- Lighter than air
gases are buoyant in airbecause they have densities lower than that of air (about 1.2 kg/m3, 1.2 g/l). Lighter-than-air gases are used to fill craft called aerostats, which include free balloons, moored balloons, and airships, to make the whole craft, on average, lighter than air. (Heavier-than-air aircraftinclude aeroplanes, gliders and helicopters.)
The density of a gas can be reduced by raising its
temperaturewhile leaving the pressureunchanged ( Charles' Law).
Heated air is widely used as a lifting gas in
hot-air balloons. (The gas in a hot-air balloon is not only heated air, but also includes the products of combustionfrom the balloon's burner.)
altitudeof a hot-air balloon is controlled by regulating lift. To increase lift, more heat is applied. To decrease lift slowly, the hot air is allowed to cool. To decrease lift quickly, hot air is vented and replaced with ambient air. Unlike balloons using low-molecular-mass gases (see below), hot-air balloons require a continual source of heat in order to remain aloft.
Low molecular mass gases
Because any given
volumeof any gas at a given temperature and pressure contains nearly the same number of molecules ( Avogadro's law), any gas with a lower molecular massthan that of air will be lighter than air (at the same temperature and pressure).
A sealed balloon expands as it rises because
air pressuredecreases with increasing altitude. As buoyancydepends on the mass of the "displaced" gas (Archimedes' principle) and because air is less dense at higher altitudes, balloons which rise to high altitudes (such as weather balloons) have to be allowed to expand as they climb so as to support the same weight. Weather balloons are made with strong elastic "envelopes" so that they do not burst as they expand.
Determining which gases are lighter than air is relatively straightforward. These gases must have a molecular mass less than 28.97 (the average molecular mass of air) and exist as a gas at atmospheric temperatures and pressures.
atomper moleculeof gas, the heaviest possible atomthat could meet these criteria is silicon, which has an atomic massof 28.1. However, silicon does not become a gas until it reaches a very high temperature. The same applies to the metals aluminum, magnesium, sodium, berylliumand lithiumand the hydrides of these. Carbonand boronhave high boiling points, but methaneand borane(the hydrides of carbon and boron) are lighter than air.
The following is a list of all stable materials with a molecular mass under 28.8 and a
boiling pointunder 100°C. Although isotopes are not considered here, it should be remembered that for the replacement of hydrogen with deuterium (or even tritium) the large relative mass difference can alter some of the properties of that specific gas (e.g. rates of reaction). Therefore ND3 can be considered a gas very different from NH3.
The acronym HAHAMICE was used to help emergency responders remember the gases which are lighter than air. It stood for::H - Hydrogen:A - Ammonia:H - Helium:A - Acetylene:M - Methane:I - Illuminating Gases (old term for natural gas):C - Carbon Monoxide:E - Ethylene
This acronym left out several gases, and was later changed to 4H MEDIC ANNA::H - Hydrogen:H - Helium:H - Hydrogen Cyanide:H - Hydrogen Fluoride
:M - Methane:E - Ethylene:D - Diborane:I - Illuminating Gases:C - Carbon Monoxide
:A - Acetylene:N - Neon:N - Nitrogen:A - Ammonia
As is noticeable, the acronyms do not include all gases lighter than air.
Many of these gases are not practical for use in balloons. The following combine poor lift with objectionable properties: carbon monoxide, hydrogen cyanide, hydrogen fluoride, diborane, ethylene and acetylene. Nitrogen has negligible lift. Neon is harmless and offers a modest degree of lift; however it costs roughly the same as helium, another noble gas with far superior lift. The four remaining gases (ammonia, methane, helium, and hydrogen) have been used as balloon gases.
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. Both provide about 9.8 N of lift (the force to lift 1 kg) per cubic meter of gas at room temperature and
sea levelpressure. Helium is preferred because it is not combustible.
The relative lifting power of hydrogen and helium can be calculated using the theory of
Thus helium is almost twice as dense as hydrogen. However, buoyancy depends upon the "difference" of the densities (ρgas) - (ρair) rather than upon their ratios. Thus the difference in buoyancies is about 8%, as seen from the buoyancy equation:
* Buoyant mass (or effective mass) = mass × (1 - ρair/ρgas)
* Therefore the buoyant mass for one liter of hydrogen in air as:
** 0.08988 g * (1 - (1.292 / 0.08988) ) = -1.202 g
* And the buoyant mass for one liter of helium in air as:
** 0.1786 g * (1 - (1.292 / 0.1786) ) = -1.113 gThe negative signs indicate that these gases tend to rise in air.
Thus hydrogen's additional buoyancy compared to helium is:
* 1.202 / 1.113 = 1.080, or approximately 8.0%
Many countries have banned the use of hydrogen as a lift gas for manned vehicles. The
Hindenburg disasteris frequently cited as an example of the risks posed by hydrogen. The high cost of helium (compared to hydrogen) has led researchers to reinvestigate the safety issues of using hydrogen as a lift gas: with good engineering and good handling practices, the risks can be significantly reduced.Fact|date=July 2008
Although pure water is not a gas at room temperature and sea level pressure, water in the vapor phase mixes readily with dry air, as do any two gases, until the
partial pressureof the water vapor reaches the saturation water vapor pressureat the current temperature. Such moist air is lighter than dry air at the same temperature, because the molecular mass of water is lower than the average molecular mass of dry air. Most hot air balloons burn propane (or some other hydrocarbon) to provide heat; the combustion products have an average molecular mass of 29.1; the "light" water vapor more or less compensates for the "heavy" carbon dioxide. Pure water vapor (steam) can be used to lift balloons; however, the question of condensationmust be addressed somehow. One route would be simply to tolerate the condensation - this supposes a rather large balloon. Alternatively, the balloon could provide insulation (for example have a double-walled structure) or the water vapour (if pure) could be maintained at least at the boiling point of water at the altitude of use (100°C at sea level; less higher) by a heating device. [There are intriguing possibilities in using a mixture of air and water vapor, at a temperature high enough for the water component to remain as vapor.] Two research efforts are currently underway to build steam-filled balloons (see external links below), taking rather different approaches; both of them have succeeded in practical demonstration of steam as a lift gas.
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, to avoid crushing by
atmospheric pressurewould require materials far stronger than any now availableFact|date=July 2008 (see unobtainium).
Hot air balloon
* [http://www.centennialofflight.gov/essay/Lighter_than_air/LTA-OV.htm Lighter-than-air - An overview]
* [http://www.blimpinfo.com/ The Lighter-Than-Air Society]
* [http://www.airship-association.org/ Airship Association]
* [http://www.flyingkettle.com/ The Flying Kettle Steam Balloon Project]
* [http://www.heidas.de/pages/MainEng.html Steam Balloon Project]
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Look at other dictionaries:
lighter-than-air — [līt′ər than er′] adj. designating or of an airship or a balloon filled with a gas lighter than air … English World dictionary
lighter-than-air — light′er than air′ adj. 1) aer. (of an aircraft) weighing less than the air it displaces, hence obtaining lift from aerostatic buoyancy 2) aer. of or pertaining to lighter than air craft • Etymology: 1900–05 … From formal English to slang
lighter-than-air — /luy teuhr dheuhn air /, adj. Aeron. 1. (of an aircraft) weighing less than the air it displaces, hence obtaining lift from aerostatic buoyancy. 2. of or pertaining to lighter than air craft. [1900 05] * * * … Universalium
lighter-than-air — weighs less than air, has very little weight, is very light … English contemporary dictionary
lighter-than-air — /laɪtə ðən ˈɛə/ (say luytuh dhuhn air) adjective 1. (of aircraft) of less specific gravity than the air. 2. of or relating to such aircraft … Australian English dictionary
lighter-than-air — adjective Date: 1903 of less weight than the air displaced … New Collegiate Dictionary
lighter-than-air — adjective relating to a balloon or other aircraft that flies because it weighs less than the air it displaces • Similar to: ↑light • Topics: ↑aircraft … Useful english dictionary
lighter-than-air aircraft — An aircraft that is supported by its buoyancy in the air. Balloons are one of the categories of lighter than air aircraft … Aviation dictionary
lighter-than-air craft — noun aircraft supported by its own buoyancy • Hypernyms: ↑aircraft • Hyponyms: ↑airship, ↑dirigible, ↑balloon … Useful english dictionary
Air pollution dispersion terminology — describes the words and technical terms that have a special meaning to those who work in the field of air pollution dispersion modeling. Governmental environmental protection agencies (local, state, province and national) of many countries have… … Wikipedia