Ballonet
A balloonet (French-English small balloon ) is a balloon-like airbag inside the envelope of a semi-rigid or impact airship , with the help of which the change in volume of the carrier gas within the envelope is compensated. The variation of the volume of the ballonet by filling it with outside air or emptying it ensures that there is always a slight overpressure inside the float , in order to keep it plump. This overpressure is only a few millibars , typically around five millibars.
This solution was first proposed in 1784 by Jean-Baptiste Meusnier de la Place (1754–1793).
theory
The lifting gas changes its volume by changing the outside temperature or the outside pressure (both weather-related and by changing the flight altitude ) according to the general gas equation :
With
- p = pressure
- V = volume
- T = temperature.
Technical design
Ballonets are inflated with air within the envelope, which is diverted from the air flow of the propeller propeller . In some model variants, however, separate fans are also used.
The size of a ballonet when fully inflated is typically about 25-40 percent of the total envelope volume.
Most airships have two ballonets, which are arranged near the bow and the stern within the hull in order to allow a static trimming of the airship around the transverse axis without additional ballast . The pilot can control the filling and emptying through appropriate valves and shift the center of gravity of the entire airship through different degrees of filling of the ballonets.
If the pressure of the carrier gas within the envelope rises above a limit value, the valves open automatically and release air into the environment to give the gas space to expand.
The height at which the ballonets are empty and the lifting gas takes up the entire volume of the envelope is called the rebound height . If an airship rises above its impact height, the overpressure inside continues to rise, since the lifting gas volume can no longer be increased when the external pressure drops. Automatic safety valves then also release the lifting gas to prevent the envelope from tearing.
One of the ballonets usually has a rip cord that allows the balloon to be opened inside the envelope. This is used when the envelope itself has a leak and lifting gas escapes. Then, after opening the ballonet, air can be blown directly into the envelope and the internal overpressure and thus the shape and controllability of the ship can be maintained until it has landed.
Examples
The Zeppelin NT as a semi-rigid airship has two ballonets.
The impact airship currently operated by WDL at Essen / Mülheim Airport has three ballonets with a total of 27% of the envelope volume. This airship is illuminated at night by two internal 50 W LED traffic lights. The partially transparent ballonets then cast sharply contoured shadows on the lower half of the shell, which are clearly visible to the eye. The variation in the degree of filling of the middle ballonet, which lies above the gondola in the envelope, does not change the trim.