report of bio-inspired wings edited 9.11

7/24/2019 Report of Bio-Inspired Wings Edited 9.11

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Aerodynamics CA2

Report of Bio-Inspired Wings

by

Benjamin Karsch G1503092F

Juhi Gurnani G1503090A

Li Jianan G1503095HRahul Sharma G1503103A

Vijay Shankar G1503104J

M.Sc. in Aerospace Engineering

Report of Aerodynamics Module

Oct to Nov 2015

Nanyang Technological University

Technische Universitat Munchen


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1 Introduction

In order to design aircrafts which can achieve optimal efficiency in various flow conditions, in-

spiration from the natural flyers such as insects and birds needs to be taken. Bio-wings are one such

feature wherein the wings of an aircraft are modified to replicate certain features of insect/bird wings

so that higher lift can be generated while reducing the amount of drag incurred in unsteady flow con-ditions.

Bio-inspired wings have unique characteristics, from morphing wings to flapping wings which

can be extracted for the development of various micro aerial vehicles (MAVs) and even UAVs. Flap-

ping wings perform very well in small spaces, are very small, and have low energy consumption,

morphing wings give an ability to change to wingspan, camber or the whole planform dynamically,

therefore it is necessary to understand these concepts from the various range of birds, insects and bats.

Computational fluid dynamics (CFD) is the use of applied mathematics, physics and computa-

tional software to visualize how a gas or liquid flows as well as how the gas or liquid affects objectsas it flows past.[1] Almost all Computational fluid dynamics solvers are fundamentally based on

solving the Navier-Stokes equations, however unsteady aerodynamic vortex models, potential flow

models are also used in combination to achieve exact characteristics of bio-inspired wings.

From the range of published papers, it is a fact that most insects are only capable of flight through

several unsteady effects induced by flapping their wings, where formation of leading edge vortices

are concluded to be the most important factor in determining the lift force produced. Hence it is nec-

essary to obtain a quantified understanding of bio-inspired wings.

It is known that the major challenges in the analysis of insect flight are the design of appropri-

ate modeling and recording techniques, and the difficulty in acquiring accurate experimental force

measurements [2]. Due to the scale of MAVs in correspondence of the insects or bats, model for

experimentation becomes really tricky and often some corrugation over the surface of the wings is

ignored, leading to a distinct change of result from the desired one and further coupled with the high

frequencies associated with wing flapping, CFD is a better option to study and analyze.

Often, despite major differences in geometry, it is found that with the simple wing (without cor-

rugation, or surface roughness) used in experimentation, identical aerodynamic characteristics were

observed. However near wing flow-field shape and pressure distributions along the chord concludes

that the simple wing cannot provide the same detailed information [2]. Similarly, for the various range

of models, based on complex bio-mechanic, bio-inspired animals, every little detail is necessary toobtain accurate relations and therefore, a study must complement both CFD analysis and experimen-

tal analysis.

2 Brief Background and Theory

2.1 Basic Terminologies

Many terminologies are the same as that for fixed-wing aircraft. Refer to figure 1[3].

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Figure 1: Basic Terminologies

Wing lengthrefers to the length from the base to tip of one piece of single wing. Wing spanmeans

the length between the two tips of the wing when they are fully stretched. Wing chordmeans the

distance from the leading edge to the trailing edge of the wing starting from any point. Attack of

angle is the angle between free stream and body x-axis. However, a more effective quantity can be

defined known as theeffective angle of attackwhich can be written as:

e= tan1

h

V (1)wheree is the effective angle of attack, is the angle of attack, h is the velocity due to plungemotion, V is the velocity of the free flow.

Upstroke and downstroke are the motions when the wing flaps upwards and downwards respec-

tively. Supinaitonmeans to put the ventral surface of the wing upward at the transition from down-

stroke to upstroke;pronationmeans to put the dorsal surface of the wing downward at the transition

from upstroke to downstroke.

Three kinds of commonly observed wing-tip trajectories are elliptical, figure-8 and double-figure-

8 as shown in figure 2 [4].

Figure 2: Wing tip trajectories

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2.2 Coordinate Systems

The simplified model for a flapping wing robot is assumed to be a rigid body with 6 degrees of

freedom. A graphical representation of the robot and associated reference coordinates are presented

in fig 3.

Figure 3: Flapping wing robot diagram with reference coordinates

To describe the dynamic model, two coordinates systems are considered, namely the body frame

{B}and the inertial frame{I}. The body frame is attached to the center of gravity near the center ofwing root. Three basic positional angles are used here. First one is stroke angle about the x-axis

in body frame. The second is the elevation or deviation angle describing the rotation of the wingabout the z-axis. The third is the angle of attackdescribing the rotation of the wing about the y-axis.Another important concept is the stroke plane. Stroke plane is defined as the plane that swept by the

wing from the wing base to the wing tip.

2.3 Governing Equations

The governing dynamic equation are 3 dimensional Navier-Stokes equations in the condition of

unsteady, incompressible flow. The governing equation can be written as:

V

t+ V V=

1

p +

2V (2)

V=0 (3)

where is the density of the fluid which can be air. is the dynamic viscosity of the fluid. pis thepressure.Vis a velocity vector which meansV= [u v w]T whereu,v,ware three component speed inx,y,z direction. is the gradient operator.

2.4 Clap and Fling Mechanism

Clap and fling mechanism are thought to be two important mechanism that could explain the thereason of high lift generated by the wings. Refer to the figure 4 [5].

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Figure 4: Clap and Fling Mechanism

AB depicts the mechanism of clap. As the wings clap ventrally, the air above the wings is accel-

erated as a consequence of suction while the air below the wings is squeezed. The accelerating air

produces less pressure while the squeezed air has higher pressure. CD explains the mechanism of

fling. As the wings stretch, the air above the wing is sucked and thus accelerated which generates less

pressure. Furthermore, in both the two mechanism, theleading edge vortex(LEV) plays an important

role. The flow above the vortex is pull in and down by the cortex to generate lift.

3 Suitability for the Research

The suitability of flapping wings especially in the MAV (micro air vehicles) domain majorlydepend upon the scaling invariance of both structural dynamics which is completely different from

the conventional fixed wing and a complete paradigm shift in fluid dynamics. Satisfying both of

the aforementioned dynamics is fundamentally difficult to scale. The study of bio mimicking and

its gamut of applications will certainly impact and revolutionize various monitoring and security

applications. Their reconnaissance and prolonged hovering with ease make them a better choice

against fixed wing MAVs. There are many challenges in mimicking bio-wings and the challenges are

further amplified due to their small size and light weight. One of the challenging aspects in the study

of bio-inspired flapping is in computational mechanics, because the flow here takes place with moving

interfaces which also encompasses fluid structure interactions. Adding to the challenge of developing

a bio-inspired wing are the highly coupled non-linearities in flight dynamics, aeroelasticity and controlsystems.

3.1 Various Research Inferences

In case of flapping wings inspired by insects, which are intrinsically anisotropic in nature, due

to their extremely thin membrane like wings which are primarily controlled by the root muscles and

the vein network spread throughout the wing, making it really difficult to mimic [7]. It is difficult in

measuring the exact force measurements of the flapping wings of insects directly owing to the cyclic

nature of the aerodynamic forces. The study of hawkmoth model by Coen van den berg et.al., [8]

revealed that vortices were formed along leading edge which moved towards the wing tip, the vortexdiameter were smaller than predicted for linearly translated wing and a strong axial flow was observed

to increase the stability of the vortex. The vortex ring velocity was good enough to provide sufficient

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lift for the weight support, it was also clear with aforementioned inferences that it is dynamic stall not

the wing rotation responsible for the leading edge vortex production.

Similarly, A. Agarwal et.al., [9] in their experiments for bio inspired synthetic wings by mimicking

the hawkmoth, the bio wing was constructed with latex similar to membrane and the veins were

mimicked with the help of nylon, rubber and carbon. Their results substantiate the improvements ofthrust by the flexible wings for all kinematic patterns in comparison to the rigid wing structures. Also

the experiment results relating to the aerodynamic significance of the flexible wings show potential

advantages relating to the performance enhancement with flexible wing designs. The results gives

adequate amount of motivation for further research, the author [9] suggests a careful aero-elastic

tailoring of the flexible wing can result to an efficient design.

3.2 Various challenges in bio-inspired wing design

The variation in the Reynolds number and its relationship with leading edge vortices (LEV) and

span wise flow structure has a substantial effect on the aerodynamic forces which are generated by theflapping wings. The advantage from the conventional fixed wing structures, the LEV in the flapping

wing structures can interact with the tip vortices (TiV) without increase in the power requirements

contrary to the fixed wings system. At the same time, there are many challenges (as mentioned in the

previous section) in practical applications of the flapping wing type MAVs a perfect wing tail coor-

dination is sacrosanct, or else there are greater chances of it falling prey to heavy gusts and making

it difficult in manoeuvring away from objects in its flight path. The modelling approach in the, paper

[14], of the kinematics of the flapping flight does not consider the rotational aspects like centripetal

and Coriolis features, while retaining the vortex dynamics. As a result of which, the resultant model

is deprived of rotational manoeuvring features.

As expressed in the earlier sections regarding the importance of the vortices which forms and

sheds according to the position of the wing during the flapping process and varied wing shape, impact

the lift and thrust of the flapping wing type MAVs to great extent. Also there are various aspects

which also account for lift and thrust, for example Fig. 4 shows the clap and fling mechanism of

the chalcid wasp, where it was found [10] that (based on steady state approximation) the generated

lift was insufficient for the body to stay afloat, similarly there are various other phenomena like

rapid pitch rotation, wake capture, LEV, TiV, and passive pitching [14]. The mentioned mechanisms

though they are unsteady due to the flapping wing nature of the system, but they form a fundamental

aspects of explaining the nature of flight. Meanwhile, we can also see how complex is the system

of flapping wing flight mechanism as compared to the conventional fixed wing, also giving greater

room for further research in various aspects of mimicking nature and simulating them for the real time

applications.

3.3 Implications and suitability of bio-inspired wing design

The primary misconception, that the bending of wings during the flight of a flapping wing sys-

tem as a function of only aerodynamic forces were investigated by various researchers [11], and it

was substantiated that the contribution of the aerodynamic forces were very small as compared to

the contribution of the inertial-elastic forces during the flapping motion. For example the corrugated

wing structure of the dragon fly brings in more local variations in the structural composition, also

it has been shown [12] [13] [6] that the corrugation in the wing increases the warping rigidity andthe flexibility, thereby improving their fatigue fracture limits, also it is an advantage having corru-

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Species Reynold no. (Re) LEV

Hummingbird and insect 104

Span wise distribution, changes with

variations in Re. wing sizing and flapping

frequency

Hawkmoth and honey bee 103-104

LEV has a spiral structure and brakes

down at the middle of the down stroke

Other insects 105-106

Doesnt improve lift, due to dynamic stall

vortex on the oscillating airfoil is found

to break away and convect elsewhere

Table 1

gated structure in wings as they become more independent to the variations in the Reynolds number.

The Reynolds number is important factor if we talk about various categories of wing shape and de-

sign, which is again dependent upon the carrier species. The following table 1 gives a brief idea of

Reynolds number regimes for various categories of species, we also know that LEV is a very impor-tant phenomena in the flapping wings aerodynamics as discussed in section 2.2, table 1 also shows

how LEV gets affected.

The author [14], states that there is a need for significant work to have better cognizance of in-

teraction between structural flexibility and aerodynamic performance under unpredictable wind gust

conditions. Thereby, refining the current state of knowledge regarding the correct balance between

kinematics and aerodynamics, and inculcating the updated research results for a better bio inspired

wing model. It is also important for researchers to have close coordinated computational and experi-

mental framework in order to explore larger domains of design space, so that they can mitigate issues

like wind gust and object collision. It is evident from the paper and the number of references thatthere is enough scope and motivation for researchers to increase the horizon of the present available

MAV to technology towards bio inspired wings, for which significant progress is still required.

4 Discussions

[Comparison to the other techniques]In this section the different techniques of wing morphing

will be compared. Focus here will be on the morphing if the airfoil control surfaces. This can replace

the conventional control surfaces and has the advantage of providing smaller drag. By doing this the

wing will be deformed. There are two basic wing deformations, such as in-plane and out-of-plane

[15],which is considered as one of the wing morphing classification. Examples are given in Fig. 5.

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Figure 5: Wing deformation modes [15]

Another classification is based on the mechanism that causes the wing to deform. First mecha-

nism, rotating the entire or only selected segments of the wing. Second, telescopic and third inflating

the entire wing or only selected segments. The approximating of the general shape based on one-

degree freedom concepts is rare and therefore, multi-degrees of freedom concepts have been proposed.

For the shape control of the wing, conventional and compliant mechanisms for one-degree freedom

have been used. Therefore, morphing was limited for only one single airfoil section. There are many

mechanisms that have been developed for wing morphing. All of them are based on the advances in

new technologies for material, actuators and sensors. This is the foundation of wing morphing. One

common smart material that has been used is shape memory alloy (SMA) miniature actuator. High

power to mass ration, frictionless actuation and silence are the most important properties why there

are used [15].

(1) The first mechanism was proposed by Krang [15, 16]. SMA wire actuator, spar, rib, wing skin

and quadrilateral frame was used to compose the wing morphing. The lower quadrilateral frame part

was fixed using spars and ribs, while the upper and lower surface where bounded by the skin. The

Actuator was installed in a way that it was linked with the front and fixed at the rear lower part of the

frame. The SMA wire is activated by temperature, if the actuation temperature is reached the wire

will shrink and the quadrilateral frame becomes deformed. Due to the frame deforming the upper

skin moves backward, while the tailing edge shifts downwards. The deformed airfoil acts like a flap.

Interesting here is that the length of the tailing edge does not change. Disadvantage of this mechanism

is the maximum current to activate the actuator to prevent overheating and that the shape change has

to be smooth to avoid aerodynamic losses due to flow separation [15, 16].

(2) Another mechanism to control the airfoil camber for wing morphing is by substituting the

traditional split flap with a smooth morphed flap. The combination of two different benefits from a

variable geometry truss and rods made of SMA are used to design the structure that is capable of

changing it shape. Advantage of this approach is that it takes into consideration, that the change in

structure has to be able to withstand the external loads while changing it shape [15, 17].

(3) Another mechanism to achieve the required morphing shape an octahedral unit cell truss struc-

ture has been developed. It is supposed to undergo large complex shape variations. Compliant joints

are used to transmit the moments. Tensioned cables were used to connect eight rigid links. One unitcell of structure consisted of these eight links. The SMAs were in a cylindrical length and created the

compliant joints. Many different morphing shapes have been achieved by modifying the dimensions

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of trusses for achieving the deflections [15, 18].

(4) One-degree freedom allows the wing to continuously morph from a planar to non-planar. The

mechanism consists of a serial chain of one-degree freedom. The chain is formed from four joints

(quaternary) links, which forms the main structure. Using a two joint (binary) links to actuate the

quaternary links. The first section of the chain is the input to the following sections through the setsof binary links. By rotating the quaternary link every other quaternary link is rotated as well. On the

quaternary links the airfoil rib sections will be mounted. The quaternary links form the main wing

structure and therefore the mechanism approximates the non-planar shape in a piece wise linear man-

ner according to the number of quaternary links. The advantage of this mechanism is that an aircraft

can achieve efficient cruise and high maneuverability [15, 19].

(5) Another one-degree of freedom concept is based on compliant ribs and intended to replace the

conventional ailerons. The structural design used compliant ribs to allow the change in shape. Since

the ribs are free of hinges and mechanisms, the deformation is achieved by the distributed compliance

of the rib structure. The skin was made out of carbon-fiber-reinforced plastic. On the lower surfacethe skin is open to allow elongation. Furthermore, stringers are used on both sides to prevent skin

buckling, limiting aeroelastic deformation and interpolating the shape between the ribs. To control

the deformation each rib was equipped with a actuator, which can either act on the skin or on internal

points of the rib trusses. Allowing to produce tensile and compressive forces. Advantages here are

that it can produce sufficient roll control, but there is a strong dependence from the generated rolling

moment coefficient and the weight of the structure [15].

(6) So far we only consider techniques based on the one-degree of freedom. Multi-degree of

freedoms concepts also have been proposed, one of them will be introduced now. The proposed two-

degree of freedom mechanism requires only two SMAs actuators to morph the structure. Importanthere is that the design allows a complete independent deformation of the two degrees of freedom. The

SMA actuators are used to contract each leg in order to produce the required position of the moving

platform. Two universal joints are connected to the actuators to allow the change in two different di-

rections. Two bars are implemented to each plane to provide structural rigidity and counter moments.

Furthermore, two springs are used to bring the mechanism to it initial position [15]. The degrees

of freedom allow a gull or sweep deformation. The combination of both brings a huge potential to

improve loiter capability, speed, range and maneuverability of an aircraft. From a prototype of this

mechanism the results were significantly good, also the simple designed that was used has advantages

for implementing it inside the structure of an airfoil [15].

All of the mechanism used a different approach to achieve the wing morphing and shows the

variety of solutions. Most of them are designed to be able to replace the conventional control sur-

faces, by deforming the section shape of the wing. It is also possible like mechanism (3), (4), (6)

to change the entire wing configuration. All of the mechanism have the advantages of being more

efficient then the conventional wings / control surfaces. The research will go into both directions and

eventually will have a combination of both morphing the entire wing and control surfaces. This will

be highly complex and change the entire wing design. As mentioned before there are advantages and

disadvantages and it is obvious that further research needs to be done and then see which mechanism

will have the best perspective for the future. Most important is that more experimental data will be

recorded for numerical simulation and calculations. By comparing it side by side it can be told that

not every mechanism considers all of the important parameters. For example, mechanism (2) alreadyconsider that when the structure changes that the experienced outsides loads can still withstand by

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the deformed structure or (5) also considered the weight of the new control mechanism. Since all

of the above mentioned mechanism work, wing morphing is a promising technology for the future.

Many different approaches exists and can be improved, but all require new materials to implement

the deforming skin, a reliable compliant mechanism and very important the connection to a fixed part

such as the connection between the wing and fuselage [20].

5 The findings by computational technologies

Wing morphing design has been continuously studied with its variable planform, camber or twist

capabilities, for example the wings of insects, birds and bats. The technique used not only focuses on

the flapping wing lift generation capabilities, but the thrust produced alongside each completion of

a cycle of flapping. The flexible structures under the analysis are that of a locust (insect) and locust

inspired Micro Aerial Vehicle. This particular design incorporate multiple wings instead of conven-

tional single pair of wings, therefore effects of planform change, camber are thoroughly analyzed

with each stroke or half-cycle of the wings. Vortex interactions were also studied and analyzed for itsbeneficial/disruptive interactions between the wings.

As we know The Deforming-Spatial-Domain/Stabilized Space Time (DSD/SST) formulation is

used to analyze each cycle, several boundary conditions are applied. However, as the researcher indi-

cated, that one of the most challenging classes of problems in computational mechanics is ows with

moving interfaces, which includes uidstructure interactions (FSI) and ows with objects or surfaces in

fast, linear or rotational relative motion. This meant using a technique capable of tracking this com-

plexity by use of moving meshes and multi fluid interactions so that the wings and their motion and

deformation can be analyzed simply.

5.1 DSD/SST-VMST Method

DSD/SST-VMST method, as the name suggests utilizes two measurement techniques, rather a

trade-off of the two. Spatial resolution and temporal resolution. Spatial resolution capable of ana-

lyzing the smallest changes, from wing planform variation to twist and camber change. On the other

hand temporal resolution capable producing/measuring quick results.

This study uses a base method to begin with, starts the computation of the base case, and on later

stages performs the computations with increased temporal and spatial resolutions compared to the

base case. However, for the sake of simplicity, we dont go through those methods in this literaturesurvey.

The study also utilized different combinations of wing configurations for the MAV and investi-

gated the beneficial and disruptive interactions between the wings and understood the role of wing

camber and twist in the MAV with respect to the Locust.

5.2 Computation of the Base Case

To obtain the data for the MAV and ultimately create the model to be analyzed, Kenji Takizawa et.al. prescribed the motion and deformation patterns of the wings [21]. This was done by extracting the

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deformation patterns and motions of the locust (insect) using high speed, multi-camera video record-

ings in a wind tunnel. Thereafter the same wing motion is applied as a reference of flight conditions

to be obtained for computational analysis of bio-inspired flapping-wing aerodynamics of an MAV.

Then the setup for the computation of the base case, mainly the surface and volume meshes, mo-

tion of the wings, and the mesh update technique was described.

5.3 Computational conditions

the length scales involved in the models used in the locust (an insect) and MAV computations and

the boundary conditions set are:

Air density 1.2251kg/m3

Kinematic viscosity 1.4606105 m2/s.

The inow velocity is 2.4 m/s.

The Reynolds number, based on the inflow velocity and the FW span of 90 mm, is 14800, which is

low enough and satisfied the actual conditions faced by the insect. And similarly all the conditions

are defined as to how much the period of flapping should be, and the duration in the form of time

steps was fed.

5.4 Method Insertion

For the first few time steps, a Cosine form was used to achieve the desired flow conditions at a par-

ticular velocity of 2.4 m/s. For the last 100 time with the steady velocity, different forms of DSD/SST

techniques in such manner to solve different order of non-linearities and iterate towards stabilized

parameters. And thereafter for the computation with flapping, the study utilized 25 spacetime slabs

(with linear basis functions) for each of the knot spans in the temporal representation of the mesh, to

obtain a specific time step to observe the flow around the flapping wing.

Throughout the analysis, it was observed that the specified velocities at different profiles were

made constant effectively instead of varied velocities with respect to chord length or span length at

certain areas. Drichlet boundary conditions were used at specific locations where the variation of

velocity and profile was critical.

5.5 Result

Three complete flapping cycles were computed and analyzed in the study, however the first cycle

of the flapping has been discussed to highlight the significance of initial conditions used.

The lift and thrust (pressure component only) are shown in Figs. 6 for the locust, and data from a

different study to compare with, and MAV.

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Figure 6: Lift(Top) and Thrust(Bottom) generated over one cycle, on right Forward wing and on Hind

wing

With regards to the first cycle, Figures 7 shows the pressure on the wing surfaces (relative to thefree-stream pressure) at different instants during the third flapping cycle.

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Figure 7: Surface Pressure on the wing surfaces (Top view on left, bottom view on the right)

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6 Conclusions

Flapping wings are a type of bio-inspired techniques used in aircrafts. These can be found in

several Micro-Aerial Vehicles (MAVs). Shyy et. al. (2010) have conducted studies on flapping wing

aerodynamics in both 2D and 3D, and have found that in contrast to the classical stationary wing the-

ory, the tip vortices produced in unsteady flows around a hovering wing contribute to lift generationrather than drag generation on the wing. This is an important finding when flapping wing designs are

being considered as unlike the stationary wing design, the elimination of wing tip vortices will not

be required in order to improve the efficiency of the aircraft and to reduce the drag. However, it was

also found that bio-inspired mechanisms such as joints and distributed actuation need to be developed

in order to able to mitigate wind gust effects on the MAVs since they are lightweight and small in size.

Takizawa et. al. (2012) also analysed flapping wings on MAVs [21]. They found that it was ben-

eficial to have flapping wings on both sides rather than just one side as it reduced the loss of lift due

to lack of pressure build up on the part of the wing close to the body of the MAV. There was a 12.5%

increase in average lift generation when flapping wings were present on both sides. In addition to thisthey also found that flat wings generated high amount of negative lift in upstroke and thus, there is a

need for the wings to be cambered and twisted. The flat wings also generated much greater drag. This

shows that flapping wing MAVs should utilize wing camber and twist in order to maintain a high lift

to drag ratio.

The flapping wing analyses done in both Shyy et. al. (2010) and Takizawa et. al. (2012) signify

that flapping wings generate more lift and incur lesser drag than conventional stationary wings when

unsteady flows are considered. They result in generation of more stable leading edge vortices that

help in avoiding flow separation over the wing surfaces. Also, as mentioned in Wang et. al. (2003)

the pressure gradient from root to tip within a vortex core drives the flow in spanwise direction and

convects the vorticity away thus, stabilizing the leading edge vortices. Another advantage of these

flapping wings is that they allow the aircraft to hover which is not possible for a fixed wing aircraft.

Such findings will help us in furthering our scientific understanding when designing aircrafts. In-

spiration from the natural flyers such as birds and insects will not only allow aircrafts to be designed

with better control but also, improved efficiency in unsteady flow conditions with the use of various

techniques such as flapping wings or corrugation. However, corrugation is a complex technique as, in

order to simulate flows past corrugated wings the internal venous structure also needs to be designed

and flow through those veins needs to be simulated as well so that a good amount of efficiency can be

achieved when compared to stationary wings.

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