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Buoyancy compensator (aviation)

The meaning of «buoyancy compensator (aviation)»

The static buoyancy of airships in flight is not constant. It is therefore necessary to control the altitude of an airship by controlling its buoyancy: buoyancy compensation.

For example, on a flight from Friedrichshafen to Lakehurst, the rigid airship LZ 126, built in 1923-24, used 23,000 kg gasoline and 1300 kg of oil (an average consumption of 290 kg/100 km). During the landing the airship had to release approximately 24,000 cubic meters of hydrogen to balance the ship before landing it. A Zeppelin of the size of the LZ 129 Hindenburg on a flight from Frankfurt am Main to Lakehurst consumed approximately 54 tonnes of diesel with a buoyancy equivalent of 48,000 cubic metres of hydrogen, which amounted to about a quarter of the lifting gas used at the start of the flight (200,000 cubic metres). After the landing, the jettisoned hydrogen was replaced with new hydrogen.

The Zeppelin NT has no special facilities to offset the extra buoyancy by fuel consumption. Compensation takes place by using a start-weight that is higher than the buoyancy lifting level at the start and during the flight, the extra dynamic buoyancy needed for lift-off and flight is produced with engines. If, during the trip, the ship becomes lighter than air because of fuel consumption, the swivel engines are used for down pressure and landing. The relatively small size of the Zeppelin NT and a range of only 900 kilometers compared to the historical Zeppelins allowed the waiver of a ballast extraction device.

With a rigid airship two main strategies are pursued to avoid the venting of lifting gas:

Only gasses have a density similar or equal to air.

Different attempts were made on hydrogen airships: the LZ 127 and LZ 129 to use part of the lifting gas as a propellant without much success, later ships filled with helium lacked this option.

Around 1905, Blau gas was a common propellant for airships; it is named after its inventor the Augsburger chemist Hermann Blau who produced it in the Augsburger Blau gas plant. Various sources mention a mixture of propane and butane. Its density was 9% heavier than air. Zeppelins often used a different gas mixture of propylene, methane, butane, acetylene (ethyne), butylene and hydrogen.[3]

The LZ 127 Graf Zeppelin had bi-fuel engines, and could use gasoline and Blau gas as a propellant. Twelve of the vessel's gas cells were filled with a propellant gas instead of lifting gas with a total volume of 30,000 cubic metres, enough for approximately 100 flight hours. The fuel tank had a gasoline volume of 67 flight hours. Using both gasoline and Blau gas, one could achieve 118 hours of cruise time.

In some airships rain gutters were fitted to the hull to collect rainwater to fill the ballast water tanks during flight. However, this procedure is weather dependent and is therefore not reliable as a standalone measure.

Captain Ernst A. Lehmann described how during World War I Zeppelins could temporarily remain at the sea surface by loading ballast water into tanks in the gondolas.[4] In 1921 the airships LZ 120 "Bodensee" and LZ 121 "Nordstern" tested the possibility on Lake Constance to use lake water to create ballast. These attempts, however, showed no satisfactory results.

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