International Journal of Scientific & Engineering Research, Volume 3, Issue 11, November-2012 1
ISSN 2229-5518
AEROSTATS- An Insight into the Artificial Eagle
Eye in Sky
Pramod Murali Mohan
Abstract— This paper is intended towards giving an insight into the advent of aerostat which is an artificial eagle eye in the sky. The basic theory, components of an aerostat system, aerostat balloon, the main parameters governing in designing an aerostat and its application are being discussed. This sheds the light on the use of aerostat and it’s dominance in coming era due to its low cost, unmanned and it’s unique ability to stay aloft days together.
Index Terms— Aerostat, balloon, Co-efficient of drag, Co-efficient of friction, drag, Lighter than air, payload
—————————— ——————————
One of the enthusiastic moments for everyone is looking at the sky. A tiddler from small age group would rejoice looking at the balloon floating in the air. One wonders why a balloon floats. It’s mainly due to the density difference between the gas filled in the ballon and the surrounding air. Lighter things tend to float in the air due to lift.
Airships also named as lighter than air (LTA) aircrafts
T is absolute temperature, K R is universal gas constant.
In aerostat the buoyancy is produced by gas which has least molecular mass. Air density depending on atmospheric condi- tions, season and altitude varies. Furthermore as per Interna- tional Standard Atmosphere, the state of atmospheric is de- termined by [1]
which came to light in the nineteenth century. One of the vehi- cles that are drawing attention and attraction is an Aerostat,
P H
1
5.256
which is big in size, low speed airship.
Aerostats are unmanned, aerodynamically shaped blimps
P
44300
4.256
(4)
P H
that are buoyed aloft, tethered to the ground by a single cable.
1
The aerostat is made of a large single fabric envelope that is
filled with lighter than air gases which provides the lifting forc-
es.
P
44300
(5)
These offer the next cutting edge technology of UAVs in lighter than air-aircraft segment. This has stricken a tryst in de- ploying it for various applications.
An aerostat balloon is an aerodynamically shaped body which is tethered to ground. It is filled with lighter than air gases and thus results in lift due to buoyancy. According to Archimedes principle, the lift FL is caused by the difference between densi- ties of air ρa and lifting gas ρg,
FL V * (a g) V * , Kg
Where V is volume of gas in aerostat (m3),
ρa, ρg are density of air and gas (Kg/m3),
Δρ is specific buoyancy of 1m3 of gas.
(1)
————————————————
The denities of air and gas can be determined by Ideal gas equation given by
Fig. 1. Components of Aerostat
Fig. 1. Show the various components of an aerostat system. They are: 1. Aerostat 2. Tether 3. Mobile mooring system 4. Mission payloads 5. Ground control station 6. Maintenance
PV RT
i.e. P RT
P is atmospheric pressure (Kg/m2)
ρ is density of gas (Kg/m3)
(2)
(3)
and officer station 7. Power generators and site handling equipments.
The aerostat balloon is tethered to a mobile mooring plat- form via winch mechanism. The payloads are carried by aero-
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International Journal of Scientific & Engineering Research, Volume 3, Issue 11, November-2012 2
ISSN 2229-5518
stat balloon. These payloads are communicated to and fro from the ground control station. The maintenance station will play a role of storage and depot/shop replacement of LRUs. The required power is supplied by the power generating sta- tion.
V = Wind velocity in m/s.
The pressure drag is governed by the shape of the aerostat.
Co-efficient of drag Cd is given by:
(8)
Cd (0.172 * (l / d )1 / 3 0.252 * (d / l)1.2 1.032 * (d / l)2.7 ) / Re1 / 6
e pressure drag is given by:
Dp 1 / 2 * *V 2 *Volume 2 / 3
Volume of oblong ellipsoid of revolution is given by:
Th
(9)
Volume = (4/3)*π*(l/2)*(d/2)2 (10) Let us consider an aerostat, i.e. Akron aerostat from Lock-
heed Martin. Where d=40.5m and l=238.95m.
Therefore volume = 205571.3433 m3.
The density of air, ρ = 1.1839 Kg/m3 at 250C.
0.14
0.12
0.1
Fig. 2. Aerostat Balloon
Aerostat balloon is an important component which carries payload filled with ‘Lighter than air’ gas and generates lift due to buoyancy. The main requirements of an aerostat balloon are high payload capacity, low blow-by, sufficient stability and fast response to winds. The total lift that is produced by buoy- ancy and aerodynamic forces is balanced by the weight of the aerostat, the tether force and payload. The buoyancy solely depends on the volume of LTA gas contained in the envelope.
0.08
0.06
0.04
0.02
0
Co-eff of friction
Co-eff of
Drag
Reynold's No.
The weight of the envelope depends on its total surface ar- ea and the density of the material that is used for manufactur- ing the balloon. Thus, to reduce the weight of the aerostat, its surface area should be reduced.
Fig. 2. Depicts the various parts of an aerostat balloon. The parts are as follows: 1. Mooring Point, 2. Bow stiffening, 3. Rip panel, 4. Flexible envelope, 5. Fin, 6. Ballonets, 7. Controlled valves, 8. Stabilizer, 9. Elevators, 10. Mounting foot, 11. Moor- ing lines, 12. Safety valves, 13. Cords, 14. Gondola, 15. Valve,
16. Fan, 17. Air duct, 18. Power plant, 19. Safety valve, 20.
Rudder, 21. Appendix.[1,2,5]
An aerostat is in flight due to the generation of aerodynamic lift and drag. The drag consists of three components: 1. The
Fig. 3. Co-efficient of Friction and drag vs Reynold's number for Ak- ron aerostat.
The co-efficient of friction and drag depends on the geo- metric parameters of the aerostat and the reynold's number. The fig. 3. shows the variation of co-efficient of friction and drag versus the reynold's number. The co-efficient stabilizes as the reynold's number increases. It can be observed that the co- efficient of drag is greater than the co-efficient of friction thus attributing that drag plays a vital role.[3,4,6,7]
40000000
35000000
30000000
25000000
pressure drag, 2. The friction drag, 3. The induced drag.
The pressure drag is due to the air displaced as the wind
rushes past the aerostat balloon. The friction drag is due to no- slip condition in the boundary layer of the flow around the
aerostat. The induced drag is very small and can be neglected.
The co-efficient of friction is given by:
20000000
15000000
10000000
5000000
0
Friction Drag (N) Pressure Drag (N)
velocity in
Cf 0.043 Re1/ 6
(6)
0 10 20 30 40 50
m/s
The friction drag is given by:
Fig. 4. Drag Forces vs Wind velocity
Df 1 2 * *V 2 * Swetted
(7)
Where Swetted = 2.33*d*l,
d= diameter of oblong aerostat balloon in m,
l= length of the aerostat balloon in m,
ρ = density of air in kg/m3,
The drag forces due to friction and pressure are depended on the wind velocity and the geometric parameters of aerostat. The fig. 4. Shows the variation of drag force in Newton versus
IJSER © 2012
International Journal of Scientific & Engineering Research, Volume 3, Issue 11, November-2012 3
ISSN 2229-5518
the wind velocity in m/s. As the wind velocity increases, due to no-slip at the boundary condition the drag force due to fric- tion is higher than the pressure drag. This shows that the drag force due to friction has to be reduced by optimizing the shape of the balloon to achieve greater lift. As the lift forces increas- es, the aerostat can attain higher altitudes during flight. Dur- ing this optimization, care has to be taken to optimize the weight of the aerostat including payload to withstand the wind blow-down.
The most efficient means to meet the needs of airborne appli- cation is a low cost, mobile and flexible system. Mobile towers are one of the means to fulfill these needs. But these are height limited, providing only short range coverage.
To fulfill all these needs and overcome the shortcoming
posed by the mobile system, aerostats are best employed and
utilized to overcome all issues. Here are few applications ex- plained below:
1. Air sampling and research: An aerostat is used to setup a research instrument that is designed to collect and store air sample, transmit and record pertinent data and provide a se- cure and sustainable research platform for atmospheric re- search.
This is to achiev an autonomous, wireless, mobile, reliable
and precise air sampling and data collection. [8]
2. Radio Communication: An aerostat acts as a tall antenna
tower for radio communications over long distances and rug-
ged terrain. The payloads are used for tactical communication,
emergency communication and disaster recovery communica- tions. The system connects antennas on an aerostat to multiple
radios on the ground. This system connects distant antennas to radios inside a secure compartmented information facility. This type of system enables persistent, inexpensive wide area radio communications. [3]
3. Disaster Relief: During the times of earthquake, floods etc. The transportation systems to the affected areas are ac- cesible. This leads to the use of other means, to supply basic necessities for people. Aerostat's play a vital role in manuever- ing along the blocked paths and roadways to rescue people, reduce casualities and losses. It helps in supplying water, medicines, equipment and supplies needed by relief workers.
These also act as surveillance means which stays over a
long period of time as compared to that of helicopter. [9]
4. Construction: Aerostat can also be used as construction
equipment. The stability of flight provided by aerostats allows
them to move and deliver construction components for more
maneuverability and efficiency for construction.
5. Aerial Early Warning: Aerostat assures crucial airspace
dominance, providing early warning and airspace control. It
generates high quality air picture including timely full and
accurate data with the help of payloads. [9]
6. Anti-terror/ Homeland Security: Aerostats enhances
homeland security against acts of mass terror. It provides im- proved airspace coverage. It helps in building up database
over time for identification of irregular events. It also provides increased co-ordination betweeen surveillance and response
units.
7. Border and Maritime surveillance: Aerostats are biggest
asset in this application as they provide cost effective solutions
for wide area surveillance and detection. It helps in monitor- ing border areas and maritime surveillance.
8. Military Intelligence: It supports collection and intercep- tion of a variety of data types and wavelength. Helps in build- ing up of intelligence database to support tactical and strategic decisions.
Other applications include up-to-the minute aerial updates of a disaster scene for emergency responders, detection of nu- clear radiation and chemical agent, oil spills, forest fires, crash sites, crowds, well equipped for night vision and stealth sur- veillance.
Aerostats are lowest total cost of ownership of any alternative airborne surveillance method. It involves lower capital in- vestment, less personnel required to operate,, less cost to maintain and operate. It involves very few mechanical parts, low fuel requirements.
It has longest duration and loiter time than any other type
of platform. Aerostats are easier and less costly to train and operate than UAVs, fixed wing or rotary wing aircraft. It has less risk of public damage or civilian injury from a system failure.
Aerostats are like an artificial eagle eye in sky and most en- vironmental friendly method of aerial surveillance. It is one of the mobile and flexible systems with quick deployment, long mission duration with wide applications.
[1] L. KONSTANTINOV, “The Basics of Gas and Heat Airship Theo- ry”, unpublished.
[2] Jonathan I. Miller∗ and Meyer Nahon†, Analysis and Design of Robust
Helium Aerostats, McGill University, JOURNAL OF AIRCRAFT Vol. 44, No. 5, September–October 2007
[3] Vinit N. Gawande*, Prakhil Bilaye*, Amol C. Gawale†, Rajkumar S. Pant‡ and Uday B. Desai§ , Design and Fabrication of an Aerostat for Wireless Communication in Remote Areas, Indian Institute of Technology Bombay, American Institute of Aeronautics and Astronautics
[4] Ir. J. Breukels, Kite launch using an aerostat, Delft University of Technol- ogy , Faculty of Aerospace Engineering, The Netherlands
[5] Amool A. Raina*, Amol C. Gawale†, Rajkumar S. Pant‡, Design, Fabrication and Field Testing of an Aerostat system, L-T-A Systems Laboratory, Aerospace Engineering Department Indian Institute of Technology Bombay, India, 400076
[6] Sagar M. Kale* , Pankaj Joshi† and Rajkumar S. Pant‡, A generic method-
ology for determination of drag coefficient of an aerostat envelope using
CFD, Indian Institute of Technology Bombay, Mumbai, Maharashtra, 400
076, India, American Institute of Aeronautics and Astronautics.
[7] C. Vijay Ram, Rajkumar S. Pant, Multi-disciplinary Shape Optimi- zation of Aerostat Envelopes, American Institute of Aeronautics and Astronautics
[8] Kosta Grammatis, A Remote data acquisition and air sampling system for quantitative and qualitative air quality research, Balloon Project
2004-2007.
[9] LIGHTWEIGHT AEROSTAT SYSTEM (LAS), A New Concept In
Security Surveillance and Communications Relay, Carolina Unmanned
Vehicles, 9 January 2009, Unclassified.
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