2005-AHS-Splst-Insect wing-with Beers

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Insect-Based Flapping Wings for Micro Hovering

Air Vehicles: Experimental Investigations

Beerinder Singh Manikandan Ramasamy

Graduate Research Assistant Post-Doctoral Fellow

Inderjit Chopra J. Gordon Leishman

Alfred Gessow Professor and Director Minta Martin Chair and Professor

Alfred Gessow Rotorcraft Center

Department of Aerospace Engineering

University of Maryland at College Park, MD 20742

Abstract

This paper addresses the aerodynamics of insect-based, biomimetic, flappingwings in hover. An experimental apparatus that incorporates flapping wingsand measures the small amount of thrust generated by these wing motions isdescribed. This methodology is used to measure the thrust generated by twowings at different wing pitch settings. Also, the effect of change in pitch phaseduring a flapping cycle is examined experimentally. To quantify the large inertial

loads acting on the wings, vacuum chamber tests were conducted. From thesetests, the temporal variation of the aerodynamic loads has been determined.Preliminary flow visualization images are presented to qualitatively compare theperformance of the two wings, and to explain the higher lift generated by onewing as compared to the other.

Notation

D dragFi inertial forceFn force normal to wing chordFx force tangential to wing chord

L liftm massr spanwise coordinateT time period of one flap cyclet time

e-mail: [email protected]

Presented at the American Helicopter Society Inter-

national Specialists Meeting on Unmanned Rotor-

craft, Arizona, January, 2004

vn velocity normal to wing chordvx velocity tangential to wing chordy coordinate along the wing chord angle of attack wing pitch angle wing flap angle

Introduction

In 1997, the Defense Advanced ResearchProjects Agency (DARPA) initiated a pro-gram to develop and demonstrate a newfamily of very small or micro air vehicles(MAVs) having a maximum dimension of 15cm and a gross weight of 100 grams. Inter-


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est in these small flight vehicles was precipi-

tated by the nearly simultaneous emergenceof their technological feasibility, alongwithan array of critical new military needs, es-pecially in urban environments.1 The tech-nological feasibility was a result of advancesin several micro-technologies, such as Micro-electromechanical Systems (MEMS), minia-ture CCD cameras, tiny infrared sensors andchip sized hazardous substance detectors.For these miniature sensors, MAVs providea highly portable platform, with low de-

tectability and low noise, capable of real-time data acquisition.

Net Force

StrokePlane

Downstroke

Upstroke

Wing Path

SectionWing

Figure 1: Insect wing kinematics

In nature, flight has evolved into two dif-ferent forms insect flight and bird flight.While both these forms are based on flap-ping wings, there are some key differencesamong them. Most birds flap their wings ina vertical plane with small changes in thepitch of the wings during a flapping cycle.As a result, most birds cannot hover be-cause they need a forward velocity to gener-

ate sufficient lift. However, the insect worldabounds with examples of hovering flight.These insects flap their wings in a nearlyhorizontal plane (Fig. 1), accompanied bylarge changes in wing pitch angle to producelift even in the absence of any forward veloc-ity. Among insects we find animals that arecapable of taking off backwards, flying side-wards, and landing upside down. Moreover,

birds like the hummingbird, which are capa-

ble of hover, have wing motions very similarto hover-capable insects. Thus, insect-basedbiomimetic flight may present a viable solu-tion for hover-capable MAVs that must beinvestigated.

The flight of insects has intrigued sci-entists for some time because, at firstglance, their flight seems impossible ac-cording to conventional aerodynamic the-ory. Ellington2 showed that a quasi-steadyanalysis of insect flight under-predicts the

lifting capability of insects. A number ofunsteady phenomena have been used to ex-plain the high lift generated by insects.Weis-Foghs clap-fling hypothesis is one suchlift generating mechanism, but it is limitedto a few species of insects and hence doesnot explain the flight of other species. Re-cent experiments conducted on a dynam-ically scaled model (Robofly) have shownthat insects take advantage of unsteadyaerodynamic phenomena to generate thrustsgreater than those predicted by quasi-steadyanalyses. 3 Figure 1 shows the typical mo-tion of an insect wing. This motion mainlyconsists of four parts: a) downstroke, inwhich the wing translates with a fixed col-lective pitch angle, b) near the end of thedownstroke the wing supinates so that theblade angle of attack is positive on the up-stroke, c) upstroke and, d) pronation at theend of the upstroke so that the angle of at-tack is positive on the downstroke. Dur-ing the downstroke and upstroke (i.e. the

translational phases) high lift is producedbecause of a leading edge vortex on thewing.4 Supination and pronation also pro-duce significant lift from rotational circu-lation (Kramer effect5). The third effect,wake capture, occurs as the wing passesthrough its own wake, which was createdduring the previous stroke.

Most of the analytical studies on the aero-

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dynamics of flapping wings have examined

either rigid wings or wings with a prescribedmotion.6, 7 Some of these studies look atornithoptic or bird-like flapping, i.e. flap-ping without the pronation and supinationphases of insect-like flapping. Some are re-stricted to small disturbances while othersare computationally intensive CFD simu-lations. DeLaurier8 developed an aerody-namic model for ornithoptic flapping whichhas been applied to the aeroelastic analy-sis of a large-scale ornithopter.9 Walker10

recently developed a simple analysis thatcan predict the translational and rotationalcomponents of the airloads on the Roboflywings. The development of a comprehensivetheory for unsteady force generation by in-sect wings is partly hindered by a lack of ex-perimental data at the chord Reynolds num-bers of interest (103 104).

An important feature of insect wings isthat they can deform greatly during flight.Also, unlike birds or bats, insect musclesstop at the wing base so any active con-trol of the wing shape is not likely.11, 12

Passive aeroelastic design is hence very im-portant for insect wings. The Roboflymeasurements are based on very low fre-quencies of motion because the fluid usedhad a high viscosity. Thus wing bendingand passive aeroelastic effects are likely tobe very small in the Robofly experiment.Tarascio and Chopra13 presented experi-mental results for a flapping wing proto-type that operated in air at high flapping

frequencies. Recently, the present authorsmeasured the thrust generated by insect-like flapping wings mounted on this flap-ping wing prototype.14 In this paper, animprovement of the thrust measurementmethodology is described and thrust mea-surements for a number of wing and strokeparameters are presented. In addition, a fewvacuum chamber test results are presented

as well as some preliminary insight into the

flow field using strobed laser sheet flow vi-sualization.

Experimental Setup

Figure 2: Flapping wing mechanism (Con-

cept by M.J. Tarascio13)

Flapping Wing Mechanism

The flapping wing test apparatus is apassive-pitch, bi-stable mechanism capa-ble of emulating insect wing kinematics13

(Fig. 2). The desired flapping and pitchingmotion is produced by a Hacker B20 31Sbrushless motor, which is controlled by aPhoenix PHX-10 sensorless speed controllerin combination with a GWS microprocessorprecision pulse generator. The motor shaft

is rigidly attached to a rotating disk, whichin turn is attached to a pin that drives aScotch yoke. The Scotch yoke houses ballends, which are attached to shafts that arefree to flap with the motion of the yoke. Asthe shaft is actively flapped, pitch actuators,which are rigidly attached to the shaft, makecontact with delrin ball ends at the end ofeach half-stroke, causing the shaft to pitch

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Figure 3: Pitch assembly

and, hence, generate the wing flip at the endof the half-stroke.

The rotation of the shaft or flip at theend of each half stroke is generated by the

pitch assembly, which also serves to fix thepitch angle of the shaft during the transla-tional phases of the wing motion. The pitchassembly consists of the main spar, whichis rigidly attached to a cam, which is inturn held in place by a delrin slider and acompression spring (Fig 3). In combinationwith the pitch stop, the entire assembly isbi-stable, in that it allows the shaft to restin only two positions. As the pitch actua-tor makes contact with the Delrin ball stopsat the end of each half-stroke, the cam is

forced to rock over to the other stable posi-tion, with the compression spring holding itin place until the next rotation.

Force and Motion Transducers

Measurement of the flapping and pitchingmotions, and the small airloads generatedby a wing mounted on the flapping mecha-nism, poses a significant challenge. To mea-sure these airloads, a load-cell has been de-

signed and built using Entran ESU-025-500piezoresistive strain gauges. The load-cellhas a narrow beam cross-section on whichtwo strain gauges are mounted to mea-sure the loads in two orthogonal directions(Fig. 4). Each strain gauge is connected ina half-bridge configuration with a dummygauge, which provides temperature compen-sation. The load-cell is mounted at the end

Figure 4: Load Cell

of the flapping shaft, with the wing beingmounted at the end of the load-cell.

Figure 5: Pitch Measurement

The load cell measures forces normal andtangential to the wing chord. To obtain thevertical and horizontal components of theseforces, the pitch angle of the shaft must bemeasured. This is done by using a Hall effectsensor in combination with a semi-circulardisk mounted on the shaft (Fig. 5). The diskhas ten small magnets arranged in a semi-circle, with the Hall effect sensor mounted

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on the pitch housing. A pitching motion

of the shaft causes the magnets to move inrelation to the Hall effect sensor, producinga change in its output.

Figure 6: Vacuum Chamber

Because strain gauges are used on the loadcell, only the moment acting at the base ofthe wing is measured. To convert this mo-ment into an equivalent force, the distancefrom the wing base at which this force actsmust be known. Because the forces on a

flapping wing are predominantly inertial innature,15,16 the point on the wing at whichthese forces act is calculated analytically,and is used to determine the forces actingon the wing. These forces are then trans-formed into vertical and horizontal compo-nents using the measured pitch angle. Themean aerodynamic thrust is calculated bytaking the average of the vertical force over

a number of flapping cycles.

Vacuum Chamber

To quantify the inertial forces acting on thewing, a small vacuum chamber has been de-signed and built using a 16 diameter, 1/2

thick acrylic cylinder (Fig. 6). At the twoends of this cylinder 1 thick acrylic platesare held by screws. The upper plate is fit-ted with a valve to connect to a vacuumpump. In addition, this plate also has a vac-uum gauge and two electrical feedthroughs

for connecting the motor, pitch sensor andforce sensors. All vacuum chamber tests areconducted at a gauge pressure of 27 of Hg,which corresponds to a 90% vacuum.

Flow Visualization

The flow visualization test stand consists ofa steel frame bolted to the ground, on whichthe flapping wing mechanism is mounted ap-proximately 4 above ground (Fig. 7). Alu-minum plates extend from ground level toapproximately 3 above the mechanism toprovide an image plane for the single wing.At the top of the aluminum plates, an alu-minum honeycomb extends 2 horizontally.The seed for the flow visualization is pro-duced by vaporizing a mineral oil into adense fog, which passes through a series ofducts before reaching a diffuser mounted ontop of the honeycomb. The diffuser reducesthe vertical velocity of the fog, while thehoneycomb helps to eliminate any swirl or

turbulence in the flow.Flow visualization images are acquired by

strobing the flow with a laser sheet gen-erated by a dual Nd:YAG laser, as shownin Fig. 8. This laser is triggered onceevery flapping cycle by a Hall effect switchmounted on the flapping wing mechanism.A charge coupled device (CCD) camera isused to capture the images.

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Figure 7: Flow Visualization Test Setup

Camera

Wing

Laser Sheet

Laser

FlappingAxis

Figure 8: Flow Visualization Schematic

Analysis

Experiments have shown that the lift anddrag coefficients on flapping wings arehigher because of the leading edge vortex.3

Previous quasi-steady analyses, such asRef. 2, did not account for this increasedperformance and hence failed to accuratelypredict the lift generating capacity of in-sect wings. However, quasi-steady analysescan explain the lift produced by an insectwing if the effects of a leading edge vor-tex, on the lift and drag coefficients, are ac-counted for. This has led to a revival ofquasi-steady models in recent years.5 How-ever, such models cannot account for theforce peaks resulting from wing wake inter-actions because these effects are unsteady innature. A blade element model developedby Walker10 is used to predict the airloadson the flapping wings. In this analysis, thewing is assumed to be rigid, i.e. the effectsof elastic bending and torsion are ignored.

X

Y

Z

i

i

i

x

y

z

1

1

1x

y

z

Figure 9: Reference Frames

The reference frames used to model themotion of the flapping wing are shown inFig. 9. The inertial reference frame XiYiZi

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has its origin at the center of rotation. The

flapping angle denotes the rotation of theflapping reference frame x1y1z1 about theZi axis as shown. The wing reference framexyz is obtained by rotating the flapping ref-erence frame by the wing pitch angle ,about the x1 axis.

At a particular instant of time t, the forcesparallel (dFx) and perpendicular (dFn) tothe wing chord, at a radial station r, aregiven by,

dFn(r, t) = dL(r, t)cos + dD(r, t)sin(1)

dFx(r, t) = dL(r, t)sin dD(r, t)cos(2)

where, dL(r, t) and dD(r, t) are the circula-tory lift and drag which depend on the angleof attack, , as given by,

= tan1vn(r, t)vx(r, t)

(3)

and where vx(r, t) and vn(r, t) are the veloc-ities parallel and perpendicular to the wingchord, respectively. As shown in Ref. 10,these velocities are determined at the 3/4chord location, which was found to givegood agreement with experimental resultsfor the Robofly wings (for lift due to transla-tion and rotation). The forces dFn and dFxwere transformed to the flapping referenceframe through the pitch angle to deter-mine the vertical and horizontal circulatoryforces. Non-circulatory forces generated bythe acceleration of the wing in a directionperpendicular to the chord were calculatedand added to the circulatory forces.

Inertial Forces

The forces acting on a flapping wing are pre-dominantly inertial in nature.15,16 The in-ertial forces acting on an infinitesimal mass

dm, located at radius r, and at a distance y

in front of the pitching axis, are given by,

Fi = dm

y cos + (r 2y sin )

y(2 + 2 cos2 ) r cos

rsin y( + sin cos 2)

T

ij

k

(4)

where, i, j and k are the unit vectors cor-

responding to the pitching reference framexyz. The inertial forces acting on a wingof arbitrary shape can be calculated usingthe above equations, provided the wing isassumed to be rigid. This analysis is usedto predict the point on the wing, where theresultant inertial force acts.

Results and Discussion

Thrust Measurement

The thrust generated by two aluminum-mylar wings has been measured for a num-ber of stroke and wing parameters. Thewing planform is based on a scaled-up fruitfly wing similar to the Robofly3 wings.These wings, made from 0.02 thick alu-minum frames, are shown in Fig. 10. InRef. 14, it was shown that Wings I andII produce the same amount of thrust butWing II can attain higher frequencies onthe flapping wing mechanism because of its

lower mass. In this paper, results are pre-sented for Wings II and III only. All theresults are based on a flapping stroke angleof 80, i.e. the angle varies from 40 to+40. Each wing was tested at two pitch an-gles of 30o and 45o. A pitch angle of 30 im-plies that the pitch is 30 during the down-stroke and then changes to 30(150) dur-ing upstroke. Similarly for the 45 case, the

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Figure 10: Scaled-up fruit fly wings

14.3 cm

. cm

4.2 cm

Figure 11: Schematic of planform showingroot cut-out

3 4 5 6 7 8 9 10 110

0.5

1

1.5

2

2.5

3

Frequency (Hz)

Thrust(grams)

Analysis (Wings I & II)

Experiment(Square load cell)

Experiment(Circularload cell)

Stroke : 80o

Pitch : +30o/30

o

Figure 12: Effect of change in load cell de-sign on measured thrust

pitch angle is 45 during the downstroke andchanges to 45(135) during the upstroke.Figure 11 shows the dimensions of the wingsand the root cut-out.

In Ref. 14, the development of a bendingbeam load cell with a square cross-sectionwas described. This load cell had smallcross-couplings between the two orthogonalaxes of measurement. Figure 12 shows thethrust generated by Wing II, when mea-sured with this square cross-section loadcell. Also shown in the figure is the thrustpredicted using the blade element analysis,as described previously. The thrust mea-sured using this load cell shows a higher-order variation as opposed to the quadraticvariation shown by the analysis. To checkwhether this discrepancy is caused by thecross-coupling present in the load cell, a

cylindrical cross section load cell was de-signed and built. On a square cross-section load cell, the strain gauges mustbe mounted with great precision if spuri-ous surface strains are to be avoided. If thegauge is off-center, it can pick up unwantedsurface strains that cause the calibrationconstants to change as the pitch angle of theload cell is varied. On a cylindrical cross-

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section, these spurious strains are mini-

mized. Figure 12 shows the thrust measuredusing this redesigned load cell. Althoughthere still remains a discrepancy betweenthe analysis and experiment, the cylindri-cal load cell shows a quadratic increase inthrust as the frequency is increased. All re-maining thrust measurements in this paperhave been made using the cylindrical loadcell. Although a cylindrical cross-sectionprovides good results, it suffers from thedrawback that the strain gauges do not have

a flat surface to bond with. This leads to ashort useful life of these load cells before thestrain gauges need to be replaced, thus in-creasing the overall testing time.

Figure 13 shows a comparison of theexperimental measurements and analyticalthrust predictions for the two wings at apitch angle of 30. Wing III pitches aboutthe 20% chord location, compared to WingII, which pitches about the 50% chord loca-tion. This change in pitching axis increasesthe thrust produced by Wing III becauseit produces more lift from rotational circu-lation during the pronation and supinationphases, as indicated by the analysis.

Figure 14 shows the thrust generated byWing II at a pitch angle of 45 along withthe thrust generated at a pitch angle of 30.At a higher pitch angle the thrust is ex-pected to increase. However, the experi-mental results show that the thrust does notchange when the pitch angle is increased forWing II. Figure 15 shows similar results for

Wing III. In this case, however, the experi-mentally measured thrust does show an in-crease when the pitch angle is increased to45. On the other hand, the predictions donot show any significant change with pitchangle. This is because, when the pitch anglefor Wing III is increased, the total changein pitch is reduced. At 45 pitch angle, thewing flips from 45 to 135, producing a to-

tal change of 90. However, when the pitch

angle is 30

, the total change in pitch is 120

as the wing flips from 30 to 150. The re-duced flip angle at 45 pitch, reduces therotational circulation for Wing III. BecauseWing III generates a significant amount oflift from rotational circulation, the net in-crease in thrust is very small at 45 pitchangle. Also, at low frequencies, the mea-sured increase in thrust is smaller. This maybe caused by a weaker leading edge vortexat slower speeds.

3 4 5 6 7 8 9 10 110

1

2

3

4

5

6

Frequency (Hz)

Thrust(grams)

Stroke : 80o

Pitch : +30o/30

o

Analysis (Wing III)

Analysis (Wing II)

Experiment (Wing II)

Experiment (Wing III)

Figure 13: Comparison of thrust generatedby Wings II and III

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1

2

3

4

5

6

Frequency (Hz)

Thrust(grams)

Wing II

Stroke : 80o

Pitch : +30o/30

o

and +45o/45

o

Analysis (45o)

Analysis (30o) Experiment (30

o)

Experiment (45o)

Figure 14: Effect of wing pitch angle onthrust (Wing II)

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3 4 5 6 7 8 9 10 110

1

2

3

4

5

6

Frequency (Hz)

Thrust(grams)

Wing III

Stroke : 80o

Pitch : +30o/30

o

and +45o/45

o

Analysis (45o)

Analysis (30o)

Experiment (45o)

Experiment (30o)

Figure 15: Effect of wing pitch angle onthrust (Wing III)

3 4 5 6 7 8 9 10 110

1

2

3

4

5

6

Frequency (Hz)

Thrust(grams)

Pitch: Baseline

Pitch: Early

Wing III

Stroke : 80o

Pitch : +45o/45

o

Figure 16: Effect of early rotation on thrust(Wing III)

Figure 16 shows the effect of a slightchange in pitch phase on the thrust gener-ated by Wing III at a pitch angle of 45.To change the pitch phase, the ball ends

shown in Fig. 2 are moved slightly towardeach other, thus causing the pitch actuatorto hit them early, producing an early pitch,(i.e. the wing flips over earlier in the flap-ping cycle as compared to the baseline case).For this case, the wing starts pitching 0.04Tearlier than the pitch starting point for thebaseline case, where T is the time period ofone flapping cycle. Insects use such changes

3 4 5 6 7 8 9 10 110

1

2

3

4

5

6

Frequency (Hz)

Thrust(grams)

Wing II

Stroke : 80o

Pitch : +45o/45

o

Pitch: Baseline

Pitch: Early

Figure 17: Effect of early rotation on thrust(Wing II)

3 4 5 6 7 8 9 10 111

0

1

2

3

4

5

6

Frequency (Hz)

Thrust(grams)

Wing II

Stroke : 80o

Pitch : +30o/30

o

Air

Vacuum

Figure 18: Thrust in air and vacuum forWing II

in pitch phase to change the lift generatedby their wings. Figure 17 shows that thethrust reduces when Wing III pitches early.An interesting observation is the nearly lin-

ear variation of thrust with frequency forthe case of early pitching. The reduction inlift is unexpected since it has been reportedelsewhere3 that early pitching may producea positive wake capture, i.e., when the wingflips early in the flapping cycle, its interac-tion with the wake created during the pre-vious cycle increases the total thrust. Fig-ure 17 shows the effect of early pitching on

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the thrust generated by Wing II. Although a

limited amount of data is available for thiscase, the thrust is nearly unchanged whencompared to the baseline case.

Vacuum Chamber Tests

Inertial forces are a large part of the to-tal forces measured using the load cell. Toeliminate these inertial loads from the totalmeasured loads, the flapping wings must betested in a vacuum. Tests conducted in a

90% vacuum show that, as expected, WingII generates a very small thrust at a pitchangle of 30, as shown in Fig. 18. However,the measurement error in the vacuum cham-ber data is larger as compared to the mea-surement error in air.

Vacuum chamber tests were used to sub-tract the inertial forces from the total forcesmeasured in air. When the wing was testedin vacuum, the frequency attained by themechanism was not the same as the fre-quency in air at the same motor supply volt-age. However, to subtract the inertial forcesfrom the total loads, the test frequencies inair and vacuum must match closely. Thiswas done by adjusting the motor supplyvoltage during the vacuum chamber teststo change the frequency. Figure 19 showsthe thrust generated in one flapping cycle byWing II, in air and in vacuum at a frequencyclose to 10.7 Hz. The frequency for the vac-uum test was 10.71 Hz, while the frequencyin air was 10.65 Hz. Because these frequen-

cies are slightly different, the results wereplotted against non-dimensional time in theflapping cycle. Figure 19 also shows the air-loads obtained after subtracting the inertialforces from the total forces, and the pitchangle measured both in air and in vacuum.It is evident from this figure that the pitchangle varies slightly in vacuum because of achange in the dynamics of the drive mech-

anism. Also, the temporal variation of air-

loads contains frequencies higher than theflapping frequency, which may be caused bythe elastic bending and twisting of the wing.It can be seen from Fig. 19 that during thetranslational phases (i.e., the portion of timewhen the pitch angle is nearly constant),there are significant durations of negativethrust. For example, for the time period0.2T-0.3T, there is a nearly constant nega-tive thrust on the wing, although the wingdoes produce significant positive thrust from

time 0.1T to 0.2T.

Flow Visualization

Preliminary flow visualization results arepresented here to show the differences be-tween Wing II and Wing III at 45 pitchangle and the baseline case of pitch phase.One of the reasons for the high lift generat-ing capability of insects, even at large pitchangles, is the presence of an attached leadingedge vortex on top of the wing. Figures 20and 21 show such a leading edge vortex onWings II and III, respectively. In these pic-tures, the wing is close to mid-stroke, thelaser sheet is at mid-span of the wing, andthe camera is placed perpendicular to thelaser sheet, as shown in Fig. 8.

Figures 22 and 23 show flow visualiza-tion images of Wings II and III, respec-tively, at a point in the flapping cycle whenthe wing was midway through the prona-tion phase. The pitch angle at this point

is nearly 90, as can be seen from the im-ages. Figures 24 and 25 show images takenwhen the wings were slightly beyond themid-pronation point. As mentioned ear-lier, Wing III generates substantially greaterlift compared to Wing II because of rota-tional circulation. The flow visualizationimages show the presence of a strong circu-lation around Wing III during the pronation

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0200

0

200Mean (Air)2.4503

Mean (Vacuum)

0.42508Thrust(gram

s)

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10

100

200

Nondimensional Time (t/T)

(

degree

s)

200

0

200

Mean airload

2.0253

Airload(grams)

Vacuum

Air

Figure 19: Airloads obtained by subtracting inertial forces

Figure 20: Leading edge vortex (Wing II) Figure 21: Leading edge vortex (Wing III)

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Figure 22: Mid-pronation (Wing II)

Figure 23: Mid-pronation (Wing III)

Figure 24: Post mid-pronation (Wing II)

Figure 25: Post mid-pronation (Wing III)

phase. For Wing II, two small vortices werevisible on the top surface of the wing. How-ever, Wing III shows a single strong vortexat the same point in the flapping cycle.

Summary and Conclusions

The thrust generated by two insect-likewings, mounted on a flapping-pitchingmechanism, has been measured for a num-ber of wing and stroke parameters. Animproved force measurement methodologyis described, which resolves some mea-surement issues that were pointed outpreviously.14 The two wings tested had thesame planform shape. One wing pitchedabout the 50% chord location (Wing II),while the other pitched about the 20% chordlocation (Wing III). The latter producesmore lift because of higher rotational circu-lation during the pronation and supination

phases. However, when the pitch angle ofthe wings is increased from 30 to 45, thethrust produced by Wing III increased butthe thrust for Wing II remained the same. Aslight change in pitch phase, so that prona-tion and supination occur early in the flap-ping cycle, reduced the thrust produced byWing III. Again, the thrust produced byWing II remained unchanged from the base-

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line case.

The inertial forces produced by Wing IIhave been measured by testing it at a vac-uum pressure of 27 of Hg (90% vacuum).These forces were then subtracted from thetotal measured loads to obtain the aero-dynamic forces on the wings. The resul-tant aerodynamic force showed high fre-quency oscillations which were suspected tobe caused by the elastic bending and twist-ing of the wing. The measured pitching mo-tion of the wing showed a small change in

vacuum as compared to the pitching motionmeasured in air at the same frequency. Thismay cause some error in the aerodynamicforce calculations.

Preliminary flow visualization tests wereconducted on the two wings at a wing pitchangle of 45. An attached leading edge vor-tex, which enables the wing to generate liftat such a high pitch setting, was observedon both wings. Qualitatively, the leadingedge vortex seems to be larger for Wing III.Also, the circulation around the wing dur-ing pronation is greater for Wing III, whichgenerates greater lift from rotational circu-lation.

Acknowledgements

Support from the Multidisciplinary Univer-sity Research Initiative (MURI) is gratefullyacknowledged. The authors also wish toacknowledge the contribution of Mr. M.J.

Tarascio (Sikorsky), who built the flappingwing mechanism.

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tics, Chap. 18, AIAA, 2001, pp. 399428.

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