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University of FloridaResearch & Engineering Education Facility (REEF)

Unsteady Mechanical Aspects of Flexible Wings: an Experimental Investigation Applied to Biologically Inspired MAVs

Roberto Albertani, UFL REEFBret Stanford, Mike Sytsma, UFL MAE

Peter Ifju, UFL MAE

MAV07, 3rd US-European Competition and Workshop on MAV, 7th European MAV Conference and Flight Competition

Toulouse, France, September 17-21, 2007

University of Florida REEF

Outline

WT Steady State Results

Motivations

Experimental Set-Up

Results:- Average Spectra- Modes ID- Damping

Aeroelastic results in WT

Current Projects

Conclusions

Outline

Wind tunnel results in steady flow Motivations Experimental Set-Up Forced vibrations results

Average spectra Modes ID Main modes frequencies and damping

Wind tunnel aeroelastic results (natural vibrations) Current projects Conclusions


Wind Tunnel Results in Steady Conditions

Six components sting balance, wing and complete vehicle, prop on/off and elevator deflections

Response surface methodology (RSM) and DOE using central composite design (CCD) and full factorial design (FFD)

Prop Off

Prop On

CL = a0 + a1α + a2δCD = a0 + a1q + a2α + a3δ + a4α

2 + a5δ2

Cm = a0 + a1α + a2δCL = a0 + a1q + a2δ + a3α + a4qα + a5δα + a6q

2 + a7α2 + a8q

2δ + a9δ2α + a10δ

3

CD = a0 + a1q + a2δ + a3α + a4qδ + a5qα + a6δα + a7q2 + a8δ

2 +

+a9α2 + a10qδα + a11q

2α + a12δ2α + a13δ

3 + a14α3

Cm = a0 + a1q + a2δ + a3α + a4qδ + a5q2 + a6δ

2 + a7qδ2

CL = a0 + a1q+ a2α + a3n + a4qα + a5qδ + a6nδ + a7αδCL = a0 + a1q+ a2α + a3α

2 + a4n + a5δ + a6α3 + a7qα + a8αδ + a9qδ

CD = a0 + a1q + a2α + a3α2 + a4n+ a5qα + a6δ + a7n

2 + a8n2 / q+ a9δ

2

CD = a0 + a1q + a2α + a3δ + a4n+ a5α2 + a6qα + a7α

3 + a8n2

Cm = a0 + a1q+ a2α + a3n+ a4δ + a5qα + a6qδ + a7nδ + a8α4

Outline


Motivations

Experimental Set-Up



Current Projects

Conclusions

Prop Off

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

0 4 8 12 16 20 24 28 32 36AOA [deg]

CL

Rigid

PR 07

BR 09

PR 12

V=13 m/sWing


Motivations and Bio-inspiration Biological inspiration

Non-isotropic structural elements (bats, birds, insects) and pliant components (cartilage)

Relevant elastic energy storage state (skin, arteries, birds primary feathers, leg joints extensor muscles in arachnides)

Dynamic tuning (campaniform sensillae in locusts, bats) Passive membrane morphing and possible interactions with BL Active morphing for BL control Structural models need to be developed and validated Direct measurements of the kinematics and dynamics of bat flight

(a) (b) (c) (d))

Figure 1. A dog-faced fruit bat (Cynopterus brachyotis) in flight. (a) Beginning of downstroke, head forward, tail backward, the wholebody is stretched and lined up in a straight line. (b) Middle of downstroke, the wing is highly cambered. (c) End of downstroke, the wing isstill cambered. A large part of the wing is in front of the head. (d) Middle of upstroke, the wing is folded toward the body.

and Dickinson 2001, 2002, Ramamurti and Sandberg 2002).Detailed fluid dynamics approaches are now being appliedto the study of bird flight as well, including the use of PIVto quantify the velocity fields in wake flows generated byhovering and forward flight (Spedding et al 2003a, 2003b;Warrick et al 2005). These studies on birds demonstrate that itis possible to analyze the structure of the flow in the wakes offlying vertebrates, and to calculate mechanical and energeticquantities directly from the wake ‘footprint’.

Although biologists and engineers have begun to scratchthe surface of insect and bird aeromechanics, studies offlight in bats have yet to make comparable progress. Mostinvestigations of vertebrate flight have assumed that animalaerodynamics can be meaningfully approximated by assumingthat bat and bird wings function in the same way as rigid,fixed wings of large, fast, human-engineered aircraft. Onthis basis, a large biological literature infers maneuverability,flight energetic and other aspects of flight performance andecology from simple metrics such as aspect ratio and wingloading, particularly for bats (Norberg 1987, Kalcounis andBrigham 1995, McLean and Speakman 2000, Stockwell 2001,Rhodes 2002, Elangovan et al 2004). This approach is areasonable place to start a quantitative analysis of animal flight,but cannot capture all of the relevant functional complexity.In contrast to most fixed-wing aerodynamics, bats fly ata low Reynolds number (104–105), have highly compliantaerodynamic surfaces and are characterized by highly unsteadyand three-dimensional wing motions (figure 1), enabled bymultiply jointed wing skeletons, thin elastic wing membranesand other specialized morphological features. Understandingthe full complexity of bat flight and the ways in which batflight differs from that of other animals requires attention tothe complex functional mechanics and architecture of wings,physiological energetic of locomotion and the kinematics andmaneuverability of flight.

As a step toward achieving this goal, we present initialexperimental data concerning the kinematics of bat flight andthe structure of the wake produced by the characteristic motionof the wings. In the long term, we will be able to apply thisinformation toward clearly defining the unique aerodynamiccapabilities of bats and understanding the relationships amongbat flight performance and the distinctive morphological andphysiological features of this group.

2. Experimental setup

All experiments were performed in an enclosed flight cagelocated at the Concord Field Station of Harvard University.The flight cage is 8 m long with a cross-sectional area ofapproximately 1 m (width) × 2 m (height). As shown infigure 2, the x-axis is defined as the bat’s flight direction,the y-axis is the transverse direction toward to the bat’s leftand the z-axis is the vertical direction. The origin of thecoordinate system is defined as the point where the bat’ssternum (approximately its center of mass) passes throughthe PIV laser sheet (discussed below).

Lesser short-nosed fruit bats, Cynopterus brachyotis, werethe subjects of all tests. We chose this species for analysisbecause they thrive in captivity, respond well to handlingand training and, at a 35–45 g body mass and 30 to 40 cmwing span, are a good size for kinematic and PIV studies.Native to many forested areas of Southeast Asia, a colony ofcaptive-bred individuals have been loaned to us by the LubeeBat Conservancy (Gainesville, Florida). All bats are female,eliminating sex-specific variation. Reflective markers areattached to key anatomical landmarks using medical adhesive.In the flight cage, the average forward speed of bats rangesfrom 2 to 3 m s−1, at the lower end of their natural range offlight speeds. A top view and a side view of the experimentalsetup are illustrated in figure 2. The bat flies through the flightcage (from left to right). Her body and wing movementsare captured by a pair of Redlake high-speed, low-lightsensitive video cameras (MotionScope PCI 1000, operating ata frame rate of 500 images s−1). Both cameras are positionedon the floor looking upwards, with advancing and recedingangles. The twin camera arrangement allows for theacquisition of the complete three-dimensional motion of thebat. Low light conditions were preferred because the batsare nocturnal and PIV measurements require a reduced lightcondition. As the bat flies through the flight cage, it trips alaser beam-break sensor which initiates the data acquisitionsequence. After a pre-set delay, typically 300 ms, calculatedto allow the bat to pass through the measurement volume,the wake flow is illuminated by a sequence of laser pulsesusing a pair of Nd:YAG lasers (5 ns, 150 mJ/pulse, typically1000 µs between adjacent images, 200 ms between adjacent

S11

Tian, Diaz, Middleton, Galvano, Isreaeli, Roemer, Sullivan, Song, Swartz and Brauer

Outline


Motivations

Experimental Set-Up



Current Projects

Conclusions


Motivations

Flexible structures are used in SUAV and MAV

Passive morphing(James Davis and Peter Ifju, UFL)

Wind Tunnel tests in steady flow (Roberto Albertani, REEF UFL)

Outline


Motivations

Experimental Set-Up



Current Projects

Conclusions


The Experimental Set-Up Open loop - Open test section WT

Test section 70 by 70 cm Max velocity 10 m/s

Structural Deformation Dynamic VIC 2 Cameras and model with

random speckle and correlation Vibrations

Scanning LDV system Shaker and piezo loadcell

Outline


Motivations

Experimental Set-Up



Current Projects

Conclusions


Results: Membrane Pre-Strain State Membrane pre tension state characterization using the VIC

Plane strain field measured on the test bench The wing was then quickly moved to the wind tunnel for

aeroelastic measurements (avoid creep phenomena)

High pre-strain state

Low pre-strain state

Outline


Motivations

Experimental Set-Up



Current Projects

Conclusions

.04 .05 .06 -.03 -.01 0 .01

Y,v

X,u

ε xx

ε xx

ε xy

ε xy

0.005 .015 .025 5 0 5 10-3

Chordwise Shear Strain


Results: Average Vibrations Spectra Average spectra with shaker structural excitation

H1 Velocity/Force [m/s / N] vs. frequency [Hz]

Membrane high pre-strain state

No pre-strain state

0 200 400 600 800 Hz

0

50

1

00

0

50

1

00

0

50

1

00

Outline


Motivations

Experimental Set-Up



Current Projects

Conclusions

Membrane low pre-strain state


Results: Modes Frequencies ID First three membrane modes for high pre-strain case

0 200 400 Hz

High pre-strain

126 Hz

266 Hz

270 Hz

Outline


Motivations

Experimental Set-Up



Current Projects

Conclusions


Low

No pre-tension

High

Results: Dolphin Mode

Relevant mode with tail dolphin

Present in all membrane pre-train states

Frequency at 105 Hz (no pre-tension case)

0 200 400 600 800 Hz

Outline


Motivations

Experimental Set-Up



Current Projects

Conclusions


Results: Modes Frequencies and Damping Main modes and damping with

experimental uncertainties Repetitions experiments Carbon fiber modes Membrane modes Higher uncertainty for damping of

low pre-strain membrane No measurable damping for no

pre-strain case

0.0

0.2

0.4

0.6

0.8

1.0

0 1 2 3 4Mode

Dam

ping

%

HighLow

050

100150200250300350400

0 1 2 3 4Mode

Freq

uenc

y [H

z]

HighNo Pre TensionLow

Carbon fiber

0

50

100

150

200

250

300

0 1 2 3 4Mode

Freq

uenc

y [H

z]

HighLow

Latex membrane

Outline


Motivations

Experimental Set-Up



Current Projects

Conclusions


Aeroelastic Results: Local Vibrations Spectra

Wing in WT natural vibrations in steady free stream at 9 m/s AOA = 150

Three chord locations Significant out-of-plane displacements Energy spike at 100 Hz membrane ONLY

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Latex membrane

Latex membrane

Carbon fiber

High pre-strain Low pre-strain No pre-strain

Outline


Motivations

Experimental Set-Up



Current Projects

Conclusions


Experimental modal analysis in vacuum chamber Decoupling viscous and structural damping Forced vibrations on wing in wind tunnel Dynamic flow measurements around wings in wind tunnel Aerodynamic forces on flexible wings in dynamic conditions

(pitching/plunging) in wind tunnel

Current Related Projects

Outline


Motivations

Experimental Set-Up



Current Projects

Conclusions

Dynamic mounting systemMAV wing on shaker


Conclusions

Experimental modal analysis is performed in ambient air on flexible wings (latex membrane and carbon fiber) for MAV applications The experiments are performed with artificial structural

excitation (shaker) and natural aeroelasticity in wind tunnel A correlation between modes shape, natural frequencies

and damping with the membrane pre-strain state is confirmed An energy spike at approximately 100 Hz in the membrane

in wind tunnel tests is observed. No structural modes have been identified at that frequency Experiments in a vacuum chamber will support the

estimation of the structural damping and viscous effects

Outline


Motivations

Experimental Set-Up



Current Projects

Conclusions


Acknowledgments

This work was supported by the Research Institute for Autonomous Precision-Guided Systems under AFOSR contract F49620-1-0170 and FA9550-07-1-0236 with Fariba Fahroo as project monitor.

The authors wish to thank AFRL at Eglin AFB for funding and Support.

University of FloridaResearch & Engineering Education Facility (REEF)

Thank You!Roberto Albertani

Unsteady Mechanical Aspects of Flexible Wings: an Experimental Investigation Applied to

Biologically Inspired MAVs

university of florida research & engineering education...

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