Ravi et al. Engels et al. The instantaneous aerodynamic forces, however, showed larger fluctuations, consistent with the flight instabilities observed in freely flying bees. For a hawkmoth, however, destabilizing effects on both yaw and roll were observed in feeding flights in vortex streets past vertical cylinders of different size [ 64 ].
Responses to large coherent structures were studied for hawkmoths flying in a vortex chamber [ 64 ] and hummingbird feeding flights [ 65 , 66 ], which indicate consistent asymmetric changes of the wing kinematics. Vance [ 67 ] carried out a comparative study on the recovery response, i.
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Ristroph et al. The trade-off between stability and manoeuvrability is considered axiomatic, however, flapping winged flyers have the potential to maintain both mutually opposing properties. During swimming and in flapping flight, reciprocating motion of fins and wings are necessary; during these motions the lifting surface produces forces in directions that are not always aligned in the direction of intended force production and tend to cancel out due to the bilateral symmetry.
Recent research on the dynamics of fish shows that the production of such antagonistic force increases manoeuvrability without sacrificing stability [ 69 ]. Similarly, the wing of insects and birds produce large instantaneous forces while flapping that are not always along the direction of mean displacement yet they continue to maintain equilibrium since these forces are cancelled by equal and opposite forces produced by the contralateral wing.
Modulation of these forces through bilateral asymmetry can produce torques that can be used for performing corrective manoeuvres in unsteady winds or to take evasive flight when nearing obstacles [ 70 ]. Additionally, flapping wings have been shown to be more resistant to gusts and freestream turbulence. Flapping-wing propulsion, inspired by flying animals, possesses potential of high lift-generating capability under low-speed flight conditions and may provide an innovative solution to the dilemma of small autonomous MAVs.
Two key issues in the biomimetic MAV design are flapping-wing mechanism and weight limitation. High power and high frequency are essential for the flapping-wing mechanism to create sufficient lift force. Light wing-body is also a must, which, however, restricts the synthesis of the actuators, power sources or materials, constraining the wing kinematic, frequencies, size or available aerodynamics forces.
Therefore, the main challenge in flapping MAV design and manufacturing, in particular, for insect or smaller size, is how to realize wing kinematics within the crucially restricted mass [ 71 ]. Electric motors and gear-crank linkage mechanisms are normally used to generate flapping-wing motions through converting motor's rotational motion to linkage's straight motion.
For larger MAVs [ 11 ], the payload can afford some avionics such as vision systems or autonomous control systems; and by processing the captured images with a ground station, autonomous obstacle avoidance can be realized. In these electric MAVs, the flapping motion is generated in terms of 1 degree of freedom, which can create feathering motion AoA owing to passive deformation of the flapping wing.
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Manoeuvring flight is commonly achieved with a rudder or elevators of the tail wing as used in fixed-wing or rotary-wing aircraft. There are also MAVs capable of actively controlling feathering motion of the wing so as to realize manoeuvring without rudders or elevators [ 71 ]. Despite the challenges for small-scale propulsion and miniaturization, researchers have successfully reproduced the bioinspired flight systems in several different ways, which may be classified in three prototypes of the four-winged or X-wing DelFly [ 9 , 10 ], the hummingbird-sized Nano-Hummingbird [ 11 ] and the insect-size Robobee [ 13 ].
Flapping micro air vehicles. Upper panel Relationship of wing span versus mass in flapping air vehicles powered by DC motor, piezoceramic actuator, rubber band, human power, etc. Lower panel Three prototype bioinspired flapping micro air vehicles: X-wing MAV [ 9 , 10 ], Nano-hummingbird [ 11 ] and Robobee [ 13 ].
The X-wing MAV has four flexible wings as two paired wings and uses the clap-and-fling mechanism achieved by the gear-crank linkage system [ 9 , 10 ]. Aerodynamically, the X-wing MAVs benefit from three clap-and-fling cycles in a wing beat, capable of augmenting the aerodynamic force generation. One of the most successful bioinspired MAVs so far is the two-winged Nano-Hummingbird [ 11 ] with a wingspan of All necessary components including actuators, power source, flapping mechanism and sensors for flight control are put on board.
Stable hovering and agile manoeuverability are performed by active modulation of the flapping wings. The insect-size MAV, Robobee [ 13 ] presents one of the smallest flapping MAVs, with a wingspan of 30 mm, a weight of 80 mg and a flapping frequency of Hz. Robobee is equipped with high-power density, piezoelectric motors, which however, restricts it to achieve a real free-flight. Manufactured by a novel methodology capable of rapidly prototyping articulated, flexure-based sub-millimetre mechanisms, the flyer can achieve a tethered but unconstrained stable hovering and basic controlled flight manoeuvres.
Enhancement of the energy transformation from motor to mechanical flapping-wing system is also very challenging but remains yet poorly understood, which may significantly regulate the wing stroke dynamics and hence passively enhance versatile aerodynamic force generation. Harne et al.
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A common structure of biomimetic flapping wings so far is composed of a single thin polymer film supported with a main leading-edge frame and some diagonal frames [ 9 — 13 ]. The configuration of the polymer film and the supporting framework without inner actuators is inspired by the wing configuration of insect wings and suitable for spanwise torsional deformation and weight saving.
For the wing membranes, polymer films are widely used because of its thin and lightweight structure, high mechanical strength and good availability. Such polymer films normally show similar properties of material with those of insect wings and bird feathers in terms of Young's modulus, which for the artificial wings such as polyester [ 9 , 13 ], polyimide and polyparaxylene [ 74 ] commonly varies from 2 to 5 GPa, in comparison with a range of 0.
While off-the-shelf the CFRP rods or strips have been previously glued manually onto the wing film, recent research is devoted to improve fabrication accuracy and to expand design space of the wing framework particularly at insect size. Microelectromechanical systems MEMS photolithography and etching process were successfully applied to a titanium-alloy wing frame while integrating to a Parylene film [ 73 ].
Alignment of strips of carbon fibre prepreg i. Further remarkable achievements are seen in MEMS fabrication of layered UV curable resin [ 80 ], as well as precise alignment and lamination of laser-cut CFRP framework, adhesive film and polymer film [ 13 , 80 ]. Such advanced micro-fabrication technologies become more important as the scale of bioinspired system shrinks down to insect size, less than 10 g, where feature size of the wing is a few hundred micro-metres or less and errors of hand working may cause fatal variation in flight performance.
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Another challenge in fabricating insect-inspired wings is how to make realistic three-dimensional shapes of insect wings, e. Recently, in order to add anisotropy in both flexural and tensional stiffness to the films, Tanaka et al. They found that tuning the micro-wrinkles enable achievement of fine stiffness control of the wing film and hence improvement on the flapping-wing aerodynamic performance.
Small autonomous MAVs normally fly at low attitudes and need complex levels of sensors to perform stable and safe trajectories. Insect-inspired sensing systems may offer an alternative solution because insects are equipped with a rich sensory system to enable them to stay aloft, perform manoeuvres and navigate long distances.
The sensory systems that mediate flight dynamics in insects can generally be divided into either inertial or visual sensors: inertial sensors halteres, antennae, etc. Inertial sensors are generally rate-based, and recent research shows that rotation rate feedback can be adequate for attitude maintenance [ 50 , 88 , 89 ]; they have low latency and thus enable insects to sense the perturbation and enable motor commands to effectuate corrective manoeuvres.
For instance, flight stabilization in fruit flies requires active response times at around 13 ms [ 68 ]. The vast majority of current unmanned aerial vehicles possess inertial measurement units IMU that combine gyroscopic sensors, magnetometers and accellerometers to stabilize flight. Over the last decade, the rapid advancement of MEMS technology has resulted in significant miniaturization of IMUs and thus facilitated the demonstration of autonomous flight stabilization on small and lightweight platforms—this includes the variety of off-the-shelf palm-sized drones.
Miniature flapping-wing platforms such as Robobee still rely on external inputs for flight stabilization [ 13 ]. Some designers have resorted to passive stabilization to alleviate the necessity for onboard IMU such as the Delft Fly [ 9 ], using an X-wing flapping configuration.
Processing the motion of images over the retina optic flow is generally considered the means through which insects navigate in the environment.
Recent research has shown that insects actively shape the temporal structure of their visual input by employing prototypical flight manoeuvres, especially to separate translational from rotational optic flow to facilitate discerning spatial information about the surroundings, see [ 90 ] for review.
The implementation of vision in aiding flight of unmanned systems has been somewhat achieved through bioinspired and mimetic approaches. The insect vision based on stereopsis and object identification has inspired some visual sensing platform in flying robots that employ horizon stabilization based flight control [ 89 , 91 , 92 ]. Implementing visual processing in MAVs is still challenging but some recent research has demonstrated its feasibility [ 93 , 94 ]. Such systems are very beneficial in indoor environment where the terrain is complex and GPS-aided navigation is unfeasible.
Flying animals are the most sophisticated and ultimate flyers on Earth, and bioinspired flapping flight systems as an integrated system [ 95 ] offer an alternative paradigm for MAVs when scaled down to insect and bird size, which, however, normally brings low-speed aerodynamics and flight control challenges in achieving autonomous flight. In the aspect of low Reynolds number aerodynamics in flapping-wing flight, we have a clear picture of the prominent features of the flapping-wing-induced large-scale vortex dynamics and its correlation with the force generation over a broad range of size and species of insects.
It has been identified that insects use an integration of multiple lift-enhancement unsteady mechanisms involving the clap-and-fling, the leading-edge vortex-based delayed stall, the rotational lift and the wake capture, which work and function differently at different sizes.
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Flexible wings in flapping flight that deform owing to interaction between aerodynamic and inertial forces in terms of span-wise bending and chord wise twist, can generate larger forces with power savings or low power consumption and hence achieve better aerodynamic performance. Wing-hinges in insects' flapping wings can also be considered capable to induce passive feathering motion due to interaction between the aerodynamic, inertial and elastic restoring forces. Analysis of passive dynamic flight stability and active control in flapping flight brings challenges in resolving a nonlinear flight dynamic system, which has been mostly performed with the averaged model-based linear theories or nonlinear models, however, both implying that flight instability probably exists in most flying insects.
A closed-loop control flight is probably achieved through integration between the passive open-loop flight dynamics and complicated motor systems in response to multimodal sensory inputs and the coordination of multiple muscles across the body, which still remains unclear. Flapping flight in unsteady natural and man-made environments is most challenges for autonomous flight but insects seem to benefit from both inherent passive and active mechanisms in stabilization and control.
Despite the challenges for small-scale propulsion and miniaturization, researchers have successfully reproduced the bioinspired flapping-wing MAVs in insect and bird scale. It is foreseen that rapid developments in biomimetic wing design and MEMS-based fabrication methods, as well as insect-inspired inertial and visual sensors along with bioinspired systematic design strategies will bring more innovative designs. Conventional fixed-wing and rotary aircraft designs are normally performed with rigid wings and bodies designed separately, and with aerodynamic theories and flight control laws developed separately.
Bioinspired flight systems are, however, an integration of different flexible structures at different levels of body, wing, wing-hinge, musculoskeleton, sensors and motors. We still know less about how these flexible structures work interactively and complementarily with active mechanisms through a closed-loop of the inner working system and external mechanical system, to achieve a systematically efficient stabilization and control in flapping flight.
This will further bring breakthroughs in integrating aerodynamics and control strategies and hence a paradigm-shift in biomimetic systematic design and miniaturization for insect-scale autonomous MAVs with capabilities to achieve controlled flight autonomy and to perform a variety of complex manoeuvres. All authors were involved in developing the scope and preparing the contents of the paper.
All authors gave final approval for publication. National Center for Biotechnology Information , U. Author information Article notes Copyright and License information Disclaimer. Accepted May This article has been cited by other articles in PMC. Abstract Insect- and bird-size drones—micro air vehicles MAV that can perform autonomous flight in natural and man-made environments are now an active and well-integrated research area. Keywords: aerodynamics, bioinspired flight system, biomimetics, flight control, micro air vehicle, sensing.
Introduction Flying animals that power and control flight by flapping their wings perform excellent flight stability and manoeuvrability, while steering and manoeuvring by rapidly and continuously varying their wing kinematics [ 1 , 2 ]. Open in a separate window. Figure 1. Figure 2. Figure 3.
Related An Introduction to Flapping Wing Aerodynamics (Cambridge Aerospace Series)
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