PARIS"]