Buoyancy compensator (aviation)
The static buoyancy of airships during a trip is not constant. It is therefore necessary to take measures to control the buoyancy and thus the altitude, the so-called buoyancy compensation.
Changes which have an effect on buoyancy
- Changes in air temperature (and thus the density of air)
- Changes in the lifting gas temperature (for example by heating of the hull by the sun).
- Accumulation of additional ballast (for example, precipitation or icing on the envelope)
- Changes in ballast (for example, during a flight maneuver or the dropping of ballast)
- Changes by consumption of fuel, especially in the large historic airships like the Zeppelins the problem of change in the buoyancy balance by consumption of fuel needed attention.
For example, the LZ 126 spent on the flight from Friedrichshafen to Lakehurst 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 to land it. An airship with the size of the LZ 129 Hindenburg spent on a flight from Frankfurt am Main to Lakehurst approximately 54 tonnes of diesel with a buoyancy equivalent of 48,000 cubic metres hydrogen which amounted for about a quarter of the used lifting gas at the start of the flight (200,000 cubic metres). After landing the jettisoned hydrogen was replaced with new hydrogen.
Compensation measures
- Particular use of the dynamic buoyancy, see lift and drag.
- Increasing buoyancy by dropping ballast. This is done mostly by the jettisoning of ballast water similar to the dropping of sandbags in ballooning.
- The reduction of buoyancy by jettisoning liftgas or adding ballast.
- Changing the density of the lifting gas by heating (more buoyancy) or cooling (less buoyancy).
- The use of vacuum/air buoyancy compensator tanks[1]
- The use of thrust vectoring using ducted fans or propellers.
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 gets lighter than air caused by 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.
Buoyancy compensation
With a Zeppelin two main strategies are pursued to avoid the jettisoning of lifting gas:
- 1. The use of a fuel with the same density as air and therefore no increase in buoyancy caused by consumption.
- 2. Adding water as ballast by extraction during the trip.
Fuel with a density close to air
Only gasses have a density similar or equal to the air.
Hydrogen
Different attempts were made on hydrogen airships, like 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 the option.
Blaugas
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 liquefied propane and butane. In density it was 9% heavier than air. The Zeppelins used a different gas mixture of propylene, methane, butane, acetylene (ethine), butylene and hydrogen[2].
The LZ 127 Graf Zeppelin had bi-fuel engines and could use gasoline and gas as a propellant. Twelve of the 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 could give 118 hours cruise.
Water as ballast
Dew and rainfall on the hull
In the airships LZ 127 Graf Zeppelin and LZ 129 Hindenburg rain gutters were attached to the trunk to collect rainwater to fill the ballast water tanks during the trip. However, this procedure is weather dependent and is therefore not reliable as a standalone measure.
Water from the ground
Captain Ernst A. Lehmann described how during World War I Zeppelins would land on the sea and pick up temporary ballast water.[3] In 1921 the airships LZ 120 "Bodensee" and LZ 121 "Nordstern" tested the possibillity on Lake Constance to use lake water to create ballast. These attempts, however, showed no satisfactory results.
Silica-gel method
The silica gel method was tested on the LZ 129 to extract water from the humid air to increase weight. The project was terminated.
Water from fuel combustion
The most promising procedure for ballast extraction during the journey is condensation of exhaust gasses from the engines which consist mainly of water (steam) and carbon dioxide. The main factors affecting gainable water are the hydrogen content of the fuel and humidity. The necessary exhaust gas coolers for this method had repeatedly problems with corrosion in the early years.
The first trials on the DELAG -Zeppelin LZ 13 "Hansa" (1912-1916) were conducted by Wilhelm Maybach. The trials were not satisfactory, resulting in an abandoned project.
The United States Navy reports the USS Shenandoah (ZR-1) (1923-25) , a helium-filled rigid airship, as the first airship with ballast water from the condensation of exhaust gas. The LZ 126/ZR-3 USS Los Angeles was refitted with helium as a lifting gas after arrival in the U.S. Exhaust gas coolers were used to prevent jettisoning of the costly helium.
Lifting gas temperature
Changes in the lifting gas temperature in relation to the surrounding air have an effect on the buoyancy balance: higher temperatures increase buoyancy; lower temperatures descrease buoyancy. Artificially changing the lifting gas temperature requires constant work as the gas is barely thermally isolated from the surrounding air. However, it was common to make use of natural differences in temperature such as thermal updrafts and clouds.
Preheated lifting gas
Preheated lifting gas was tested to offset the higher weight of the Zeppelin. One variation tested on the LZ 127 Graf Zeppelin was to blow heated air on the lifting gas storage cells with the aim to gain buoyancy for launch.
See also
References
- ^ Walrus
- ^ Gas Fuels for Airships: The manufacture of blau gas, with details of some possible alternatives doi 10.1108/eb029368
- ^ Lehmann, Ernst A.; Mingos, Howard. The Zeppelins. The Development of the Airship, with the Story of the Zepplins Air Raids in the World War. Chapter VI THE NORTH SEA PATROL -- THE ZEPPELINS AT JUTLAND "A sea anchor is cast out and ballast tanks in the cars, which are almost as seaworthy as boats, are filled with water"