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Military


Airship Lift

Any gas or gas mixture that is "lighter than air", and therefore has a lower density (weight per cubic meter) is suitable as a lifting gas. Under standard conditions (0C, 1bar) air has a density of 1.28 kg/m.

Air that has been heated to about 70C has a lower density than atmospheric air. Its density is approximately 1.02 kg/m and therefore significantly higher than that of hydrogen or helium. This results in less lift (only approx. 260g/m). A great disadvantage is that heat has to be added to the air constantly because it cools down and consequently loses lift (density increases with cooling). Hot air is used with small hot air-airships and of course with hot air balloons. A large-scale technical application of hot air for airships is impossible, because high heating costs and the low amount of lift make it uneconomical.

Hydrogen is a low-density gas and easy to produce. It exists as the H2 molecule and is therefore larger than helium. This is relevant for the gas tightness of the airship envelope. Under standard conditions (0, 1bar), one cubic meter of hydrogen has a density of 0.09 kg/m. However, together with oxygen, hydrogen is flammable and in a certain mixing ratio with air can form oxyhydrogen which is highly explosive. Therefore, hydrogen is no longer used for commercial airship operations.

Helium is an inert gas and therefore chemically inert, not flammable and appears as a single atom He. It is found in the earth's crust and is a by-product of natural gas extraction. At 0.18 kg/m, its density is clearly greater than that of hydrogen, but still much smaller than that of air. Because of the small size of the helium atom, it was a problem for a long time to manufacture gas-tight envelope materials. With modern materials it is now possible to better confine the diffusion within the envelope material. CargoLifter envelope material loses less than 1% of helium per year.

Prior to the Great War airships and balloons almost invariably employed Hydrogen. Coal gas is cheaper and more universally available. It is sometimes used for free ballooning, but has a lifting power of only about half that of hydrogen. Helium, although having only 93 % of the lifting power of hydrogen of equal purity, is totally non-flammable and has, therefore, signal advantages for airships exposed to attack with incendiary bullets.

Apart from hostile incendiary action the risk of fire in the air is small and is mainly due to the petrol. It is thought that the use of heavy oil fuel would give added safety. The heavy oil engine involved prohibitive weight, but a Diesel engine capable of burning only 38 lb. of fuel per H.P.-hour would, on the basis of too hrs. flight, justify an increase of machinery weight of 12 lb. per H.P. over the 5 lb. per II.P. of the petrol machinery which burns -5 lb. per H.P. hour.

During a long (light the consumption of petrol so reduces the weight of the ship that, in order to restore her static equilibrium for landing or to avoid the increase of resistance if she is flown very light, it is necessary either to discharge a quantity of hydrogen or to acquire weight. The latter can be done by condensing the steam in the exhaust gas. Petrol produces steam equivalent to some 140% of its weight, and the proportion of this which can be collected depends upon the temperature and humidity of the issuing gas. The chief difficulty in the condensation is due to the fouling of the cooling surfaces with an oily deposit.

Attempts had been made to burn, as supplementary fuel, the hydrogen, which must otherwise be discharged. When burning hydrogen alone in an engine with a compression ratio of about 5:1 it is not possible to develop more than 25 % of the engine's full power without serious detonation. When petrol and hydrogen are burnt together the proportion can be so adjusted that any fraction up to full power can be developed. A few of the smaller airships were fitted in this way but the system was abandoned on account of increased risk of fire.

Buoyant lift is the influence on the weight of a body created when the body density differs from that of the medium surrounding it. A person in the environment, for example, experiences lift that corresponds to the amount of air volume he takes in. This buoyancy lift is utilized in airship aviation. The balloon envelope is filled with a gas (see lifting gas), that has a lower density than air. It is "lighter than air". If, for example, one were to take a balloon that has a volume of 1 cubic meter and fill it with helium, one would be able - under simplified assumptions and deducting the weight of the balloon - to lift approximately 1 kg of service load. (In this example the air density is assumed to be at 1.28 kg/m, the helium density at 0.18 kg/m, the weight of the balloon at approximately 100 g.)

The lift created through flow around bodies is called positive lift. One example is the lifting surface of an airplane. Because of the special profile with a more prominent curvature on the upper side an air particle has to cover a greater distance, and therefore flow around the profile at a higher speed. This creates force which in turn works on the profile i.e. the body. In pre-war airship aviation, positive lift, especially with rigid airships, did not play a big role. Today positive lift is applied selectively. Most airships today start "heavier than air" and also use positive lift for take-off. One aspect of airship operations that is not technically obvious is buoyancy compensation. When an airship takes off with neutral buoyancy the aerostatic lift produced by the helium is equal to the total weight of the vehicle-the combined weight of the structure, payload, and fuel. As fuel is burned en route, however, the total weight of the airship decreases but the aerostatic lift remains the same. If nothing is done, over time the ship will gain significant positive buoyancy. As this is undesirable from both a control and structural viewpoint, the airship must have a mechanism for buoyancy compensation.

Hydrogen-filled airships such as the Graf Zeppelin and Hindenburg simply vented excess hydrogen into the atmosphere to compensate for the weight of fuel burned. This was an acceptable solution because hydrogen was both inexpensive and easily generated wherever the ships were scheduled to land and refuel. Not so for helium, however, which is considerably more expensive and cannot be generated locally. It must be shipped in heavy steel cylinders from where it was originally mined or subsequently stored. Helium-filled airships such as the Akron and Macon were constructed with an apparatus on the engine exhaust to condense and recover the water it contained. The water was then stored to compensate for the weight of fuel burned. While a seemingly elegant solution to the en route buoyancy compensation problem, water recovery apparatus was heavy, at least initially unreliable, and the condensers mounted on the skin of the ship added drag. While the equipment improved over time, "the water recovery problem as a whole remained the bte noire of the helium-inflated rigid airship."

The other aspect of the buoyancy compensation problem occurs when cargo is offloaded at destination. If an airship arrives at a destination with neutral buoyancy and offloads 30 tons of cargo, it immediately has 30 tons of excess lift. For an airship in commercial operations this is addressed by onloading equivalent ballast, either outbound cargo, water, or both, as the inbound cargo is removed. It can be problematic for a military airship however, as there is often no outbound cargo during a buildup at a forward operating base.





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Page last modified: 11-07-2011 15:29:12 ZULU