micro air vehicle full report
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MICRO AIR VEHICLES
Have you ever thought of a class of air vehicles which are less than 15 cms in size (most prominent linear dimension), can hover for two to three hours in air at a velocity of 20 to 30 mph at an altitude of 600 meters or even higher. This seminar and presentation will give you an idea of such vehicles called MAVâ„¢s (micro air vehicles). A dwell into the design and aerodynamics of such vehicles would be quite interesting. It should be noted that its aerodynamics resembles the aerodynamics of insects rather than that of conventional aircrafts. The aerodynamics and design procedure will be elucidated in this seminar and presentation. This seminar and presentation will also provide you an account of control, propulsion, communication and navigation of micro air vehicles.
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MICRO AIR VEHICLES
Â¢ SMALL AIR CRAFTS
Â¢ USES DIFFERENT LIFT GENERATING MECHANISM
Â¢ MECHANISM MIMICKS THE FLIGHT OF INSECT ENCARISA FORMOSA
Â¢ FOUNDED FROM NATURE
PRAMOD RAMAKRISHNAN (Lecturer,Dept.of Mech.Engg )
ANOOP A S
A KIND OF MAV
COMPARISON WITH OTHER
BENEFITS THAN ORDINARY
Â¢ WEIGHT IS VERY SMALL (50 grams)
Â¢ SIZE IS ABOUT 15-30 CENTIMETERS
Â¢ IT CAN BE HOVERED
Â¢ CAN BE PASS THROUGH SMALLER AREAS
FUNCTIONS OF MAV
Â¢ USED FOR TAGGING,TARGETING
Â¢ HELPS SOLDIERS
Â¢ USED FOR MISSIONS IN CANYONS,INTERIOR OF BUILDING Etc
Â¢ RESCUE OPERATIONS
Â¢ TRAFFIC MONITORING
Â¢ WILD LIFE SURVEYS
Â¢ POWER LINE INSPECTION
Â¢ AERIAL PHOTOGRAPHY
TRACKING BY MAV
Â¢ ALL THE SYSTEMS BASED ON MICRO ELECTRONICS
Â¢ FLIGHT CONTOL IS DIFFICULT
Â¢ STABILISATION AND GUIDANCE REQUIRE HIGLY AUTONOMOUS CONTROL SYSTEMS
Â¢ IMITATES THE INSECT ENCARISA FORMOSA
Â¢ FLAPPING WINGS
Â¢ WINGS HELPS HOVERING
Â¢ THE LIFT IS PROPORTIONAL TO
WING AREA AND SQUARE OF VELOCITY
LIFT IS GENERATED BY
*AIR FLOW BY VEHICLE SPEED
*WING FLAPPING TO SUPPORT -
IT CAN BE OPERATED AT VERY LESS VEHICLE SPEED
WIES FOGH'S 2-D MODEL
(a) (b) © (d)
(e) (8 00
a-b clapped together; c-e starts opening; f-h separation
Â¢ IN FLYING MECHANISM IMBALANCES ARE DAMPED DUE TO VISCOUS INTERACTIONS IN THE LOW
REYNOLDS NUMBER REGIME
Â¢ ALL HORIZONTAL FORCES CANCELLED BY OPPOSITELY WORKING MECHANISMS FOR THE 2 WINGS
Â¢ LIFT IN TERMS OF LIFT COEFICIENT,CL CL= (1.38 Wc)/U,WHERE W=ANGULAR VELOCITY OF THE WING c=CHORD LENGTH U=FORWARD VELOCITY
Â¢ REDUCE THE WEIGHT
Â¢ ACHIEVE MORE STABILIZATION
Â¢ MINIMIZE SIZE
Â¢ INCREASE SPEED
Â¢ MAXIMIZE BATTERY LIFE
Â¢ NEW DEVELOPING TECHNOLOGY
Â¢ DERIVED BY OBSERVING NATURE
Â¢ NEW MECHANISM FOR WINGS
Â¢ FLAPPING DOES'NT GENERATE SUFFICIENT LIFT
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In the last decade researchers have developed increasingly sophisticated unmanned air vehicles (UAV) for military applications. Increased demands for intelligence are spawning the development of a smaller next-generation UAV called the micro air vehicle, or MAV. Small enough to fit in the palm of your hand, an MAV would have an operating range of several kilometers and transmit detailed pictures back to a portable base station. Potential applications for MAVs, both military and civilian, are numerous. For most military applications, MAVs would be controlled by local users, operating covertly, to supply real-time data. This article focuses on a military surveillance application that uses either visible or mid-wavelength infrared imaging sensors. As designed, the MAV would fly in a low Reynolds-number regime at airspeeds of 10 to 15 m/sec. Propulsion would be provided by a combination of an electric motor with either an advanced lithium battery or fuel cell, or by a miniature MEMS engine, which is a more efficient option. Because of the close coupling between vehicle elements, system integration would be a significant challenge, requiring tight packaging and multifunction components to meet mass limitations. Next, we see the robust schemes for the detection of extreme MAV attitudes, where no horizon is visible, and for the detection of horizon estimation errors, due to external factors such as video transmission noise.
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MICRO AIR VEHICLES
“IT IS A NEW STEP OF DEVOLOPING TECHNOLOGY”
Kiran S Nayanar
Department Of Mechanical Engineering
College Of Engineering, Trivandrum
Micro Air Vehicles.ppt (Size: 879 KB / Downloads: 303)
TYPES AND WORKING.
ABOUT THE FLAPPING WINGS USING WEIS FOGH’S MODEL.
MECHANISM AND ITS FEATURES.
WHAT ARE MAVS(AV’S)?
Multi functional, militarily capable, small flight vehicles.
size should be less than15cms.
Reynolds's no < 10^5.
For a Primarily intended and developed for defence applications.
WHY MAV’S? WHY NOT SOMETHING BIGGER?
Keeps security personnel out of harms by providing situational awareness right down to platoon level.
Can be individually controlled
Can be used for a wide range of new missions _ (even unthought-of before)
COMPARISON WITH OTHER AIR VEHICLES
Wildlife study &photography
Tracking criminals & illegal activities
inspection of pipes
Operation in constraint environment.
Bio chemical sensing.
Which uses propellers.
Similar to that of normal aircrafts or
Which uses flapping wings.
CHARACTERISTICS OF THE FLAPPING WINGS
Lift is generated by the airflow created by the vehicle speed.
And also the wing flapping will support the weight of the vehicle.
Can be operated at very less vehicle speed.
WEIS FOGH’S 2D MODEL
IMPORTANCE OF THIS MODEL
Compared to ordinary flapping this model will be generating more lift, (Circulation).
Opening of the wings by rotational movement.
Separation of the wings by translation movement.
Flapping of the wings.
To design the mechanism the three positions of the wings are utilized.
First of all, the flapping movement of the wings is designed assuming the positions of the fixed pivots, O1 and O2.
The position of points A and B are determined.
The criterion for a feasible solution is that the links should be in the range of 1cm 4cm.
The steps are repeated until all the conditions are satisfied.
Next the driving mechanism O3CAO2 is designed.
Trying many times, the movements of the wings are obtained as required, in specified dimension limits.
The dynamic and the resultant loads were analyzed assuming the linkages to be made of steel. (Width 5mm and thickness 1mm)
The maximum acceleration occurred at position of about 1200 of link O1B.
Assuming the link O2A is moving with average angular velocity of 100 Hz and zero angular acceleration.
The driving mechanism O3CAO2 has not included in this analysis.
This analysis is used to compute the forces on the pin joints and the links.
From the analysis, the maximum force is on joint O2 and is of magnitude 31.38 N.
When the mechanism is oscillating, there are unbalance forces acting on it.
In a flying mechanism the imbalances are damped due to viscous interactions in the low Reynolds number regime.
All horizontal forces will be cancelled by oppositely working mechanisms for the two wings.
The lift generated by the above mechanism is evaluated in terms of lift coefficient (CL).
CL = (1.38 ** c)/U where,
is the angular velocity of the wings.
c is the chord length and U is the forward velocity.
The angular speed = 60 rpm.
Chord length c = 15 cm.
This will give a lift coefficient of 1.38 when the forward speed of the vehicle is 1 m/s.
This is comparable with the lift coefficient of well-designed airfoil at an angle of attack of about 6-8o.
Micro Air Vehicles are new development of the technology by which a variety of operations are done.
Simple flapping of the wings does not generate sufficient lift.
So special mechanism is designed to get the required lift using Weis Fogh’s model.
The future attempts will be for designing the mechanism for a forward movement.
And reducing the weight of the vehicle using lighter material.
Then to achieve high flight speeds, better stability in air.
And also to maximize the battery life.
S. MUKHERJEE, S. SANGHI; “Design of a Six-Link Mechanism for a Micro Air Vehicle”, Defense Science Journal, Vol. 54, No. 3; July 2004.
JAMES M. MC-MICHAEL, COL. MICHAEL S. FRANCIS; “Micro Air Vehicles: Toward a New Dimension in Flight; darpa.mil
ON Aerodynamic modelling of an insect-like flapping wing in hover for micro vehicles.
Phil. Trans. R. Soc. Lond. A 2002 360, 273-290
DESIGN AND DEVELOPMENT OF A MICRO AIR VEHICLE (μAV)
CONCEPT: PROJECT BIDULE
Mr T. Spoerry1, Dr K.C. Wong
School of Aerospace, Mechanical and Mechatronic Engineering
University of Sydney
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INTRODUCTION OF MICRO AIR VEHICLES
There has been recent interest in unmanned air vehicles with a largest linear dimension no greater than 6 inch Micro air vehicles (MAVs) are intended to operate in close proximity to a point of interest without being detected and should provide surveillance teams with critical information in a rapid-deployment urban-environment mission scenario.
Micro Air Vehicle is a small flight vehicle that uses lift-generating mechanism different from the mechanism used for larger aircraft. These machines are used to perform a variety of mission including reconnaissance, surveillance, targeting, tagging etc in hazardous locations and for bio-chemical sensing in defence sector. The design features and the configurations of MAVs are different from that of normal aircrafts. The speed of MAV is very low and the size is less than 38.10 cm length, width or height. MAVs are not the small versions of ordinary aircrafts but are affordable fully functional, military capable, small flight vehicles in a class of their own.
The mechanism used for the flapping wing is a six-link mechanism, which is designed from nature i.e. from the motion of the wings of a type o¬f insect called Encarisa Formosa.
The advantage of this special mechanism is that, it is having more lift compared to a fixed wing mechanism. Also it can increase the lift without increasing the vehicle speed. Since the proposed mechanism helps for lift generation and not for the full 3-d movement this model have not been used for more functions. The future work will be for designing for the forward motion and also for reducing the weight by using light materials.
COMPARISON WITH OTHER AIR VEHICLE
The low Reynolds number regime is significant in that it project and implimentations a fundamental shift in physical behaviour at MAV scales and speeds - an environment more common to the smallest birds and the largest insects. While naturalists have seriously studied bird and insect flight for more than half a century, our basic understanding of the aerodynamics encountered here is very limited. Neither the range - payload performance of bees and wasps nor the agility of the dragonfly is predictable with more familiar high Reynolds number aerodynamics traditionally used in UAV design. And if our understanding of low Reynolds number effects is limited, our ability to mechanize flight under these conditions has been even more elusive.
With the small size of the MAV come high surface-to-volume ratios and severely constrained weight and volume limitations. The technology challenge to develop and integrate all the physical elements and components necessary to sustain this new dimension in flight will require an unprecedented level of multi functionality among the system components. The traditional "stuffing the shell" paradigm of conventional aircraft design is not likely to be workable for MAVs. Yet to be developed, Micro Air Vehicles will be roughly one-tenth the scale of the Sender, and the weight of a six-inch, fixed-wing MAV may be only 50 grams or so, just one one-hundredth the weight of the Sender. The above figure illustrates the difference in size. Yet MAVs must be capable of staying aloft for perhaps 20 to 60 minutes while carrying a payload of 20 grams or less to a distance of perhaps 10 km. finding high density sources of propulsion and power is a pivotal challenge. And while the Sender is a conventional, moderate aspect-ratio, fixed wing aircraft, MAVs may require more unusual configurations and approaches ranging from low aspect-ratio fixed wings to rotary wings, or even more radical notions like flapping wings.
COMPARISON BETWEEN FIXED WING, ROTARY-WING AND FLAPPING-WING DESIGNS
Micro air vehicles are either fixed-wing aircraft, rotary-wing aircraft (helicopter), or flapping-wing (of which the ornithopter is a subset) designs; with each being used for different purposes. Fixed-wing craft require higher, forward flight speeds to stay airborne, and are therefore able to cover longer distances; however they are unable to effectively manoeuvre inside structures such as buildings. Rotary-wing designs allow the craft to hover and move in any direction, at the cost of requiring closer proximity for launch and recovery. Flapping-wing-powered flight has yet to reach the same level of maturity as fixed-wing and rotary-wing designs. However, flapping-wing designs, if fully realized, would boast a manoeuvrability that is superior to both fixed- and rotary-wing designs due to the extremely high wing loadings achieved via unsteady aerodynamics
The Tamkang University (TKU) in Taiwan recently (in Jan., 2010) realized automatically control of the flight altitude of 8-gram flapping-wing MAV. The MEMS Lab in the TKU has been developing MAVs for several years and since 3 years ago the Space and Flight Dynamics (SFD) Lab has joined the research team for the development of autonomous flight of MAVs. The MAVs developed in the TKU are 20 cm in wing span and 8 gram in weight. Instead of traditional sensors and computational devices, which are too heavy for most MAVs, the SFD combines stereo-vision system and ground station to control the flight altitude. This seems to be the first flapping-wing MAV of weight under 10 grams which realizes autonomous flight (or partly autonomous flight).
The TU Delft University in the Netherlands has developed the (in 2008) smallest ornithopter fitted with a camera, the Delfly Micro, the third version of the Delfly project and implimentation that started in 2005. This version measures 10 cm and weighs 3 grams, slightly larger (and noisier) than the Dragonfly it was modelled after. The importance of the camera lies in remote control when it is out of sight and eventually completely autonomous flight. This version has, however, not yet been successfully tested outside, although it performs well indoors.
Robert Wood at Harvard University developed an even smaller ornithopter, at just 3 cm, but this craft is not autonomous in that it gets its power through a wire and is led along a rail.
The concept of micro-sized Unmanned Aerial Vehicles (UAVs) or micro Air Vehicles (MAVs) has gained increasing interest over the past few years, with the principal aim of carrying out surveillance missions. The primary payload of these tiny aircraft (~15centimetres or 6 inches wingspan) is usually a miniature image sensor.
The history of micro air vehicles really began with the development of model airplanes in the 19th century and the development of radio controlled model airplanes in the 20th century. The improvement in propulsion systems from rubber bands to liquid fuel internal combustions engines and later battery powered electric motors made it possible to produce longer and longer flights. The development of miniature radio receivers and control components in the 1990s also had a large impact on the ability to design a very small flying vehicle. Once the aerodynamics and control of small aircraft models with a mass less that 100 grams were better understood, the micro-air-vehicle was born.
After the Montgolfier brothers made their first unmanned balloon flight in Paris in 1783, experimental aviation accelerated by 1799, Sir George Cayley produced the first design for the modern airplane configuration. Cayley tested a boy-carrying glider in 1849, and by 1853 he made a controlled flight of a man carrying glider. Cayley became known as the “Father of Aeronautics” due to these experiments and corresponding publications. Alphonse Penaud, another Frenchman, began selling rubber band powered pusher-propeller ready-to-fly model airplanes in 1871. Penaud recognized that longitudinal stability was crucial. In 1879, Victor Tatin flew a tethered monoplane model in France using a compressed air motor. Nikola Tesla, a Serbian immigrant and inventor, allegedly arrived in New York in 1884 with plans for a remotely controlled unmanned airplane. Fourteen years later he entered a remotely controlled four-foot-long boat in the Electrical Exposition in Madison Square Garden. Tesla manoeuvred his boat to stop or go, turn left or right by blinking its lights using different radio frequencies. Before Tesla showcased his boat in Madison Square Garden, Louis Brennan, an Irish inventor, demonstrated his wire-guided torpedo in 1888. Two decades later, Rene Lorin, a French Artillery Officer, proposed a jet powered flying bomb that would be controlled from a manned escort. Because of the possible military applications, a growing interest in remote controlled vehicles emerged in a number of countries.
The first appearance of winged insects is shrouded in the past, but they probably took to the air almost 350 million years ago (Wotton, 1981; Ellington, 1991a). Wingspans of the early fossils ranged from 10 to 710 mm, and the form of the wings suggests a variety of adaptations in flight style. The Protodonata, which were the ancestors of dragonflies, were among the early fliers; their wings were similar enough to modern forms to suggest comparable flight capabilities, although perhaps with less refinement. Through natural selection, the insects have been experimenting successfully with wings, kinematics, aerodynamics, control and sensory systems for hundreds of millions of years. Much more recently, interest has developed in small autonomous flying vehicles, largely motive by the need for aerial reconnaissance robots inside buildings and confined spaces. Industry, commerce and the military have all identified potential roles for such micro-air vehicles (MAVs). Research on MAVs is primarily conducted by aerodynamic and robotic engineers who are attempting to improve the performance at small sizes of conventional fixed wings and rotary wings. However, there already exists a very successful design for intelligent MAVs with much better aerodynamic performance than conventional wings and rotors.
The Black Widow is the current state-of-the-art MAV and is an important benchmark. It is the product of 4 years of research by Aerovironment and DARPA. The Black Widow has a 6-inch wingspan and weighs roughly 56 grams. The plane has a flight range of 1.8 kilometres, a flight endurance time of 30 minutes, and a max altitude of 769 feet. The plane carries a surveillance camera. In addition it utilizes computer controlled systems to ease control.
The Black Widow is made out of form; individual pieces were cut using a hot wire mechanism with a CNC machine allowing for greater accuracy.
The University of Florida has been very successful over the past five years in the MAV competitions. In 2001 they won in both the heavy lift and surveillance categories. Their plane was constructed of a resilient plastic attached to a carbon fibre web structure. This resulted in a crash resistant airfoil. Figure shows a picture of the University of Florida design for the heavy lift competition.
Brigham Young University has been involved with the MAV competition for a number of years now, and their concept has been different. They have chosen to increase the wing area without increasing their largest dimension by making a biplane. The two wings double the surface area without affecting the maximum dimension of the aircraft. Another technique that was employed by the Brigham Young team was to bring multiple planes to the competition.
Some of these planes were much larger than others. The idea behind this was that the large plane could be flown to guarantee a qualification for ranking, and once that was done; they would attempt to make their smaller planes fly for the necessary two minutes to qualify. Figure2.4 shows some of the MAVs that were designed by Brigham Young.
Arizona State University has also been involved with the competition for a number of years. Their design featured a simple swept, flying wing design. It also uses the COX .01 TEE-DEE engine for thrust. The control surfaces are at the trailing edge of the wing, and it uses an aileron system for turning. The 2001 MAV from Worcester Polytechnic Institute placed 4th in the 2001 competition. Their plane was constructed out of a carved block of polystyrene. The electric propulsion system, control system, and payload were stored in a rectangular fuselage below the wing. The team also used a folding propeller to gain an advantage on its maximum dimension. The rules have since been changed to stipulate that the largest dimension is measured in the flight condition. There are some similarities in many of these aircraft. Almost all of the designs use the COX .01 TEE-DEE engine. Almost all of the designs also use a leading edge that is swept back from the propeller, which allows for a slightly more efficient thrust from the engine, or motor.
Notre Dame’s MAV design was entered in both the heavy lift and the surveillance competitions also. The plane was constructed out of balsa and foam. It should be noted can also see that the design has a large fuselage, which would increase the drag greatly, however this design also utilized the COX .01 TEE-DEE. Control was provided by an aileron/elevator system mounted below the wing and a rudder on a vertical tail above the wing.
CONSTRUCTIONS AND WORKING DETAILS
Thus far we have relied on an Edisonian approach to design our aircraft. No mathematical models would have lead to the development of our new concept. In order to use this approach we made some significant advances in the construction methods so that design iterations could be made quickly and each design could be thoroughly tested.
Following step-by-step construction techniques used to fabricate a MAV fuselage and wing are described here.
Step1. A drawing is made of the fuselage panels to act as a guide for placement of the carbon fibre.
Step2. The drawing is taped onto the fiat tool.
Step3. Transparent skin material (Vacuum bag material) is then placed over the drawing.
Step4. Unidirectional carbon fibre tape is cut into long narrow tacky strips.
Step5. The carbon fibre strips are placed on the transparent skin material manually using the drawing as a guide or by automated process. Multiple layers are used in places where high stiffness is required. Overlap at the corners assures a mechanically sound joint.
Step6. Hinge material is placed between layers of carbon fibre at the location of the control surfaces.
Step7. Nonporous Teflon release film is then placed over the assembly.
Step8. The assembly is then placed into a vacuum bag and subsequently into a vacuum oven for cure.
Step9. Aider the cure cycle is complete; the parts are removed from the bag. The fuselage panels are then cut out.
Step10. The parts are glued together with cyanoacrylate adhesive.
Step11. Kevlar thread is used to lash the parts together. Without lashing, the glue joints would fail.
Step12. The motor, servos, receiver, control rods and horns are installed.
Step1. A drawing is made of the wing plan form to act as a guide for carbon fibre placement. Step2. The drawing is taped onto a curved tool.
Step3. A layer of nonporous Teflon release film is placed over the drawing.
Step4. Unidirectional carbon fibre tape is cut into long narrow tacky strips.
Step5. The carbon fibre strips are placed on the release film using the drawing as a guide. Multiple layers are used in places where high stiffness is required. Overlap at the comers assures a mechanically sound joint.
Step6. Nonporous Teflon release film is then placed over the assembly.
Step7. The assembly is then placed into a vacuum bag and subsequently into a vacuum oven for cure.
Step8. After the cure cycle is complete, the carbon fibre wing skeleton in separated from the tool.
Step9. Spray mount adhesive is applied to the skeleton.
Step10. Thin latex rubber material is then applied to the wing.
Step11. Cyanoacrylate adhesive is used to reinforce the bond line.
Step12. Excess latex rubber is trimmed away.
Early on the decision was made to pursue electric propulsion over an internal combustion engine. Preliminary calculations of fuel consumptions for the smallest available combustion motor; furthermore, an electric solution has the weight advantage of powering all components off one common power source. In addition, an electric motor is more reliable and does not pose the difficulties of a moving center of gravity caused by an emptying fuel tank. Micro-R/C motors have a larger power output than the commercial motors we considered but at the cost of increased weight
Most R/C vehicles use rechargeable nickel cadmium (Ni-CAD), nickel zinc (Ni-Zn), or nickel metal hydride (Ni-MH) batteries because of their high current draw, long life and very slight recharge memory. Rechargeable batteries are preferred for this type of application because they are cost effective and would not need to be removed from the plane. However these batteries are very heavy and are designed for long life. The ideal batteries would fit completely inside the wing of the MAV. To fit inside the wing the battery would need to be very thin, less than 8mm in one dimension. The only batteries of that size and shape did not meet the electrical requirements of the motor. These batteries are available in 3 and 6 volts with different sizes and capacities.
There are two dimensions that are used to describe propellers, diameter and pitch. These two dimensions can be seen in Figure below. Diameter is simply the length from tip to tip of the propeller. The pitch is the length the propeller would move forward in one revolution in an ideal fluid. It can be explained as similar to the wavelength of a wave, the distance moved in one cycle. The pitch of the propeller is dependent on the angle of twist of the blades on the prop. On most propellers however the angle of the blades changes with the position on the radius of the prop.
The propeller supplies the thrust for the propulsion system. The thrust that is created by the propeller is difficult to predict because there a several inter-related parameters that effect the thrust. Parameters include are prop dimensions, flow speed, prop angular velocity, air density, and prop efficiency. The equation for propeller efficiency is:
Where V is the plane velocity, T is the output thrust, and the P is the power output of the motor.
© TESTING AND SELECTION
Many propellers are designed for specific motors, or rpm, while others are designed for specific size planes. This necessitates experimental testing in selecting a prop for a plane. At the start of the component selection process the propeller selection was driven by both the weight and diameter, which affect the maximum dimension of the MAV. When motors were first purchased for the MAV, several different propellers, in the correct size range, were also purchased for testing.
A receiver is the part of the plane that receives the information from the pilot controlled transmitter. Receivers have an antenna attached to them and also have a power input plug and output plugs to the servo flap controllers. The type of receiver required by the Hi-Tech transmitter was FM channel 37. The weight was the most important parameter to the design, providing the receiver was compatible with the transmitter. Some of these receivers have a speed controller built into them for controlling the motor speed. Speed controllers are not entirely necessary for a model plane but do allow the pilot to "throttle back", which can save battery life significantly.
The antenna that comes with a receiver is usually a 39 inch light wire antenna soldered to the receiver. This antenna is designed to hang off the back of the plane and receive the communications. It seemed however that this type of antenna would create a noticeable drag force at this scale of aircraft. For that reason alternate antenna designs were researched. Upon further reading it was found that the antenna was stiff and was 5 inches long. The normal antenna is just a wire connected to the receiver. This antenna is simply an inductor and small capacitor wired in parallel at the base of the wire antenna. The capacitor and coil effectively tune the receiver to pick up a 72 MHz signal from the transmitter, at a much shorter length than normal.
Servo motors are small electric motors that move the control surfaces of the plane. Servos are plugged directly into the receiver, which powers and controls the servo motors. The lightest servos found were the Wes-Technique 2.4 gram linear servo. The Wes-Technique servos work quite well but are rather fragile. Several servos were broken during initial flight tests. A 3 gram model of the same design was tested as well and seemed to be much more crash resistant that the smaller servos. The 2.4 grams servos are ideal however and will last if they are mounted and protected adequately.
3.3 WORKING PRINCIPLE
Newton’s first law states a body at rest will remain at rest or a body in motion will continue in straight-line motion unless subjected to an external applied force. That means, if one sees a bend in the flow of air, or if air originally at rest is accelerated into motion, there is force acting on it. Newton’s third law states that for every action there is an equal and opposite reaction. As an example, an object sitting on a table exerts a force on the table (its weight) and the table puts an equal and opposite force on the object to hold it up. In order to generate lift a wing must do something to the air. What the wing does to the air is the action while lift is the reaction.
Let’s compare two figures used to show streams of air (streamlines) over a wing. In figure 3.6 the air comes straight at the wing, bends around it, and then leaves straight behind the wing. We have all seen similar pictures, even in flight manuals. But, the air leaves the wing exactly as it appeared ahead of the wing. There is no net action on the air so there can be no lift. Figure 3.7 shows the streamlines, as they should be drawn. The air passes over the wing and is bent down. The bending of the air is the action. The reaction is the lift on the wing.
As Newton’s laws suggests, the wing must change something of the air to get lift. Changes in the air’s momentum will result in forces on the wing. To generate lift a wing must divert air down; lots of air.
3.3.1 FORCES ACTING ON AN AIRCRAFT
There are four forces acting on the airplane all the time during airplane is flying. The four forces are (1) Lift, (2) Gravity force or Weight, (3) Thrust, and (4) Drag. Lift and Drag are considered aerodynamics forces because they exist due to the movement of the Airplane through the Air.
LIFT: It is produced by a lower pressure created on the upper surface of an airplane's wings compared to the pressure on the wing's lower surfaces, causing the wing to be lifted upward. The special shape of the airplane wing (airfoil) is designed so that air flowing over it will have to travel a greater distance and faster resulting in a lower pressure area (see illustration) thus lifting the wing upward. Lift is that force which opposes the force of gravity (or weight).
Lift depends upon (1) shape of the airfoil (2) the angle of attack (3) the area of the surface exposed to the airstream (4) the square of the air speed (5) the air density.
WEIGHT: The weight acts vertically downward from the center of gravity (CG) of the airplane.
THRUST: It is defined as the forward direction pushing or pulling force developed by aircraft engine. This includes reciprocating engines, turbojet engines and turboprop engines.
DRAG: It is the force which opposes the forward motion of airplane. Specifically, drag is retarding force acting upon a body in motion through a fluid, parallel to the direction of motion of a body. It is the friction of the air as it meets and passes over an airplane and its components. Drag is created by air impact force, skin friction, and displacement of the air.
3.4 HOW TO CONTROL MICRO AIR VEHICLES
Pilot reports and video recording through a small on-board camera indicate that our flexible wing micro air vehicles have unusually smooth flying characteristics. Most of our test flying is done using conventional RC equipment at close range, keeping the vehicle in continuous visual contact. Because of the small size of the vehicles, flying at distances greater than about 100 feet can quickly cause loss of orientation unless the pilot is flying by monitoring the video output from an on-board camera. The RC transmitter produces a radio frequency signal that causes the RC receiver carried in the vehicle to develop a series of pulses of varying pulse widths (pulse width modulated or PWM) that are delivered to the control surface servos as the command signals for the desired positions of these surfaces. On the equipment we use, the pulses are generated at a constant frequency of 40 Hz. To capture this control input information, we have developed a simple system that uses a second RC receiver on the same RC frequency as the flight unit to monitor the pulse widths of the servo signals on the various servo terminals on the receiver and store the data on a notebook PC. With this system, stick input data is recorded without any contact or interference with the pilot or the micro air vehicle and the recording can be done at any reasonable range within the operating range of the RC system (at least 1100 meters).
INTRODUCTION OF FLAPPING WING MECHANISM
When we go through the flying creatures of the nature all of them are capable of initiating lift in flight by flapping of wings. Thus the wing flapping is a universal method of sustained biological flight propulsion. There are flying creatures weighing up to 15 Kg, which effectively use this wing flapping for flying. Thus the idea of adopting this feature to the Micro Air Vehicle was a good one.
Consider the case of MAV with fixed wing and propeller. The lift for flying is created by the airflow from the propeller of the vehicle towards the wings and the wings will support its weight. This lift is proportional to the wing area and square of the velocity of airflow over the wings. Smaller the linear dimension of the vehicle the less lift it can apply because the area of the wing decreases as square of the linear dimension. One solution to counter this effect is to increase the velocity of the vehicle. But the velocity cannot be increased in situations such as indoor missions where a MAV makes the most sense. So for creating more lift the flapping wings are used. By the flapping wing mechanism the vehicle can hover in the air like insects. Only helicopter presently achieve hovering. But micro helicopters are not feasible because of the mechanical problems of balancing them with tail and gyroscope etc.
4.1CHARACTERISTICS OF FLAPPING WING MECHANISM
In the micro air vehicles with flapping wings the lift is generated in two ways,
(1) By the air flow created by the vehicle speed.
(2) Wing flapping to support the weight of the vehicle.
So if the scale is reduced, i.e. the size of the vehicle is reduced, the frequency of beating can be increased without affecting the minimum velocity of the vehicle. Since this design is not affected by the scale changes the size of an air vehicle can be reduced to a size of millimetres as observed in nature.
Another feature of this flapping-wing mechanism relates to the minimum speed of the vehicle and the ability to perform short take offs and landings. Actually a vehicle with flapping wing provided with enough power can take off and land vertically. So the fixed wing mechanism is very limited in low speeds, thus requiring significant distance to increase its speed before attaining flight speeds. The lift is generated in the flapping wings mainly by shedding of vortices. Initially, as the two wings starts motion from the clap position at the back, according to Kelvin’s circulation theorem, the net circulation around the insect body is zero. Then as the insect beats its wings back and forth, high lift coefficient is built up due to vortices shedding from the trailing edge to generate necessary circulation around the wings.
In ordinary insects, this build-up of high lift coefficient is delayed, because it takes some time after flapping is initiated for the insect to attain the required velocity of wing beating and the circulation around the wing to be developed. Hence to counter the delay, one species of insects, Encarsia Formosa, precedes each beat with a special movement, which causes the necessary circulation to occur immediately and avoid any delay in built up of maximum lift. This model, named after the discoverer Weis Fogh, consists of a pure rotation, followed by a translation and a rotation of the wings.
4.2 WIES FOGH’S 2-D MODEL
For a body starting to move in a fluid at rest retains zero circulation of fluid around it. This retards the generation of lift on the body. But when the body brakes into two pieces, there may be equal and opposite circulation, around these, each suitable for generating the lift required.
This model is based on the sequence of motions of the wings of an insect, Encarsia Formosa. The figure given above illustrates the wings movements, which constitute the Weis Fogh mechanism.
At certain instant in the wing flapping cycle, the insect performs a clap, i.e. both the wings are clapped together [(a), (b)]. The two wings then starts to fling open but are still connected to each other at the bottom so that these effectively form a single body [©—(e)]. Then the wings break apart and are completely separated [(f), (g)]. When the wings break apart, the circulation around each wing is in the opposite directions to generate enough lift. The motion of the wings described above is not one of pure rotation, or pure translation. To generate this class of motion accurately, multiple drives with feed back position control on each axis are required.
4.3 FLAPPING MECHANISM
The mechanism used for flapping of the wings is a six-link mechanism. This is designed according to the position of the wings in the Weis Fogh’s model. The positions (a), (e) and (h) are the three main positions of the wings, on which synthesis is done by kinematic inversion method.
4.3.1 DESIGN STEPS
The guidance mechanism is a double rocker. To power this, Weis Fogh’s mechanism using a continuously driven motor, a crank rocker arrangement has been designed. In the figure O3CAO2 represents the driving crank rocker arrangement, with O3C as crank and O2A the rocker. O1BAO2 represents the rocker-rocker pair and the coupler BA has the wing attached to it. The procedure for developing the mechanism is as follows. First the flapping mechanism O1BAO2 is designed. For the known positions of the wing, assume positions of fixed pivots O1 and O2. Subsequently, if a good solution is not obtained iteration with alternate solutions are needed.
Inverting about the extended coupler (wing), the points A and B on link coupler are determined. The links O1A and O2B are the driver and the follower of classical four-link mechanism, respectively. The criterion for a feasible solution is that the links should be in the range 1 cm to 4 cm for ease of manufacture and miniaturization. Hence if the criterion is not satisfied then steps 1 and 2 are repeated until this criterion is satisfied for all the links.
Next the driving mechanism, O3CAO2 is designed. In this case the position of link O2 is known from the design of flapping mechanism. It is also known that the link O3 is a crank. A position for O3 is first chosen on the bisector of the angle formed by the two extreme positions of link O2. Iterate for some lengths of O3C, and different positions of O3. A good design, satisfying the conditions in step 3, was obtained after six iterations.
ADVANTAGES, DISADVANTAGES AND ITS APPLICATION
a) It can be packed to 1/10 of original size.
b) It has Low mass.
c) It requires Low power requirements.
d) It has High reusability.
e) Outdoor NBC emergency reconnaissance
f) It has High stability and control.
g) It has High lift and slow landing speed.
h) Direct connectivity.
i) It can be individually controlled.
j) It can be used for a wide range of new missions.
a) The most important problem encountered so far is the quality of images taken from onboard TV equipment.
b) Micro air vehicles have small range of communication with controller.
c) It is very difficult to design flapping wing mechanism.
d) It is very difficult to control it in bad weather condition such as heavy rains etc.
e) It is very expensive to design micro air vehicles.
f) Lithium batteries which are generally used in MAVs are relatively expensive, particularly with reference to the common alkaline batteries with their widespread commercial use.
In contrast to higher-level reconnaissance assets like satellites and high altitude UAVs, MAVs will be operated by and for the individual soldier in the field as a platoon-level asset, providing local reconnaissance or other sensor information on demand, where and when it is needed. MAVs may also be used for tagging, targeting, and communications, and may eventually find application as weapons, as well. The reconnaissance application is a primary driver behind the first generation of MAVs. Micro sensors like those mentioned earlier suggest the possibility of reduced latency and greatly enhanced situational awareness for the small unit or individual soldier. Additionally, the MAV's ability to operate in constrained environments like urban canyons and, eventually, even the interior of buildings, gives these systems a level of uniqueness unmatched by other concepts. MAVs are not replacements for previously manned air vehicle missions; because of their size, they will be capable of completely new missions not possible with any existing systems.
MORE ON MISSIONS
Micro Air Vehicles will be capable of a wide range of useful military missions. The one most often identified by users is the textbook, "over the hill" reconnaissance mission illustrated in Figure 5.1. The current concept suggests that reconnaissance MAVs need to range out to perhaps 10 km, remain aloft for up to an hour, reach speeds of 10 to 20 m/s (22 to 45 mph), and be capable of real time day/night imagery.
In contrast, some surveillance applications may require less range - payload performance. In these instances, the MAV would relocate to a suitable vantage point and serve as a fixed, unattended surface sensor with capabilities ranging from imagery to seismic detection. At the same time, MAVs must be launched and operated relatively simply with an easy-to-operate ground station. Ground stations may employ directional antennas to maintain contact with the MAV at long range.
In urban operations (see Figure 5.2), MAVs, acting in small, cooperative groups, will enable reconnaissance and surveillance of inner city areas, and may serve as communication relays.
They may also enable observations through windows, and sensor placement on vertical and elevated surfaces. Their application to building interiors is the most demanding envisioned. The capability to navigate complex shaped passageways, avoid obstacles and relay information will require yet another level of technology.
Biochemical sensing, illustrated in Figure 5.3 is another potential mission for MAVs. With gradient sensors and flight control feedback, MAVs will be able to map the size and shape of hazardous clouds and provide real time tracking of their location.
MAVs may also find application in search and rescue operations. An MAV could be packed into the ejection seat mechanism on fighter aircraft. If the pilot has to "punch out", the MAV is released from the ejection seat and lingers in the air for up to an hour, providing the downed pilot with reconnaissance information, or sending a signal to rescue vehicles.
A large number of potential commercial applications also exist. These include traffic monitoring, border surveillance, fire and rescue operations, forestry, wildlife surveys, power-line inspection and real-estate aerial photography.
5.4 FUTURE WORK
Development of a mechanism that simulates Wies Fogh’s model is the first step towards developing an MAV. A robust model of the MAV using this concept is being fabricated to measure the resulting lift. The future work involves the design for forward moving, reducing the weight of the device using lighter materials, including the structure of the device, the controllers, the gyroscope for stabilizing, and developing small power sources. Also there will be design improvements to achieve high flight speeds better stability in air, etc. The overall aim will be to minimize the size and weight, to increase the speed, and to maximize the battery life for this MAV.
Micro Air Vehicles are a class of UAVs whose time has just about come. Micro Air Vehicles are the new development of the technology by which a variety of operations are done. An approach to design a flying mechanism different from the approaches being followed by the researchers around the world has been described. Simple flapping, which is an oscillatory motion of the wings about a fixed axis does not generate sufficient lift. Although biological have reported the efficiency of the Wies Fogh model the design using this model has not been attempted so far. In the seminar and presentation, we highlight the recent research and development in establishing a flexible-wing-based technology for MAVs. Our effort addresses the entire scope comprehensively, including basic concept, novel fabrication approaches, aerodynamics investigations, and flight test data. The outcome is improved understanding of the key issues related to robust and stable flight and vehicle durability.
Joined: Apr 2012
27-07-2012, 09:26 AM
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