flapping-wing micro air vehicles - wit press

Flapping-wing micro air vehicles

M.F. Platzer & K.D. JonesAeroHydro Research & Technology Associates, Pebble Beach, CA, USA.

Abstract

This paper presents an overview of the two types of flapping-wing micro air vehicles that havebeen developed in recent years. Biomimetic designs imitate nature while biomorphic designs aremerely inspired by nature, but incorporate other design features not found in nature. A recentsuccessful biomimetic flapping-wing micro air vehicle—the Microbat, developed by the Aero-Vironment Company—is described. Also, the major design features of a biomorphic micro airvehicle, developed by the authors, are presented in some detail. This design incorporates a fixedwing and two flapping wings arranged very close behind the trailing edge of the fixed wing.

1 Introduction

Flapping-wing model aircraft date back at least to 1874, when Alphonse Penaud built a rubber-band-powered ornithopter. The great aeronautical pioneer Otto Lilienthal [1] was fascinated bythe flight of birds and tried to make them the basis for his flight experiments in the 1890s. Thecourse of aeronautical history has shown that fixed-wing and rotary-wing aircraft have distinctadvantages over flapping-wing aircraft.Yet, the challenge of developing flapping-wing air vehicleshas continued to attract model airplane enthusiasts and aeronautical engineers. For example, theX-wing canard flapper, built in 1985 by Frank Kieser [2], set a new record for indoor free-flightendurance and variations of this design still hold the record. This highly innovative design isshown in Fig. 1. In 1992 DeLaurier & Harris from the University of Toronto [3] successfully flewan engine-powered remotely piloted ornithopter for almost 3 min and DeLaurier still hopes to flya manned ornithopter [4].

The initiative launched by the Defense Advanced Research Projects Agency (DARPA) in 1996to develop micro air vehicles has given new impetus to flapping-wing research because flap-ping wings may be superior to rotary-wing or propeller-driven air vehicles at the small scalerequired by DARPA. For example, propellers generate torque and create helical slipstreams, bothof which degrade vehicle performance and handling. Moreover, the efficiencies of small-scalepropellers and rotors are adversely affected by the flow separation effects encountered at low

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394 Flow Phenomena in Nature

Figure 1: Frank Kieser’s X-wing flapper [2].

Reynolds numbers. Flapping wings have larger actuator areas and therefore lower actuator load-ings, thus enabling better efficiencies.

In principle, one distinguishes two types of flapping-wing vehicles, i.e. biomimetic and biomor-phic designs. Biomimetic designs imitate nature while biomorphic designs are merely inspiredby nature, but incorporate other design features not found in nature.

2 Biomimetic designs

Until Frank Kieser’s design in 1985, virtually all flapping-wing fliers were biomimetic. TheAeroVironment Company in Simi Valley, CA, took this one step further by exchanging the rubberband motor for an electric motor, producing the elegant Microbat, shown in Fig. 2. The latestversion of the Microbat uses a two-cell lithium polymer battery, a three-channel radio, and hasa 23 cm span and total weight of 14 g; it has made 25 min flights. While many aeromodelingenthusiasts had previously built successful gas and electric-powered ornithopters, the Microbatseems to be the smallest radio controlled biomimetic flapper.

3 Biomorphic designs

As already mentioned, Frank Kieser deserves the credit for a very original design. We took adifferent approach in our design, shown in Fig. 3. It consists of a fixed wing and two flappingwings arranged very close behind the trailing edge of the fixed wing. Clearly, this is a configurationnot found in nature, although insect biplanes existed many millions of years ago, albeit withouta fixed wing. As noted by Wootton & Kukalova-Peck [6], the Homoiopteridae were an ancientgroup of large, sometimes gigantic insects which had forewings and hind wings that overlappedextensively. The same appears to have been the case for some members of the family Lycocercidae.No such overlapping is found any longer in modern insects, leading to the interesting questionfor the reason for the disappearance of biplane insects.



Flapping-Wing Micro Air Vehicles 395

Figure 2: AeroVironment’s Microbat [5].

Figure 3: Authors’ model with a fixed wing and two flapping wings.

Our own reason for choosing the biplane configuration with wings that flap in counterphase wastwofold. First, the common center of gravity of wings that are flapping in counterphase remainsunaffected by the flapping motion of the individual wings in contrast to the use of single wings.Hence the vehicle is dynamically balanced and pitch oscillations of the whole vehicle are eli-minated. This provides an inherently more stable platform for use as a surveillance vehicle.Birds have evolved to compensate for these oscillations and their neck is highly articulated suchthat their head is inertially stable for improved vision. The second equally important reason forchoosing the biplane configuration stems from the aerodynamic benefits that accrue from its use.The lift enhancement that is caused by flying a fixed wing close to the ground is well known.




Similarly, birds flying close to the water surface achieve an increase in thrust and propulsive effi-ciency by flapping their wings in the ground effect. Using the panel code described in reference[7] these benefits can be quantified and are shown in Fig. 4. The thrust coefficient and the propul-sive efficiency are plotted in the upper and lower figures, respectively, as a function of reducedfrequency (defined as the product of the circular frequency of flapping and the wing chord dividedby the flight speed). It is readily seen that flight in the ground effect, or equivalently flight withtwo wings flapping in counterphase, is quite beneficial. Furthermore, birds have no choice but toflap their wings such that the flap amplitude varies along the span from zero amplitude close to thebody to maximum amplitude at the wing tips. Hence the inner part of the wing contributes littleto thrust generation, but the bird has no alternative. A wing that flaps with a constant amplitudealong the span would seem to be preferable. Therefore, we chose the arrangement shown in Fig. 3which makes it possible to drive the wings with constant amplitude along the span.

Finally, we decided to make use of the fact that flapping wings act as two-dimensional propellers.Arranging conventional propellers along the trailing edge of a wing energizes the flow over theupper wing surface because of the propellers’ suction effect. As a consequence, wing stall isdelayed to a higher angle of attack than possible with a more conventional arrangement. In contrastto conventional propellers, flapping wings are ideally suited for such an arrangement.An additionalargument for its use was the greater tendency toward flow separation in micro air vehicles becauseof the much lower flight Reynolds numbers encountered in such vehicles (typically 30,000 for themain wing in the model shown in Figure 3). We explored the effectiveness of flapping wings insuppressing flow separation in a series of water tunnel tests. The cusped trailing edge of an airfoiltogether with a small airfoil mounted close to the trailing edge is shown in Fig. 5. This smallairfoil could be oscillated in plunge. The flow over the cusped trailing edge is fully separated dueto the large adverse pressure gradient in this region. However, the flow could be made to reattachby the plunge oscillation of the small airfoil, as shown in Fig. 5. Therefore, we decided to mountthe two flapping wings very close to the trailing edge of the fixed wing.

Figure 4: The predicted benefits of the ground effect [8].




It is seen that this decision process led us to a vehicle configuration that is truly biomorphic.At first appearance it would seem that the design separates the lift and thrust generators, as iscommonly done in conventional aircraft. However, all three wings form a system that togetherefficiently produce lift and thrust, but their efficiency depends on mutual interference. The biplaneflapping wings need each other in order to get the benefit of flight in the ground effect, and the mainwing needs the downstream flapping wings in order to maintain the attached flow for efficient lift.There are perhaps other bio-inspired facets of the design. For example, many birds and insectsdo not actively control the angle of attack or twist of the wings, but rather they are deflectedmerely by aerodynamic loading—an aeroelastic deflection. Similarly, the flapping wings on ourmodel are connected to the flapping mechanism with tiny elastic members which allow for passiveaeroelastic pitching. A wind tunnel variant of the model in Fig. 3 was built and is shown in Fig. 6.Using flow visualization with a smoke wire, the effectiveness of the flapping wings in suppressingflow separation is seen in Fig. 7, where the wings are not flapping in the image on the left, withmassive flow separation from the leading edge, and with the wings flapping in the image on theright, the flow is seen to be almost completely attached. Impressively, the video frame shown onthe right is just four frames after flapping started, with a flapping frequency of about 30 Hz, whichcorresponds to four flapping strokes or about 1/8th of a second to transition from fully separatedto fully attached flow.

Figure 5: Flow reattachment due to airfoil oscillation [8].

Figure 6: The wind tunnel test model [8].




Figure 7: Flow separation control by means of flapping wings [8].

4 Design and development of the fixed/flapping-wing micro air vehicle

For the implementation of the above-described design concepts into a viable flying vehicle theenergy source and the motor are critical issues. Fortunately, the requirements of the cell phoneindustry for batteries of high energy density and for tiny electric motors (so-called pager motors—the motors which vibrate cell phones and pagers) pushed the state of the art to the point where itbecame feasible to start assembling a flight vehicle. In late 2002, the available lithium polymercells weighed 4.2 g, the motor/gear assembly about 2.5 g, the DC–DC step-up circuit needed toincrease the battery voltage to 5 V about 0.7 g and the three-channel receiver for the radio gearabout 2 g. The weight of the structure itself was estimated to be about 5 g, yielding a total weightof about 14 g. Using the best aerodynamic data for wings of this scale, a main wing with a spanof 30 cm and a chord of 14.5 cm was built. The first flight took place in December 2002, lastingabout 3 min, when the unguided model landed high in a tree. The flight speed was only about2 m/s, but the ability of the micro air vehicle to fly at very high angles of attack without stallingfully confirmed the effectiveness of the above-described flow separation control method. With thepower off, the micro air vehicle stalled quite easily in response to gusts, but turning the flappingwings on restored control right away. A second micro air vehicle was built a few months later,which had a slightly smaller span of 27 cm and a weight of 13.4 g. The third micro air vehicle wasstill slightly smaller with a 25 cm span and a weight of 12.4 g. The smallest micro air vehicle, builtfor the 8th International MAV Competition, had a span of 23 cm and a weight of 10.5 g. The flightspeeds of all the models varied between 2 and 5 m/s, and battery capacities allowed for flights upto 20 min duration. The very low flight speed made the model ideal for flight in confined areas.We routinely flew them in lecture halls as part of our presentation.

5 Summary and outlook

Stimulated by DARPA’s initiative in 1996 significant progress has been achieved in the develop-ment of micro air vehicles. In this paper we have described two flapping-wing micro air vehicleswhich have reached flight status, i.e. the biomimetic Microbat, designed and developed by theAeroVironment Company, and our biomorphic fixed/flapping-wing micro air vehicle. Both vehi-cles benefited from the advances made in the fixed and flapping-wing aerodynamics for micro airvehicle applications, as reviewed in reference [9]. However, both developments were still largelybased on ‘cut and try’ methods. Hence, much more experimental and computational work in the




field of low Reynolds number aerodynamics is required in order to optimize future designs. Theamazing recent advances in battery and motor technology, driven by the cell phone industry, havemade it possible to achieve success. These advances, together with the further miniaturizationof the avionics components, are certain to continue. Hence the design and development of newmicro air vehicles is likely to pose many further exciting challenges for the aeronautical engineer.

References

[1] Lilienthal, O., Der Vogelflug als Grundlage der Fliegerkunst, R. Gaertners Verlagsbuch-handlung: Berlin, 1992.

[2] Ornithopter Media Collection, published on the web at http://www.ornithopter.org/indoor.shtml

[3] DeLaurier, J.D. & Harris, J.M., A study of mechanical flapping wing flight. AeronauticalJournal, 97(968), pp. 277–286, October 1993.

[4] Larijani, R.F. & DeLaurier, J.D.,Anonlinear aeroelastic model for the study of flapping wingflight (Chapter 18). Progress in Astronautics and Aeronautics, Vol. 195, American Instituteof Aeronautics and Astronautics, 2001.

[5] Keennon, M.T. & Grasmeyer, J.M., Development of the black widow and microbat MAVsand a vision of the future of MAV design, AIAA/ICAS International Air and Space Sympo-sium and Exposition—The Next 100 Years, AIAA 2002-3327, July 2003.

[6] Wootton, R.J. & Kukalova-Peck, J., Flight adaptations in Palaeozoic Palaeoptera. BiologicalReview, 75, pp. 129–167, 2000.

[7] Jones, K.D., Dohring, C.M. & Platzer, M.F., Experimental and computational investigationof the Knoller-Betz effect. AIAA Journal, 36(7), pp. 1240–1246, 1998.

[8] Jones, K.D., Bradshaw, C.J., Papadopoulos, J. & Platzer, M.F., Improved performance andcontrol of flapping-wing propelled micro air vehicles, AIAA 2004-0399, January 2004.

[9] Mueller, T.J., Fixed and Flapping Wing Aerodynamics for Micro Air Vehicle Applications,Progress in Astronautics and Aeronautics, Vol. 195, American Institute of Aeronautics andAstronautics, 2001.



flapping-wing micro air vehicles - wit press

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