2006-Challenges Facing Future Micro-Air-Vehicle Development

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JOURNAL OF AIRCRAFTVol. 43, No. 2, March–April 2006

Challenges Facing Future Micro-Air-Vehicle Development

Darryll J. Pines and Felipe Bohorquez

University of Maryland, College Park, Maryland 20742

Nomenclature

Ar = rotor disk areaC D = sectional drag coefficientC D0 = zero-lift drag coefficientC lα = lift-curve slopeC P = power coefficientC Pi = induced power coefficientC P0 = profile power coefficientC T = thrust coefficientc = chord length D = drag force D. L . = disk loading L = lift force

m = massP. L . = power loadingSF = separated flowT = rotor thrustV = local wind velocity perceived by flapW = weightW f = final weightW o = gross takeoff weightα = blade section angle of attackη = efficiencyµ = dynamic viscosityρ = air densityσ = rotor solidity = flapping amplitude (peak to peak)

I. Introduction

O NE-HUNDREDyears afterthe Wright Brother’sfirst poweredhistoric flight at Kitty Hawk, North Carolina, on 17 December 1903, aerospace engineers still face challenges in understanding andharnessing the physics of flight. Whereas the 20th century usheredin the era of manned atmospheric and spaceflight, it is likely that the21st century will be characterized by the emergence of autonomouscomputer-controlled uninhabited flight. Driven by a host of civilian,homeland security, and military objectives, uninhabited air vehicles(UAVs) have emerged as the platform of choice for warfightersconducting survelliance and reconnaissance operations in hostileenvironments.

Because of their relatively low cost and propensity for provid-

ing accurate survelliance information, numerous UAV development

Dr. Pines is a Professor at the University of Maryland in the Department of Aerospace Engineering. He is currently on leave at the DARPA in

the Defense Sciences Office. Dr. Pines research interests include smart structures, structural health monitoring and micro and nano air vehicle flight

physics. He is a fellow of IOP and an associate fellow of AIAA. He is also a member of ASME and AHS.

Felipe Bohorquez is doctoral student at the University of Maryland working on micro air vehicle research. He is a student member of AIAA and

the American Helicopter Society.

Received 2 September 2003; revision received 28 February 2005; accepted for publication 28 February 2005. Copyright c 2005 by the American Institute of Aeronauticsand Astronautics, Inc. All rights reserved. Copies of this paper maybe made forpersonal or internal use, on condition that thecopier pay the$10.00

per-copy fee to the Copyright Clearance Center, Inc., 222 Rosewood Drive, Danvers, MA 01923; include the code 0021-8669/06 $10.00 in correspondencewith the CCC.

programs have been initiated worldwide.1,2 As indicated in Fig. 1,typical UAVs have wing spans greater than 1 m and gross takeoff weights (GTOW) greater than 5 kg. However, in the past decade anew class of UAV has emerged, which is at least an order of magni-tude smaller in length and two orders of magnitude lighter in weightthan previously developed aircraft. This new class of UAV is calleda micro air vehicle (MAV) or sometimes referred to as a µUAV.These vehicles have been defined to have no length dimensiongreater than 6 in. with gross takeoff weights of approximately 200 gor less.

As a new class of air vehicle, these systems face many uniquechallenges that make their design and development difficult. For example, micro air vehicles operate in a very sensitive Reynolds-number regime. In this regime, many complex flow phenomenatakeplace within the boundary layer. Separation, transition, and reat-tachment can all occur within a short distance along the chordlineof a wing or rotor and can dramatically affect the performance of the lifting surface. Hence, designing vehicles that can efficientlyfly in this flight regime represents an entirely new challenge toaerospace design engineers. In fact because of the lack of knowl-edge about the fundamental flow physics in this regime, the de-velopment of small-scale flying vehicles parallels the developmentof the practical powered aircraft developed by the Wright Brothersin 1903. After the initial development of the Wright Flyer, engi-neers and scientists struggled to develop analytical tools as wellas gather enough experimental wind-tunnel and flight-test data tohelp develop better design tools that would improve the perfor-mance of fixed-wing aircraft. A similar trend is evolving in the

development of small-scale mechanical flying machines. There arefew if any analysis tools to help MAV designers accurately modelthe steady and unsteady environment that MAVs encounter while inflight.

Fortunately, the development of larger-scale UAVs over the past30 years provides some insight and guidance into the anticipatedperformance of microscale fixed-wing and rotary-wing MAV de-signs. For instance, usingdata from previously developed UAVs, it ispossible to extrapolate performance parameters such as endurance,wing span, payload mass, and range to much smaller scale lengths.Figure 2 illustrates such scaling trends for UAV payload weightand endurance vs GTOW. Surprisingly, over a broad range of UAVGTOW, the payload mass scales linearly on a log-log scale. Ex-tending this linear scaling trend to a micro air vehicle with aweight of approximately 100 g provides a first guess of the po-

tential payload weights for a small-scale fixed-wing mechanical

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PINES AND BOHORQUEZ 291

Fig. 1 Scale of uninhabited air vehicles.2

Fig. 2 UAV GTOW and payload weight vs wing span.

flyer. This scaling suggests an approximate payload weight of 10 g. Similarly, extending the linear curve fit for enduranceprovides some insight into expectations for the endurance of small-scale length fixed-wing UAVs. Fixed-wing UAV endurancedata have a much greater scatter as efficiencies in propulsionand aerodynamics become more uncertain as size and weightdecrease.

Although these scaling trends offer some insight into expectedperformance, data and design tools for small-scale aircraft [Wingspans


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292 PINES AND BOHORQUEZ

Fig. 3 Great Flight diagram.

Table 1 MAV design requirements

Specification Requirements Details

Size


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Table 2 Design and performance parameters of some representative MAVs8−15

Black Widow Hoverfly LUMAV MicroStar Microbat MICORVehicle properties (Aerovironment) (Aerovironment) (Auburn Univ.) (Lockheed-Martin) (CalTech) (UMD)

GTOW, g 80 180 440 110 10.5 103Cruise speed, m/s 13.4 15–20 5 13.4–15.6 5 2

Wing loading, N/m2 40.3 —— —— 70.9 40 ——

Disk loading, N/m2 —— 70 185 —— —— 25Wing span or rotor 15.24 18 15.24 22.86 15.24 15.24diameter, cmMax L/D 6 N/Aa N/A 6 N/A 5Endurance, min 30 13.2 20 25 2 min 16 s 3Hover endurance N/A 7.3 N/A N/A N/A 3Power source Lithium-ion Lithium-ion 2-stroke Lithium-ion Sanyo NiCad Lithium-ion

batteries batteries IC engine batteries N-50 cells batteriesEnergy density, W-h/kg 140 140 5500 methanol 150 100 150Hover power, W N/A 24.5 70 N/A N/A 11Hover FM N/A 0.39 0.41 N/A N/A 0.55

aN/A= not available.

Fig. 4 Endurance or hover time of current MAVs.

flight, it is important to minimize the power required to increase the

endurance. This suggests that from Eq. (1) one would like to havehigh wing loading, high lift coefficient, low drag coefficient, highpropellerefficiency, andfly as low as possible.Althoughit is difficultto find all of these parameters for existing fixed-wing MAV designs,it is still possible to compare the endurance performance based onmeasured flight times. Figure 4 displays the GTOW weight of vari-ous MAV designs vs endurance or maximum hover time. Althoughthese represent substantial progress in the field, the fact that nonehas been able to achieve true long-loiter times (>60 min) or effi-cient hovering flight is a testamentto thedifficulty of flying extendedmissions withsmall vehicles. Careful inspection of these vehicle de-signs reveals a variety of technical challenges for aerospace designengineers. For example, a detailed breakdown of the mass fractionsof three of these vehicles reveals a number of shortcomings whencompared to full-scale systems. Figure 5 displays the mass fractionsof three microflyers compared to a full-scale Boeing 767 commer-cial jetliner. Notice that for the small-scale flyers the mass fractionof the propulsion system (batteries/power and motor/transmission)

is in excess of 60% of the total vehicle mass. In contrast a jetliner

has a propulsion/fuel system mass fraction of approximately 40%.It appears that this 20% savings at full scale is used entirely for payload because the payload mass fraction is 29% for the 767 and just 9% for the University of Maryland’s MICOR (Micro CoaxialRotorcraft) and CalTech/Aerovironment’s Microbat, respectively.Additionally, there is a wide variation in the mass fraction of thestructure required to support flight of the three small-scale vehiclescompared in Fig. 5.

2. Rotary-Wing/VTOL MAV Hover Performance

In the case of rotary-wing/VTOL designs, another metric of in-terest is hover efficiency. Although the hover performance of moreconventional full-scale rotorcraft configurations is well documentedin theliterature, thehover performance of micro “hovering” airvehi-

cles in hover at low Reynolds numbers is relatively unknown.

16,17,†

†Data available online at http://www.faulhaber.de.

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Fig. 5 Mass fraction of MAVs compared against a Boeing 767.

Fig. 6 Rotary-wing MAV performance.

To compare the hover performance of various rotary-wing andVTOL MAV designs, it is important to examine the metric for comparing hover efficiency. This is accomplished by comparingthe actual power required to hover with the ideal power required tohover.18 This leads to the rotor figure of Merit (FM) given by

FM = ideal power actual power

= T vP

(2)

where P is power supplied to the rotor and v is the rotor inducedvelocity. The larger the value of FM, the smaller the power re-quired to produce a given thrust, or the larger the thrust per unitpower. For an ideal rotor FM should equal 1. By rearrangingEq. (3), it is not difficult to derive the following equation relat-ing power loading to rotor disk loading from simple momentumtheory:

P. L . = 0.638√

D. L .

FM

(3)

where P . L . (Power/Thrust) is power loading and D. L . (Thrust/Disk

Area) is disk loading. This expression measures the aerodynamicand power efficiency of hovering vehicles.

Figure 6 plots P. L . vs D . L . for the three hovering MAV config-urations (Kolibri, Hoverfly, MICOR) listed in Table 2. Upper- andlower-bound limits are illustrated for figure-of-merit values of 0.40and1.0.Thosevehicle designs lyingabove thecurve fora FM= 0.40require more power to stay aloft and are therefore less efficient thanother hovering designs. For the designs considered in this study,it appears that MICOR has the lowest power loading for a givendisk loading at this scale length. However, it is instructive to ex-amine the vehicle’s figure of merit vs various thrust coefficients.This is displayed in Fig. 7. Notice that in comparison to full-scalerotorcraft, MICOR’s 8% cambered airfoils achieve at best a FMvalue of 0.55 (Ref. 19). Full-scale rotors achieve FM values in therange from 0.65 to 0.85. In a full-scale rotor 30% of the power is consumed by the profile losses and 70% by the induced losses.However, at low Reynolds numbers the profile power has a muchlarger influence over the total power required by the rotor. At high

Fig. 7 Figure of merit vsthrust coefficient.

thrust coefficients the contribution of profile power goes up to 45%.In addition, section lift-to-drag ratios for typical airfoil geome-tries tend to range from 2 to 8. Thus, scale size, airfoil design,thickness, and surface roughness appear to have a significant effecton the hover perform-ance of micro rotary wing designs. Hence,MAV designers are faced with significant challenges as length scaledecreases.

In terms of efficient MAV design, several technical questions re-main unanswered. For example, is it possible that MAV designerscan improve the overall efficiency and performance of MAVs? Are

there lessons to be learned by studying the efficiency of biolog-ical flyers? Unlike subsonic fixed-wing aircraft with their steady,almost inviscid flow dynamics, biological flyers such as insects andsmall birds fly in a sea of vortices when they flap their wings.These vortices can be used to keep MAVs aloft, especially inthe case of hovering flight. Recently, it has been demonstratedby computational-fluid-dynamics (CFD) analysis20 that bumble-bees flap their wings in a complex kinematic figure-eight pat-tern to generate lift and thrust. Similarly, hummingbirds are well-known masters of hovering flight by flapping their wings in excessof 20 Hz.

Closer examination of biological fliers reveals that existing MAVdesigns cannot match the aerodynamic performance (stability, ma-neuverability, and efficiency) of insects and small birds. This shouldcome as no surprise because design tools at this scale of flight are

not available. In addition, the underlying physics that are respon-sible for the achievements of nature’s great flyers is still not wellunderstood. For example, how an insect can take off backwards, flysideways, and land upside down21 cannot be explained using con-ventional aerodynamic theory. It appears that insect wings producelift more efficiently than one would expect based on conventionalsteady-state aerodynamic theory.22−25

III. Fundamental Physics Limiting MAV Performance

One of the greatest challenges for researchers is determining howinsects and small birds can generate forces that can range from 2to 12 times their body weight. Conventional steady-state aerody-namic theory is unable to explain this phenomenon. Thus, some-thing about the complexity of the wing pitching/plunging/lagging

motion increases the lift produced by a wing above and beyond thatwhich it could generate under steady flow conditions or that canbe predicted by conventional steady-state aerodynamic theory. Thereasons for this remain research topics, but some conclusions areemerging. Recent work by experimental biologists indicates thatthe pitching/plunging motion of the insect wing can improve ef-ficiency by enabling the recovery of wake vorticity.26 Other stud-ies indicate that birds can increase their aerodynamic efficiency vialarge-scale morphing of theirwing geometries.27 A remarkablywiderange of changes can be affected including variations in anhedral,dihedral, planform, camber, aspect ratio, wing sweep, and wingwarping.

Given that the performance of the current generation of MAVs isvastly inferior to that of birds, insects, and even full-scale UAVs, itseems logical that subsequent generations of MAVs could improvetheir performance by mimicking at least some aspects of biologi-cal flight. Thus, if MAVs are to approach and possibly exceed theperformance of biological flyers, advances are required in several

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fundamental areas including 1) low-Reynolds-number aerodynam-ics ( 105 the performance of smooth air-foils greatly improves. This was primarily because of the underly-ing highly viscous laminar flow physics present at low Reynoldsnumbers.30 A more recent study by Baxter and East31 in thisReynolds-number regime, steady-state aerodynamic analysis re-veals that as the Reynolds number decreases the minimum power condition for straight and level cruising flight can be approxi-

Fig. 8 L/Dmax vs Reynolds number for smooth and rough airfoils(McMasters and Henderson).

mated by

C 2 L min Power =

3C d o

b( Reref / Re)

0.5 (6)

where Reref is the reference Reynolds number value of 105. Thus,

it can be seen from this expression that as the Reynolds number decreases the profile drag increases. Baxter concluded that the min-imum drag-minimum power configuration of MAVs requires vehi-cles with lift coefficients in excess of three. These results indicate

that as the profile drag coefficient increases relative to the induceddrag coefficient, the operating C L at which minimum drag andmini-mumpower are obtained aresignificantly higher than those requiredat more conventional flight Reynolds numbers (>105). Similar tofixed-wing airfoil studies, rotary-wing rotor aerodynamics suffer from the same flow physics that limit the performance of fixed-wing MAVs. In a recent study by Bohorquez et al.,19 blade elementmomentum theory (BEMT) coupled with a uniform inflow modelwas used to determine the two-dimensional lift and drag propertiesof several simple airfoil shapes at a tip Re= 3× 104. They foundthat estimated maximum rotor lift-to-drag ratios ranged from 4 to10 (see Fig. 9). In addition, they found that the profile drag for rotor blades at Re < 3× 104 constitutes a higher percentage of the totalpower required to hover than found in full-scale rotors. PreliminaryCFD and fluorescent oil-based flow-visualization experiments (see

Fig. 10) revealed large-scale flow separation with only a fractionof the rotor having attached flow. This is largely because the flowin this regime is laminar, but also unstable and tends to separateeasily from the surface of airfoils. This is because the low inertialforces in the fluid boundary layer render it unable to stay attached

Fig. 9 Lift-to-drag ratio for several rotors with aspect ratio of 6 at atip Re = 3×× 104 calculated using BEMT.

Fig. 10 Oil flow-visualization Results on 6-in. rotors with tip Re = 3×× 104 and 8% curvature.

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in the face of the adverse pressure gradients after the suction peak.Under certain conditions, the separated shear layer can reattach tothe surface, forming a laminar separation bubble (LSB).32−35 Morecommonly, the shear layer can transition to turbulent flow and onlythen have the kinetic energy to be able to reattach thereby forminga transitional separation bubble.

In addition, a number of complex and nonlinear flow phenomenacan cause the LSB to move back and forth over the airfoil as well asincrease and decrease in size. Although the discussion of these flow

physics is beyond the scope of the present paper, there have beennumerous techniques used to mitigate the effects of the LSB on theairfoil’s aerodynamic performance. These techniques include bothpassive and active flow control methods to inject disturbances intothe boundary layer at the proper scale length. A summary of thesetechniques is presented in a recent paper by Gad-el-Hak.36 The goalof flow control is to improve the overall aerodynamic efficiency of airfoils and to transition laminar flow to turbulent flow as a meansof reducing the skin-friction drag and causing the flow to reattachover an airfoil.

This limit in aerodynamic efficiency using conventional steady-state aerodynamic theory has prompted many researchers to searchfor the unsteady aerodynamic mechanisms that might explain thehigh forces produced by insects and small birds. 33−44 Pioneeringresearch by Lighthill,37 Rayner,38,39 and Pennycuick40,41 has pro-

vided some insight into avian flight. However, until recently littlewas known regarding the complex kinematics of insect flight. Bi-ologists Ellington and Dickinson have made significant advancesin understanding the aerodynamic physics of insects. According toEllington42 (see Fig. 11), the wing stroke of an insect is typicallydivided into four kinematic portions: two translational phases (up-stroke and downstroke), when the wings sweep through the air witha high angle of attack, and two rotational phases (pronation andsupination), when the wings rapidly rotate and reverse direction.For comparative purposes, flap-ping flight of insects in hoveringmode is characterized by wing-beat frequency and mean Reynoldsnumber. Ellington42 approximates mean Reynolds number as

Re = 4 f R2/υAR (7)

where AR is the aspect ratio, f is the wing-beat frequency, R is thewing length/span, and is the wingbeat amplitude (peak to peak).Using geometrical similarity and scaling relationships for wing-beat frequency, researchers discovered that the Reynolds number increases as a function of mass m0.42. For various sized insects,Reynolds number varies from 10 to 104. Over this wide range of

Fig. 11 Diagram of the vortex system during the complete wing-beat cycle. The shaded area at pronation denotes the morphological lower wing

surface on the insect diagram (insets). A large leading-edge vortex (LEV) with strong axial flow is observed during the downstroke. This LEV is stillpresent during supination, but turns into a hook-shaped vortex. A small LEV is also detected during the early upstroke and gradually grows intoa large vortex in the latter half of the upstroke. This LEV is still observed closely attached to the wing during the subsequent pronation, where atrailing-edge vortex (TEV) and a shear-layer vortex (SLV) are also formed, together forming a complicated vortex system (Ref. 42, 99).

Reynolds number for flapping flight, the flow regime is primar-ily laminar with several unsteady mechanisms contributing to theelevated performance of insect flight. The unsteady mechanismsthat have been proposed to explain the elevated performance of in-sect wings typically emphasize either the translational or rotationalphasesof wing motion. However, thefirst unsteady effect to be iden-tified wasa flapping mechanismtermed the“clapand fling,”43 whichis a close apposition of the two wings preceding pronation that ac-celerates the development of circulation during the downstroke.44

Although the clap and fling can be important, especially in smallspecies, it is not used by all insects and thus cannot represent a gen-eral solution to the phenomenon of force production. Recent studiesby Dickinson and colleagues45−48 and Liu et al.49 suggest that sev-eral unsteady mechanisms might explain how insect wings generatesuch large aerodynamic forces over a complete wing-beat cycle.

A more detailed look at these aerodynamic mechanisms can beexplained by examining the flow physics associated with any airfoilwith increasing angle of attack. The high adverse pressure gradientsthat build up near the leading edge under dynamic translating (flap-ping) conditions cause flow separation to occur there. Experimentalevidence suggests the formation of a shear layer that forms justdownstream of the leading edge, which quickly rolls up and forms avortex. Notlongafterit is formed, this vortex leavesthe leading-edgeregion and begins to convect over the upper surface of the airfoil.

This induces a pressure wave that sustains lift and produces airloadswell in excess of those obtained under steady conditions. Althoughdelayed stall might account for enough lift to keep an insect aloft, itcannot easily explain how many insects can generate aerodynamicforcesthat exceed twice their body weightwhilecarryingloads. Sev-eral additional unsteady mechanisms have been proposed, mostlybased on wing rotation. Depending on the Reynolds number (seeFig. 12), these mechanisms include delayed stall, wake capture,50

rotational circulation, and bound circulation. Dickinson et al.46 haveproposed that the flight of a drosophilia fly relies on complex kine-matic motion of the insect’s wings. This kinematic motion givesrise to unsteady lift and drag forces that exceed lift forces under steady aerodynamic loads at the same Reynolds number. Althoughthese recent advances in understanding aerodynamic physics havegiven researchers a clearer picture of low-Reynolds-number flight,researchers still have not been able to demonstrate a direct connec-tion between mechanisms and the forces generated in flight.

Thus, themajor obstacle in realizingtruly efficient micro air vehi-clesin the


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Fig. 12 Summary of unsteady aerodynamic mechanisms governing the flight regime of insects and small birds (http://socrates.berkeley.edu/ flymanmd/).

Table 3 Proposed microengines58

Properties MIT UCB UMICH1 UMICH2

Mass, g 4 22.1 54.4 39.3Power, w 10 30 21.1 22.3Type: gas Wankel Swing Swing ——

turbine engine engine

range 50


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As the wing span andaspect ratio increase, the lift-to-drag ratio tendto increase, affecting the glide ratio of the aircraft. Thus, birds withlong wing spans and high aspect ratios are more akin to dynamicsoaring. Because most MAVs have short wing spans and low aspectratios, one would not expect for these vehicles to have great glideor soaring properties. These performance limitations are a result of span requirements imposed under the original Defense AdvancedResearch Projects Agency (DARPA) MAV program that limited thewing span to 15.24 cm (6 in.).

Figure 15 displays the average wing loading (N/m2

) values for birds as a function of body mass. Specific birds are displayed bythe “x” symbol. The wing loading for three MAVs is also displayedin this figure with the “o” symbol. Notice that the wing loading for MAVs is significantly higher than the equivalent size bird. This sug-gests that MAVs must flyfaster in comparison to birds of comparablegeometric size, aerodynamic properties and weight to stay aloft. Toaccomplish this goal, MAVs must expend more power to overcomethe induced aerodynamic drag. In addition, a higher wing loadingmeans lower agility and maneuverability. Finally, the more loadedthe vehicle the less efficient it is because more power is needed tocarry the same unit load.

2. Wing Design

Most fixed-wing MAV designs are based on rigid-wing technol-

ogy (see Fig. 16) consisting of conventional rib/spar elements tosupport aerodynamic and structural loads. These structural elementshave been constructed from a number of different types of materialsincluding basal wood, foam core, and fiberglass skin combinations.In full-scale aircraft, rigid composite wings are used to support theair loads and to minimize structural dynamic effects so as to avoidunstable aeroelastic interactions. Material for these types of wingsmust consist of properties with high tensile strength and stiffness,but low structural weight. More importantly, wings at full scale do

Fig. 15 Wing loading vs mass.

Fig. 16 MAV fixed-wing design evolution.

not flap. However, for small-scale vehicles it is clear that naturesuggests that flapping flexible wings may be advantageous to en-hance the performance of small-scale microflight. The advantagesof the use of flexible wings include passive adaptive washout con-trol that extends the range of the aerodynamic lifting surface. Suchwings have the effect of suppressing wind gusts and restoring con-stant thrust and lift over the airfoil. Researchers at the University of Florida63 have developed numerous flexible fixed-wing designs andmanufacturing methods to understand the performance enhance-

ments of flexible wings for fixed-wing MAVs.To further understand the potential benefits of thin flexible wingsfor small-scale flight, one simply has to examine the form and func-tion of insect wing designs.64 Insect wings are comp. of thin cuticu-lar structures supported by a series of veins filled with insect bloodto provide a level of structural rigidity and flexibility to the wing.Patterns of wing venation are often highly complex. For a typicalinsect wing (see Fig. 17),65 the major veins originate at the axil-lary apparatus, running distally and toward the trailing edge of thewing. As displayed in Fig. 17, the median flexion line represents aradial groove or region of increased flexibility along which the wingcan deform to achieve variable camber. Two other flexion lines, theclaval furrow and jugal fold, help to achieve wing folding againstthe body.

Thus, MAV wing design represents one of the major challenges

to efficient flight in the low-Reynolds-number regime. In the caseof flapping flight, it is important that mechanical wing designshave features of high specific strength, low specific modulus, andlow elastic modulus. The actual design choices will depend on thespecific wing kinematics and dynamics that are desired for the flap-ping air vehicle. However, researchers at Cranfield (Shrivenham)54

a)

b)

Fig. 17 Detailed wing morphology illustrating veins and folding lines.

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and supracoracoideus muscles. The pectoral muscle is used for thedownstroke of a bird’s wing and is significantly larger in mass frac-tion than the supracoracoideus muscle that is used for the upstrokemotion of the wing. In comparison, birds have approximately threetimes the mass fraction of muscle found in humans. Thus, this largemuscle mass fraction coupled with the elasticity/flexibility and lowinertia of a bird’s wing provides the necessary power and lift-to-weight ratios required for efficient low-Reynolds-number flight.

Although there is at least one example of a truly biomimetic

power system that converts chemical energy in ATP to mechanicalwork, practical biomimetic devices for converting stored energy tomechanical work are at least a decade away. However, there is avariety of energy conversion and propulsion systems that can de-liver 10 to 15 W of power to MAVs. These include microengines,battery/electricmotor technology, microfuel cells, and internal com-bustion engines. The status of some of these areas as they relate toMAV propulsion is discussed next.

1. Microengines

A promising,but technically challenging propulsionand/or power source is the micro turbine engine. These devices are capable of producing 10 to 50 W of power in a volume less than 1 cm3 whileconsuming only 7 grams of fuel per hour. Towards this goal, the

Massachusetts Institute of Technology (MIT)68

Gas Turbine Lab-oratory has been developing a microelectromechanical-systems(MEMS) scale device under DoD funding, which consists of a1-cm-diam engine with a centrifugal compressor and radial inflowturbine, separated by a hollow shaft for thermal isolation, and sup-ported on air bearings. A working combustor has been built, butthe compressor, generator, and bearings have yet to be perfectedat the microscales. The program goal is to produce 13 g of thrustwith thrust-to-weight ratios approaching 100:1 (compared to 10:1for today’s modern jet fighter).

The United Kingdom’s Defense Evaluation and Research Agencyhas also successfully produced and demonstrated their own variantof a turbine engine at small scale. The device is 1.3 cm by 0.5 cmand weighs less than 2 g. It uses a hydrogen-peroxide-kerosenefuel mixture and has achieved 6.4 g of thrust for up to an hour of

operation.Otherproposed microengines include thoseunder development at

University of California, Berkeley and the University of Michigan.Cadou et al.69,70 summarize the expected performance parametersof various microengines in Table 3.

2. Battery/Electric Motors

Many of the MAVs listed in Table 3 have relied on battery power to drive efficient electric motors. Compared to a rechargeable Nicadbattery of the same weight, a lithium ion or polymer batteries de-liver several times more energy. However, lithium ion and polymer batteries suffer from low dischargerates and insufficient energy den-sity (150 W · hr per kg) to sustain long-duration MAV flight. Recentadvances include the development of thin flexible/conformal sheetsof lihium ion battery technology. This new development enablesthe battery to double as a multifunctional structure supporting aero-dynamic lift and as a source of energy. This can enable the nextgeneration of MAVs to stay aloft longer.

3. Micro Fuel Cells

Although fuel-cell technology71 is less mature, it is anticipatedthat progress will result in energy densities that are at least two tofour times greater than lithium-ion battery technology. Recently,MTI, Inc, has developed a direct methanol micro fuel cell for thewireless consumer market. These fuels cells have energy densitiesof approximately 240 W · h/kg. Although the energy density ap-pears to meet the needs of MAV designers, size limits their use inMAV applications. IGR Enterprises, Inc, was funded by DARPAto develop a MAV fuel cell weighing less than 30 g and producingapproximately 20 W of power for 1 h. However, this technology isprobably a few years from finding itself on MAVs.

Fig. 20 Comparison of MAV endurance associated with various en-ergy storage/conversion systems.69

Fig. 21 Estimated efficiency of various hydrocarbon-fueled ICengines.69

4. Internal Combustion Engines72,‡

In thenear term, Fig. 20 shows that energy storage andconversiontechnology based on hydrocarbon fuels and small reciprocating en-gines that exist today can provide acceptable levels of performancefor 200-g class MAVs, that is, if the estimates of engine power and efficiency are correct. Although their thermal efficiencies atMAV scales are low, their power densities are quite high, typically1 w/g. Suchengineshave already beendeveloped for the hobby-classcommunity such as the Cox Tee Dee 0.01, which is only 0.01 in. 3

in volume but can produce about 20 W of power.One problem is thatthese estimatesof engine power andefficiency

are based on manufacturer’s published data,70 which is rudimentary.Manufacturers usually report only peak power at a particular oper-ating speed, which means that fuel consumption information and

efficiency must be estimated. Actual data showing power curvesand efficiency (specific fuel consumption) information are never re-ported because these engines are primarily produced for hobbyists.Moreover, manufacturers’ reported data are often highly suspect.There are many reports in the literature about engines with highpower whoseperformance in model aircraft/cars/boats is worse thanengines with lower reported power. Although this apparent anomalycould be the result of intentional deception by manufacturers, itcould also result from the lack of standardized testing proceduresbetween manufacturers. Cadou shows that efficiency estimates (seeFig. 21) for small reciprocating engines vary widely and seem toshow only weak scaling with engine size in comparison to full-scale engines. Clearly, reliable data produced using standardizedtesting procedures are required to verify the performance capability

‡Data available online at http://www.globalsecurity.org/intell/library/ reports/2001/uav0401.htm.

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Fig. 22 Estimate of mass fractions computed for birds.

of existing IC engines and to understand how to optimize perfor-mance for use in biomimetic MAVs.

Another problem apparent from Fig. 20 is that building efficientIC engines becomes more difficult as engine size is reduced. Ther-mal and frictional losses scale directly with surface/volume ratio

and therefore increase as the size of the engine is reduced. Severalresearch teams building microengines in the 10-Watt/5-g class haverecognized this problem but exactly how these losses scale withdevice size is not well understood. Although this information iscritical to designers of MAVs who need to be able to make realisticestimates of the performance capability of a particular size power system before it is actually built, there has been very little work ondeveloping reliable scaling laws.

D. Summary of Status and Performanceof Current MAV Development

In summary, it appears that the major reason for the across-the-board failure to achieve true long-loiter, highly maneuverable, andhovering flight in a single configuration is that all of the config-urations/designs depicted in Fig. 4 rely on conventional steady-state aerodynamic mechanisms that become inefficient at the lowReynolds numbers associated with smooth airfoils and rotors. Theinefficient utilization of aerodynamic lift coupled with relatively in-efficient systems for storing and releasing energy (power systems)are the two most significant factors inhibiting the development of efficientminiature mechanicalflying machines. From a systems per-spective, it appears that biological flyers achieve remarkable massfractions (Nature’s Great Flyers reference) (see Fig. 22) when com-pared to miniature mechanical flyers. Thus, future MAV designswill undoubtedly take some lessons from nature to achieve efficientoperating mechanical flying machines.

IV. Emerging MAV Research and Technology Trends

The next generation of MAV designs that show the greatest

promise are the ornithopter configurations.Up to now, therehas beenlittle research toward the development of these types of configura-tions. Researchers at Caltech and Aervironment, with its prototypeMicrobat, have placedtheir focus on the wing design, where flexiblethin membranes, similar to bat wings are used. Because reproducingthe complex kinematics of biological flyers is challenging, one of theprimary problems faced when using simplifiedmovements is theincreased drag in the upstroke. To reduce this drag, MEMs electro-static actuator valves have been incorporated to parylene wings inorder to control the flow through the membrane.73,74 Without elec-trostatic actuation air can move freely from one side of the skin tothe other side trough the vent holes. With actuation, these vent holesare sealed, and the airflow is controlled. The membrane behaves asa complete diaphragm. Actuation is done on the downstroke andstopped on the upstroke reducing the drag of the wings over theflapping cycles. To aid the design of ornithopters, one can againexamine the characteristics of biological creatures. Studies of birdsby Rayner 39 and Norberg67 have revealed the following scaling re-

lationships characterizing the flapping wingbeat frequency used toacquire thrust and lift:

f min = 2m−0.1667 (11) f max = 8m−0.333 (12)

f hummingbird = 1.32m−0.6 (13)

These expressions offer some insight into the potential range of

future practical flapping wing MAV designs. For insect-sizevehiclesin the milligram to tens of grams weight class, wing-beat frequencyscales with m−0.24, albeit with significant scatter in the data. Theseflapping frequency scaling relationships offer some guidelines for achieving flapping flight; however, the details lie in the developmentof efficient lightweight flapping mechanisms that harness similar wing-beat kinematics and unsteady aerodynamics found in insectsand small birds.

A. Flapping Mechanisms

To achieve flapping flight, researchers must develop monolithiclightweight devices that can generate large-scale kinematic angular motion with minimal mechanical effort. Insects and small birds ac-complish flapping flight in different ways but rely heavily on their

anatomy. In the case of an insect, thestructural components requiredfor flight derive from the morphological exoskeleton of insect bodydesign. The key element in the insect anatomy is the thorax. Flex-ibility is essential for the insect thorax, as internal muscular forcescause the thorax to deform and either directly or indirectly transmitforces to the wings. Elastic return of stored energy is also impor-tant to minimize the total effort required during a wing-beat cycle.Figure 23 shows a cross-sectional view of the thoracic segmentillustrating the intrinsic flight muscles that are used for flappingflight. It is believed that two different sets of thoracic muscles areprimarily responsible for complex wing motions. These muscles

a)

b)

Fig. 23 Cross section of thorax revealing indirect/direct flight musclesresponsible for insect flapping flight.

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a) Mockup of Robofly

b) 4-bar flapping mechanism

Fig. 24 University of California, Berkeley’s Robofly.74,75

act either directly on the wing base and sclerites of the auxiliaryapparatus65 or can act indirectly to move the wings via indirect tho-racic deformation. Although the details of the interaction of these

muscles that enable flapping flight remain a research topic in thebiological community, researchers at a number of institutions haveeither developed or are developing complex mechanical devices thatcan generate simultaneous pitching, flapping, and translatingmotionof a small-scale wing.

Recently, a team at the University of California, Berkeley, has de-veloped a piezoelectric wing-driven flap actuation mechanism75,76

that can generate the kinematic motion required for mimicking theaerodynamic performance of a fruitfly ( Drosophila). The mechani-cal equivalent is named “Robofly” and attempts to mimic the kine-matics of a fly. A conceptual view of this concept is displayed inFig. 24a. This vehicle was designed for a Reynolds-number flightregime of 150 and weighs approximately 350 mg. The flappingmechanism wasdesignedusing a four- andfive-barlinkage schemesdriven by piezoelectricactuators. A picture of oneof theearly mech-

anism designs is displayed in Fig. 24b. Unfortunately, this conceptwasnever able to achieve enoughlift force to support its own weighteven with tethered power.

DeLaurier and his students at the University of Toronto workingclosely with Kornbluh and his colleagues at SR International§ haveinvestigated the clap and fling unsteady aerodynamic mechanismcommonly found in insects andsmallbirdsto generate lift. Kornbluhattempted to develop a mechanism that generates the unsteady clap-fling motion using electroactive polymer actuators. These elas-tomeric actuators produce large stroke at reasonable frequencies,but require high voltage to drive them. The high voltage requiredby the actuators precluded their use in a flapping vehicle designcalled Mentor. Nevertheless, a larger than MAV scale flapping ve-hicle (∼500 g) that harnessed the clap-fling flapping motion was

§Data available online at http://www.erg.sri.com/automa-tion/actuators.html.

Fig. 25 MENTOR flying vehicle.

Fig. 26 Biomimetic rotating/flapping/pitching mechanism.§

developed with both electric and internal combustion-driven power.

The MENTOR vehicle was the first of its kind to fully hover under biologically inspired flapping wing propulsion (see Fig. 25).At the University of Maryland, the author and his students 77 de-

veloped a rather unusual flapping, pitching, and rotating device for evaluating the effects of unsteady rotor motion on the hover perfor-mance of small-scale rotorcraft designs. In the spirit of generatinglarge-scale kinematic motion, a biomimetic mechanical mechanismhas been developed, which rotates like a conventional rotary-wingconfiguration but is capable of undergoing large angular changes inits pitch and flap degrees of freedom at high frequency. The pitchand flap degrees of freedom can be independently controlled to gen-erate unsteady air loads. A conceptual view of this mechanism isdisplayed in Fig. 26. Preliminary results of unsteady pitching at 1/4perrev frequencies show a significant performance gain in the figureof merit near the onset of dynamic stall.

Finally, in the United Kingdom, Zbikowski (personal communi-cation) and his colleaguesat Cranfield (Shrivenham) have developeda flapping mechanism for evaluating the wide range of motion asso-ciated with flapping insect flight. However, there are little publisheddata on this device and its ability to mimic the unsteady physics thatinsects and small birds use to acquire efficient aerodynamic lift inhover and forward flight.

Intricate actuation of the flapping mechanism is another potentialapplication for smart material technology; the use of conventionalelectric motors requires the transformation of rotating motion intoa linear reciprocating movement. This involves a certain mechan-ical complexity, added weight, and more important, a fixed move-ment pattern, that makes it difficult to changethe flapping amplitudeand hence restricts the control schemes and experimental degreesof freedom. Smart linear actuators can alleviate these problems byprovidingcontrollable stroke amplitude at high frequencies. In addi-tion, theefficient transduction of mechanical power will be essentialfor achieving long endurance and reliable flapping flight.

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Fig. 27 Aerovironment WASP MAV.81

B. Morphological Shape Changes

Although large-scale kinematic flapping motion is of importance,radical shapechangesalong the span,chord, and thicknessdirections

could also be quite significant to small-scale vehicle performance.For example camber modification along the chord could be used toadapt the airfoil to different flight conditions. In addition wing-tipeffects can significantly affect the aerodynamic performance in-cluding the wake structure several chord lengths removed from theunsteady vortex interactions. So the simultaneous use of shape trans-formations could provide enough control authority to command theMAV. Towards this goal, morphing structures technology78−81 is inits initial stages of development by researchers at NASA LangleyResearch Center at a much larger length scale; however, such tech-nology might find its way into small-scale flying machines.

C. Multifunctional Structures

Another emerging technology is the integration of additional

functionality into a wing structure, for example, including beingable to support air loads of an air vehicle wall also providing power,sensing, or communication. Recently, researchers from the NavalResearch Laboratory in conjunction with Aerovironment and Tel-cordia flight tested a multifunctional wing that was integrated withconformal lithium-ion battery technology under DARPA¶support.The wing while acting as the main lifting surface also served as asource of battery power in the form of conformal lithium-ion bat-tery packs. A picture of the MAV is displayed in Fig. 27. The MAVweighed approximately 150 g and had a wing span of 14 in. Thefixed-wing MAV had a measured flight time of 107 min. Thus far,this has exceeded the endurance of all other fixed-wing/ and ro-tary air vehicles listed in Table 3. Other opportunities for the use of multifunctional structures technology in MAV applications includeconformal antennas mounted to the aerodynamic wing or fuselage

of an MAV to provide line-of-sight communication to the ground,as well as the use of self-consuming or “autophagous” structuresthat provide aerodynamic lift but are consumed as power for flightis extracted from the wing material.

D. Emerging Propulsion/Power Technology

Emerging propulsion technologies‡ include the following:1) Beaming energy to the aircraft for conversion to electricity us-

ing either microwaves or lasers eliminates the need to carry propel-lant onboard, but requires a tremendous transmit-to-receive power ratio (microwaves) or very precise pointing (lasers) and limits flightto within line of sight of the power source (both). Microwave beam-ing would take 100 kW (134 hp) of transmit power to run just amicro-UAV at a range of 0.6 miles, let alone a more substantiallysized aircraft, whereas a laser would only require around 40 W

¶Data available online at http://www.darpa.mil/body/news.html.

(0.05 hp) of power. Recently, a team of researchers from NASAMarshall Space Flight Center and the Universityof Alabama demon-strated that a small-scale aircraft can fly under laser power.

2) Reciprocating chemical muscles (RCMs) are regenerative de-vices that use a chemically actuated mechanical muscle (ionomers)to convert chemical energy into motion through a direct, noncom-bustive chemical reaction. Power generatedvia an RCMcan be usedfor both propulsion (via wing flapping) and powering of onboardflight systems. RCM technology could power future generations

of MAVs, providing vertical takeoff and landing as well as hover capabilities.3) Improvements in electric battery technology such as the de-

velopment of lithium sulfur batteries have higher specific energy(>400 W · h/kg) than lithium ion technology.

V. Notable Firsts in the MAV Flight Regime

Similar to the 100-year history of powered human flight, therehave been several notable firsts that have been achieved by re-searchers working in the MAV field based on DoD’s MAV designrequirements. An incomplete, but growing list includes:

1) The first battery-powered electric motor open-loop controlledflapping flight was by Microbat-CalTech/Aerovironment.

2) The longest endurance (30 min of a fixed-wingMAV by Black Widow Aerovironment.

3) The first autonomous MAV flight (global-positioning-systemwaypoint navigation) was by Microstar–Lockheed Martin.

4) The first open-loop controlled hovering flight of a biologicallyinspired flapping vehicle was by MENTOR-SRI.

5) The longest-endurance flapping flight (


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new vehicles cost? These questions are sure to be answered over thenext decade by researchers working at the intersection of biologyand engineering. Thus, in order to make future advances in MAVtechnology it might be wise to heed the words of the Wilbur Wright:“It is possible to fly without motors, but not without knowledge andskill.”

Acknowledgments

The author would like to thank several colleagues at the Uni-

versity of Maryland including Chris Cadou for providing informa-tion on propulsion/power issues at small scale. A special thanks toJ. Gordon Leishman for his insights on low-Reynolds-number flowphysics and performance of rotary-wing airfoils. Finally, the au-thor would also like to thank Inderjit Chopra and Norman Wereley,as well as graduate students, Felipe Bohorquez, Jayant Sirohi, PaulSamuel, Joseph Conroy, Ron Perel, Falcon Rankins, Berinder Singh,Sylvain Morel-Fatio, and Julie Blondeau for numerous discussionson micro-air-vehicle technology. This work was supported in partby the National Rotorcraft TechnologyCenter under Contract Num-ber NC-2944, with Yung Yu serving as contract monitor, and by theArmy Research Office, under Contract Number W911NF0410176,with Gary Anderson serving as contract monitor.

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2006-Challenges Facing Future Micro-Air-Vehicle Development

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