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Transcript of gnana-atio11- 07
Design-Build-Fly of OSIRCA: Oblate Spheroid Indoor Remotely Controlled Airship
K.Gnanaprakash1, K.P.Kamalraj1, T.A.Aswath Saravanan1, C.Gopalakrishnan1and D. Balaji2
Hindustan College of Engineering, Chennai, India
and
Rajkumar S. Pant3
Indian Institute of Technology Bombay, Mumbai, India
Introduction
It is well known that a remotely controlled airship is perhaps much more viable than a remotely
controlled aircraft for missions involving prolonged aerial observation of any activity occurring on the
ground, (e.g. aerial coverage of Sporting events, or security patrolling), long endurance loiter, product
promotion, or precise dropping of payload (e.g., showering of flowers during religious and confetti
during inauguration/closing ceremonies). Airships can also be gainfully utilized in remote sensing a
particular area as well as scientific experimentation such as collecting air or gas samples,
measurement of local wind profiles, and solar irradiation, and agricultural applications such as
spraying insecticides. A remotely controlled airship can be used for both indoor and outdoor
applications.
Design development and flight testing of prototypes of several Lighter-Than-Air systems for various
scientific applications has been carried out at IIT Bombay since 2002 [1]. Some recent projects
include indoor remotely controlled airships for neural network control hardware implementation [2],
aerostats as platforms for low-cost re-locatable wireless communications systems [3], outdoor
remotely controlled airships for product promotion [4], aerial river ferry using superheated steam-
filled balloons [5], and aerostats for snow cover evaluation [6].
This paper aims at sharing the experience gained in design, fabrication, and flight testing of an oblate
spheroid indoor airship (OSIRCA) and brings out the advantages of a spherical envelope over
conventional shapes. This paper is a part of our capstone design course in aircraft design. The
conceptual design, sizing, envelope fabrication and initial testing was carried out at LTA systems lab
of IIT Bombay, and fabrication of gondola, system integration and testing was carried out at
Hindustan College of Engineering, Chennai. The paper will be divided in three sections covering the
three key themes of this project, viz., Design, Build and Fly. The section on Design will focus on
selection of key parameters, viz., envelope surface area, volume, and arc length and envelope fineness
ratio. It will also explain the tradeoffs during envelope material selection, and explain the procedure
followed for geometric design of the envelope petals. This section will also explain how to account
1 Senior Undergraduate Student, Department of Aeronautical Engineering, Non-Member, AIAA2 Lecturer, Department of Aeronautical Engineering, Non-Member, AIAA3 Associate Professor, Aerospace Engineering Department, Member, AIAA
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for operational issues while sizing the envelope, viz., off standard temperature and density
corrections.
The section on Build will provide the details obtained from design process and the procedure followed
for fabrication of hull and gondola. It will outline the key fabrication procedures such as preparation
of petal diagrams, cutting of envelope petals with tolerance, hooks, and provision of adequate hooks
for gondola attachment. This section will also highlight various issues involved in Radio Frequency
(RF) sealing technique that was employed for envelope fabrication.
The section on Fly will give some insights into the operational issues related to the tuning of various
parameters such as balancing, ballasting, additional tasks to ensure successful flights will be provided.
The systematic procedure followed for leakage detection and removal will be described. The paper
will also include some suggestions for improvements for better performance of the remotely
controlled airship. Figure 1 shows a photograph of OSIRCA in a small hanger in at HCE, Chennai.
Figure 1: OSIRCA: Oblate Spheroid Indoor Remotely Controlled Airship
Key Design Requirements:
OSIRCA was designed to meet certain design and operational parameters like payload, maximum
speed, endurance, and operating altitude. The maximum speed of the airship was limited to less than
2m/s since we do not need much velocity for indoor purposes. Moreover if any video transmission is
to be done from this airship, it should remain stationary so that the video may not get distorted due to
high velocity. The altitude is fixed at 15m anticipating most of the indoor stadium will not exceed this
height. It will also be a tough task to bring down the airship using power plant if it exceeds this
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height. Since the weight of the battery is around 100 gms power available can be utilized only to
lesser extent which gives rise to a design constraint (endurance). Table 1 lists the key requirements
and design related parameters of OSIRCA.
Table 1: Key design and operational requirement of OSIRCA
Design Requirements Value
Payload 1 kg
Ballast weight < 0.1 kg
Max. Speed < 2 m/s
Operating Altitude
(AGL)
< 15 m
Endurance ~ 15 min
Duration of Trials/Demo One week
Total System Cost < US $ 500
Design methodology:
The design methodology followed was broadly on the lines proposed by Pant [7] and Sangole et al.
[2], and comprised of distinct activities related to determination of preliminary design data, selection
of envelope material, and design of the envelope profile. The lift and drag calculations including the
effects of off standard temperature and density corrections were carried out using the formulae listed
by Cheeseman in [8].
The envelope consists of eight identical petals, and the coordinates for plotting the petal profile were
obtained using the scheme listed by Sangole et al. [2], shown in Fig. 2.
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Figure 2: Flowchart for sizing procedure
Figure 3: Petal profile Generation
To begin with, the volume, surface area and arc length calculations were carried out, and the envelope
profile coordinates were generated to determine its shape, since it is the major lift generating body in
an airship. The envelope also needs to have sufficient amount and properly located hooks for gondola
attachment.
Most radio-controlled airships are non-rigid and their envelope is usually in the shape of a cigar that
maintains its shape due to internal overpressure. Rigid models often encounter serious weight
problems because of an adverse weight to volume ratio, especially for small sizes. The most difficult
task is to choose a material which makes the envelope light, tough, and most important of all, gas-
tight. It is very important to decide an appropriate shape for the envelope, since the drag experienced
(and hence the power required) is a strong function of the envelope profile.
Envelope design:
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The first critical decision was the selection of Spheroid shape from the various choices of
conventional profiles. The main reason for this was ease in fabrication, as well as increased
maneuverability, tighter turn radius, and increased reliability of fabric. These requirements and the
perceived advantages of spheroid shape in meeting them will be elaborated in the final paper.
After selecting the envelope shape, one must deduce all the preliminary design parameters for the
fabrication of the same. Since the airship was to be used indoors, the constraints on the overall
dimensions of the envelope were quite dominant. These constraints automatically put certain
limitation on the volume of the envelope and thus, the net lift available as a result. Hence appropriate
length and fineness ratio of the envelope was decided in an iterative fashion so that maximum
possible buoyant and dynamic Lift was produced, without increasing Drag at the same time. The
analysis includes determination of hoop and longitudinal stresses to ensure that the shapes are
structurally capable of handling these loads, using the methodology outlined by Gupta and Malik [9].
Using all this data and information, the coordinates of each point in the envelope profile of OSIRCA
were determined. This helped in the development of Petal profile coordinates which were later plotted
to form a Petal template. Figure 4 shows the size of a single Petal of OSIRCA; a photograph of a
Template for Petal profile, and two petals which were fabricated in the Lighter-Than-Air Systems
Laboratory at Indian Institute of Technology Bombay are also shown.
Figure 4: OSIRCA Petal profile and photograph showing Petal Template and two Petals
Gondola layout:
Designing a compact and low weight gondola from locally available materials with sufficient space to
install the motors and equipment and the required payload turned out to be a huge design challenge.
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Details of this activity will be highlighted in the paper. After a detailed analysis, the finalized Gondola
Layout was arrived, as shown in Fig. 5.
Figure 5: Gondola configuration of OSIRCA
One of the perennial problems of most indoor airships has been their lack of control power, especially
at low speeds. In order to alleviate this shortcoming, it was decided to provide thrust vectoring control
(TVC) in OSIRCA. Moreover, thrust vectoring has some peculiar advantages when used on airships
with spheroid envelopes, which will be outlined in the paper. The easiest method of thrust vectoring is
by turning the propellers by rotating the connecting rod to the vector angle. Three approaches for
TVC on OSIRCA were examines, the merits and demerits of each will be discussed in the paper.
Propulsion System:
Two choices were available for powering OSIRCA, viz., fuel operated IC engines or battery powered
electrical motors; each has its own advantages and disadvantages. For indoor airships, the use of IC
engines is impractical, and a wide range of DC motors are available. The issue of concern was the
selection of appropriate motor and required power packs that could run the motor for sufficient
amount time without much loss in Thrust. Based on the requirements and scrutiny of locally available
motors, 2 “speed-180” motors rotating at a maximum rpm of 3000 and producing a thrust of 250
grams and one “speed-120” motor rotating at a maximum rpm of 1000 and producing a thrust of 25
grams are selected. Futaba S-148 standard servo was used for rotating the connecting rod. One Ni-MH
type rechargeable battery is used to power up the whole set up. Battery was selected based on the
design requirement (endurance).The capacity of the battery used on OSIRCA was 8.4V, 650mAh,
which is able to deliver adequate power for about 20 minutes on full charge. Three propellers were
used out of which two for forward thrust and the other, tail propeller for direction control. Propulsion
system also has a tilting rotor mechanism so that direction of thrust can be changed up and down. In
this mechanism the two propellers are attached to a connecting rod which can be tilted.
Fabrication:
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This section of the paper will describe the procedure followed to fabricate a light-weight gondola and
the spheroid envelope from the petal profiles. After an extensive market search, coated PVC was
selected as the most appropriate material for the envelope.
The selected materials that are prescribed in the gondola design section were assembled in a frame as
per the design configuration selected. Proper space is provided for accommodation of payload (say a
camera). Both the hull and the gondola are attached together with the help of attachment hooks. Thus,
with the inclusion of procedures for shape and structural analysis of the envelope, the RC airship
design methodology became more flexible and robust to size the airship as per given requirements at
low cost.Two photographs of OSIRCA’s gondola are shown in Figure 6.
Figure 6:Photographs of Gondola of OSIRCA
The joining of Petals was carried out using the Radio Frequency welding machine available in LTA
Systems Lab. of IIT Bombay, as shown in Figure 7.
Figure 7:OSIRCA envelope under fabrication using RF welding machine
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Circular patches were also sealed to the envelope to enable proper hooks to be attached, which are
used to attach the gondola, as well as provide means for anchoring the airship while on ground. Many
special issues have to be kept in mind while fabricating such envelopes, as well as key precautions
have to be taken, these will be highlighted in the paper.
The key components of the OSIRCA are shown in Fig 8.
SERVO PICTURE TRANSMITTER PICTURE
MIXER PICTURE RECEIVER PICTURE
Figure 8: Photograph of key components of OSIRCA
A Servo is a small device that has an output shaft. This shaft can be positioned to specific angular
positions by sending the servo a coded signal. As long as the coded signal exists on the input line, the
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servo will maintain the angular position of the shaft. As the coded signal changes, the angular position
of the shaft changes.
A four channel transmitter consists of four different controls. In our case the four controls are thrust,
differential speed control, tail rotor and tilting of main rotor. The transmitter produces pulses with
different length at each position of the control. These pulses are produced by a digital oscillator inside
the transmitter. These signals are sent by the antenna to the receiver.
The receiver circuit receives the modulated frequencies by the antenna. Here the frequencies are
demodulated and corresponding electrical impulses are produced. These impulses are transmitted to
the servos for mechanical movements corresponding to the deflections given in the transmitter.
The mixer does the job of mixing the thrust and the differential speed control. The thrust control is
channelized to the transmitter control in such a way that the thrust level can be fixed in a particular
position. It takes the fixed thrust level as the mean and increases the speed on one main rotor and
decreases on the other corresponding to the control movements.
A wireless VGA camera having the transmitting capability of 50m was used to transmit the
videography. Normal television set can be used to display the surveillance/video coverage. A single
battery was used for both power plant and the transmitting system since it may include additional
weight to OSIRCA or the need for an extra battery can be compensated.
Table 2: Key sizing parameters & Mass Breakdown of OSIRCA
Key Parameters Value Key Component Value
Envelope Fineness Ratio 1.56 Envelope (including hooks
and patches)
2.99 kg
Envelope Surface Area 17.4m2 Gondola (including
receiver, mixer, servo and
camera and transmitter)
0.65 kg
Envelope volume 5.23m3 Power plant System
(including battery, 3
motors and 3 propellers)
0.45 kg
Max. Height m Ballast + Payload 1.1 kg
Max. Width m All Up Mass 5.23 kg
Table 3: Cost breakdown of OSIRCA
Components Cost
Envelope Material Rs. 2000 per 1.3 sqm (USD 50 per 1.3 sqm)
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Envelope Fabrication Rs. 6000/- (USD 130)
Relocation Expenses&
internship fee
RS 10000/- (USD 250)
Helium (Two Cylinders) 6500 per cylinder (USD 150)
Gondola and Power plant
System
Cost of engine, battery, 3 propellers, one servomotors
Rs. 9000/- (USD 200)
Remote Controller Rs. 2500/- (USD 50)
Wireless Camera& VTU Rs. 10,000/- (USD 250)
Miscellaneous items Rs. 1000/- (USD 25)
Total System Cost 50000/- (USD 1100)
While testing Hydrogen was used due to low cost, less density and easy availability. Lift available
while using Hydrogen will be the same as the volume of the envelope. But in application perspective,
the OSIRCA would be used for indoor purposes where safety is major concern it is safe to use Helium
even if it is budgeted in operating expenses.
Testing:
The paper will also outline the procedures followed for testing the envelope leakages and proper
rigging to ensure control effectiveness. Schemes to be followed for proper filling of envelope with
LTA gas, and precautions to be taken during flight operations will also be elaborated upon. Initially
while filling the pressure available from the cylinder will be more because of high cylinder pressure.
But after sometime the velocity in which the gas entering the envelope will gets reduced due to the
reducing cylinder pressure and sometimes the flow may get reversed. So this has to be monitored
while filling the envelope. During this process there may be leakage, normally the leakage was
considered as 3liter per square meter per day. Based on this, for one week demo the number of
cylinder requirement can be decided.
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Figure9: Photograph of OSIRCA while testing
After the attachment was done, the tethers were tied to a digital weighing machine for measuring the
additional lift. This additional lift had to be nullified to ensure neutral buoyancy, by adding ballast
weights in the gondola in such a way that it does not affect the location of center of gravity.
Appropriate amount of downward thrust was then applied so that the airship could ascend and gain
the desired altitude. Several test runs were carried out to check all the controls for proper functioning
and to ensure adequate control power during the motion of the airship in all the directions. It is better
to have a leakage test every time the airship is being used so that the leakages can be reduced as much
as possible.
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Figure 10: Some OSIRCA team members at HCE and IITB
Based on the experience gained in carrying out this exercise, some suggestions for improvements in
the design and fabrication of OSIRCA will also be listed, e.g., procurement of better material, use of
catenary cables, and introduction of small rigid members for additional strength.
Work in progress:
The further development of the project can be done as follows:
For Envelope:
1. Better-suited material can be procured. Properties that must be sought are low surface density,
low Hydrogen / Helium permeability as well as high strength. This will bring down the
weight of the envelope considerably and maximise payload.
2. Use of internal catenary curtain and suspension cables can be thought of. This will improve
the lenticular shape of the envelope.
3. Internal members like bars or rods can be used, converting the non-rigid airship into a semi-
rigid one. But, it will improve the lenticular shape and strength of the envelope.
4. A device can be designed and built to measure the actual drag on the envelope, to verify the
calculated values of the same.
For Gondola:
1. Use of a zip fastener to attach gondola to the envelope.
2. Use of a safety mechanism which would cause the airship to descent when a radio control is
lost or when the battery power falls below a predetermined level
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3. Use of individual motor speed and direction control
4. Use of Ball Bearing to reduce friction for connecting rods.
If the videography is needed to be telecasted then it is better to have an advanced camera so that the
clarity of the video can be improved. And the range of the transmitter can be increased for both the
camera and the power plant so that it would be used for long distance surveillance.
References:
1. Gawale, A. C. and Pant, R. S., “Design, Fabrication And Flight Testing Of Remotely Controlled Airships”, Proceedings of National Conference on LTA Technologies, Aerial Delivery R&D Establishment, Agra, October 2002.
2. Sangole, R., Agashe, S., Palshikar, K., Nakanekar, G., “Design, Fabrication and Testing of Remotely Controlled Indoor Airship employed for Neural Networks Hardware Experimentation”, Undergraduate Project Thesis, Mechanical Engineering Department, PVG College of Engineering, University of Pune, India, 2006
3. Gawande, V.N., Raina A. A., Bilaye, P., Pant, R. S., and Desai, U. B., Design and Fabrication of an Aerostat for Wireless Communication in Remote Areas, AIAA-2007-7832, Proceedings of AIAA’s 7thAviation, Technology, Integration, and Operations (ATIO) Conference and 17th Lighter-Than-Air Systems Technology Conference, Belfast, Northern Ireland, UK, Sep. 2007.
4. Gawale, A., Raina, A. A., Pant, R. S. and Jahagirdar, Y. P., Design Fabrication and Operation of Low Cost Remotely Controlled Airships in India, AIAA-2008-8853, Proceedings of AIAA’s 8th Aviation Technology Integration and Operations (ATIO) Conference, Anchorage, Alaska, USA, Sep. 14-19, 2008.
5. Banerjee, S., Raina, A. A., and Pant, R. S., Low Cost Trans-river Aerial Ferry, AIAA-2008-8851, Proceedings of AIAA’s 8th Aviation Technology Integration and Operations (ATIO) Conference, Anchorage, Alaska, USA, Sep. 14-19, 2008.
6. Bhandari, K. M., and Raina A. A., Conceptual Design Of A High Altitude Aerostat For Study Of Snow Pattern, Proceedings of International Symposium on Snow & Avalanches (ISSA-09), SASE, Manali, India, 6-10 April 2009.
7. Pant, R. S., A methodology for arriving at the baseline specifications of a non-rigid airship,Technical Note, AIAA Journal of Aircraft, Vol. 45, No. 6, pp. 2177-2182, Nov-Dec 2008.
8. Cheeseman, I., “Chapter 2: Aerodynamics,” In Airship Technology, Khoury G.A., Gillet, J. D., eds., Cambridge Aerospace Series 10, Cambridge Univ. Press, New York, 1999, pp. 25–38.
9. Gupta, S., and Malik, S., “Envelope Details for Demo Airship”, PADD Project Report, Aerial Delivery R&DEstablishment, Jan-2002.
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