Encyclopedia of Aerospace Engineering Volume 7.3 MICRO AIR VEHICLES 7. 3.04 STRUCTURE AND MATERIALS: 2 – MAV -Motivated

Encyclopedia of Aerospace Engineering Volume 7.3 MICRO AIR VEHICLES 7. 3.04 STRUCTURE AND MATERIALS: 2 – MAV -Motivated

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   Encyclopedia of Aerospace Engineering , Online © 2010 John Wiley & Sons, Ltd.This article is © 2010 John Wiley & Sons, Ltd.This article was published in the  Encyclopedia of Aerospace Engineering  in 2010 by John Wiley & Sons, Ltd. MAV-Motivated Structures and Materials Kwang-Joon Yoon, Joon-Hyuk Park, and Sriyulianti Widhiarini Konkuk University, Seoul, Korea 1 Introduction 12 Flapping-wing MAV Development 13 Design and Manufacture of BiomimeticFlapping-wing MAV 44 Performance Test and Evaluation 85 Summary 11Acknowledgments 11References 11 1 INTRODUCTION Nature’s flyers, such as birds, bats, and insects have amazedhumankind for long with their remarkable flight charac-teristics, which triggered the development of manmade airvehicles. The study of flight can be traced back to the earliestdesignofthemanmadeflyingmachine,byLeonardodaVinciin1500sintheformaofflappingvehicle.However,thedevel-opmentdriftedeversincethesuccessfulpoweredflightusingfixed-wings by Wright brothers. More remarkable innova-tions later followed this success, and they were all focusedon fixed wing aircraft.Recently, active air vehicle research, inspired by nature’sflyers, has led to the development of micro air vehicles(MAVs).Itisnowawell-integratedresearchareainthedevel-opment of smaller flight vehicles for environmental monitor-ing, surveillance, and assessments in hostile environments.As the sizes of the MAVs are getting smaller, they have beendeveloped to mimic nature as closely as possible. They areexpectedtohavehighmaneuverabilityandverylowspeedca-pability,butatthesametime,shouldbeabletoreachthoseca-pabilitieswithhighpowerandhighaerodynamicefficiencyinalowReynoldsnumberenvironment,from50000to150000.Consequently, MAVs will experience increasing drag andloss of efficiency due to the low Reynolds number. Due tothe limitations of size, their wings also have a small aspectratio which will reduce the effective angle of attack andinducethe3Dflowstructurecreatedwhentheyfly.Moreover,they are also sensitive to wind gusts, and become moredifficult to control.MAVscanbegenerallycategorizedintothreekindsbasedon their mechanism of producing lift: fixed, rotary, andflapping-wing. Research on fixed-wing MAV is the most de-velopedareaamongothercategories.Thesecondcategoryof MAV is rotary-wing. Compared to the earlier type, this MAVhas a relatively lower endurance due to its smallscale rotorscausing highly effective power requirements for the hover.Next is the flapping-wing MAV, its research comes fromthe inspiration of mimicking the mechanism and features of birds and insects. The aerodynamics of this vehicle is highlyunsteadyduetoaccelerationanddecelerationofthewingdur-ingitsupanddownmovements,inadditiontothehighlyrela-tiveviscouseffectatlowReynoldsnumberthatalreadyexists. 2 FLAPPING-WING MAVDEVELOPMENT 2.1 Overview MAVswitha15cmwingspanseemtohavethecharacteristicsbetween the borders of two groups of nature’s flyers because DOI: 10.1002/9780470686652.eae405   Encyclopedia of Aerospace Engineering , Online © 2010 John Wiley & Sons, Ltd.This article is © 2010 John Wiley & Sons, Ltd.This article was published in the  Encyclopedia of Aerospace Engineering  in 2010 by John Wiley & Sons, Ltd. 2 Micro Air Vehicles Figure 1.  Size of natural flyers. Reproduced from Pornsin-Sirirak  et al.  (2000) c  IEEE. itliesbetweenthesetwogroupsasshowninFigure1.Smallerflyers, like insects and hummingbirds are able to hover yetthey cannot soar. On the other hand, bigger flyers like birdsandbatscannothoverbuttheycansoar.Thesmall-sizeflyersusetheflapping-wingmechanismtogeneratelifttoovercometheir own weight.RelatedtothedevelopmentofMAV,thesmallerthesizeof the vehicle, the more it resembles the features of the nature’sflyers, in this case, large insects. Then, it will be more effi-cienttoapplytheflappingmechanismforthistypeofvehiclecomparedtousingthefixed-wing.Theairfoilperformanceof afixed-wingMAVwillreduceseverelyforReynoldsnumberinthisrange.Sincealowerliftcoefficientwillbeobtained,thewings will have lower loading capability. Moreover, due tothehigherdragcoefficientresultingfromtheviscosity,higherpowerinputisnecessary.Flowseparationonthewing,whichcauses stall at low angles of attack, will also reduce the per-formance and maneuverability of the wings (Pornsin-Sirirak  et al ., 2000). These reasons have encouraged more researchon flapping-wing mechanism to emerge.One of the earliest developments of flapping-wing MAVwas Aerovironment Microbat, a palm-sized 10g ornithopter,capableofflyingaroundfor5minutes(Pornsin-Sirirak  etal .,2000). Several research teams from different universitiesalso took part in the development of flapping-wing MAVs.University of Arizona has been developing ornithopter since2000, with its noticeable design, a 15-inch wingspan, and47g RC ornithopter (Olson  et al ., 2005). TU Delft cre-ated Delfly, inspired by the flight of a dragonfly, capableof performing a straight and horizontal flight and transformto a very slow, hovering flight (http://www.delfly.nl/?site=DIII%26menu=home%26lang=en). MAV research teamin Konkuk University, Korea has also been developingflapping-wing MAVs, since 2002 for the participation in In-ternational Micro Air Vehicle Competition (IMAVC) andEuropean Micro Air Vehicle (EMAV) competition.IMAVandEMAVflightcompetitionsaretwomajorMAVcompetitions. The IMAVC consists of three major cate-gories: (i) Ornithopter competition of the smallest radio-controlled ornithopter, (ii) Surveillance, and (iii) Endurance.On the other hand, EMAV competition is divided into indoorand outdoor competitions, each section comprising flightdynamics and autonomy categories.Recent studies on MAVs cover various fields, fromaerodynamics to navigation and control. In aerodynam-ics, the studies have been conducted to reveal the secretsbehind the flight mechanisms of both birds and insects. Theflight of birds is characterized by the capability of soar-ing and gliding, whereas insects are more toward hovering.Research conducted to analyze the unsteady flight mecha-nism can be characterized into four categories: rotation cir-culation(Dickinson,Lehman,andSane,1999),clapandfling(Weis-Fogh, 1973), wake capture (Dickinson, Lehman, andSane, 1999), added mass (Ellington, 1984), and leading edgevortex (Ellington  et al ., 1996). Each mechanism will be re-lated to the performance of the main aerodynamic and powersource, particularly the wings. Hence, the development of structural design of the wings becomes necessary.The on-going researches of flapping-wing MAVs are be-ing focused on reducing the size of the vehicle as a way tomimic the natural flyers closely. Therefore, the developmentand optimization of the structural design of the vehicle arenecessary. One important breakthrough is the minimizationof the weight of the MAV by using composite material forthe vehicle structure. The use of these advanced materials isexpected to reduce the weight of the MAV without signifi-cantly affecting the flight performance of the vehicle. There-fore, more studies on lightweight composite structures arebeing conducted to demonstrate the possibility.Due to the fast response of direct and reverse piezoelec-tric effects, more research studies have also been conductedto cover the needs of a high-density power source to re-place the motor. Piezoelectric materials are promising can-didates for this. One of the most significant works of insect-mimicking MAV design was made at the University of Cal-ifornia at Berkeley (Fearing  et al. , 2000). A micromechan-ical flying insect (MFI) actuated by two unimorph piezo-electric actuators produced a flapping angle of 80 ◦ at flap-ping frequency of 275Hz and average lift of 1400  N froma single wing, including the inertia force (Sitti, 2001; Steltz,Avadhanula and Fearing, 2007). Konkuk University has alsosuccessfully demonstrated a flapping device actuated by aunimorph actuator which can passively create wing rotationand produce a 2.7g vertical force and a 0.83g forward force DOI: 10.1002/9780470686652.eae405   Encyclopedia of Aerospace Engineering , Online © 2010 John Wiley & Sons, Ltd.This article is © 2010 John Wiley & Sons, Ltd.This article was published in the  Encyclopedia of Aerospace Engineering  in 2010 by John Wiley & Sons, Ltd. MAV-Motivated Structures and Materials  3 at a driven peak-to-peak voltage of 300V pp  and a flappingfrequency of 9Hz (Park   et al. , 2004; Syaifuddin, Park andGoo, 2006; Nguyen  et al ., 2007). The latest development is a60mg flapping-wing system introduced by Harvard Univer-sitywhichwasactuatedbyasmallunimorphpiezoelectricac-tuator (Wood, 2007). However, even the latest breakthroughin the use of piezoelectric actuator still cannot offer an inde-pendent flight system. Therefore, embedding a lightweightsystem in a part of the vehicle is also necessary, so that mini-mization of the weight will be more effective. The structuralsystem of the wing as the main part of the MAV also needsparticular attention. By modifying the shape and structuralsystem to be similar to that of the biological counterparts,the vehicle is expected to be lighter yet have a good flightperformance for a flight-capable flapping-wing MAV. 2.2 KU flapping-wing MAV development With the aim to create an RC-controllable flapping-wingMAV,theMAVresearchteaminKonkukUniversityhasbeendeveloping different kinds of MAVs. The development of those vehicles was first started to find out the best design of MAV for the IMAVC. However, in the long run, the studywas also diversified to analyze the biomimetic structure of flapping-wing inspired by birds and insects, to be applied tothe design of a flight-capable MAV. This is motivated by thefact that the study for the wing structure of a flight-capableflapping-wing MAV has not been specifically conducted inany other research.Figure 2 shows the shape of KU-Ornithopter with a 28cmwingspan developed in 2006. It could fly for more than 8minutes by a remote-control at 2–3ms − 1 wind condition.It was the primitive development of the flapping-wing vehi-cle, and its wing was not particularly designed to follow anyspecific biological insect or bird. Instead, it only focused onkeeping stiff leading edge and flexibility at the rest of thewing. The vehicle was designed to complete the mission inthe 2006 International Micro Air Vehicles Competition. Themission was to fly 8 or O shaped turns between two poles12m distance apart, as many times as possible in a limitedtimeperiodtogetahighscore.Sincesharpandrapidturningswere needed, the focus was also on producing high maneu-verability. It was ranked 2nd for the ornithopter mission atthe 10th IMAVC in 2006.Since then, the development of KU-Ornithopter hascontinued. Many studies and experiments were conductedto enhance the flight performance of the vehicle. Figure 3showsthe36cmflappingwingornithopterdevelopedin2007withanonboardmicro-videocamera.Thetailstructureoftheearliest development was still maintained, however it wasmodified by a rudder and elevator to enhance the control of the vehicle.For out-of-sight control and guidance, the micro-videocamera was mounted at the front nose of the MAV with atransmitter in order to collect the vision images and send itto the ground station. It was placed pointing to the ground30 ◦ downward from the flight direction, which intended tocapture the ground images during the level flight. Althoughsomefuzzyandvibratingimageswerereceivedduetothevi-bration from flapping motion, it could transmit the pilot viewreal-time video image within 200m remote control range,which enables vision/image-based control. The landing gearsystem was employed in the MAV for takeoff and landingat flat ground less than 20m without hand-launching assis-tance. Three wheels were attached to the fuselage with 22 ◦ incidence angle from the horizontal plane. The indoor flightdemonstration for this vehicle was successfully performed atthe MAV07 in Toulouse, France in 2007.Besidesenhancingthevehicle’sflightperformanceforthecompletion of IMAVC and EMAV flight missions, the devel-opment was also made to the wing structure of the vehicle.The 36cm flapping-wing MAV was modified by mimickingthe structure of nature’s flyers. This study was carried out to Figure 2.  KU-Ornithopter-06 with 28cm wingspan and 1st prototype composite wing structure. DOI: 10.1002/9780470686652.eae405   Encyclopedia of Aerospace Engineering , Online © 2010 John Wiley & Sons, Ltd.This article is © 2010 John Wiley & Sons, Ltd.This article was published in the  Encyclopedia of Aerospace Engineering  in 2010 by John Wiley & Sons, Ltd. 4 Micro Air Vehicles Figure 3.  KU-Ornithopter-07 with 36cm wingspan, biomimetic wing structure, and onboard camera. Figure 4.  15cm KU-Ornithopter-08 with biomimetic wing structure. analyze the significance of biomimetic structure to the flightefficiency.ThewingofCicada( Cic`adidae )waschosenasthebase for the artificial wing development. The main featuresofCicadawereappliedtothewings,suchasthecamberwinginspanwisedirection,thicknessdifferencethroughwingrootto wing tip, and a cell-type wing structure. More details willbe given in the following section.The development of the flapping wing vehicle contin-ued further to reduce the size of the vehicle with an im-proveddesign.In2008,the15cmKU-Ornithopterwasdevel-oped.Somechangesweremadetothealtitudecontrol,whichchanged the flapping frequency by controlling the RPM of the motor and tail structure and by removing the horizontalstabilizer, as shown in Figure 4. The total force generated bythe wing was not sufficient to fly under wind velocity over2ms − 1 , so most of the flight tests were conducted indoor. Itwas ranked 3rd in the indoor dynamic mission category atthe EMAV flight competition in 2008 held at Braunschweig,Germany. 3 DESIGN AND MANUFACTURE OFBIOMIMETIC FLAPPING-WING MAV 3.1 Biomimetic wing development Wings play the most important role in flapping wing flightnot only to change the direction of the movement, but also toundergo deformation, which may lead to aerodynamic forcegeneration during the flight. More specifically, there is noinsect that flies by using only the simple flexural vibrationof the wings. Insects are able to fly only through complexflight mechanisms, such as delayed stall, rotational circula-tion,andwakecapture.However,thereisnoreportonadevel-oped artificial flapping mechanism which can perform thosemechanisms at the same time. Modifying the wing structureof a flapping-wing vehicle to resemble closely that of thenature’s flyers, thus became the focus of this research. Toprove that such a modification will affect flight efficiency,the biomimetic structure was applied to the vehicle and some DOI: 10.1002/9780470686652.eae405   Encyclopedia of Aerospace Engineering , Online © 2010 John Wiley & Sons, Ltd.This article is © 2010 John Wiley & Sons, Ltd.This article was published in the  Encyclopedia of Aerospace Engineering  in 2010 by John Wiley & Sons, Ltd. MAV-Motivated Structures and Materials  5 Figure 5.  Wing shape and structure of Cicada. simplifications were made during the fabrication by main-taining the main features of the insect’s wing.By analyzing the wings of Cicada, three main featuresof its wings were identified: camber in chord and span-wisedirection, thickness difference throughout the wing root towingtip,andcell-typemembranes.Thesefeaturesareshownin Figure 5. Camber in chord and span-wise directions isimportant to efficiently control the wing deformation duringthe flapping motion. The cell-type membrane was a resultof the wing vein structure throughout the whole wing areaand the vein thickness variation, which stiffens the insect’swings.Bykeepingthesemainfeatures,theartificialwingwascreated with modifications.The thickness variation occurs as a result of the veins,which surrounds the cell membranes; thickness is reducedfrom the wing root to the wing tip. These features showedthat the insect wing has an efficient wing structure for un-steady flight (Park and Yoon, 2008). Moreover, camber onbothspanwiseandchorddirectionsarealsoimportanttoeffi-cientlycontrolthewingdeformationduringflappingmotion.Keeping these features of insect wing in mind, attempts weremade to mimic the insect features.The biomimetic wing development was done initially byadding more frames to the srcinal structure and adoptingcamber in the spanwise direction. In order to compare the Table 1.  Dimension and properties of materials.Thickness WidthMaterial Part (mm) (mm)Carbon/epoxy Wing frame 1 (6 layers) 1.2 2fabric frame Wing frame 2 (4 layers) 0.8 2  E  1 = 59.1GPa;  E  2 = 55.7GPa;  G 12 = 4.7GPa;  12 = 0.06Carbon/Epoxy Diameter (mm) 1.2Rod  E  1 = 100GPa;  E  2 = 7.0GPa;  G 12 = 4.0GPa;  12 = 0.3PET Film Thickness (mm) 0.02  E  = 1.8GPa;   12 = 0.3 flightcharacteristicsbetweentheartificialandnormalwings,we set two types of wings for the experiment. Both wingswere composed of carbon/epoxy frame for the main sparand polyethylene terephthalate (PET) for the wing skin. Thedimension and properties of these materials are shown inTable 1. Type A, the 1st prototype wing design is a flat wingwith one frame placed across the wing area as shown inFigure 6a. Type B, the biomimetic wing, is a cambered wing Figure 6.  Wing structure and materials: (a) 1st prototype wing and (b) biomimetic wing. Reproduced from Park and Yoon (2009) c  Springer. DOI: 10.1002/9780470686652.eae405
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