Lighter than air refers to gases which are buoyant in air because they have densities lower than that of air (about 1.2 kg/m, 1.2 g/l). Some of these gases are used as lifting gases in lighter-than-air craft, which include free balloons, moored balloons, and airships, to make the whole craft, on average, lighter than air. (Heavier-than-air aircraft include aeroplanes, gliders and helicopters.)
Usage as lifting gas
Ammonia has sometimes been used to fill weather balloons. Due to its relatively high boiling point (compared to helium and hydrogen), ammonia could potentially be refrigerated and liquified aboard an airship to reduce lift and add ballast (and returned to a gas to add lift and reduce ballast).
Methane (the chief component of natural gas) is sometimes used as a lift gas when hydrogen and helium are not available. It has the advantage of not leaking through balloon walls as rapidly as the small-moleculed hydrogen and helium. (Most lighter-than-air balloons are made of aluminized plastic that limits such leakage; hydrogen and helium leak rapidly through latex balloons.)
Hydrogen and helium
Hydrogen and helium are the most commonly used lift gases. Although helium is twice as heavy as (diatomic) hydrogen, they are both so much lighter than air that this difference is inconsequential. Hydrogen has about 8% more buoyancy than helium (as shown here). Both provide about 9.8 N of lift (the force to lift 1 kg) per cubic meter of gas at room temperature and sea level pressure. Helium is preferred because it is not combustible.
First proposed by Italian monk Franceso de Lana in 1670, the vacuum balloon would be the ultimate expression of displacement lift power. A frequent topic of blue sky thinking, the basic principle has remained the same: A container strong enough to preserve a vacuum that displaces sufficient air to lift the container and an additional load. However, construction of a container strong enough to withstand atmospheric pressure while at the same time being lighter than the air it displaces is currently far from the realm of possibility given current technology (see unobtainium). Although this is perhaps an example of linear thinking: plasma windows could be used to contain a vacuum. Less exotically, magnetic fields could be used to keep the balloon from collapsing. A vacuum balloon would be able to lift 1.292 (g/L) at sea level making it 16% more efficient than helium and 7% more efficient than hydrogen.
Published in July 2009.
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