Subject:
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RE: LEGO RCX Skydivers
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Newsgroups:
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lugnet.robotics
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Date:
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Mon, 8 Nov 1999 15:07:05 GMT
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Original-From:
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Jim Thomas <jim.thomas@trw#AvoidSpam#.com>
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Viewed:
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815 times
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Well, my class built a blimp for my Senior project and we were shooting for
20lbs of lift. Here is a paper I wrote on it for another class. It is long
(for here) but has all the pertinent information to this discussion. Hope
it helps.
JT
(journal paper for Professional Non-Fiction Writing)
James Thomas : LTA TELEVISION CAMERA PLATFORM
LTA JOURNAL, VOL. 16, NO. 11, NOV. 9,
1989
Lighter-Than-Air Television Camera Platform
James Thomas UNIVERSITY OF REDLANDS
ABSTRACT -- This paper examines the design and construction of a
nonrigid lighter-than-air blimp. The payloads of this blimp
consist of a television camera and transmitter. A standard model
airplane radio-control unit operates the blimp. This paper
explores all facets of blimp design, including aerodynamics,
lift, material selection, and construction techniques.
1 INTRODUCTION
1.1 Background
Traditionally blimps have carried passengers and performed a
variety of tasks. The military used them as observation
platforms and plans to use them in the future as radar platforms.
Goodyear sends its blimps to sporting events to take overhead
footage for television. These large blimps carry a pilot and
other passengers. A small blimp (20 feet or smaller) cannot
carry passengers, but a compact television camera would make a
useful and interesting payload. This paper describes the design
and construction of a blimp meeting several criteria including:
lift capability, flying speed, and maneuverability.
1.2 Project Goal
We began the project began with several goals. These goals
include capabilities such as:
Filming and transmitting of video images to an ordinary
television set
Filming straight ahead and straight down
Having a maximum velocity of five miles-per-hour or more
Flying under full power for more than one hour
Turning 360o in less than 20 seconds
These goals lead to many potential uses for the blimp. These
possible uses include:
Filming of college football games and other sporting events
Providing aerial footage of parades and outdoor events
Aerial footage for reality agencies to promote properties
1.3 Paper Organization
Section II examines the requirements of payloads, flight
performance, and safety. Section III examines the payloads:
camera, transmitter, RC unit, propulsion system, and gondola.
Section IV discusses the envelope design: lift, aerodynamics,
thrust requirements, and construction. Section V discusses
flight performance and operation.
2 DESIGN SPECIFICATIONS
2.1 Payload Requirements
The blimp must have enough lift to carry the following
essential load items:
camera
transmitter battery
servos and speed controls
motors and fans
gondola
transmitter
RC receiver
RC battery
motor battery
Table 1 contains weights for these items. The total weight of
these items is 9.3 pounds. To provide a margin of error and to
allow extra capacity for future payloads, we use a total payload
weight of 20 pounds for all calculations. This large margin of
error is essential when designing a blimp that must fly on the
first try.
2.2 Flight Performance Requirements
To make an effective camera platform the blimp must maneuver
smoothly and rapidly. Important maneuvers which must occur
rapidly include:
Turning
Altitude changing
Acceleration/Deceleration
The camera mount pivots in the vertical plane, but it does not
rotate horizontally; therefore, the blimp must rotate
horizontally for it. Thus, the propulsion system must provide a
way to rotate the blimp rapidly. Ideally the blimp should rotate
on its own axis, and one rotation should not exceed 20 seconds in
duration. For straight flight the blimp should attain a minimum
speed of five miles-per-hour. The blimp should also have enough
battery power to maintain this speed for at least one hour. A
long battery life provides versatility in blimp operations.
2.3 Camera and Transmitter Requirements
The camera system must refocus and adjust its aperture while
in the air. The camera should pivot automatically to provide a
variety of views. The Transmitter's range should exceed one
mile. This range permits the blimp to travel away from the
receiver and behind obstacles without loosing the video signal.
As a camera platform the blimp should benefit greatly by having
these basic features.
2.4 Safety Requirements
To operate without posing a hazard to people the blimp must
have safety devices. Some situations where safety measures must
provide a backup include:
Power loss to the propulsion system
Lost radio-control from interference or lost power
Sudden gusts of wind
A blimp equipped with these safety precautions will not endanger
surrounding people or vehicles.
3 PAYLOAD DESIGN
3.1 Camera and Transmitter
There are only three loads in this blimp project not
required for flight -- the camera, the transmitter, and the
transmitter battery. Because the blimp is unmanned the camera
must have all of its settings controlled remotely or
automatically. These settings include: focusing, brightness,
contrast. Most small cameras made are fully automatic which
makes this alternative attractive. The Ci-20 automatic video
camera manufactured by Canon suits this application while
weighing only 18 ounces. A TV transmitter must broadcast the
video signal to the ground. A two watt transmitter broadcasts
its signal two miles, which meets the one mile minimum broadcast
range requirement. Figure 1 shows the transmitter circuit
design.
3.2 Radio-Control system
Radio-control (RC) systems built commercially for hobbyists
come in a variety of configurations. RC systems have between two
and ten channels, each operating one servo or speed control.
The design specifications for the blimp require at least four
channels for operation and two additional channels for the safety
system. Thus the project requires a six channel RC system.
MOSFET speed controls operate the electric propulsion motors.
These speed controls use transistor technology to vary the
voltage applied to the motors. One of the servos controls the
angle of the fans causing the blimp to ascend or descend. Figure
2 shows a simplified diagram of the RC system and its operation.
3.3 Propulsion System
Two electric motors turning fans provide the required
propulsion. The fans pivot vertically to cause ascension and
descent. Figure 3 shows a diagram of the motor and fan
configuration. The motors chosen provide a maximum of 10 watts
of power apiece at 7 volts, thus drawing about 2.8 amps (1.4 amps
* 2 motors) from the battery. The two motors create a turning
action by varying the speed of each fan blade.
Integration of the speed control stick and the steering
control stick provides a natural flight control system. The
steering command and the throttle command combined together
control the speed of each motor. This requires an op-amp network
to combine the signals from the RC receiver and send appropriate
signals to the two motor speed controls. Figure 4 shows a flow
diagram of steering-throttle control system. Figure 5 shows the
op-amp circuitry required to implement this flow diagram. Table
1 is a parts list for this circuit.
3.4 Safety Devices
Because the blimp flies unteathered and operates remotely
the blimp must have safety precautions should something go wrong.
The first safety device is a helium release valve. Should a
problem arise the operator activates a servo and releases some
helium. If the motors loose power the release valve provides an
effective means of bringing the blimp down to the ground.
The greatest problem in using an RC system comes from radio
interference causing loss of control. Should the RC receiver
loose contact with the RC transmitter the blimp must not fly away
on its own. An effective solution makes the blimp circle until
the RC system regains contact. A RC unit equipped with a
built-in signal loss backup system provides this solution. Upon
loss of the signal the receiver automatically enters a shutdown
state. The shutdown state for the blimp sets full thrust on the
left motor and no thrust on the right motor. After five minutes
the helium release valve activates. Operation returns to normal
if the RC system regains contact.
A third safety feature intends to safeguard against wind.
Should a strong breeze start unexpectedly the blimp may drift
away out of control. To protect against this a coiled nylon cord
with a small weight at the end sits on a hatch door in the floor
the gondola. A servo opens the door causing the cord to uncoil
to the ground. A member of the operating crew then grabs the
cord and pulls the blimp in.
In all the safety devices require two channels and the
special RC unit. Considering the cost of the camera and the RC
system, however, the expense and effort to incorporate these
safety devices into the design pale in comparison.
3.5 Gondola Enclosure
The gondola construction uses brass tubing and balsa wood.
The design of the brass frame provides extra support under the
camera and batteries. The balsa wood acts as a covering only and
does not carry much weight. The camera's swivel mount attaches
directly to the frame allowing easy attachment of the camera to
the gondola. The batteries sit directly on cross-members of the
frame. The fan swivel also attaches directly to the frame.
Brazing the frame joints makes solid, lightweight, connections.
Overall the gondola frame and covering weighs a total of 20
ounces. Figure 3 illustrates the gondola frame design.
3.6 Payload Design Summary
Figure 4 shows the gondola with all of the payloads
included. We placed the transmitter and motor batteries near the
center of the gondola because they comprise nearly half of the
payload weight. Placing the camera in the front of the gondola
allows it to view both foreword and downward. Placing the
propulsion units in the rear of the gondola generates a moment
which the motors use to fly the blimp up and down.
4 ENVELOPE DESIGN
4.1 Lift Requirement
The basic physical principal used in lighter-than-air
vehicle design is the difference in gas densities. Using the
ideal gas law the density (p) of a gas is:
p=MP/RT
where M is molar mass, P is pressure, T is temperature, and R is
the universal gas constant. The difference in density (dp)
between helium and air is:
dp = MairPair/RTair - MHePHe/RTHe
Now we must find MHe and Mair. Helium has a molar mass (MHe) of
4 grams per mole. Air, however, consists of a mixture of gases
so the molar mass must be approximated using its composition.
Air is 75.51% N2, 23.14% O2, and the other 1.35% is mostly H20.
Therefore the molar mass of air is:
Mair = .7551(28.0) + .2314(32.0) + .0135(18.0) (grams/mole)
so
Mair = 28.8 grams/mole
The buoyancy (B) of a given volume of helium (VHe) is:
B = dp * VHe
We use this formula for buoyancy to determine the appropriate
size for the blimp.
An ellipsoid with a length-to-width ratio of 3:1 was chosen
as a test shape since it resembles the cigar-teardrop shape of a
blimp. The volume of an ellipsoid is:
V = 4/3 * PI * a2 * b
where PI is 3.14, a is the minor radius, and b is the major
radius. A computer program generated a table of blimp sizes
based on the temperature and pressure of the two gases. Table 2
shows blimp sizes which can lift 20 pounds. Notice that the size
depends on air and helium temperatures, internal pressure, and
skin weight. For the selected temperature and pressure
conditions the size of the blimp measures about 6x18 feet. All
envelope calculations use these dimensions.
4.2 Envelope Aerodynamics
All aerodynamic calculations use an ellipsoidal shape with a
short radius of 3 feet and a long radius of 9 feet. The actual
blimp shape resembles a teardrop. A teardrop shape has a lower
drag coefficient than an ellipsoid of the same dimensions.
Therefore the blimp speed performance should exceed that of the
design.
The total drag (DT) on a body in a fluid flow is:
DT = 1/2 * CD * pair * U2 * A
where CD is the coefficient of drag for the body, pair is the
density of air, U is the velocity of the fluid flow, and A is the
cross-sectional area perpendicular to the flow. For an
ellipsoid:
A = PI * a2
To find CD we must first calculate the Reynolds number (ReL) for
the flow:
ReL = (U * L) / v
where U is the flow velocity, L is the length of the ellipsoid,
and v is a constant which is 15.89 x 10-6 for air. Using U=5 mph
and L=18 feet ReL=8.53 x 105. For this value of ReL, CD for a
3:1 ellipsoid is 0.05. Using pair=0.0744 lb/ft3 and a=3 the
total drag is 0.51 newtons. Therefore the blimp needs 0.51 N of
thrust to achieve a velocity of 5 mph.
4.3 Tail-Fin Drag
To approximate the drag on the tail-fins we chose a size of
3x3 feet. There are four square tail-fins. The drag on the fins
is predominantly frictional.
The Reynolds number for the flow across the tail-fins is:
ReL = (U*L)/v = (7 ft/sec * 3 ft) / 15.89x10-6 ft2/s
ReL = 1.7x105
For this value of ReL the coefficient of frictional drag (Cf) is:
Cf = 0.074[ReL]-1/5 = 6.9 x 10-3
The surface area of one tail-fin is:
Asurface = 2 * 3 ft * 3 ft = 18 ft2
The frictional drag (Df) of one tail-fin is:
Df = 1/2 (0.0744 lb/ft3)(7.0 ft/s)2(18 ft2)(6.9x10-3)
Df = 4.5 x 10-2 Newtons
Since there are four tail-fins the drag force becomes:
Df = 4 * 4.5 x 10-2 Newtons = 0.18 Newtons
Because the pressure drag is negligible compared to the
frictional drag the total drag for all four fins is:
Dt = Df = 0.18 Newtons
4.4 Gondola Drag
The total drag on the gondola is also approximated by its
frictional drag. The dimensions of the gondola are:
4 in wide x 4 in tall x 1 foot long
The total area of the exposed surfaces -- the top surface is not
exposed -- is:
Asurface = 2(.33 ft)(.33 ft) + 3(.33 ft)(1 ft)
Asurface = 1.22 feet2
We must find the Reynolds number for the flow over the gondola.
ReL = (0.0744 ft/s)(1.0 ft)/ 15.89x10-6 ft2/s
ReL = 4.4 x 104
For this value of ReL
Cf = 1.328[ReL]-1/2 = 6.3 x 10-3
Finding the frictional drag force:
Df = 1/2 (8.3 lb/ft3)(7.0 ft/s)2(1.22 ft2)(6.3x10-3)
Df = 3.3 x 10-3 Newtons
thus,
Dt = 3.3 x 10-3 Newtons
4.5 Total Blimp Drag
Summing the envelope, tail-fin, and gondola drags yields the
total drag for the blimp.
Dt = 0.51 N + 0.18 N + 3.3 x 10-3 N
Dt = 0.69 Newtons
This figure for the drag is used in the propulsion design to
determine the power required for flight. A thrust of about 0.7
newtons will propel the craft at a velocity of 5 miles per hour.
4.6 Propulsion Requirements
The drag calculations determined that the propulsion system
must produce 0.7 newtons of thrust. The propulsion system deals
in power not thrust; therefore, we need a connection between
power and thrust. In theory the power required to maintain a
given speed against a given drag (thrust required) is:
P = U * D
where U is velocity and D is the drag force. This equation for
power required yields the propeller power required, not the
electrical power required. Both the electric motor and the
propulsion fan are not 100% efficient in converting power from
electricity to thrust. Using standard, conservative,
efficiencies for the electric motor (70%) and propulsion fan
(30%), we find a realistic electrical power requirement. The
electrical power (Pe) is:
Pe = P / (.70 * .30)
Pe = U * DT / (.70 * .30)
For U=5 mph and DT=.7 N, Pe = 5.5 watts. This equals the actual
power drawn from the battery. If the motor battery must last at
least two hours it needs
(5.0 watts / 7 volts) * 2 hours = 1.42 (amp*hours)
of capacity.
4.7 Envelope Material and Construction
Mylar possesses the perfect weight and strength properties
for use as the envelope's material. The durable and lightweight
Mylar comes in rolls of 8x100 feet. Figure 6 shows the shape of
the template used to make the blimp. We cut 10 sections of the
Mylar using the teardrop template. Heat-sealing attaches the
sections together along their edge. Turning the envelope
inside-out once completely sealed provides a smooth exterior
surface with all seams in the interior of the blimp. A valve
attached near the rear of the blimp provides an easy inflation
and deflation point.
4.8 Gondola Attachment
The gondola attaches to the envelope with Velcro. We glued
the "male" half of the Velcro to the envelope using a flexible
silicone glue. The Velcro encircles two-thirds of the blimp's
circumference. Figure 7 shows the exact placement of the Velcro
strips on the blimp. The "female" half of the Velcro strips
attach to the gondola. To attach the gondola we line up the
Velcro strips and press them together. Tests made to determine
the strength of this attachment scheme found that the Velcro held
up under loads of 50 pounds. The gondola weighs less than 20
pounds making the chance of it becoming dislodged extremely
slight. To add increased stability during operation we placed
additional strips of Velcro on the front and rear of the gondola.
These extra strips help keep the gondola from shifting while the
fans provide thrust.
4.9 Envelope Design Summary
Figure 8 shows the overall design of the envelope, placement
of tail-fins, and the gondola attachment system. The drag
calculated may or may not reflect the final drag experienced
during flight. The teardrop shape of the blimp has less drag
than the ellipsoidal shape used in the calculations. Therefore
the blimp should fly faster than originally planned. The actual
drag of the gondola and tail-fins could also differ from the
calculations. Those calculations only provide an estimate of the
gondola and tail-fin drag.
5 DISCUSSION
5.1 Flight Operation
The blimp flies like a plane in most respects. The control
stick operates the turning and altitude adjustments. The
throttle stick operates the blimp speed and camera positioning.
During flight the control stick operates like the control stick
of an airplane. When the operator moves the stick foreword the
blimp descends. When the operator pulls the stick back the blimp
climbs. Moving the stick left and right makes the blimp turn
accordingly.
5.2 Flight Performance
When not moving the blimp turns on its own axis in five to
ten seconds. This satisfies the design requirement of 20
seconds. The turning time depends heavily on the direction and
velocity of any wind present.
The blimp reached a maximum speed of seven miles-per-hour
during routine testing. At this speed the blimp still turns in
less than 20 seconds.
The blimp climbs or descends under full power at a rate of
about three feet per second. This allows the blimp to reach an
operating altitude of 50 feet in under ten seconds.
5.3 Camera Operation and Picture Quality
The camera operation simply requires aiming it up and down.
An ordinary VCR receives the images transmitted. The image
quality compares to that of an ordinary TV image. Because of the
small size of the camera, however, the picture does not quite
have the same crisp picture as an ordinary TV image.
The two watt transmitter has an effective range of about one
mile without any noticeable noise or interference in the picture.
No problems arose in reciveing the signal even when the blimp
moved behind buildings and other obsticals.
5.4 Safety Equipment Operation
All of the safety systems tested out successfully and
functioned properly. The helium release valve caused the blimp
to descend at a 0.25 ft/sec rate. The automatic shutdown system
also worked well. When we turned the RC transmitter off the
blimp began circling immediately and the helium release valve
activated after 5 minutes. The safety rope deployed properly
when activated and held the blimp properly. The safety systems
on board the blimp proved effective and adequate to prevent
mishaps which could result in damage to the blimp and other
vehicles.
5.5 Operation Costs and Considerations
The helium used to fill the envelope represents the only
cost to operate the blimp. The batteries recharge and therefore
do not create additional costs. One cylinder of helium costs
approximately 40 dollars and can fill 291 ft3. The volume of the
6 x 18 foot blimp is 339.3 ft3 and would require more than one
cylinder to fill completely. Pressurizing the blimp to 0.5
atmospheres requires about 500 ft3 of helium to fill it.
Therefore the cost to fill the blimp reaches 80 dollars if the
extra helium in the tank goes unused.
6 CONCLUSION
The blimp performed as well or better than all of the
requirements. As a result of over-designing the blimp exceeds
the size necessary for the actual loads. The final weight of the
entire blimp -- envelope, gondola, payloads -- measured only
twelve pounds. To lift this weight only 161 ft3 of helium would
suffice. The volume of the blimp built measures about 339.3 ft3
which lifts a total load of 25.25 pounds (including the envelope
weight). A 3:1 ellipsoid with a volume of 161 ft3 measures 4.7 x
14.1 feet -- considerably smaller than 6.0 x 18.0 feet. Finding
the exact weight of the gondola and envelope prior to
construction allows the final blimp to have the smallest size
possible. If the blimp design originally had a smaller margin of
safety it would fly faster and turn more quickly than the one
constructed. We overbuilt the blimp to ensure that it would
operate on the first try.
Any future blimp projects should take into consideration the
benefits and losses associated with over-design. By rebuilding
the envelope for this project a reduction in envelope size would
result. A blimp of smaller dimensions (5 x 15 feet) would
provide the required lift.
Blimp design and construction requires an iterative
approach. Figure 8 shows a flow chart of the iterative design
process. As the flowchart shows the process contains many
interdependent parameters. The only way to deal with this is to
over-design or make one large equation that relates all of these
dependancies together. We urge others to construct computer
models of these equations and publish their results.
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Message has 1 Reply:
 | | RE: LEGO RCX Skydivers
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| Jim, this is very interesting. Is the complete paper (with tables etc) available anywhere on the Web?? Is it in a form where it could be output in Postscript on some computer, and then changed to a .PDF file? I could help with that if it is in some (...) (25 years ago, 8-Nov-99, to lugnet.robotics)
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