Airship development began with non-rigid, pressurized airships. However, the efficiency of the airship increases markedly with increasing size. When larger airships were required, they were built using the rigid, non-pressurized design This was necessary because there were no materials available at that time that were suitable for large, non-rigid pressurized airship envelopes. The rigid non-pressurized airship design allows the use of different materials for the envelope and the gas cells. With modern materials, such as aramid fiber cloth and neoprene rubber, large, non-rigid, pressurized airships can be built and the many advantages to this type of design can be utilized.
The rigid airship is very heavy, with the weight of the structure, gas cells and outer envelope weighing about 70% of the gross lift. This can be accomplished in the non-rigid design for about 35% of the gross lift. This, of course, results in a corresponding increase in the payload. The possibility of structural failure is also much more likely to occur with a rigid airship.
The single cell design was chosen because the multi-cell design adds much complication and very little advantage. The airship is generally in balance and has little capability to increase the lift available or decrease the lift required. The failure of a single cell on a multi-cell airship will, therefore, cause the airship to descend. Also, the single cell airship has been flying for many years, and this type of accident has not been a problem. Excessive pressure from surging of the lifting gas from one end of the airship to the other could be a problem if the pitch attitude is allowed to change. The present invention eliminates this possibility by providing for large static longitudinal stability.
One of the main obstacles to airship development has been the lack of a practical and reliable method of controlling the altitude of the airship and of having adequate control during the critical phases of take-off, landing and ground handling.
Most airships prior to the present invention have used changes in the pitch attitude to produce positive or negative lift through aerodynamic forces on the airship hull to vary the altitude.
In order to prevent the required longitudinal control forces from becoming excessive, it was necessary to design for a low static and dynamic longitudinal stability. As a result of this low longitudinal stability, unexpected updrafts and down-drafts caused these airships to pitch up or down and to exceed the altitude envelope limits of the ground and pressure height before the crew could regain control.
The large rigid airships were more vulnerable to this type of accident because they generally had a greater length to diameter ratio than the non-rigids. A cylinder is unstable when traveling against the wind in the direction of its major axis.
The large rigid airships were kept inside large buildings while on the ground, and many accidents occurred while bringing the airships in and out of these buildings.
The present invention provides adequate stability and control of the airship during all phases of flight and while on the ground.