Fig. 75.—Early-type Airship.
A. Gas envelope; B. Car suspended below envelope; C. Motor, which drives propeller (D) through a shaft; E. Small horizontal plane for rising or descending; F. Fixed fin, or keel plane, to give stability; G. Rudder.
Fig. 76.—Airship “ballonette.”
A. Gas envelope; B. The interior
ballonette, or collapsible chamber,
which is filled with air
through the pipe C.
A typical craft, representing the first of those navigated with any certainty, is shown in [Fig. 75]. A gas-containing envelope, made of a light, strong, varnished fabric, is kept taut by the pressure of the gas within; the car, constructed of wood or metal tubing, is suspended by ropes from the envelope, and contains engine and crew, with a two-bladed propeller revolving astern. Such a machine, in its control, had an elevating-plane and rudder, upon the same principle as those of the aeroplane. One of the difficulties to be overcome was the expansion and contraction of gas in the envelope owing to differences in altitude and temperature. When the craft ascended, its envelope completely inflated, the gas began to dilate owing to the outer air becoming less dense; and some had to be allowed to escape through automatic valves. Then, should the machine descend to a lower level, there was not sufficient gas in the envelope to keep it tightly stretched, and it tended to sag at the bow as it was driven through the air. To prevent this kinking, which would have reduced the speed of the airship, and made it difficult to control, an interior chamber, called the “ballonette,” was fitted to the envelope, as shown in [Fig. 76]. When the gas-container was tightly filled, this ballonette lay empty upon its lower surface; but, should the envelope tend to become flaccid, through a loss of gas, a fan pumped air into the ballonette; and, swelling out within the balloon, it compensated for the gas which had escaped, and prevented the envelope from losing its shape.
The craft shown in [Fig. 75] is of the non-rigid type; its car, that is to say, is hung by ropes from the envelope; and when the envelope is deflated it can be detached from the car and the machine packed away in a relatively small space. But as airships were built larger, and greater speeds were obtained, it became necessary to strengthen the envelopes with some form of keel; and this led to a type which is known as the semi-rigid, and is developed successfully in France. [Fig. 77] illustrates an airship of this build. Along the lower side of its envelope is placed a light, rigid framework or keel, and from this is suspended the car which contains engines and crew.