Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
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 1 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. 2. 3. 4. 5. 6. 7. 8. 9. Surveillance radars of all sizes and capabilities Signal Intelligence (SIGINT) collection equipment Gyro-stabilized daylight 3 low-light level and infra-red video cameras Direct television broadcast and relay FM radio broadcast and relay VHF/UHF Ground Control Intercept (GCI) Microwave communications, and Environmental monitoring equipment. 2 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. 3 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. 4 5 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. 6 4)Airships Free-flying buoyant aircraft that can be propelled and steered. Some airships obtain aerodynamic lift via the shape of their envelope or through the addition of fins or other shape. These types of craft are called hybrid airships. 7 Usage as lifting gas Lighter than air refers to gases that are buoyant in air because they have densities lower than that of air (about 1.2 kg/m3, 1.2 g/L). Some of these gases are used as lifting gases in lighter-than-air aircraft, which include free balloons, moored balloons, and airships, to make the whole craft, on average, lighter than air. Hot air Hot air is frequently used in recreational ballooning. Hot air is lighter than air at ambient temperature. Neon Neon is lighter than air and will lift a balloon. However, it is relatively rare on Earth, expensive, and is among the heavier of the lifting gases. Water vapor The gaseous state of water is lighter than air, and has successfully been used as a lifting gas. It is generally impractical due to high boiling point and condensation. Ammonia 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 8 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 smallmolecule hydrogen and helium. (Many 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. In a practical dirigible design the difference is significant making a 50% difference in the fuel carrying capacity of the dirigible and hence increasing its range significantly. 9 Effect of envelope shape on payload capacity The envelope shape affects the payload capacity in the following ways 1) Surface Area: The envelope weight is decided by the Total Surface Area (TSA) of the envelope. Wenv = TSA * ρmatl where ρmatl = density of the envelope material For a fixed volume, the surface area varies greatly with the shape. It is widely known that the minimum surface area for a given volume is obtained for a spherical shape. 2) Envelope Stress: The difference in internal and external pressure on the aerostat envelope generates stress on the membrane. For a given pressure difference, the stress is a function of the envelope shape. If the stress is low, a material of low ultimate strength which is expected to be lighter can be used. On the other hand for a higher stress, a stronger material which is expected to be heavier (higher ρmatl) will have to be used to make the envelop. Thus shape directly influences the self weight of the aerostat. 3) Fin Weight: The envelope shape decides the aerodynamic force and moments generated on the envelope. The size of fins required to trim the aerostat at a given angle of attack and to provide the required stability is thus decided by the shape of the aerostat. 10 4) Tether weight: The effect of shape on drag has been well established through past studies. Drag causes blow by on the aerostat causing it to draw a longer tether for the same height of operation. Thus the weight of the tether supported by the aerostat increases for a shape causing higher drag. This additional tether weight is carried at the expense of useful payload weight. To increase the payload capacity, it is thus necessary to reduce the drag on the aerostat. Drag on aerostat The ambient wind on the aerostat produces drag which tends to displace it along the direction of flow. This displacement is called blowby. Blow-by reduces operational height and may also give rise to functional disadvantages depending on the application eg:- it produce errors in station keeping. To maintain the height of operation, a longer tether will have to be released at the expense of a decrease in payload capacity. Therefore a low coefficient of drag is an essential requirement of an aerostat envelope. The necessity of low drag also places demands on the trim angle and stability margin of the aerostat. Since CD increases with angle of attack, it is essential to keep the angle of attack for the aerostat as low as possible. It is also necessary to keep the static margin high so that the aerostat shows quick response to wind disturbances and the angle of attack is maintained. 11 CONCLUSION The project is based on the procedure given in the design project books. It includes designing of introduction, types of aerostat, light weight air, etc. The project contain detailed layout of literature survey are provided. Assessment of hard ware and soft ware requirements and initial purchase of materials and fabrication towards the realization of the project. This will meet the current day requirements; these performance statements say that it is suitable to meet demands of airlines. These operational statements say that safety level is high. 12 References Amool A Raina, Rajkumar S. Pant “Design and Shape Optimization of Aerostat Envelopes”, Presented at 10th AIAA, Indian Institute of Technology Bombay, Mumbai Maharashtra, 400076, India 2. Lutz, T., and Wagner, S., “Drag Reduction and Shape Optimization of Airship bodies,” Journal of Aircraft, Vol. 35, No. 3, May- June 1998, pp 345-351. 3. Kanikdale, T. S., Marathe, A. G., and Pant, R. S., “Multidisciplinary Optimization of Airship Envelope Shape”, AIAA-2004-4411, Presented at 10th AIAA/ISSMO Multidisciplinary Analysis and Optimization Conference, Albany, USA, 2004 4. Kale, S. M., Joshi, P., and Pant, R. S., “A Generic methodology to estimate drag on an aerostat envelope”,AIAA-2005-7442, presented at 5th ATIO Conference, 16th LTA and Balloon Systems Conference, Arlington, Virginia, USA, September 2005. 1. Net source 1. http://www.public.iastate.edu/~zjw/papers/AIAA-2005-2968.pdf 2. http://www.ilcdover.com/products/aerospace_defense/supportfiles/ AIAA2003-6630.pdf 3. http://www.unols.org/publications/winch_wire_handbook__3rd_ed /10_single_drum_winches.pdf 13