Download Drag on aerostat

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
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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
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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.
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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.
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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.
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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.
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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.
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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
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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.
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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.
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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.
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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.
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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.
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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
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