Having settled upon the size and shape, there must be an appropriate means of attaching the car to carry the power plant, its accessories and control and the crew. While apparently a simple matter, this involves one of the most important elements of the design that of stability. A long envelope of com paratively small diameter being necessary for the reasons given, it is essential that this be maintained with its axis horizontal. In calm air, the balloon, or container, is subjected to the action of two forces. One is its weight, ap plied to the centre of gravity of the system formed by the balloon, its car and all the supports; the other is the thrust of the air, applied at a point known as the centre of thrust and which will differ with different designs, according as the car is suspended nearer or farther away from the balloon. If the latter contained only the gas to inflate it, with no car or weight to carry, the centre of gravity and the centre of thrust would coincide, granting that the weight of the envelope were negligible. As this naturally cannot be the case, these forces are not a continuation of each other. But as they must necessarily be equal if the balloon is neither ascending nor descending, it follows that they will cause the balloon to turn until they are a continuation of each other. As this would be neither convenient nor compatible with rapid propulsion, it must be avoided by distributing the weight along the car in such a manner that when the balloon is horizontal the forces represented by the pressure above and the weight below must be in the same perpen dicular. This is necessary to assume a hori zontal position while at rest. But the longi tudinal stability of the airship as a whole must be preserved. Its axis must be a tangent to the course it describes, if the latter be cur vilinear, or parallel with the direction of the course where the course itself is straight. This is apparently something which should be.taken care of by the rudder, any tendency on the part of the airship to diverge from its course being corrected by the pilot. But a boat that needed constant attention to the helm to keep it on its course would be put down as a boat, in other words, of faulty design in the hull. A dirigible having the same defect would be difficult to navigate, as the rudder alone would not suffice to correct this tendency in emer gencies. Stability of direction is accordingly provided for in the design of the balloon itself, and this is the chief reason for adopting the form of a large-headed and slender-bodied fish. This brings the centre of gravity forward and makes of the long tail an effective lever which overcomes any tendency of the ship to diverge from the course it should follow. However, the envelope of the balloon itself would not suffice for this, so just astern of the latter. °stabilizing surfaces are placed, consisting of vertical planes fixed to the envelope. These form the keel of the dirigible and are analogous to the keel of the ship.
In addition to being able to preserve its static equilibrium and to possess proper longitu dinal stability, the successful airship must also maintain its dynamic equilibrium — the equilib rium of the airship in motion. This may be made clear by referring to the well-known ex pedients adopted to navigate the ordinary spherical balloon. To rise, its weight is dimin hhed by gradually pouring sand from the bags which are always carried as ballast. To de scend, it is necessary to increase the total weight of the balloon and its car, and the only method of accomplishing this is to permit the escape of some of the gas, the specific lightness of which constitutes the lifting power of the balloon. The balloon, or the container itself,
being merely a spherical bag, on the upper hemi spherical half of which the net supporting the car presses at all points, the question of defor mation is not a serious one. Before ore it assumed proportions where the bag might be in danger of collapsing, the balloon would have had to come to earth through lack of lifting power to sustain it longer. Owing to its far greater size, as well as to the form of the surface which it presents to the air pressure, such a crude method is not applicable to the dirigible. Dy namic equilibrium must take into account not only the dirigible's weight and the sustaining pressure of the air, but also the resistance of the air exerted upon its envelope. This resistance de pends upon the dimensions and the shape of that envelope, and in calculations the latter is always assumed to be invariable. Assume, for instance, that to descend the pilot of a dirigible allowed some of the hydrogen gas to escape.
As the airship came down, it would have to pass through strata of air of constantly increasing pressure as the earth is approached. The rea son for this will be apparent as the lower strata bear the weight of the entire atmosphere above them. The confined gas will no longer be suffi cient to distend the envelope, the latter losing its shape and becoming flabby. As the original form is no longer retained, the centre of resist ance of the air will likewise have changed to gether with the centre of thrust, and the initial conditions will no longer obtain. But as the equilibrium of the airship depends upon the maintenance of these conditions, it will be lost if they vary. In the design of a successful dirigible, the proper location of the propulsive effort with relation to the balloon is a very im portant factor. Theoretically, this should be applied to the axis of the balloon itself, as the latter represents the greater part of the resist ance offered to the air. In theory, the balloon offers such a substantial percentage of the total resistance to the air that the area of the car and the rigging were originally considered practi cally negligible by comparison. Actually, how ever, this is not the case. Calculations show that the sum of the surface of the suspending rigging, passengers, air pump and other trans verse members and exposed surfaces, total an equivalent of a quarter or even a third of the transverse section of the balloon itself. To in sure the permanently horizontal position of the ship under the combined action of the motor and the air resistance, a position of the propel ler at a point about one-third of the diameter of the balloon below its horizontal axis will be necessary. Without employing a rigid frame like that of the Zeppelin and the Pax, however, such a location of the shaft is a difficult matter for constructional reasons. Consequently, it has become customary to apply the driving effort to the car itself, as no other solution of the problem is apparent. This accounts for the tendency common in the dirigible to afloat high forward," and this tilting becomes more pro nounced in proportion to the distance the car is hung beneath the balloon. The term °deviation° is employed to describe the tilting effect pro duced by the reaction of the propeller. Con flicting requirements are met with in attempting to reduce this by bringing the car closer to the balloon as this approximation is limited by the danger of operating the gasoline motor too close to the huge volume of inflammable gas. The best solution of the problem has been found by placing the motor in the car, and driving a shaft so located between the car and the balloon by means of a chain.