17-12-2012, 06:33 PM
aerostat
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INTRODUCTION
An aerostat is a craft that remains aloft primarily through the use of buoyant lighter than air gases, which impart lift to a vehicle with nearly the same overall density as air. Aerostats include free balloons, airships, and moored balloons. An aerostat's main structural component is its envelope, a lightweight skin containing a lifting gas to provide buoyancy, to which other components are attached. Aerostats are so named because they use "aerostatic" lift which is a buoyant force that does not require movement through the surrounding air mass. This contrasts with aerodynes that primarily use aerodynamic lift which requires the movement of at least some part of the aircraft through the surrounding air mass.
An aerostat is a pressurized, completely flexible structure. Its hull is filled with the inert lighter-than-air, non-burning gas helium. Inside the lower part of the hull is an air compartment called a ballonet. An automatic system of sensors, switches, blowers and valves controls the super-pressure within the hull to maintain the external aerodynamic shape. There is associated power and housekeeping equipment. The hull is an aerodynamically-shaped balloon, fabricated from a high-strength multi-layer fabric and designed for long term use in all types of environments. Thermally bonded together, the completed flexible structure exhibits an exceptionally low helium loss rate. The multi-layer laminate provides significant resistance to ultraviolet radiation, chemicals and oxidation, while offering a field-proven life expectancy of 10 plus years.
An aerostat is an aerodynamically shaped tethered body, belonging to the family of Lighter-than-air vehicles. Aerostat envelopes are filled with a „lighter than air‟ gas (which is Helium or Hydrogen in most cases) and thus generate lift due to buoyancy. The envelope is gimbaled at the tether confluence point, so that it can freely align with the direction of the ambient wind. Adequately sized fins are provided on the envelope to impart it stability during wind disturbances. Payloads in modern day aerostats are usually radars, surveillance cameras or communication equipment. In order to deploy more sophisticated equipment on Aerostats, it is always desirable to increase their payload capacity, without compromising on their operating altitude. This paper also provides details of a methodology for arriving at the optimum shape of the envelope of an aerostat, keeping in mind the aerodynamic and structural considerations, while incorporating some constraints imposed from manufacturing considerations.
Aerostats have been successfully employed by commercial companies to carry payload such as
1. Surveillance radars of all sizes and capabilities
2. Signal Intelligence (SIGINT) collection equipment
3. Gyro-stabilized daylight
4. 3 low-light level and infra-red video cameras
5. Direct television broadcast and relay
6. FM radio broadcast and relay
7. VHF/UHF
8. Ground Control Intercept (GCI)
9. Microwave communications, and Environmental monitoring equipment.
Types of aerostat
1)Moored balloon
Systems that are connected to the surface via one or more tethers. In contrast to the other types of aerostats, moored balloons are non-free flying. A notable example of moored balloons are barrage balloons. Some moored balloons obtain aerodynamic lift via the contours of their envelope or through the use of fins. Moored balloons are also used for sight seeing and advertising. Aerophile SA has made the first one in 1994 and have sold so far more than 50 of them in 25 countries becoming the world's largest lighter than air carrier ever with 300 000 passengers flown every year.
2)Helikite
A trademarked name given to a patented combination of a helium balloon and a kite to form a single, aerodynamically sound tethered aircraft, which exploits both wind and helium for its lift. The attached balloon is generally oblate-spheroid in shape although this is not essential. A Helikite is not a moored balloon, because a Helikite is not a balloon. A Helikite is a tethered aerostat. The US Customs classifies a Helikite as "other non-powered aircraft" not as balloons. The British Civil Aviation Authority's Air Navigation Order gives Helikites its own classification as "Helikites" as opposed to "kites" and "balloons". A Helikite is not just a kite because Helikites fly in nil wind and kites need wind to fly. A Helikite is not just a balloon because Helikites can fly even if weighed down to be heavier than air whereas balloons will never fly if heavier than air. A Helikite is a new type of tethered aerostat with its own official classification. Trials have shown that Helikites fly to greater altitudes than tethered balloons and in far higher winds. They stay stationary and steady in the air in more conditions and for longer than any other type of aerostat. If the word aerostat comes from the Greek "aer" + "statos" then Helikites are a pure form of aerostat.
3)Free balloons
Free-flying buoyant aircraft that move by being carried along by the wind. Types of free balloons include hot air balloons and gas balloons.