micro air vehicle and its flapping mechanism full report
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17-02-2010, 06:28 PM

.doc   MICRO AIR VEHICLES AND ITS FLAPPING MECHANISM.doc (Size: 1.28 MB / Downloads: 483)

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Micro air-vehicle is a small flight vehicle that uses a lift-generating mechanism that is different from the mechanism used for large aircrafts. In micro air- vehicles, one of the methods for creating lift is a special flapping mechanism, which consists of translation and rotational motions of wings. This is adopted from one species of insect Encarsia Formosa. For this purpose, a 6-link mechanism is designed for three positions of wings by synthesis using kinematic inversion, and dynamic force analysis is done and the lift coefficient calculated is comparable with the lift of a well-designed airfoil.
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 defense 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 for lift generation in these smaller vehicles is of different types like using rotary wings and using flapping wings.
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 of 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 with out 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.
The low Reynolds number regime is significant in that it project and implimentations a fundamental shift in physical behavior 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 multifunctionality 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.
In contrast to higher-level reconnaissance assets like satellites and high altitude UAVs, MAV will be operated by and the individual soldier in the field as a platoon-level asset, providing local reconnaissance or other sensor information on demand, where 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 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. Micro Air Vehicles will be capable of a wide range of useful military missions.
Micro air vehicle are capable of a wide range of useful military missions. One of the most important functions is Ëœover the hillâ„¢ reconnaissance illustrated in the figure below.
In this concept 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). For controlling and launching the MAVs directional antennas are used.
In urban operations 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.
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.
In the case of a challenge of developing self propelled MAVs assisted propulsion can be used i.e., unpropelled MAVs can be launched from over head flight vehicles so that they can target above an area giving information to the operators.
A large number of potential and commercial applications also exist. These include traffic monitoring, border surveillance, fire and rescue operations, forestry, wild life surveys, and power-line inspection and real estate aerial photography.
Biochemical sensing, illustrated below 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.
According to the use of the micro air vehicles they are classified into different types. And also they show a difference in the mechanism used to fly the vehicle. One of them uses a propeller for creating the lift for flying and another type uses a flapping wing mechanism for getting more lift for more effective flying.
The development and fielding of militarily useful MAVs will require overcoming a host of significant technology and operational obstacles.
The physical integration challenge is believed to be the most difficult problem, the degree of which increases dramatically with decreasing vehicle size or increasing functional complexity. At and below the 15 cm scale size, the concept of "stuffing" an airframe with subsystems - our conventional approach to hardware integration - becomes extremely difficult.
Many of the system functions depicted will be provided by microelectronics-based components. Even so, separate modules for each function would consume more volume than may be available. From an electronics perspective, the on-board processor and communications electronics form the core of the vehicle. They provide critical links between the sensor systems and the ground station, and they are vital to the flight and propulsion control systems. In the scheme depicted in Figure 4, Power generation and propulsion subsystems support critical electronics and flight control functions in addition to flight propulsion power. The multifunctionality required by the MAV weight and power budgets may be achieved only by a highly integrated design, with physical components serving multiple purposes, or accomplishing multiple and often diverse functions. For example, the wings may also serve as antennae or as sensor apertures. The power source may be integrated with the fuselage structure, and so on. The degree of design 'synergy' required has never been achieved in a flight vehicle design.
Flight control is the single technological area, which harbors the largest numbers of unknowns for the MAV designer. The laminar-flow-dominated flight environment can produce relatively large forces and moments, and they are difficult to predict under all but the most benign flight conditions. Unsteady flow effects arising from atmospheric gusting or even vehicle maneuvering are far more pronounced on small scale MAVs where inertia is almost nonexistent, that is, where wing loading is very light. Platform stabilization and guidance will require rapid, highly autonomous control systems.
One common trend in aircraft and in nature is that smaller flyers travel slower and tend to have a higher ratio of wing area to vehicle weight. Given the limited wingspan available, MAVs may have to achieve high relative wing areas by having larger chords, i.e. by using configurations with low aspect ratio (wingspan divided by chord), more like flying wings, or butterflies. So MAVs may have to cope with fully three-dimensional aerodynamics. Here, there are even less low-Reynolds number data available than there are for two-dimensional airfoils. To make matters worse, MAVs will experience highly unsteady flows due to the natural gustiness (turbulence) of the atmosphere. Interestingly, natural flyers of the same scale use another source of unsteady aerodynamics, flapping wings, to create both lift and propulsive thrust. For some applications, MAVs may ultimately have to do the same.
Small-scale propulsion systems will have to satisfy extraordinary requirements for high energy density and high power density. Acoustically quiet systems will also have to be developed to assure covertness. The power required can be reduced considerably by having low wing loading, achieved in MAVs by having large wing areas and lightweight vehicles. The Gossamer Albatross had a huge wing area (and low weight) so that a very weak engine could power it. But this was done with huge wingspans. In contrast, the 15 cm limitation means MAVs may have to maximize area by increasing the wing chord, leading to low aspect ratio configurations. Finally, there is nothing more effective than low weight to reduce power requirements. High energy density (i.e. light-weight) power sources are essential. Battery-based systems will likely power the first generation MAVs, but more exotic technologies like fuel cells are being developed for follow-on systems.
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.

In the micro air vehicles with flapping wings the lift is generated in two ways,
By the air flow crated by the vehicle speed.
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 millimeters 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 Kelvinâ„¢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 vorticity shedding from the trailing edge to generate necessary circulation around the wings.
In ordinary insects, this buildup 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.
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 below 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 [©â€(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.
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 Foghâ„¢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. This mechanism is driven by an electric motor, and the output is an extension attached to the coupler AB.
The guidance mechanism is a double rocker. To power this, Weis Foghâ„¢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.
The dynamics and the resultant loads were analyzed. The links were assumed to be of steel (width 5mm and thickness 1mm). The maximum acceleration occurred at the position of about 1200 of link O1B. This corresponds to the position of wing given in fig 7(e) and the second reference position of the wing in the design. Hence a combined static and the inertia force analysis of the flapping mechanism at this position are done. The assumptions are as follows.
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.
The analysis is used to compute the forces on the pin joints and the links. From the analysis, the maximum force is on the joint O2 and is of magnitude 31.38 N. When the mechanism is oscillating, there are unbalanced forces acting on it. A further detailed analysis would have helped in balancing the unbalanced forces on this mechanism. In a flying mechanism, imbalances are damped due to viscous interactions in the low Reynolds number regime. All the horizontal forces will be cancelled by oppositely working mechanisms for the two wings.
An estimate of the lift, which would be generated by such mechanism, has been evaluated. The lift per unit width, L is evaluated in terms of the circulation, , present around each wing and then calculated as the product of the density, , chord length, c, and the forward velocity, U of the device. For the type of motion presented, the circulation is found to have a value given by = 0.69 c2, where is the angular velocity of the wing. The lift on the wing can be expressed in terms of lift coefficient CL, defined as
CL= L/(0.5* U2)
This leads to
CL= (1.38 c)/U
For the designed apparatus, the angular speed will be about 60 rpm and c would be around 15-cm. This would give a lift coefficient of 1.38 for a forward speed of 1 m/s. This is comparable to the lift on a well-designed airfoil at an angle of attack of about 6-80.
Development of a mechanism that simulates Wies Fogh 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 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.
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
TECHNICAL CHALLENGES--------------------------------------------------------6
INTRODUCTION TO FLAPPING WING-----------------------------------------9
CHARACTERISTICS OF FLAPPING WING-----------------------------------10
WEIS FOGHâ„¢S 2D MODEL----------------------------------------------------------11
DESIGN STEPS-------------------------------------------------------------------------13
ESTIMATED LIFT--------------------------------------------------------------------15
FUTURE WORK-----------------------------------------------------------------------16
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06-07-2012, 04:53 PM

add more details on micro aerial vehicle's drawbacks... and how to over come themDodgy

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