Car of the Zeppelin
(From the Transactions of the American Society of Mechanical Engineers)
Stability
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.
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.